Airport and Air Traffic Control System

January 1982

NTIS order #PB82-207606
Library of Congress Catalog Card Number 82-600545

For sale by the Superintendent of Documents,

U.S. Government Printing Office, Washington, D.C. 20402
Foreword

Air transportation is expected to continue growing during the next two decades. In

dealing with this growth it will be important to ensure safety and minimize the costs of

the system to the Government and airspace users. Large investments are now anticipated in both airports and air traffic control systems, investments that require unusually long leadtimes. For these reasons the House Committee on Appropriations has requested that OTA conduct an assessment of airport capacity and related air traffic control issues.

This subject is, more than most, a moving target. There have been rapid changes

in Federal Aviation Administration (FAA) plans in recent years, and these plans have

been further complicated by airline deregulation and the aftermath of the Professional

Air Traffic Controllers Organization strike. These events affect future plans because

they influence the rate of growth and where that growth will occur. There also continue to be rapid and significant changes in the aviation, telecommunications, and

data-processing technologies on which the system relies. In addition, these plans are

coming before Congress during a period of increasing budgetary constraints.

This assessment is intended to provide a perspective on both airport development

aid and FAA’s proposed air traffic control system modernization. In both areas there

are questions of how much improvement will be needed, how soon it will be needed,

and how the funding of improvements will be allocated among airspace users.

Director

. . .

Ill
Airport and Air Traffic Control Advisory Panel Members

Raymond L. Bisplinghoff, Chairman

Vice President and Director of R&D, Tyco Laboratories

Jesse Borthwick

Executive Director

National Association of Noise Control Officials

Secor D. Browne

Secor D. Browne Associates, Inc.

Jack Enders

President

The Mitre Corp.

Matthew Finucane

Aviation Consumer Action Project

William T. Hardaker

Assistant Vice President, Air Navigation/Traffic

Control

Air Transport Association

William Horn, Jr.

National Business Aircraft Association, Inc.

Jack D. Howell

Air Line Pilots Association, International

Alton G. Keel, Jr.

Assistant Secretary of the Air Force

Research, Development and Logistics

Clifton A. Moore

General Manager

Department of Airports

City of Los Angeles

Thomas L. Oneto

Planning Officer

Aircraft Owners and Pilots Association

Robert E. Poli

President

Professional Air Traffic Controllers Association

Gilbert F. Quinby

Consultant

Janet St. Mark

President

SMS Associates

David S. Stempler

Airline Passengers Association

Richard Taylor

Vice President

Boeing Commercial Airplane Co.

David Thomas

General Aviation Manufacturers Association

iv
Airport and Air Traffic Control System Project Staff

John Andelin, Assistant Director, OTA

Science, Information, and Natural Resources Division

William Mills, Project Director

Marsha Fenn M. Karen Gamble Larry L. Jenney Paul B. Phelps Zalman Shaven

Contractors

Adib Kanafani, Institute of Transportation Studies, University of California at Berkeley

Vincent Volpicelli, Port Authority of New York and New Jersey

Jerry D. Ward

Robert Simpson, Flight Transportation Laboratory, Massachusetts Institute of Technology

John Heritage, Editor

R. Bryan Harrison

OTA Publishing Staff

John C. Holmes, Publishing Officer

John Bergling Kathie Boss Debra M. Datcher Joe Henson
Contents

Chapter

1. Executive Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2. Introduction and Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. The National Airspace System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4. Aviation Growth Scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

S. Technology and the Future Evolution of the ATC System . . . . . . . . . . . . . . . . . . .

6. Airport Capacity Alternatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7. Policy Implications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page

3

9

25

45

67

101

125
ACRONYMS

AATF

ACARS

ACAS

ADAP

AERA

ANCLUC

ARINC

ARTCC

ARTS

ASR

ATA

ATARS

ATC

ATCRBS

BCAS

CDTI

CFC

DABS

DARC

DME

DOD

DOT

DPI

F&E

FAA

FAR

FSS

GA

GPS

ICAO

IFR

Airport and Airways Trust

Fund, trust fund

ARINC /Communications Addressing Reporting System

Airborne Collision Avoidance

System

Airport Development Aid Program

automated en route air traffic

control

airport noise comparability and

land use

Aeronautical Radio, Inc.

air route traffic control center

Automated Radar Terminal

System, a computer-driven

display system used in terminal areas

airport surveillance radar

Air Transport Association

Automatic Traffic Advisory

and Resolution Service

air traffic control

Air Traffic Control Radar

Beacon System

Beacon Collision Avoidance

System

cockpit display of traffic information

central flow control

Discrete Address Beacon System (Mode S)

Direct Access Radar Channel

distance measuring equipment

Department of Defense

Department of Transportation

disposable personal income

facilities and equipment

Federal Aviation Administration

Federal Air Regulation

flight service stations

general aviation

Global Positioning System

International Civil Aviation

Organization

Instrument Flight Rules

ILS

INS

ITU

MLS

Mode S

NASCOM

NAS

NASP

NOTAMs

O&M

OMB

PANCAP

PATCO

PIREP

PMS

PSR

RCAG

RE&D

ROI

RNAV

SACDRS

SMSA

SSR

TACAN

TCA

TCAS

TRACON

TRB

Tri-Modal BCAS

VFR

VOR

VORTAC

Instrument Landing System

inertial navigation system

International Telecommunication Union

Microwave Landing System

a digital data link system

(formerly DABS)

National Airspace Communications System

National Airspace System

National Airport System Plan

Notices to Airmen

operation and maintenance

Office of Management and

Budget

practical annual capacity of an

airport

Professional Air Traffic Controllers Organization

Performance Measuring System

primary surveillance radar

remote communication airground

research, engineering, and

development

return on investment

area navigation

Standard Air Carrier Delay

Reporting System

Standard Metropolitan Station

Area

secondary surveillance radar

Tactical Control and Navigation System

terminal control area

Traffic Alert and Collision

Avoidance System

terminal radar approach control

Transportation Research Board

a variation of the Beacon Collision Avoidance System

Visual Flight Rules

very high frequency omnirange

transmitters

A TACAN colocated with a

VOR station

. . . Vlll
Chapter 1

EXECUTIVE SUMMARY
Contents

Page

Aviation Growth Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Airport Capacity Alternatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Air Traffic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Funding Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Response to Future Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Chapter 1

EXECUTIVE SUMMARY

.

The National Airspace System includes about

6,500 public-use airports connected by a network of air routes defined by navigational aids.

Aircraft operating along these routes and in terminal areas near airports are monitored and controlled by a system of ground-based surveillance

and communications equipment—the air traffic

control (ATC) system—operated by the Federal

Aviation Administration (FAA).

In 1980, the 435 airports with FAA towers

handled some 180,000 takeoffs and landings per

day, or roughly 66 million per year, of which 74

percent are general aviation flights and 4 percent

are military. The remaining 22 percent of operations are commercial flights (air carrier, commuter, and air taxi) and are heavily concentrated in a few large airports. The 66 top airports

handle 77 percent of commercial operations and

88 percent of passenger enplanements; the 10

largest handle 33 percent of operations and 47

percent of passengers.

This concentration of air traffic at a few large

hubs creates congestion and delay, which in turn

increases airline operating costs and, ultimately,

the cost of air travel for the public. As air traffic

and fuel prices increase, the cost of these delays

will be magnified. General aviation users of major hubs also feel the effects of delay in the form

of access restrictions imposed during peak hours

to deal with airport congestion.

Concern about these problems, and about the

feasibility and cost of the proposed solutions,

prompted the House Committee on Appropriations (Subcommittee on Transportation) to request that OTA undertake an assessment of airport and terminal area capacity and related ATC

issues. The Senate Committee on Commerce,

Science, and Transportation endorsed the request of the House Committee on Appropriations, which directed OTA to concentrate on

four major topics:

● scenarios of future growth in air transportation;

• alternative ways to increase airport and terminal area capacity;

● technological and economic alternatives to

the ATC system modifications proposed by

FAA; and

• alternatives to the present ATC process.

OTA’s major findings are presented below.

AVIATION GROWTH SCENARIOS

FAA expects air traffic to increase considerably over the next 10 to 20 years, and with it the

demand for ATC services. Its plans for modernizing and expanding the National Airspace System are predicated on accommodating continued rapid growth. A key assumption in FAA’s

Aviation Forecasts has been that there will be no

constraints on future growth and that new facilities and equipment will be deployed where and

when needed to meet demand. FAA forecasts

have consistently exceeded actual demand in

the past, however, with lo-year projections of

growth as much as 50 percent higher than actually occurred. This raises questions about the

usefulness of FAA forecasts as a basis for longterm planning and about how quickly FAA

needs to proceed with capacity-related improvements in its 1982 National Airspace System Plan

(NASP).

Most other aviation forecasts generally support FAA’s projections, but some do not. This is

not surprising in light of the uncertainty about

the factors that may affect future traffic growth.

The Air Transport Association and a major aerospace firm have suggested that the U.S. airline

industry may already be approaching its mature

size, which would mean that air carrier operations may level off or even decline by the end of

the century. Airline deregulation has destabil3
4 ● Airport and Air Traffic Control System

ized market structure and airline profitability,

leading to questions about the ability of the industry to finance badly needed new equipment.

There are questions about the future price and

availability of aviation fuel and about the longterm impacts of the Professional Air Traffic

Controllers Organization walkout.

There is also uncertainty about the future distribution of operations among user groups and

among airports. FAA expects general aviation

users to account for 75 percent of the increase in

demand, but there are large uncertainties about

the continued growth of the general aviation

fleet. One such uncertainty is the future price

and availability of the aviation gasoline used by

small personal aircraft. As for air carriers, market forces and the restrictions imposed following

the strike have already resulted in a redistribution of operations away from congested hubs to

second-tier airports that have excess capacity.

This new trend, in combination with improved

facilities for general aviation traffic at reliever

airports, could make it possible to accommodate

some increases in aggregated operations within

existing system capacity.

AIRPORT CAPACITY ALTERNATIVES

At any given airport, delay occurs when demand for terminal airspace or runways approaches the capacity to handle aircraft safely.

Some delay is normal and inevitable, especially

during peak traffic hours or when capacity is

reduced because of adverse weather. At some

major airports, however, the level of demand is

now such that delay is chronic and severe. These

delays inconvenience passengers, increase airline

operating costs, and waste over a hundred million gallons of fuel each year.

One way to deal with delay is to increase the

capacity of hub areas, either by adding runways

to an existing airport or by building a new airport to relieve other, overcrowded airports.

Large amounts of land are required, however,

and there are strong community objections to

airport noise. These factors have made major

airport construction and expansion rare in the

past decade. In addition, building new runways

or airports requires years of planning (and, in

some cases, litigation) before it can be implemented. At some airports, however, independent “stub” runways for propeller aircraft could

increase effective capacity and minimize landuse and noise problems.

A more immediate way to alleviate delay is to

manage traffic so that demand fits within existing capacity. This could be done through

economic measures, such as differential pricing

schemes to help divert traffic from peak to offpeak hours, or perhaps from congested to underutilized airports. Administrative measures, such

as hourly quotas or user restrictions, could induce a similar reallocation of demand.

Improved ATC technology could also help

ease airport congestion. Automated terminalarea metering and spacing, to smooth and expedite the flow of traffic, and the Microwave

Landing System, to permit more flexible use of

crowded airspace close to the airport, might permit existing capacity to accommodate more operations. The magnitude of the potential benefits

varies widely with local conditions, runway

configuration, and traffic mix.

There is no single “best” way to increase capacity or reduce delay. A variety of measures—

economic, administrative, and technological—

will be needed and the optimum solution for any

given airport will be determined largely by local

conditions.
Ch. 1—Executive Summary ● 5

AIR TRAFFIC

FAA is planning a program of technological

improvements intended to enable the National

Airspace System to handle a higher volume of

traffic with increased efficiency and safety. This

new technology will replace present equipment

—some of which has been in use for over 40

years—with a modern integrated system that

will be more reliable and productive. This

should allow new or improved forms of service

to be offered to airspace users. Operating costs

should be lower than with the current generation

of ATC equipment, but there would also be major capital cost requirements. Many of these improvements can be implemented during the next

10 years, but the full modernization program

will not be completed until the late 1990’s.

Two technologies are at the heart of the new

generation of ATC: 1) advanced computers; and

2) a two-way digital data link between aircraft

and the ground. Advanced high-speed computers and new software will permit the ATC system to improve the overall management of traffic flow, as well as to formulate tactical measures

that will ensure conflict-free, expeditious, and

fuel-efficient flight paths for individual aircraft.

Replacement computers will be installed first in

en route ATC centers, then in terminal areas,

and finally in a central flow control facility that

will manage air traffic on a national basis. In addition to safety and capacity benefits, these computers will permit a level of automation in ATC

that will greatly reduce the workforce needed to

handle future traffic loads.

The improved data link between aircraft and

ground facilities will permit a rapid and extensive exchange of information and instructions

without relying exclusively on voice radio for

communication—for example, transmittal of

clearances and weather information. FAA also

proposes to use this data link as the basis for the

Traffic Alert and Collision Avoidance System

(TCAS) which will provide aircraft with an

independent, airborne supplement to groundbased separation assurance.

In terminal areas, the use of the Microwave

Landing System (MLS) will provide more precise

CONTROL

and reliable guidance for landing in adverse

weather conditions. In combination with procedural changes, MLS could also lead to more

efficient use of airport capacity because it allows

aircraft to follow any of several curving or segmented approach paths to the runway, thereby

easing some of the constraint imposed by the

present Instrument Landing System (ILS), which

provides only straight-line guidance along a

single path.

In general, OTA finds that the ATC system

improvements proposed by FAA are technologically feasible and desirable with respect to safety, capacity, and productivity, although there

are alternatives that might be equally effective.

In most of the programs reviewed, detailed cost

and benefit information is not yet available,

making it difficult to judge the cost effectiveness

of the FAA proposals in relation to the possible

alternatives. For the same reason, it is not yet

fully clear whether the overall benefits will exceed the capital expenditures needed to effect the

improvements, how the benefits will be distributed among user groups, and how system cost

will be allocated. Further information will be

needed on implementation plans and specific

costs and benefits throughout the Congress’ consideration of the FAA’s 1982 National Airspace

System Plan.

Funding Issues

Based on information available at the end of

1981, OTA estimates that the costs of airport

development grants-in-aid, modernization of

ATC facilities and equipment, and related research and development could average roughly

$1.5 billion per year over the next 10 years,

about 50 percent higher than the level of recent

years. Congress has several options to provide

funding for these programs. One would be to

cover these expenditures by general fund appropriations. This option, while it would afford

the Congress continuing close control of FAA

programs through the annual appropriations

process, might not provide the assured continuity of funding needed for undertaking a 10-year

program of the scope envisioned by FAA.
6 ● Airport and Air Traffic Control System

Alternative options involve reestablishing, in

one form or another, the Airport and Airways

Trust Fund which expired in October 1980. Possible approaches to reinstituting the trust fund

include: 1) a user tax structure and tax rates similar to those that existed before; 2) higher user tax

rates—raised either uniformly or selectively by

type of user; or 3) a different scheme of taxation

that would levy fees in proportion to benefits

received or costs imposed by each type of airspace user.

All of these options are controversial, and the

search for a solution is complicated by many

long-standing issues about the equity of user

charges and the appropriate distribution of trust

fund revenues. Other issues that could emerge in

the debate are how to use the present uncommitted balance in the trust fund (amounting to

about $3 billion) and whether to use trust fund

moneys to help meet operating and maintenance

costs. In the past, trust fund allocations derived

from user fees have covered only about 15 percent of these costs, and many feel that users

should pay a larger share of them. Others argue

that trust fund moneys should be reserved exclusively for capital improvements and R&D expenses,

RESPONSE TO FUTURE GROWTH

Basically, there are three forms of action that

can be taken to affect growth: regulatory, economic, and technological. Regulatory actions include measures imposed by the Government that

would restrict the use of airspace or the availability of ATC services according to user class or

types of activity. Economic measures are those

that would affect the cost of using the airspace

or that would allow the market forces of competitive pricing to determine access to facilities

and services that are in high demand. Technological responses include not only improved

forms of ground-based and avionic equipment

to increase the efficiency of airspace use, but

also increases in airport capacity through the

construction of new or improved landing facilities. All three approaches are likely to be used;

the issue is not which to adopt, but what combination and with what relative emphasis. Ultimately, the measures adopted to deal with

growth will reflect a more fundamental policy

decision: is growth to be accommodated wherever and whenever it occurs; or is it to be managed and directed so as to make the most effective use of existing resources, with the costs fairly borne by the beneficiaries.
Chapter 2

INTRODUCTION AND

OVERVIEW
Contents

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Trends and Forecasts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The Airport Capacity Problem. ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The ATC Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

The Committee Request . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

OTA’s Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Technological Improvements. ....... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Control Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Freedom of Airspace Use. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Automation and Controller Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Funding and Cost Allocation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

LIST OF FIGURES

‘9

9

11

12

14

14

14

15

16

16

18

19

20

Figure No. Page

1. Profile of U.S. Airports, 1980. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2. FAA Budget and Funding Sources, 1971-80. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Chapter 2

INTRODUCTION AND OVERVIEW

BACKGROUND

The National Airspace System (NAS) includes

about 6,500 public-use airports serving nearly all

cities and small communities in the United

States. Connecting these airports is a network of

air routes, defined by navigational aids, that

channeI the flow of traffic. Flight along these

routes, as well as operations in the terminal

areas surrounding airports, is monitored and

controlled by a system of ground-based surveillance equipment and communication links—the

air traffic control (ATC) system.

With two exceptions (Washington National

Airport and Dunes International Airport), * U.S.

airports used by commercial flights are owned

and operated by local, regional, or State authorities. Many general aviation (GA) aircraft also

use these commercial air carrier airports, but

most are served by smaller public airports and

by roughly 10,000 privately owned fields. The

air route system and the ATC system are operated by the Federal Aviation Administration

*Washington National and Dunes International are owned by

the Federal Government and operated by the FAA.

(FAA), which has responsibility for assuring the

safe and expeditious movement of aircraft in

U.S. airspace and contiguous areas. FAA is also

responsible for coordinating the use of airspace

shared by military and civil aviation.

In all, the NAS accommodates about 180,000

operations (takeoffs and landings) per day at airports with FAA control towers, or roughly 66

million per year. Of these, 22 percent are commercial flights (scheduled air carrier, commuter,

and air taxi), 74 percent are general aviation,

and 4 percent are military. Most of the commercial operations are concentrated at the top 66

airports, which account for over 77 percent of

commercial operations and 88 percent of passenger enplanements. Within this group, airline

traffic is even more highly concentrated at a few

major hubs. As shown in figure 1, the 10 largest

hubs handle 33 percent of all operations and 47

percent of all passengers.l

‘FAA Statistical Handbook of Aviation, Calendar Year 1980

(Washington, D. C.: Federal Aviation Administration, 1981),

passim.

TRENDS AND FORECASTS

The use of NAS, as measured by aircraft operations at airports with FAA towers, has grown

at an annual rate of about 4 percent in recent

years, due almost entirely to the rapid growth of

the GA sector.2 FAA expects the rate of growth

to slow to about 3 percent per year in the next

decade, but this would still mean that the congestion now experienced at the 5 or 10 largest

airports may spread to 10 or 15 additional airports by the year 2000. This growth would also

lead to substantial increases in the workload of

the ATC system. FAA workload forecasts indicate that there may be both capacity* and —

‘FAA Aviation Forecasts, Fiscal Years 1981-1992 (Washington,

D. C.: Federal Aviation Administration, 1980), passim.

*In a general sense, capacity refers to the number of aircraft that

can be safely accommodated in a given period of time. Airport casafety problems arising from the growth in demand for ATC services, problems that will not

be confined to major airports or commercial

operations. Projections show the demand for

ATC services by GA users could increase by as

much as 70 percent over the next 10 years.

The accuracy of these forecasts depends on

factors that are difficult to predict reliably, For

example, the growth in aviation is extremely

pacity is defined as the maximum number of aircraft operations

(takeoffs and landings) that can be accommodated in a given period of time on a given runway (or set of runways) under prevailing

conditions of wind and weather and in conformance with established procedures for maintaining safe separation of aircraft. Similarly, airspace capacity is defined as the maximum number of

flights that can be allowed to pass through a volume of airspace

during a given period of time without violating minimum separation standards.

9
10 ● Airport and Air Traffic Control System

Figure 1.— Profile of U.S. Airports, 1980a

alncludes heliports, STOL ports, seaplane bases, and mllltary-cwll joint.use fields, excludes facllltles tn Puerlo RICO, Vlrgln Islands, and Paclflc

Territories.

SOURCE FAA Stat/s r/ca/ Handbook, 7980

sensitive to the state of the national economy.

The price and availability of fuel could be a serious constraint on all classes of aviation. The

long-term effects of airline deregulation are uncertain but they could have an important influence on the profitability and competitive structure of the industry. Thus, while there is a consensus that air activity as a whole will continue

to grow, it is not certain how much growth to

expect, where it will occur, or what strategies

should be adopted to accommodate it. It does

seem clear, however, that growth of aviation,

even at a rather slow rate, gives rise to concern

about future airport capacity, terminal area congestion, and the safety and efficiency of the ATC

system.

Photo credit: Bill Osmun, Air Transport Association

A crowded terminal
Ch. 2—Introduction and Overview • 11

THE AIRPORT CAPACITY PROBLEM

Concentration of air traffic at a few large

hubs, brought about by the economics of air

transportation and by the general increase in air

travel, creates congestion and delay. * The cutback in scheduled flights following the air traffic

controllers’ strike has caused the problem to

abate temporarily, but congestion can be expected to recur when operations return to normal levels, and with it the associated problem of

safely handling a growing volume of air traffic.

Congestion results in delays that increase airline

operating costs and, ultimately, the cost of air

travel for the public. If fuel prices increase, the

cost of these delays will become magnified.

Commuter airlines and air taxi services are even

more vulnerable to delay costs than trunk airlines, since they have a much smaller base of

passengers across which to spread these costs.

*Delay occurs whenever aircraft must wait beyond the time they

are scheduled to use an airport or a sector of airspace. In practical

terms, delay is usually defined as occurring whenever some percentage of aircraft must wait longer than a specified period of time,

e.g., 80 percent of the aircraft must wait 4 minutes or longer. Congestion occurs as demand (the desired number of operational approaches capacity. An increasing number of aircraft seeking to use

an airport or an airspace sector at the same time causes queues to

build up among aircraft awaiting clearance to proceed.

GA users of major hubs also feel

delay in the form of restrictions

busy airports imposed during peak

with congestion.

the effects of

on access to

hours to deal

Expanding airport capacity, either through

construction of new airports or enlargement of

existing ones, is an obvious but far from easy solution. The availability of land for airport expansion is severely limited in major metropolitan areas, and the cost of available land is often

prohibitive. There is also rising community

resistance to airport expansion and construction

on the grounds of noise, surface congestion, and

the diversion of land from other desired purposes. Even where these obstacles could be overcome, increasing capacity by building a new airport is at best a long-range solution—the leadtime from conception to beneficial use of a new

airport is often a decade or more.

To deal with the problem of congestion in the

near term, and in a less capital-intensive way,

two management approaches may be used. One

is to shift some of the demand for use of the airport from peak to off peak hours by administratively imposing quotas or by applying differenPhoto credit: Neal Callahan

Congestion and delay
12 ● Airport and Air Traffic Control System

tial pricing for airport access according to the

time of day. This solution tends to work to the

advantage of major air carriers and against the

commuter and air taxi operators, and even more

heavily against GA users, who complain that

quotas or peak-hour pricing might effectively

preclude them from using major airports at all.

An alternative strategy is to divert some traffic

to another airport—for example, from a large

metropolitan hub to GA reliever airports in the

vicinity. In several cities the problem is not a

general shortage of capacity but a disproportionate demand at one airport, while excess

capacity exists at nearby airports that could

serve as satellites or relievers. The difficulty

arises in determining who is to be diverted, since

few potential users of reliever airports would

willingly accept diversion, especially if it imposes inconvenience or extra cost. One way to

make diversion more attractive would be to improve the ground transportation links between

hubs and reliever airports.

The intractability of the congestion problem

and the difficulties of increasing airport capacity

or making more efficient use of capacity through

managerial techniques have prompted some

people to look to the ATC system for an alternate solution. Through procedural changes or

technological improvements, the ATC system

might be able to make more efficient use of the

airspace in crowded terminal areas, thereby expediting the flow of traffic to and from runways.

THE ATC PROBLEM

The task of controlling air traffic in congested

terminal areas is greatly complicated when traffic consists of a mixture of large and small,

piston and jet aircraft. Arriving and departing

traffic, which is descending and climbing along

various paths and at different speeds to and

from en route altitudes, may consist of a combination of IFR and VFR traffic. * This traffic

mixture is inherently difficult to manage. Efficiency dictates that aircraft be moved to and

from- the runway as expeditiously as possible

and that gaps in traffic be kept to a minimum.

Safety, on the other hand, requires a regular

traffic pattern to prevent conflicts, and a

minimum safe separation distance to prevent

fast aircraft from overtaking slower ones. Air

turbulence in the form of wake vortices,**

which are more severe behind heavier aircraft,

requires even greater separation between aircraft

than would be needed if all were a uniform size.

The overall result is that ATC procedures necessary to assure safety and to manage the workload also contribute to delays in terminal areas.

—.—

“Aircraft operating under Instrument Flight Rules (IFR) and Visual Flight Rules (VFR).

**Eddies and turbulence, generated in the flow of air over wings

and fuselage, can upset the stability of following aircraft. Wake

vortices, which are invisible, cannot now be accurately detected,

and their movement and duration cannot be reliably predicted.

Technological improvements to the ATC system could help make fuller use of the physical

capacity of the airport and reduce controller

workload. Among these improvements are new

surveillance, communication, navigation, and

data processing equipment that could enhance

the controllers’ ability to separate and direct

traffic. The Discrete Address Beacon System

(previously know as DABS and now designated

as Mode S) is a new generation of radar equipment that permits aircraft to be interrogated individually for information about identity, position, and altitude. Mode S also provides a twoway data link that could reduce dependence on

the present voice radio channels and provide a

much more rapid and extensive exchange of information between air and ground. Various

forms of proposed airborne systems to detect

and avoid potential collisions would provide a

supplement to present separation assurance

techniques and reduce some of the controller’s

burden in handling a high volume of traffic. It

may also be possible to provide computer analysis of flight plans in advance that would help

resolve conflicts in terminal areas, expedite traffic flow, and permit more direct and fuel-saving

routing from origin to destination. Another proposed improvement is the addition of special

cockpit displays that would provide a picture of
Ch. 2—Introduction and Overview • 13

traffic in terminal areas and thereby permit

pilots to cooperate more effectively with the

controller or to assume some of the controller’s

present responsibility for separation assurance

and determining flight path in terminal areas.

Finally, the Microwave Landing System (MLS)

would not only improve the ability to land in

conditions of severely reduced visibility, but

also permit multiple or curving approach paths

to the runway instead of the single-file, straighten approach required with the present Instrument Landing System (ILS). In the longer term,

proposed new ATC technology might replace

the present system of ground-based radar and

radio navigation and surveillance capabilities.

These proposed improvements, if adopted,

would require very large investments over the

next two decades. These investments would be

made by the Federal Government, but some of

the funds could be provided by taxes on airspace

users, who might also have to purchase new

avionics equipment to supplement or replace

what they already have. Managing the transition to a new generation of ATC would also require careful attention, both to assure continuity

of service and to avoid the penalties of excessive

cost or unexpected delay. It therefore seems

especially important to select an evolutionary

path that does not foreclose options prematurely

and does allow flexibility in the choice between

competing technologies.

These prospective ATC improvements raise

important issues for airspace users. If the required new avionics systems become mandatory

for access to terminal areas or for general use of

controlled airspace, some GA, small commuter,

Photo credit: Federal Aviation Administration

Air controller and screen
14 • Airport and Air Traffic Control System

and air taxi operators may find the cost pro- of the present system as possible. Some possible

hibitive. New civil aviation requirements may improvements might ultimately have to be renot be entirely compatible with the missions or jected, despite of their potential for increasing

capabilities of military aircraft that share the capacity or enhancing safety, because of the cost

airspace. There will probably be pressure to pro- to users or infringement of the right of access to

long the transition period and to retain as much the airspace.

THE COMMITTEE REQUEST

Concerns about these problems and about Specifically, the Committee on Appropriate feasibility and cost of proposed solutions tions requested that OTA make an independent

prompted the House Committee on Appropria- assessment in four major areas:

tions,- Subcommittee on Transportation, to re- ●

quest that OTA undertake an assessment of airport and terminal area capacity and related ATC ●

issues. Subsequently, the Senate Committee on

Commerce, Science, and Transportation also ex- ●

pressed interest in these issues and endorsed the

request of the House Committee on Appropriations. ●

scenarios of future growth in air transportation;

alternative ways to increase airport and terminal area capacity;

technological and economic alternatives to

the ATC system modifications proposed by

FAA; and

alternatives to the present ATC process.

OTA’s APPROACH

This assessment considers the growth of air

transportation over the remainder of this century. Particular attention is given to large hub

airports, where most of the congestion and delay

is expected to occur. For the ATC system, the assessment focuses on improvements that would

affect the safety and capacity of terminal airspace, but developments in other parts of the

ATC system (en route and flight information

services) are also considered, Effects of these

changes on airspace users (commercial operators, passengers, general aviation, and the military services) are also examined. Policy options

and alternative development plans are identified

and analyzed.

The results of this assessment are presented in

the following five chapters:

Chapter 3. Description of the functions, organization, and operation of NAS with emphasis on ATC.

Chapter 4. Analysis of possible long-range

trends in air activity and the effect they

might have on technical, investment, and

management decisions.

Chapter 5. Examination of prospective new

technologies and organizational alternatives for the ATC system.

Chapter 6. Analysis of various ways to increase airport capacity and their advantages

and disadvantages.

Chapter 7. Discussion of the policy implications that arise from alternative approaches

to increasing airport capacity and improving the ATC system.

ISSUES

Expanding, improving, and maintaining the of the Federal Government from the earliest

national system of airways, airports, and air days of aviation. There have been undeniable

traffic control has been an important objective benefits to airspace users and the general public
Ch. 2—Introducflon and Overview • 15 ——.———

from the greater speed and regularity of air

transportation and from the remarkable record

of safety that has been achieved over the years.

The rationale for Federal involvement in the development and operation of NAS has traditionally rested on two grounds: 1) promotion and

regulation of interstate and foreign commerce;

and 2) enhancement of the capability for national defense. It has been argued on both

grounds that the Federal Government must take

an active role to coordinate the development

and to manage the operation of the system. The

system that has evolved under Federal sponsorship and direction is not without its flaws,

however, and some observers believe that future

development should be directed along lines

other than those of the past. Many of their concerns are embodied in the summary of major

issues which follows; these issues will be treated

in greater detail in subsequent chapters of the

report.

Growth

There is basic agreement among aviation experts that civil aviation in the United States will

continue to grow, thereby increasing the overall

demand for airport use and ATC services. There

is considerably less agreement about the rate of

growth, the distribution among airspace users,

the demands on various types of facilities and

the kinds of services that will be required. As a

result, there are sharp disputes about how to accommodate this growth or to influence the form

and direction it may take.

FAA’s projections have led it to conclude that

severe capacity restrictions will manifest themselves in terminal areas and some parts of the en

route system and that perhaps as many as 20 airports may be saturated by 2000. To accommodate this expected growth, the FAA proposes the

addition of new airport capacity and ATC facilities designed to handle higher traffic volumes.

However, past FAA forecasts have consistently

projected higher rates of growth than have actually materialized, casting doubt on the current

FAA forecasts and the expected demand for

ATC services through the remainder of this century. Some observers see trends already developing in a different way. They argue that recent

changes such as airline deregulation, the growth

of commuter service, sharp rises in fuel cost, and

slower economic growth will either dampen

growth or cause it to develop in a pattern

significantly different from that of the past. For

example, one suggestion is that in an unregulated environment, market forces will cause a

redistribution of traffic as users find that delay

costs outweigh the benefits of operating at congested hub airports.

GA is the sector of aviation where growth has

been the most rapid and where there is most serious concern about accommodating future demand. Twenty years ago, GA accounted for

only a small fraction of instrument operations;

today it represents slightly over half of all instrument operations at FAA facilities, and most

forecast; show GA demand for ATC services increasing at rates far higher than those of commercial air carriers. Measures to restrict GA

activity at major hubs or to divert it to reliever

airports or offpeak hours are certain to be controversial. GA users feel that reservations, quotas, or differential pricing schemes, would unfairly deny them access to and use of the airspace system. On the other hand, some believe

that GA flights into congested terminal areas

should be limited because they typically carry

very few passengers and so provide less public

benefit than commercial aviation per operation

or per unit of airspace use.

At a more general level, the prospects of traffic growth and capacity limitations raise the

issue of strategic response to accommodating

future demand. In the past, the approach has

been essentially to accommodate demand wherever and whenever it occurred, i.e., the aim has

been to foster growth in civil aviation. Some

question whether this approach is still desirable,

arguing that demand and the growth of air activity should be managed and directed in ways to

make the most productive use of airspace and

the most efficient use of existing facilities.

Basically, there are three forms of action that

can be taken to influence growth: regulatory,

economic, and technological. Regulatory ac -

tions include measures imposed by the Government that would control the use of the airspace

or the availability of ATC services according to
16 ● Airport and Air Traffic Control System

user class or types of activity. Economic measures are those that would affect the cost or price

of using the airspace or that would allow market

competition to determine access to facilities and

services that are in high demand. Technological

responses include not only improved forms of

ground-based and avionic equipment to increase

the efficiency of airspace use, but also increases

in airport capacity through construction of new

or improved landing facilities. All three approaches are likely to be used, and the issue is

not which to adopt but what combination and

with what relative emphasis. Ultimately, the

choice of measures will reflect a more fundamental strategic decision about how to meet increasing demand. Chapter 4 presents a further discussion of future growth, and chapters 5 and 6 examine the various responses to growth.

Technological Improvements

The many technological improvements of the

ATC system being contemplated by FAA fall

into four classes:

● navigation and guidance systems;

● surveillance;

● communication; and

● process improvements.

These potential improvements have three major

characteristics: 1) most are technologically

sophisticated and require further development

and testing before they can be operationally

deployed; 2) they will entail very large expenditures by the Federal Government to put them in

place and—in most cases—additional costs to

airspace users who will have to equip their aircraft with special avionics; and 3) many years

will be required for full deployment.

There are several controversial aspects of

these technologies. First, there are purely

technical and engineering questions that need to

be answered: will these new systems work as intended, what are their advantages and disadvantages compared to existing technology, and how

can their development be managed so that options are not foreclosed prematurely? As decisions are made and implementation proceeds, it

will be necessary to coordinate the program

carefully in order to provide an orderly transition and to avoid the costs that could result from

delay or unexpected technical setbacks.

Beyond these technical and managerial matters, there are more fundamental questions

about the role of FAA in planning and carrying

out technological programs of this nature. Congress, for example, has questioned FAA’s proposed handling of the program for modernization of its en route computer system, as have

other members of the aviation community. They

are concerned that FAA is not consulting adequately with specific user groups and not taking

advantage of relevant expertise available outside

the aviation community. Some of them foresee a

time when air traffic may have to be curtailed

simply because the technology to handle increased traffic with an acceptable level of safety

has not been properly planned, developed, and

deployed.

On the other side, there are those who defend

FAA’s general strategy for ATC modernization

and approve the way in which particular technological programs are being handled. They argue

that deployment must proceed at a cautious pace

both because of the enormous uncertainties that

must be overcome and because there must be

continuity of operations throughout the transition. In their view, the potential consequences of

abrupt changes or premature decisions are more

serious and, in the long run, more harmful to

aviation than temporary curtailments that may

have to be imposed while technological difficulties are being resolved.

Chapters examines some of the technological

issues surrounding proposed system improvements, and chapter 7 addresses strategy and

policy options for managing the transition.

Control Philosophy

Perhaps the most fundamental issue underlying the proposed improvements in the ATC system is that of control philosophy—the principles

that should govern the future operation of the

system. The philosophy of the present system

for controlling IFR traffic is embodied in three

operational characteristics: the system is primarily ground-based, highly centralized, and places

great emphasis on standardized (i.e., predict-
Ch. 2—lntroduction and Overview • 17

able) behavior by airspace users. In contrast,

VFR traffic has little contact with the ATC system, except with flight service stations and control towers at airports, and operates much as it

did in the early days of aviation, even though it

shares airspace with IFR traffic in some instances.

As ATC technology evolved the locus of decisionmaking under IFR began to shift from the

cockpit to the ground. Routes were determined

by the placement of ground-based navigation

aids; surveillance was accomplished by reports

to ground centers and later by search radar; and

observers in airport towers began to direct aircraft in landing and takeoff patterns. As the density of air traffic increased, ground-based ATC

personnel began to take more and more control

over the altitude, route, and speed to be flown.

To some extent this transfer of responsibility

was the inevitable consequence of the technology employed, but organizational reasons also

dictated ground-based control. Decisions concerning not the movement of individual aircraft

but the pattern of traffic as a whole can best be

made by a single person who is in a position to

observe all flights operating throughout a

volume of airspace over a span of time. Coordination and direction of several aircraft required that a single individual have authority

over others—a role that the pilot of a single aircraft could not be expected to assume or that

other pilots would accept.

Ground basing implies concentration of control at relatively few locations, and the trend has

been for centralization to increase over time.

Again, the reasons are both technological and

organizational: centralization is organizationally

advantageous because it consolidates functionally similar activities and allows technical specialization, both of which lead to greater efficiency

and reliability of operation. For example, en

route traffic in continental U.S. airspace is now

controlled from 20 regional centers (ARTCCs,

and proposed ATC system improvements would

lead to even further consolidation, with en route

and terminal control eventually merging into a

single type of facility. A similar trend toward

centralization can be observed in FAA’s plans to

consolidate flight service station activities at

about 60 sites, compared to the present dispersion at over 300 locations.

Perhaps the best example of the trend toward

centralization is the growing importance of the

Central Flow Control (CFC) facility at FAA

headquarters in Washington, D. C., which acts

as a nerve center for the entire airspace system.

With the aid of computers, CFC reviews the national weather picture and anticipated aircraft

operations for the coming day and determines

the incidence and cost (extra fuel consumed) of

delays that could occur because of weather and

air traffic demand. This results in a daily operational master plan that smooths demand among

airports and allows delays to be taken on the

ground at the point of departure rather than in

holding patterns at the destination. The value of

this capability was demonstrated when capacity

quotas were imposed as a consequence of the

August 1981 air traffic controllers’ strike. CFC

allowed a national airspace utilization plan to be

developed, with detailed instructions to airports

and en route centers on how to manage traffic

and minimize the adverse effects of the capacity

restrictions,

A system characteristic that accompanies

ground-based centralization of control authority

is standardization of performance. FAA operating procedures specify the behavior of pilots and

controllers in every circumstance, which increases the reliability of system operation by

reducing uncertainty and by routinizing nearly

every form of air-ground transaction. Safety is

the prime motivating factor, but capacity and efficiency are also highly important considerations. Controller workload is reduced when the

range of possibilities they have to deal with is

limited, and this in turn permits a given volume

of traffic to be handled with less stress or, alternately, an increase in the number of aircraft each

controller can safely handle. Either way, the efficiency of the ATC system (measured in terms of

hourly throughput or controller productivity) is

increased, with a corresponding reduction in

system operating cost.

Despite the advantages of ground-basing, centralization, and standardization, there are complaints about the control philosophy of the pre-
18 • Airport and Air Traffic Control System

sent system. Pilots complain that a groundbased system detracts from their control over

the conduct of the flight. Centralization may

also be a problem if, by concentrating control

facilities or flight services, the personnel on the

ground are less able to provide particularized instructions or to take action based on localized

knowledge of flight conditions. Standardization,

by definition, limits the flexibility of response

and the freedom to pursue individual or special

courses of action.

The prospective changes in ATC technology

are viewed with mixed feelings by airspace users

and air traffic controllers. Technology that

would increase the level of automation could, on

one hand, promote greater centralization and

standardization of control functions and could

lead to increases in safety, capacity, or efficiency, On the other, automation could serve to increase ground authority still further and to

reduce the flexibility of the system in dealing

with nonroutine events. Technology like collision avoidance systems or cockpit displays of

traffic information could give back to the pilot

critical information (and hence control responsibility) and might enhance the pilot’s ability to

cooperate more effectively with the groundbased controller. At the moment, these devices

are thought of as backups in the event of controller or system error, but their prospective use

also raises the possibility of independent pilot

actions that might contravene controller instructions or disrupt the overall pattern of traffic.

Chapter S, which deals with these and other

forms of advanced aviation technology for

ground-based and airborne application, treats

the issues that arise from prospective changes in

distribution of control between the air and the

ground or from further centralization of ATC

functions and services.

Freedom of Airspace Use

The rising demand for ATC services and the

prospect of congestion at more and more major

airports are the basic stimuli for many of the

technological improvements and procedural

changes now being sought by the FAA. However, the very measures that might ease capacity

problems or assure the safety of high-density

airspace are often controversial with some categories of users because they are perceived as infringements on their freedom to use NAS. GA

users feel particularly threatened, but air carriers

and commuter airline operators have also voiced

concern. The military services as well are wary

of some new forms of ATC technology and the

procedures that may accompany their use because they may interfere with military missions

or be incompatible with performance requirements for combat aircraft.

As the complexity of ATC technology has increased, so has the amount of equipment that

must be carried on the aircraft and the amount

of controlled airspace from which VFR flight is

excluded unless the aircraft is equipped with a

transponder to allow identification and tracking

by the ATC system. Restrictions on airport use,

especially at large and medium hubs, have also

grown more confining for VFR flights, and the

airspace around many of the busiest airports is

now designated as a “terminal control area” in

which all aircraft are subject to air traffic control

and may operate only under rules and equipment requirements specified by FAA. GA, the

principal user of the VFR system, finds itself

pressured in several ways. Uncontrolled airspace

is shrinking and may disappear altogether; it is

becoming increasingly difficult to use metropolitan airports because of equipment requirements;

and the cost of equipping the aircraft with IFR

avionics and acquiring an instrument rating are

often out of economic reach for the personal GA

pilot. Prospective technological improvements—such as the Traffic Alert and Collision

Avoidance System (TCAS), data link, or MLS—

are viewed by many GA users as further restrictions on their access to airports and airspace.

Many of them feel that, while this new technology may be desirable or even necessary for air

carriers and larger business aircraft, it should

not be required of all GA users or made a prerequisite for IFR services or access to commercial

airports.

Commuter airline operators share some of

these GA concerns. Virtually all commuter and

air taxi operators are equipped for IFR operation

and find their needs well served by the present
Ch. 2—Introduction and Overview ● 19 —

ATC technology. They see little further advantage in new technology and are concerned about

the expense of having two sets of equipment

serving the same purpose—advanced avionics

needed for a high-density terminal at one end of

the flight and present-day equipment that may

be useful for many years to come at small community airports. They are also concerned that

the more advanced avionics might eventually

lead to more restrictive rules of operation or access to terminal areas. Thus, many commuter

and air taxi operators would favor a dual-mode

system that allowed them to retain their present

IFR avionics even though more advanced forms

were in use by other types of aircraft operators.

Military aviation operates under the civil

ATC system in all shared airspace and under

military control in areas restricted to military

use. In flying through civil airspace to and from

training areas, military aircraft must often follow circuitous routes or observe altitude and

speed restrictions that lengthen transit time. The

military services would prefer an arrangement

that allows more direct access to training areas

and avoids operation in mixed airspace. Air carriers have a different view: the most direct

routes for trunk airlines are often blocked by

restricted military areas, and the air carriers

argue for procedures that would allow them to

traverse these areas in the interest of shortening

flight time and saving fuel.

Another issue has to do with new technology

that might be adopted for civil aviation, which

in most cases would be extra equipment for military aircraft. For combat aircraft, particularly

fighters, the space for avionics and antennas is

often at a premium. While careful coordination

of military and civil requirements can eliminate

some of these problems, certain basic incompatibilities are likely to remain and to produce continuing controversy.

The issues of freedom of airspace access and

use are discussed further in chapters in connection with specific forms of new aviation technology.

Automation and Controller Functions

Despite the vast complex of ground-based

equipment and facilities for surveillance, communication, and data processing, ATC remains

a highly labor-intensive activity. FAA is keenly

aware of this and has sought for some time to

find ways to automate selected ATC functions.

However, most of the automation that has been

instituted so far has been to assist air traffic controllers rather than replace them. Decisionmaking and communication—two major elements of

controller workload—have not been automated

to any appreciable degree, and the ratio of controller work force to aircraft handled has remained relatively constant. In addition, the

present method of backup to automated control

functions involves reversion to manual procedures used in the previous generation of ATC

equipment; this method of assuring service in

the event of outages has tended to perpetuate the

team size and staffing patterns of the previous

generation.

Plans for an advanced generation of ATC call

for automation of several manual controller

functions: conflict prediction and resolution,

terminal area metering and spacing, flight plan

approval and issue of clearances, and communicating routine control instructions to individual

aircraft. Such forms of automation could lead to

substantial increases in controller productivity

and might eventually provide the basis for a

more extensively automated system in which

most routine control functions are carried out by

computers, with the human controller acting in

the role of manager and overseer of machine

operation.

This path of evolution raises three important

groups of issues. First, there are questions about

the feasibility and advisability of replacing the

human controller to such an extent. ATC now

relies heavily on judgment and awareness of the

dynamics and subtleties of the air traffic situation. Some observers doubt that all of these

characteristics could be dependably incorporated into computer software in the foreseeable
20 . Airport and Air Traffic Control System

future. The proponents of automation argue

that much of the routine, repetitive, or predictive work of ATC is ideally suited to computers,

and that an incremental approach to automation

will help solve many of the problems since each

new step can build on successful previous advances.

A second major set of issues is the reliability

of automated systems and the backup methods

to be used when the inevitable equipment

failures occur. Experience with the present

automated ATC equipment indicates that computer failure rates are a cause for concern, and

the loss of computer-supplied data may mean

that ground personnel lose effective control of

traffic until manual backup procedures are instituted—a process that may take several minutes to complete. Computer experts maintain

that equipment and software reliability can be

greatly improved and that automated systems

can be designed to be more failure tolerant.

These experts also contend that present experience with manual procedures as backups to

outages of automated equipment indicates a fundamental flaw in design philosophy because the

proper backup to an automated system is not

manual operation, but another automated system. Critics of automation question the acceptability of a system in which the human controller has no effective means of intervening in

degraded states of operation.

A third issue is whether some of the responsibility that now resides with the ground-based

system ought not to be transferred to, or at least

shared with, the cockpit. A pilot in an aircraft

equipped with an airborne collision avoidance

system and a display of the immediately surrounding air traffic might be in a superior position to select the appropriate maneuver in case

of conflict; in effect, such an airborne system

would create a mode of IFR operation similar to

the present VFR system. The chief disadvantage

of this concept is that it could lead pilots to make

a series of short-term tactical responses that

might not be consistent with the overall scheme

of managing traffic in congested airspace. In this

case, the ground system would still have to act

in the capacity of referee, and some contend that

it would be better to keep all control of individual flight paths under one authority.

Chapter 5 contains a further examination of

the issue of automation in connection with the

discussion of the proposed en route computer replacement program and the mechanization of

the Mode S data link and TCAS systems.

Funding and Cost Allocation

The expenditures that are likely to be required

for ATC system improvements over the coming

years could be considerably higher than those of

past years. For the period 1971 to 1980, the

amounts budgeted for facilities and equipment

(F&E) and associated research, engineering, and

development (RE&D) have averaged $397 million and $106 million respectively (in constant

1980 dollars).3 Future improvements of the en

route and terminal area ATC system and related

programs for flight service station, navigation,

and communication facility modernization may

call for spending at twice this annual level or

more. At the same time, operating and maintenance (O&M) costs are expected to rise, at least

until modern labor-saving equipment is installed

and productivity gains begin to be realized.

Since creation of the Airport and Airways

Trust Fund in 1970, FAA has had two sources of

funding. F&E, RE&D, and airport grants-in-aid

have been covered wholly by appropriations

from the trust fund. In addition, the trust fund

has covered about 15 percent of O&M expenses,

although this proportion has varied considerably from year to year. The balance of O&M

costs, about $1.9 billion per year (1980 dollars),

and all other FAA budget items have been from

general fund appropriations. Overall, trust fund

outlays have met about 40 percent of annual

FAA expenses. The major source of revenue for

the trust fund has been a tax levied on domestic

and international airline passengers (see fig. 2).

In October 1980, the Airport and Airways Development Act expired, and Congress declined

to pass reauthorizing legislation. At that time

the trust fund had an uncommitted balance of

3

0TA calculations based on FAA budget data, 1971-80.
Ch. 2—Introduction and Overview • 21

—————————— — .

Figure 2.— FAA Budget and Funding Sources, 1971-80

General

fund

SOURCE: Off Ice of Technology Assessment, based on FAA budget data, 1971-80.

$2.9 billion, the equivalent of about 2 years’ expenditure at the then prevailing rate. Since that

time some of the user taxes contributing to the

trust fund have still been collected (but at reduced rates of taxation), and these revenues

have been deposited partly in the General Fund

and partly in the Highway Trust Fund. If these

revenues are included and if authorizations from

the trust fund during fiscal year 1981 are deducted, the uncommitted trust fund balance

stood at roughly $3 billion at the beginning of

fiscal year 1982.
22 ● Airport and Air Traffic Control System

In considering sources of funding for future

airport and ATC system improvements, Congress will encounter three broad and long-standing areas of controversy. In the absence of a

trust fund or some other form of user charges to

support capital improvement programs, these

parts of the FAA budget would have to be

funded from general revenues, which is certain

to raise the issue of whether civil aviation and

the airport and ATC system should be subsidized by the general public. The argument that

the recipients of a service should pay the costs

for the Federal Government to provide that service (a position strongly supported by the present

administration), holds that capital improvements of facilities and equipment and the O&M

costs of running the airport and ATC system

should be borne by airspace users through various specific taxes. On the other hand, it can be

argued that civil aviation, like other modes of

transportation, provides a general benefit and

therefore deserves support with public moneys.

Other modes of transportation receive subsidy

from the Government, and some members of the

aviation community contend that there is no justification for singling out civil aviation for full

recovery of capital and operating costs.

The resolution of this issue that has prevailed

for the past 10 years has been a combination of

special users taxes and General Fund financing,

with the former going for capital expenditures

and a small share of operating costs and the latter for the balance of FAA costs. A perpetuation

of this scheme, through reestablishment of the

Airport and Airways Trust Fund, could embroil

Congress in another issue—what is the “fair”

amount to be paid by various user classes. Most

people concede that each user should pay roughly in proportion to the cost that they impose on

the system, but there is violent disagreement

within the aviation community as to what these

costs are and how they are to be reckoned. Cost

allocation studies conducted by the Department

of Transportation and the FAA have generally

concluded that, under the tax structure that existed before October 1980, commercial aviation

paid nearly all (88 percent) of the cost of services

provided to them. On the other hand, general

aviation taxes returned at almost one quarter of

allocated costs.4 GA representatives have disagreed strongly with these findings, arguing that

there is a substantial public benefit of aviation

that has been undervalued in these cost allocation studies and that GA is charged for facilities

and services that are neither required nor used

by a major part of GA operators. Congress has

shown little inclination to alter the user charge

structure, and most of the proposed legislation

to reestablish the trust fund would have little effect on the distribution of user charges that existed previously.

The third area of controversy concerns how

the collected levies should be applied to costs.

By congressional action, the use of trust fund

moneys is restricted largely to capital expenditures and research and development activities,

with some contribution toward operating expenditures. There are two major points at issue:

1) how should expenditures for capital improvements be allocated between airports and ATC

facilities and equipment (and among airports

and ATC facilities used by various types of aviation); and 2) should the allocation be broadened

to cover a substantial part (or perhaps all) of

O&M costs.

Resolution of these issues will become especially important when FAA presents its longrange plan for ATC system improvement. Increased expenditures for facilities and equipment

and associated R&D will be called for, and operating expenses will probably remain high. FAA

will be seeking a long-term commitment and an

assured source of funding, but it will face strong

opposition from segments of the aviation community if paying for FAA’s programs and operating costs entails an increase in user taxes or a

reallocation of the share to be borne by various

classes of airspace users.

‘J. M. Rodgers, Financing the Airport gnd Air-way System; Cost

Allocation and Recovery, FAA-AVP-78-14 (Washington, D. C.:

Federal Aviation Administration, November 1978).
Chapter 3

THE NATIONAL AIRSPACE SYSTEM

Photo credit U S Department of Transportafton
Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Airports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

International Airports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Domestic Air Carrier Airports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Commuter Airports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Reliever Airports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

General Aviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Air Traffic Services. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Landing Aids... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Flight Planning and Advisory Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Air Traffic Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System Organization and Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ATC Sectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ATC Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Airspace Users. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

List of Tables

Table

l. Airports Included in National Airport System Plan, 1980 . . . . . . . . . . . . . . . . . . .

2. U.S. Pilot Population, 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3. Summary of Aviation Activity, 1980 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

List of Figures

Figure

3.

4.

5.

6.

7.

8.

Airspace Structure . . . . . . . . .

Typical Flight Service Station

. . .

CommunicationLinks......................:...

Air Route Traffic Control Center Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Connections of aTypical ARTCC With Other Facilities. . . . . . . . . . . . . . . . . . . . .

ATC Activities for a Typical IFR Flight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ATC Facilities and Equipment at a Typical Large Airport . . . . . . . . . . . . . . . . . . .

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Chapter 3

THE NATIONAL AIRSPACE SYSTEM

The National Airspace System (NAS) is a see how the system operates and to identify faclarge and complex network of airports, airways, tors that may shape its future development. For

and air traffic control (ATC) facilities that exists explanatory purposes, it first considers the goals

to support the commercial, private, and military of the system and then describes the system

use of aircraft in the United States. This chapter under three major headings: airports, air traffic

examines the major parts of the system, both to services, and airspace users.

GOALS

NAS is designed and operated to accomplish

three goals with respect to civil aviation:

1. safety of flight;

2. expeditious movement of aircraft; and

3. efficient operation.

These goals are related hierarchically, with safety of flight the primary concern. The use of airport facilities, the design and operation of the

ATC system, the flight rules and procedures employed, and the conduct of operations are all

guided by the principle that safety is the first

consideration.

Without compromising safety, the second

goal is to permit aircraft to move from origin to

destination as promptly and with as little interference as possible. This involves preventing

conflicts between flights, avoiding delays at airports or en route, and eliminating inefficient or

circuitous flight paths. It also entails making

maximum use of airport and airway capacity in

order to satisfy demand, so long as safety is not

compromised. If safety and capacity utilization

are in conflict, the Federal Aviation Adminstration’s (FAA) operating rules require that the volume of traffic using the system be reduced to a

level consistent with safety.

The third goal is to provide airport and ATC

services at low cost. This entails minimizing the

costs to users—not only monetary costs but also

the penalties of delay, inconvenience, or undue

restriction. It also entails operating the system as

efficiently as possible so as to reduce transaction

costs and to increase productivity, i.e., to handle more aircraft or to provide better service to

those aircraft with a given combination of runways, controllers, and ATC facilities.

Whereas safety cannot be compromised in the

interest of cutting costs, capacity and cost may

be traded off for the sake of safety. The special

measures adopted to deal with disruption of the

system as a result of the air traffic controllers’

strike in August 1981 illustrate the hierarchal relationship of safety, capacity, and efficiency. In

order to continue safe operation in the face of

work force reductions, the number of aircraft allowed to use certain crowded airports and air

ways at peak demand hours was reduced to a

level that could be handled safely. These measures reduced capacity (the number of aircraft

that the system could accommodate) and increased cost (delays, canceled flights, adherence

to quotas), but an effort was made to allow the

remaining capacity to be used effectively and

keep costs within reasonable limits. For example, limits on the number of air carrier flights

were imposed only at the 22 busiest airports,

and restrictions were later eased at those airports

where more operations could be accommodated.

Airlines were allowed to use larger aircraft so as

to provide as much seat capacity as possible but

with fewer flights, and wherever possible flow

control procedures were employed to ensure

that aircraft were delayed on the ground rather

than in flight, so as to minimize waste of fuel.

Other restrictive measures were applied to cut

back on general aviation (GA) flights. The military services voluntarily reduced flight operations.

25
26 • Airport and Air Traffic Control System —.—

The anticipated growth of air traffic and the capacity. Before turning to examination of these

demand for ATC services over the next two dec- problems, however, it is first necessary to look

ades poses several problems, and the need to at the major parts of the NAS and to consider

maintain a dynamic balance among system goals the factors that could shape their course of remotivates the search for improved methods of velopment.

ATC and better utilization of airway and airport

AIRPORTS

Airports are the first major part of NAS. They

are any place designed, equipped, or commonly

used for the landing and takeoff of aircraft. This

definition covers a broad variety of sites: many

of the sites designated as airports by the FAA are

merely dirt strips or seaplane moorings near

open water; at the opposite end of the spectrum

are complex air terminals serving major metropolitan areas, like the 5,000-acre JFK International Airport in New York. About 60 percent of

the 15,000 U.S. airports are private or military

fields and not available for public use. Of the

roughly 6,500 civil airports open to the public,

almost 90 percent are used exclusively by small

GA aircraft. The remaining 780 airports (about 5

percent of all U.S. airports) are served either by

scheduled air carriers or by commuter and air

taxi operators (see table 1).

FAA, in compliance with the Airport and Airway Development Act of 1970, maintains a master list of airport development needs for the next

decade. This compilation, which is periodically

revised, is known as the National Airport System Plan (NASP). It identifies categories of airports that are of Federal interest and that are

eligible for Federal funds under the Airport Development Aid Program (ADAP), and the Planning Grant Program administered by FAA.

NASP categorizes public use airports according

to the type of aviation activity they accommodate: international, domestic air carrier, commuter, reliever, and general aviation. This does

not imply that GA aircraft use only GA airports;

in fact, there are GA operations at all categories

of airports. Rather, the GA classification denotes that such airports serve only GA and not

other types of users.

International Airports

An international airport regularly serves air

carrier flights operating between the United

States and foreign countries. International airports tend to be among the best equipped airports in terms of runways, landing aids, and

ATC facilities. In 1980 there were 76 such airports.

Table 1 .–Airports Included in National Airport System Plan, 1980a

Type of service Conventional Heliport Seaplane Total

Air carrierb 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . 603 1 31 635

Commuter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 — 6 145

Reliever. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 — 155

General aviation. . . . . . . . . . . . . . . . . . . . . . . . 2,198 4 22 2,224

Total NASP airports. . . . . . . . . . . . . . . . . . . 3,095 5 59 3,159

Total public-use airports not in NASPc 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3,360

Total. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.519

alncludes airports in Hawaii and Alaska.

blnclude5 76 airports designated as ports of entrY.

cEntirely general aviation.

SOURCE: Federal Aviation Administration, National A/rPort Sysfern Plan, 1980-89, 1980,
Ch. 3—The National Airspace System . 27

Domestic Air Carrier Airports

In 1980, NASP included 603 airports served

by domestic air carriers, a figure that includes all

of the international airports described above but

excludes 1 heliport and 31 seaplane facilities

served by scheduled air carriers. These airports

are classified by FAA according to the size of the

traffic hub they serve, where a hub is defined

as a Standard Metropolitan Statistical Area

(SMSA) requiring air service. The hub classifications are:

Percentage of total

Hub classification: airline passengers *

Large (L) 1.00 or more

Medium (M) . 0.25 to 0.99

Small (S) . . . . . . . . . . . . . . . . 0.05 to 0.24

Nonhub (N) . . . . . . less than 0.05

*Passengers eplaned by domestic and foreign carriers at U S airports

A hub may have more than one air carrier airport, and the 25 SMSAs presently designated as

large hubs are served by a total of 38 air carrier

airports. The distribution of aviation activity at

domestic air carrier airports is highly skewed,

with progressively greater percentages of flights

and passengers concentrated at fewer and fewer

airports. In 1980, for example, the 486 nonhubs

handled only 3 percent of all passenger enplanements; the 76 small hubs handled 8 percent; the

41 medium hubs handled 18 percent; and the 25

large hubs handled 70 percent. To carry this

point one step further, the top five air carrier airports (Chicago, Atlanta, Los Angeles, Denver,

Photo credit: Federal Aviation Administration

All filled up

Photo credit: Federal Aviation Administration

Room to grow

and Dallas/Fort Worth) handled about onequarter of all passenger enplanements and onefifth of all airline departures. This means that air

traffic congestion tends to center at a very small

fraction of airports; but because of the volume

of traffic handled at these airports, it affects a

large percentage of all aircraft and passengers.

Commuter Airports

Until the Airline Deregulation Act of 1978,

many commuter and air taxi airlines were not

certificated as scheduled air carriers by the Civil

Aeronautics Board (CAB), and NASP classified

airports served exclusively by commuter and air

taxi in a separate category. Since airline deregulation, the number of airports in this category

has fluctuated widely, showing sharp increases

in 1979 and 1980 as commuter airlines sought to

open up new markets and an almost equally

sharp drop in 1981 as these markets failed to

materialize. Commuter airports, typically located in small communities, handle a very low

volume of traffic, 2,500 to 5,000 passenger enplanements per year. The major concern about

this category is not capacity but keeping the airport in operation so as to provide essential air

service for the small communities in which they

are located.

Reliever Airports

Reliever airports are a special category of GA

airport whose primary purpose is to reduce congestion at air carrier airports in large and medi-
28 ● Airport and Air Traffic Control System

urn hubs by providing GA users with alternative

operational facilities and aircraft services of

roughly similar quality to those available at hub

airports. The criteria for classification as a reliever airport in NASP are 25,000 itinerant operations or 35,000 local operations annually,

either at present or within the last 2 years. The

reliever airport must also be situated in a SMSA

with a population of at least 500,000 or where

passenger enplanements by scheduled airlines

are at least 250,000 annually. There were 155

airports designated as relievers in the 1980-89

NASP.

General Aviation

GA airports are either private use or public

use, but only the latter are eligible for Federal

development or improvement funds under

NASP. There were approximately 2,200 GA

public-use airports in the 1980 NASP. Capacity

is usually not a concern except at the largest GA

airports, such as Long Beach, Van Nuys, Teterboro, or Opa-Locka, which may require improvements similar to those contemplated at

major hub airports. For most GA airports the

chief concern is upgrading and extending airport

facilities and ATC services so as to accommodate larger and more sophisticated aircraft and

to allow operation under adverse conditions.

These improvements are being sought both to

support the expected growth of GA and to provide facilities comparable to air carrier airports,

thereby permitting diversion of some GA operations from congested hubs.

AIR TRAFFIC SERVICES

The ATC system—the second major part of

the National Airspace System—offers three

basic forms of service: navigation aid (including

landing), flight planning and in-flight advisory

information, and air traffic control.

Navigation

Aid to navigation was the first service provided to civil aviation by the Federal Government. At the end of World War I, the Post

Office undertook to set up a system of beacons

along the original airmail routes to guide aviators at night and in times of poor visibility. By

1927, this airway extended from New ‘fork to

San Francisco, with branches to other major

cities.

In the 1930’s, ground beacons for visual guidance were replaced by two types of low-frequency radio navigation aids—nondirectional

beacons and four-course radio range stations.

The nondirectional beacon emitted a continuous

signal that allowed the pilot to navigate, in a

manner analogous to using a light ground beacon, by homing on the signal with an airborne

direction finder. The radio range station was a

further improvement in that it emitted a directional signal, forming four beacons alined with

respect to the compass, each defining a course.

Pilots listened to a radio receiver and followed

these radio beams from station to station along

the route. The four-course radio range system

was phased out beginning in 1950, after reaching

a maximum deployment of 378 stations. Lowfrequency nondirectional radio beacons are still

in limited use in the United States and widespread use in other parts of the world. *

The technology that supplanted the low-frequency four-course range as the basic navigation

system for civil aviation was very high frequency omnirange (VOR) transmitters, which

were first put in service in 1950. This system had

several advantages over low-frequency radio.

VOR is less subject to interference and aberrations due to weather; it is omnidirectional, permitting the pilot to fly on any chosen radial

rather than only the four courses possible with

the radio range station; and the addition of a

cockpit display freed the pilot from the need to

listen to radio signals continuously. The major

disadvantage of VOR is that signals are blocked

● In 1981, there were 1,095, nondirectional radio beacons in

service in the United States, including 54 military and 734 non-Federal installations.
Ch. 3—The National Airspace System . 29

at the horizon, and navigational signals from a

station can be received over a much smaller area

than low-frequency radio. To provide the same

geographical coverage as the older low-frequency radio system, therefore, a great many

more VOR stations were required. At present,

there are 1,039 VOR stations in operation (930

FAA, 42 military, 67 non-Federal), providing extensive but not complete coverage of the contiguous 48 States and Hawaii and limited coverage of Alaska.

In the 1960’s, the basic VOR system was supplemented by distance measuring equipment

(DME) that permitted measurement of range as

well as direction to a station. The DME used the

distance-measuring portion of a military Tactical Control and Navigation System (TACAN),

colocated with a VOR station to create what is

called a VORTAC. This is the standard airway

navigation aid in use today, and at present all

commercial air carriers have VOR/DME equipmerit. ’ Over 80 percent of GA aircraft are also

equipped with VOR receivers, and over onethird of these also have DME. In addition to the

Federal investment in VORTAC facilities (on the

order of $250 million), there is a very large private investment (roughly $300 million) in airborne navigation equipment to use the present

VORTAC technology. As a result, both the Federal Government and the aviation community

have a strong incentive to protect this investment by prolonging the operational life of their

VORTAC equipment and the airway route

structure based on it.

Nevertheless, VOR—which relies on 30- or

40-year-old technology-has some inherent disadvantages. Because it is a ground-based system, it does not provide coverage of oceanic

areas. Because it is a line-of-sight system, VOR is

of limited usefulness at low altitudes or in mountainous areas. The VOR route structure concentrates traffic along rather narrow channels and

produces a potential for conflict at intersections

where airways cross. Further, navigation from

one fix (intersection) to the next does not always

● Military aircraft are equipped with TACAN, VOR/DME, or

both.

produce the most direct routing from origin to

destination.

Several alternative navigational systems (developed principally for military aviation) are

available, and some are already used in auxiliary

applications by civil aviation. The Omega system, developed by the U.S. Navy, is a low-frequency radio system that provides global coverage. It has been purchased by some airlines for

transoceanic flights. Loran-C (also low-frequency radio), operated by the Coast Guard, is a

maritime navigation system that also covers

most of the continental United States; it affords

very good accuracy and low-altitude coverage,

even in mountainous areas. Some airline and

corporate jet aircraft have self-contained airborne navigation systems such as Doppler radar

or Inertial Navigation System (INS), which are

accurate and are usable worldwide. All of these

new systems permit “area navigation” (RNA V),

whereby the pilot can fly directly between any

two points without restriction to a VOR airway.

There are also available RNAV systems that permit the aircraft to follow direct routings using

VOR as a reference.

Many commercial air carriers and more than 7

percent of GA aircraft (largely business and corporate aircraft) have RNAV capability. Since

1973, FAA has been gradually implementing

RNAV routes in the upper airspace and instituting approach procedures at selected airports

to accommodate aircraft equipped with such

systems. Phasing out the current airways structure and converting to a more flexible system of

area navigation is a process that will require

many years to complete. At present, FAA is

committed to upgrading VORTAC stations to

solid-state equipment at a cost of roughly $210

million (fiscal year 1980 dollars) over the next 10

years. At the same time, FAA must face the

question of adopting new navigation technology

to conform to new international standards

scheduled for consideration by the International

Civil Aviation Organization in 1984. The issue is

not so much selection of a single new navigation

system to replace VORTAC as it is a question of

adopting procedures for worldwide navigation
30 ● Airport and Air Traffic Control System

(especially RNAV) that will be compatible with

several possible technologies.

Landing Aids

A guidance system for approach and landing

is simply a precise, low-altitude form of navigation aid with the additional accuracy and reliability needed for landing aircraft in conditions of

reduced visibility. The standard system now in

use, the Instrument Landing System (ILS), was

first deployed in the early 1940’s although a prototype system was first demonstrated by James

Doolittle in 1929.

ILS provides guidance for approach and landing by two radio beams transmitted from equipment located near the runway. One transmitter,

known as the localizer, emits a narrow beam

alined with the runway centerline. The other

transmitter, the glide slope, provides vertical

guidance along a fixed approach angle of about

3°. These two beams define a sloping approach

path with which the pilot alines the aircraft,

starting at a point 4 to 7 miles from the runway.

Because the ILS is generally not accurate or reliable enough to bring the aircraft all the way onto

the runway surface by instrument reference

alone, the pilot makes a transition to external

visual reference before reaching a prescribed

minimum altitude on the glide slope (the decision height). The decision height varies according to the airport and the type of ILS installation: 200 feet for most airports (category I), but

100 feet on certain runways at some airports

(category II). At present there are 708 category I

and 44 category II ILS installations in commission in the United States. * FAA plans call for installation of ILS at additional sites, primarily

commuter airports, and for modernization of

some 250 existing sites by converting to solidstate equipment and, in the process, upgrading

69 of them to category II capability.

ILS has two major limitations, both of which

affect airport capacity. First, since the ILS does

not provide reliable guidance all the way to

touchdown, there are times and conditions when

the airport must be closed. Such severely reduced visibility occurs less than 1 percent of the

time for U.S. airports as a whole, but when this

happens at a busy airport, traffic can be backed

up not only at the affected airport but also at

alternate landing sites and at airports where traffic originates. The other limitation is that it provides only a single fixed path to the runway—in

effect, a conduit extending 4 to 7 miles from the

runway threshold through which all traffic must

flow. This has an even greater affect on capacity. When visibility is such that the ILS approach

must be used, traffic must be strung out along a

single path and the rate at which landings can be

effected is constrained by the speed and spacing

of aircraft in single file.

The Microwave Landing System (MLS),

which has been under development by FAA for

several years and is now ready for initial deployment, could overcome these limitations of

ILS, which in turn could help improve the flow

of traffic in terminal areas by allowing more

flexibility in segregating and sequencing the arrival of aircraft on the runway. The magnitude

of the resulting capacity gains is subject to some

dispute, however, and not all agree that MLS

would play a major part in reducing terminal

airspace congestion. The MLS is discussed further in chapter 5.’

Flight Planning and

Advisory Information

Timely and accurate information about

weather and flight conditions is vital to airmen,

and FAA perceives this aspect of system operation to be a prime benefit, particularly to the GA

community. Flight planning and information

services take several forms and are provided

partly by FAA and partly by the National Oceanic and Atmospheric Administration (NOAA)

of the Department of Commerce. NOAA publishes maps, aeronautical charts, and related

documents from information furnished by the

FAA. The National Weather Service of NOAA

provides weather maps and reports. FAA pub-

● In addition, there are 48 non-FAA facilities that have category I

ILS installations.

‘Microwave Landing Transition Plan, APO-81-1 (Washington,

D. C.: Federal Aviation Administration, 1981).
Ch. 3—The National Airspace System • 31

lishes manuals, instructions, and notices to airmen (NOTAMs) to help pilots in planning and

executing flights. FAA operates a national

weather teletype network, disseminates weather

information by radio broadcast and recorded

telephone messages, and provides weather briefings. FAA also disseminates to airmen, both preflight and in flight, information concerning the

status of navigation aids, airport conditions,

hazards to flight, and air traffic conditions. FAA

personnel are also available to help pilots in preparing and filing flight plans and to disseminate

these flight plans to other ATC facilities along

the intended route and at the destination.

All of these planning and advisory services are

intended to guide the airman in making use of

the airspace under either of two basic sets of

rules—Visual Flight Rules (VFR) and Instrument

Flight Rules (IFR)—which govern the movement

of all aircraft in the United States. * In general, a

pilot choosing to fly VFR may navigate by any

means available to him: visible landmarks, dead

reckoning, electronic aids (such as VORTAC),

or self-contained systems on board the aircraft.

If he intends to fly at altitudes below 18,000 ft,

he need not file a flight plan or follow prescribed

VOR airways, although many pilots do both for

reasons of convenience. The basic responsibility

for avoiding other aircraft rests with the pilot,

who must rely on visual observation and alertness (the “see and avoid” concept).

In conditions of poor visibility or at altitudes

above 18,000 ft, pilots must fly under IFR. Many

also choose to fly IFR in good visibility because

they feel it affords a higher level of safety and

access to a wider range of ATC services. Under

IFR, the pilot navigates the aircraft by referring

to cockpit instruments and by following instructions from air traffic controllers on the ground.

The pilot is still responsible for seeing and avoiding VFR traffic, when visibility permits, but the

ATC system will provide separation assurance

from other IFR aircraft and, to the extent practical, alert the IFR pilot to threatening VFR aircraft.

● Similar visual and instrument flight rules are in force in foreign

countries that are members of the International Civil Aviation Organization (ICAO). In many cases, ICAO rules are patterned on

the U.S. model,

Photo credit Federal Aviation Administration

A display of air traffic as it appears to a controller

The distinction between VFR and IFR is basic

to ATC and to the safe and efficient use of

airspace, since it not only defines the services

provided to airmen but also structures the

airspace according to pilot qualifications and the

equipment their aircraft must carry. VFR flights

over the contiguous 48 States may not operate at

altitudes above 18,000 ft, which are reserved for

IFR flights. The altitudes between 18,000 and

60,000 ft are designated as positive control

airspace; flights at these levels must have an approved IFR flight plan and be under control of

an ATC facility. Airspace above 60,000 ft is

rarely used by any but military aircraft. Most of

the airspace below 18,000 ft is controlled, but

both VFR and IFR flights are permitted.

The airspace around and above the busiest

airports is designated as a terminal control area

(TCA) and only transponder-equipped aircraft

with specific clearances may operate in it regardless of whether operating under VFR or IFR. All

airports with towers have controlled airspace to

regulate traffic movement. At small airports

without towers, all aircraft operate by the seeand-avoid principle except under instrument

weather conditions. Figure 3 is a schematic rep-
32 ● Airport and Air Traffic Control System

Figure 3.—Airspace Structure

*

45,000 ft.

Continental (FL450)

control

area

Positive Transponder

I Jet control with altitude

routes

I

area encoding

I

required

1—18,000 ft MSL 1

A

14,500 f’ MSL

—.—— ——..——

v

--— -12,500 ft MS L

- -

-

Control areas and transition areas

--3,000 f’ AGL ~

v7

– – 1,200 ft AGL

Airport transition area ‘ 1-

traffic

area

r 700 ft AGL

. . -. ,>. !

AGL - Above ground level

MSL - Mean sea level

FL - Flight level

SOURCE: Federal Aviation Administration.

resentation of the resulting airspace structure; as have a radio if he elects to file a VFR flight plan

the general rule, VFR flights are permitted everywhere except in positive control airspace although clearances are required to operate within

TCAs and at airports with control towers.

The IFR/VFR distinction also governs avionics and pilot qualifications. A VFR flight taking off and landing at a small private field and

flying only in uncontrolled airspace needs little

or no avionic equipment, although a pilot must

or land at an airport with a control tower. Aircraft flying under IFR, on the other hand, are required to have radio and avionics equipment

that will allow them to communicate with all

ATC facilities that will handle the flight from

origin to destination. They must also be instrumented to navigate along airways and to execute

an IFR approach at the destination airport.

These requirements apply to all IFR aircraft, and

Federal Air Regulations also specify additional
Ch. 3—The National Airspace System ● 33

equipment requirements and pilot qualifications

for various classes of air carrier aircraft. In addition, both IFR and VFR aircraft must have transponders that automatically transmit their identity and altitude when they are in TCAs* or at

altitudes above 12,500 feet.

The VFR/IFR distinction also determines the

type of ATC facility that will provide service to

airspace users. There are three general types of

facilities operated by FAA: air route traffic control center (ARTCC), which serve primarily IFR

traffic; airport traffic control towers, which

serve both IFR and VFR aircraft; and flight service stations (FSS), which primarily serve VFR

traffic.

FSS serves three primary purposes: flight

planning and advisory information for all GA

aircraft; the dissemination of flight plans (VFR

and IFR) to other facilities along the intended

route; and operation of teletype networks to furnish information on weather and facility status

to civil and military users. FAA encourages but

does not require pilots flying VFR to file a flight

plan; IFR flights must file a flight plan and obtain clearance to use the airspace. Personnel are

on duty to provide direct briefings and assistance in filing flight plans (counter service), but

most FSS contacts are by telephone or by radio.

If a VFR flight encounters weather or restricted

visibility en route, the pilot (provided he is rated

for instrument flight) can change to an IFR flight

plan while in the air and be placed in contact

with the ATC system. The FSS handles these requests and coordinates changes with towers or

ARTCCs. * *

FSS personnel are also ready to aid VFR pilots

who experience in-flight emergencies. If a pilot is

lost, the FSS will assist him by means of direction-finding equipment or arranging for tracking

by an ATC radar facility. FSS personnel provide

weather reports to pilots aloft and receive and

relay pilot reports on weather and flight conditions. In more serious cases, such as engine trouble or forced landing, the FSS will attempt to

*Altitude-encoding transponders (Mode C) are required only in

Group I TCAs, of which there are nine at present.

● *In the interest of reducing controller workload, this service

was suspended following the controllers’ strike in August 1981.

pinpoint the location and coordinate search and

rescue operations. Flight service stations also

make periodic weather observations and transmit this information by teletype network to

other ATC facilities and U.S. weather reporting

services. Thus, FSS is essentially a communications center, serving general aviation directly

but also providing information services for all

airspace users. Figure 4 illustrates the communication links and the types of facilities that are in

contact with a typical FSS.

FAA operates 317 FSSs, mostly at airports

with VORTAC installations. Since traffic operates out of thousands of airports, much of FSS’s

work is done by means of transcribed messages

and standardized briefings. The importance of

FSS as an onsite facility at airports may thus be

diminishing, and FAA has plans to consolidate

FSSs into about 60 centralized locations. Concurrent with the reduction in the number of

FSSs, FAA plans to increase the amount and

type of on-call and remote services, including

methods for semiautomatic filing of flight plans.

FSS personnel would, however, be available—

but usually at a remote location—to provide

emergency services or to provide direct assistance to airmen. This proposed consolidation of

FSS facilities has been the subject of controversy

in the aviation community because it is feared

that the quality and extent of services might be

diminished and that observations for the National Weather Service might be curtailed.

Air Traffic Control

The essential feature of air traffic control service to airspace users is separation. The need for

this service derives from the simple fact that,

under IFR conditions, the pilot may not be able

to see other aircraft in the surrounding airspace

and will therefore need assistance to maintain

safe separation and reach his destination. Historically, this need came about gradually with

the increasing use of the airspace as the airlines

began to operate under instrument flight conditions in the 1930’s. In 1934 and 1935, the airlines

organized a system for controlling traffic within

roughly 100 miles of Newark, Chicago, and
SOURCE: Federal Aviation Administration.

Cleveland. In 1936, the U.S. Government assumed responsibility for these centers and established five more “airway” centers within the following year.

This “first generation” of separation service

relied solely on radio and telephone communication. At established points along the airways,

pilots were expected to report their time of arrival and altitude and their estimated time of arrival over the next checkpoint. In the ATC center controllers wrote the message on a blackboard and tracked flights by moving a marker

on a tabletop map. In a later improvement,

paper strips marked with flight data were posted

in the order of their estimated arrival at each

reporting point or airway intersection. This

flight-strip system is still available as a backup

system in the event of radar surveillance equipment failure, since it requires only radio communication between the pilot and the controller. To

provide direct pilot-controller contact, especially as traffic density grew, it became necessary in the 1950’s to establish remote communication air-ground stations at distances over 100

miles from ATC centers to relay messages from

pilots to the controller handling their flights.

This greatly improved the safety, capacity, and

efficiency of the control process. In the first

generation system, aircraft flying in the same

direction and altitude were kept 15 minutes

apart in their estimated arrival times at reporting

points. This separation standard depended on

the accuracy of position information and—

equally important—on the speed and reliability

of communicating instructions to resolve potential conflicts. Since the capacity of the ATC system increases as separation standards are reduced, progress therefore depended on further

improvements in both communications and surveillance equipment as the ATC system developed.

The second generation of separation service

came with the introduction of radar after World

War II. In the 1950’s, airport surveillance radars

(ASRs) were introduced at major airports to

provide data on arriving and departing aircraft

within roughly 50 miles* At about the same

time, the Civil Aeronautics Authority (predeces-

● FAA now operates 195 ASRs.
Ch. 3—The National Airspace System ● 3 5

sor to FAA), in coordination with the Air Force,

began purchasing long-range (200-mile) radars

for the en route centers with a view to establishing complete radar coverage of the continental

United States. This was completed in 1965, with

the exception of some gaps in low-altitude

coverages, and today data from multiple radar

sites are relayed to ATC centers, so that radar

contact can be kept with almost every IFR flight.

The introduction of radar allowed continuous

monitoring of actual aircraft progress and the

detection of potential conflicts or hazard situations. The controller, under a process known as

“radar vectoring, ” could direct aircraft away

from thunderstorms, around slower aircraft or

downwind for spacing in the approach area. In

so doing, however, the controller began to

preempt control of heading and altitude from

the pilot for short periods of time. Radar separation standards were greatly reduced from those

of the first generation: 3 miles on approach or

about 2 minutes at piston aircraft speeds.

Despite these improvements, there were still

two major deficiencies in a surveillance system

that relied on raw radar return: the altitude of

the aircraft was not measured; and the identity

of the aircraft could not be established from

radar return alone. In 1958, the newly formed

FAA began development of a so-called “secondary” radar surveillance system in which the

radar beam, as it rotated in the scan of azimuth,

triggered a positive, pulsed-code reply from a

“transponder” (or beacon) on board the aircraft.

This pulse contained information on the identity

and altitude of the aircraft which could be correlated with primary radar return. This development program, known as Project Beacon, led to

adoption of the secondary radar system in 1961,

and it is the standard surveillance method in use

today for separation assurance. All commercial

air carriers and about two-thirds of GA aircraft

are now equipped with transponders* and the

primary radar system has become a backup for

use in the event of equipment malfunction. The

introduction of transponders and the simultaneous development of digitized information

systems and computer-driven traffic displays led

● Slightly less than 30 percent of GA aircraft have altitudeencoding (Mode C) transponders.

to a reduction of controller workload. Automated flight plan processing and dissemination,

introduced at about the same time, further

reduced controller workload by facilitating

handoffs of aircraft from one en route sector to

another and between en route and terminal area

controllers. Collectively, these technological

changes constitute the third generation of air

traffic control.

All of these improvements have simplified and

speeded up the acquisition of information

needed to provide separation service, but they

have not substantially altered the decisionmaking process itself, which still depends upon the

controller’s skill and judgment in directing aircraft to avoid conflicts. In recent years, attempts

have been made to automate the decisionmaking

aspects of separation assurance or to provide a

backup to the controller in the form of computer-derived conflict alerts. Computers can

now perform a simplistic conflict alert function

by making short-term projections of aircraft

tracks and detecting potential conflicts that the

controller may have missed. Since the technique

depends upon all aircraft being equipped with

transponders, however, it does not provide separation assurance between unequipped aircraft.

The introduction of two-way digital communication rather than voice would mark the beginning of a new generation of separation service. In 1969, the Air Traffic Control Advisory

Committee recommended the introduction of an

improved form of radar known as the Discrete

Address Beacon System (DABS). This system

provides selective identification and address and

a two-way, digital data link that allows improved transmission of data between ground

and aircraft, so that much of the routine ATC

information can be displayed in the cockpit for

the pilot. DABS would thus provide more complete and rapid exchange of information than the

present voice radio method. DABS would improve separation service in other ways as well. It

could provide more accurate position and track

data and could lead to more comprehensive

forms of automated conflict detection and resolution. Further, because DABS can interrogate

aircraft selectively it can avoid the overlap of

signals in areas of high traffic density.
36 . Airport and Air Traffic Control System

Another method for providing improved separation assurance is by means of collision avoidance systems on board the aircraft, which would

alert the pilot to converging aircraft and direct

an avoidance maneuver. Airborne collision

avoidance systems, while conceived as a backup

to ground-based separation service, would effectively transfer back to the IFR pilot some of the

see-and-avoid responsibility that now governs

VFR flight. Still another approach to separation

assurance is the use of techniques to meter or

space the movement of aircraft traffic into terminal areas from the en route portion of the system. These are strategic rather than tactical

measures, in that they are directed not at avoiding conflicts per se but at preventing the congested conditions in which conflicts are more

likely to occur. Traffic metering, spacing, and

sequencing techniques are now used by controllers to prevent traffic buildup or undesirable

mixes of aircraft, but for some time FAA has

been seeking to develop automated methods that

will accomplish this smoothing and sorting of

traffic flow without intervention by controllers.

Success of these efforts will depend upon development of computer prediction and resolution

routines that will detect conflicts among flight

plans (rather than flight paths) and issue appropriate instructions before actual conflict occurrs.

SYSTEM ORGANIZATION AND OPERATION

The third major part of the National Airspace

System is the facilities and operational procedures for managing air traffic.

ATC Sectors

From the controller’s viewpoint, the ATC system is made up of many small sectors of airspace, each defined in its horizontal and vertical

extent and each manned by a controller with one

or more assistants. Each sector has one or more

assigned radio frequencies used by aircraft operating in the sector. As the flight moves from sector to sector, the pilot is instructed to change

radio frequencies and establish contact with the

next controller. On the ground, the controller

must perform this “hand off” according to strict

procedures whereby the next controller must indicate willingness to accept the incoming aircraft

and establish positive control when the pilot

makes radio contact before relieving the first

controller of responsibility for the flight.

Since the number of aircraft that can be under

control on a single radio frequency at any one

time is limited to roughly a dozen, sector boundaries must be readjusted to make the sectors

smaller as traffic density grows. At some point,

however, resectorization becomes inefficient;

the activity associated with handing off and receiving aircraft begins to interfere with the routine workload of controlling traffic within the

sector. To help manage this workload, the sectors around busy airports are designed in such a

way that arriving or departing traffic is channeled into airspace corridors, in which aircraft

are spaced so as to arrive at sector boundaries at

regular intervals. While this procedure facilitates

the task of air traffic control, it results in longer

and more fuel-consuming paths for aircraft,

which have to follow climb and descent paths

that are less than optimal. To this extent, the

performance characteristics of the ATC system

aggravate the effects of congestion in busy

airspace and detract from the overall efficiency

of airspace use.

ATC Facilities

Organizationally, the facilities that control air

traffic are of three types: en route centers, terminal area facilities (approach/departure control and airport towers), and flight service stations. The first handles primarily IFR traffic; terminal area facilities and flight service stations

handle both IFR and VFR flights. In addition,

flight service stations perform information collection and dissemination activities that are of

systemwide benefit.
Ch. 3—The National Airspace System ● 37

The en route portion of the ATC system consists of 20 ARTCCs, * each reponsible for a major geographic region of the continental United

States (see figs. 5 and 6). An ARTCC contains

between 12 and 25 sectors which control traffic

on the airways within the region, and ARTCC

airspace is further divided into low-altitude sectors primarily used by propeller aircraft and

high-altitude jet sectors. When aircraft are in

level cruise, management of traffic is relatively

simple and problems are infrequent. The sectors

that are difficult to control are those where

flights are climbing or descending around a major airport. Since these en route sectors are feeding aircraft into and out of terminal areas, the

task of control also becomes complicated if the

airport is operating near capacity. En route controllers may be required to delay the passage of

aircraft out of their sector in order to meter traffic flow into terminal areas.

At smaller airports, aircraft leaving control of

an ARTCC pass directly to control by the air-

*In addition, there are two ARTCCs located outside the continental United States, in Hawaii and Puerto Rico.

port tower. At major hubs, however, there is an

intermediate ATC facility called terminal radar

approach control (TRACON) located at the airport. The TRACON (or “IFR room”) handles arriving and departing traffic within roughly 40

miles of the airport—sequencing and spacing arrivals for landing on one or more runways, and

sometimes at more than one airport. The

TRACON also vectors departing aircraft along

climbout corridors into en route airspace. The

approach and departure controllers at a

TRACON exercise a high degree of control over

aircraft and must monitor the progress of each

aircraft closely, as well as coordinate their activities with the ARTCCs from which they are

receiving traffic and with the towers that are

handling the takeoffs and landings at the airport

itself.

Tower personnel control the flow of traffic to

and from the runways and on ramps and taxiways connecting to the terminal. Tower controllers are the only ATC personnel that actually

have aircraft under visual observation, although

at larger airports they rely heavily on radar for

surveillance. Figure 7 illustrates the activities of

Figure 5.— Air Route Traffic Control Center Boundaries

n

SOURCE: Federal Aviation Administration.
38 • Airport and Air Traffic Control System

Figure 6.—Connections of a Typical ARTCC With Other Facilities

SOURCE: Federal Aviation Administration.

ATC terminal and en route facilities handling a

typical IFR flight.

There are currently 431 airports with towers

operated by FAA, of which 234 are approach

control towers and the remainder are nonapproach control towers. An approach control

tower, with its associated TRACON, provides

separation and instrument landing services for

IFR traffic and is also responsible for integrating

VFR traffic into the approach Pattern. Figure 8

available at a large airport with an approach

control tower. A nonapproach control tower is

responsible for assisting traffic by providing

weather, traffic, and runway information for all

arrivals (VFR or IFR), but does not provide ILS

or separation assurance.

Airspace Users

The users are the fourth major part of the National Airspace System. They cover a wide specillustrates the equipment and facilities typically trum in skill and experience, types of aircraft
Ch. 3—The National Airspace System ● 39

Figure 7.—ATC Activities for a Typical IFR Flight

Chicago O’Hare

International Airport

At the departure gate, pilot confirms altitude, speed, route and

estimated flight time with controller in the Chicago tower at

O’Hare. After flight clearance,

pilot contacts Chicago ground

control for taxiing instructions

and proceeds to runway.

Thirty miles farther in the flight,

the departure controller transfers

responsibility by instructing the

pilot to contact a particular controller at the en-route Chicago

Center, located in Aurora, Ill.

The controller at Chicago Center

tracks the plane as it climbs to approximately 23,000 feet, then

hands over the flight to another

controller at the center who

handles flights above that height.

The airplane reaches cruising altitude of 33,000 feet about 100

miles east of Chicago.

The plane continues its descent

and New York Center hands off

responsibility for the flight to the

local New York approach-control

facility at Garden City, N. Y.,

where a controller lines up the

plane for its final approach to La

Guardia Airport.

When ready for takeoff, pilot once

again contacts controller in the

Chicago tower who, using radar

and his own view from the tower,

clears airplane for takeoff.

One mile away from takeoff point,

the controller in the Chicago

tower transfers responsibility for

the fright to a departure controller, also at O’Hare airport, who

directs the pilot to the proper

course for the first leg of the

flight.

The next handoff takes place as

Chicago Center passes responsibility to the en-route Cleveland

Center in Oberlin, Ohio. One controller tracks the airplane and

transfers responsibility to a colleague as the fright passes from

one sector to another.

About 6 miles from the runway,

responsibility passes to the tower

at La Guardia, where a controller

monitors the aircraft’s instrument

landing. The last handoff of the

flight is made from tower to

ground control, which directs the

plane to its assigned gate.

SOURCE Newsweek

flown, and demands for air traffic services. They

can be grouped in three categories—commercial,

GA, and military—with GA exhibiting the

greatest diversity. Table 2 is a summary of the

U.S. pilot population in 1980 according to the

type of license held and the percentage with instrument ratings, i.e., those qualified to use the

airspace under IFR. The table shows that about

42 percent of all pilots are now IFR qualified; 10

years ago the percentage was about 30 percent.

Almost all of this growth has occurred in the

private (GA) category.

Table 3, which is a breakdown of aviation activity according to type of aircraft and hours

Table 2.-U.S. Pilot Population, 1980

Instrument

Pilot group Number rated Percent

Private (GA):

Student. . . . . . . . . . . 199,833 0 0

Private license ... , . 357,479 39,347 11

Commercial:

Commercial a . . . . . . 183,422 147,741 81

Airline transport

Iicense b

. . . . . . . . . 69,569 69,569 100

Total (excluding

students) . . . . . 610,490 256,547 42

‘A cO~mercla\ license allows the holder to work aS a pilot and operate on air

craft providing passenger service for hire.

bA more advanced rating required of pIlols for air Carrier airlint3S.

SOURCE: FAA Statist/ca/ I-/arrdbook of Aviation, 7980.
40 ● Airport and Air Traffic Control System

Figure 8.—ATC Facilities and Equipment at a Typical Large Airport

SOURCE: Federal Aviation Administration.

Table 3.–Summary of Aviation Activity, 1980

Number of Percent Estimated hours flown (millions)

User group aircraft IFR-equipped a

Total IFRa Percent IFRa

Commercial air carrier:

Piston. . . . . . . . . . . . . . . . . 595 100 0.48 0.48 100

Turboprop . . . . . . . . . . . . . 682 100 1.11 1.11 100

Turbojet . . . . . . . . . . . . . . . 2,526 100 6.63 6.63 100

Rotorcraft . . . . . . . . . . . . . 2 100 <.01 <.01 100

Total . . . . . . . . . . . . . . . . 3,805 100 8.22 8.22b100

General aviation:

Piston (single-engine). . . . 168,435 34 28.34 2.83 10

Piston (multiengined) . . . . . 24,578 91 6.41 2.82 44

Turboprop . . . . . . . . . . . . . 4,090 99 2.24 1.66 74

Turbojet . . . . . . . . . . . . . . . 2,992 100 1.33 1.22 92

Rotorcraft . . . . . . . . . . . . . 6,001 2 2.34 <.01 0

Total . . . . . . . . . . . . . . . . 206,096 42 40.66 8.53 21

Military (all types) . . . . . . . . . 18,969 N.A. 5.26 N.A. N.A.

aEStirnateS based on 1979 survey of general aviation aircraft.

blncludes 7,00 million hours for air carriers (all classes); 0.09 million hours for air taxi; 0.99 dlliOn hours fOr COmmuWrS; and

0.14 million hours for air cargo.

SOURCES: FAA Statistical Handbook of Aviation, 1980; General Aviation Activity and Avionics Survey, 1979, FAA-MS-B1-1,

January 1981.

flown, indicates the relative airspace use and de- a class, general aviation aircraft (98 percent of

mand for IFR services among user categories. the civil fleet) fly only about 1 hour in 5 under

Commercial air carrier aircraft (including com- IFR, but this figure is deceptive. Turboprop and

muters and air taxis) make up less than 2 percent turbojet GA aircraft (those with performance

of the civil aviation fleet, but they account for characteristics and usage most like air carrier airabout 17 percent of hours flown and almost half craft) are virtually all IFR-equipped and log a

of the total IFR hours flown in civil aviation. As very high percentage of their flight hours under
Ch. 3—The National Airspace System ● 41

IFR. The growing numbers and increasing tendency of these more sophisticated GA aircraft to

operate under IFR has caused the general increase in ATC system workload over the past 10

years. At present, GA aircraft account for 51

percent of all IFR flight hours, 30 percent of IFR

aircraft handled by ARTCCs and 45 percent of

instrument approaches at FAA control facilities.

Commercial air carriers are the most homogeneous category of airspace users, although

there are some differences between trunkline

operators and commuter or air taxi operators in

terms of demand for ATC services. Certificated

route air carriers follow established schedules

and operate in and out of larger and better

equipped airports. They have large, high-performance aircraft that operate at altitudes above

18,000 feet en route, where they have only

minimal contact with aircraft not under the positive control of the ATC system. In terminal

areas, however, they share the airspace and facilities with all types of traffic and must compete

for airport access with other users. Airline pilots

are highly proficient and thoroughly familiar

with the rules and procedures under which they

must operate. All air carrier flights are conducted under IFR, regardless of visibility, in

order to avail themselves of the full range of

services, especially separation assurance.

Commuter airlines also follow established

schedules and are crewed by professional pilots.

However, they characteristically operate smaller

and lower performance aircraft in airspace that

must often be shared with GA aircraft, including

those operating under VFR. As commuter operations have grown in volume, they have created

extra demands on the airport and ATC systems.

At one end of their flight they use hub airports

along with other commercial carriers and so may

contribute to the growing congestion at major

air traffic nodes. Their aircraft are IFR-equipped

and can operate under IFR plans like other

scheduled air carriers, but this capability cannot

be used to full advantage unless the airport at

the other end of the flight, typically a small community airport, is also capable of IFR operation.

Thus, the growth of commuter air service creates pressure on FAA to install instrument landing aids and control facilities towers at more

smaller airports.

GA aircraft include virtually all types, ranging

from jet aircraft like those used by scheduled air

carriers to small single-engine planes that are

used only for recreation. Most are small, lowperformance aircraft that operate only at low altitudes under VFR, and many use only GA airports and never come into contact with the en

route and terminal control facilities of the ATC

system. However, there is increasing use of more

sophisticated, IFR-equipped aircraft by businesses and corporations, many of whom operate

their fleets in a way that approximates that of

small airlines. By using larger aircraft and equipping them with the latest avionics, the business

portion of the GA fleet creates demands for

ATC services that are indistinguishable from

commercial airspace users.

It is the disparate nature of GA that makes it

increasingly difficult to accommodate this class

of users in NAS. The tendency of GA aircraft

owners at the upper end of the spectrum to upgrade the performance and avionic equipment of

their aircraft increases the demand for IFR services and for terminal airspace at major airports.

In response, FAA finds it necessary to increase

the extent of controlled airspace and to improve

ATC facilities at major airports. These actions,

however, tend to crowd out other types of GA,

typically VFR users who would prefer not to

participate in the IFR system but are forced to do

so or forego access to high-density terminal

areas. The safety of mixed IFR-VFR traffic is the

major concern, but in imposing measures to separate and control this traffic, the ATC system

creates more restrictions on airspace use and

raises the level of aircraft equippage and pilot

qualification necessary for access to the airspace.

Military operations can be placed in two

broad categories. Many operations are similar to

GA, but others involve high-performance aircraft operating in airspace where they are subject to control by the ATC system. Front an operational point of view, military flight activities

comprise a subsystem that must be fully inte-
42 • Airport and Air Traffic Control System

grated within NAS; but military aviation has

unique requirements that must also be met, and

these requirements sometimes conflict with civil

aviation uses. Training areas and low-level

routes that are used for training by military aircraft are set aside and clearly indicated on the

standard navigation charts. The military services would like to have ranges located near their

bases in order to cut down transit time and maximize the time aircrews spend in operational exercises. Civilian users, on the other hand, are

forced to detour around these areas at considerable expense in both time and fuel. FAA is

charged with coordinating the development of

ATC systems and services with the armed

forces, so that a maximum degree of compatibility between the civil and military aviation can

be achieved.
Chapter 4

AVIATION GROWTH SCENARIOS

Photo credit: Federal Aviation Administration

A busy airport terminal
Contents

Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

FAA Aviation Forecasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Baseline Scenarios: Procedures and Assumptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Alternative Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Other Aviation Forecasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

Forecast Structures and Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Comparison and Critique of Forecasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Factors Affecting Traffic Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

U.S. Economic and Regulatory Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Deregulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Industry Maturity and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Fuel and Labor Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Financing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Substitution for Air Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Strike Impacts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Implications for Airport Congestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Continued Growth and Airport Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Redistribution of System Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Expanded Capacity and Improved Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

List of Tables

Table Page

4. Comparison of Selected Economic Assumptions and Aviation

Growth Predictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

5. Aviation Growth Assumptions for ’’Redistribution” Scenarios, Domestic

Service, 48 States. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

List of Figures

Figure Page

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22*

FAA Tower Workload, Actual and Forecast 1960-93 . . . . . . . . . . . . . . . . . . . . . . 46

FAA En Route Workload, Actual and Forecast 1960-2000 . . . . . . . . . . . . . . . . . . 47

FAA Flight Service Workload, Actual and Forecast 1960-2000. . . . . . . . . . . . . . . 48

Tower Operations, Actual and Forecast 1974-93 . . . . . . . . . . . . . . . . . . . . . . . . . . so

Instrument Operations, Actual and Forecast 1974-93 . . . . . . . . . . . . . . . . . . . . . . 51

IFR Aircraft Handled by En Route Centers, Actual and Forecast,

1974-93 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Total Flight Service Station Activities, Actual and Forecasq 1943-93 . . . . . . . . . . 52

Projected U.S. Certificated Air Carrier Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Possible Long-Term Impacts of PATCO Strike on ATC Workload Levels . . . . . 58

Activity at Top 50 Commercial Airports-48 States, 1978. ......, . . . . . . . . . . . 60

Airport Airside Capacity Perspectivte-Low Economic Growth Scenario . . . . . . 60

Airport Airside Capacity Perspective-Average Economic Growth Scenario . . . 61

Airport Airside Capacity Perspective-High Economic Growth Scenario . . . . . . 61

Number of Commercial Airports Overcapacity-Year2000,

48 Contiguous States. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Chapter 4

AVIATION GROWTH SCENARIOS

INTRODUCTION

There is a general consensus that domestic

aviation activity will increase over the next 10

to 20 years, and with it the demands placed on

the Nation’s airports and air traffic control

(ATC) system. There is far less agreement, however, about how much growth there will be,

how it will be distributed, and how it will affect

the future characteristics of the National Airspace System (NAS). As a result, there is uncertainty about where system improvements will be

needed, and how soon.

Federal Aviation Administration (FAA) plans

for the modernization and expansion of the NAS

are predicated on the continued rapid growth of

air traffic and ATC workloads. Preliminary figures for the most recent FAA “Aviation Forecasts” indicate that the number of aircraft using

the system will double by 2000 and that, between 1981 and 1993, total operations will increase by 56 percent at en route ATC centers, by

60 percent at FAA-towered airports, and by 88

percent at flight service stations.

Accommodating this anticipated demand

growth has been a primary justification for proposed investments in system improvements, but

FAA’s forecasts have consistently proven to be

too high in the past. In part, this is due to the

way in which they are made: FAA makes its

forecasts on the assumption that present trends

will continue, that there will be no constraints

on growth, and that proposed improvements

will in fact be made.

Comparison with other aviation forecasts is

difficult, since only FAA projects ATC workloads, but it is of interest that some recent forecasts of other measures of demand have been

higher than FAA’s. In all such projections, however, there is considerable uncertainty about a

number of factors that might affect future

growth and system requirements, such as U.S.

economic growth, fuel prices and availability,

airline profitability, new technology, and the

possibility of significantly higher aviation user

fees. Industry maturity may lead to a levelingoff of airline operations, and changes in route

structure may lead to a more even distribution

of these operations throughout the system. Even

greater uncertainty surrounds the effects of

airline deregulation and the long-term impacts

of the Professional Air Traffic Controllers

(PATCO) walkout.

As a result of these uncertainties, there are

valid questions about the accuracy and usefulness of any projection of aviation activity over

10 or 20 years. At present, no individual projection—including FAA’s-should be considered

more than a broad estimate. Collectively, such

projections indicate a likely range of possible

futures for NAS and its ATC requirements; but

because they are based on similar assumptions

and similar forecasting procedures, they may

also be subject to similar errors.

This chapter examines and compares a number of projections, but its main focus is on the

procedures and assumptions underlying the

aviation forecasts on which FAA will base its

1982 system plan. The purpose of this examination is to provide some sense of the range of possible future demand for aviation facilities and

services, in order to assist Congress in making its

decisions about long-lived investments in both

airports and ATC equipment.

FAA AVIATION FORECASTS

FAA is the most continuous, comprehensive,

and detailed source of aviation projections. Its

“Aviation Forecasts” are made annually by the

Office of Aviation Policy and Plans (OAPP) in

support of current operations and as a basis for

long-range planning. Many other organizations

45
46 • Airport and Air Traffic Control System

also use FAA’s forecasts as the basis for their

own long-range planning activities.

However, FAA has a poor forecasting record:

over the past 15 years its predictions have consistently been too high, often by 50 percent or

more. Figures 9, 10, and 11 compare past forecasts with actual levels of operations at FAA

towers, en route centers, and flight service stations. They show that the workloads originally

forecast for fiscal year 1981 were between 50 and

180 percent higher than what actually occurred;

in more recent forecasts this level of demand on

the ATC system is not expected until the 1990’s

or later.

Several unforeseeable events combined to

cause these errors, including the 1973 oil embargo, sharp increases in fuel prices, rising inflation and interest rates, and airline deregulation.

These factors and other pertinent changes in historical trends are now reflected in FAA forecasts, but current expectations may once again

be betrayed by unanticipated developments in

the future. If key assumptions are overly optimistic, the resulting projections will once again

be too high.

Three sets of FAA forecasts were compared in

detail for this review: those of September 1978,

which predate the Airline Deregulation Act, and

those of 1979 and 1980. The year-by-year forecasts for 1982-93, due in October 1981, were

“sent to the shredders instead of the printers” (in

the words of the Director of OAPP) because the

Figure 9.— FAA Tower Workload, Actual and Forecast, 1960-93

I

i *1971a

160

140 - 1967 *

*1975

120 -

Difference between

actual and forecast 1979 *

100 - 1977* * * *

1978 1980

60 -

60 -

40 “

1981

20

t

I

I

I

o

I I I I I 1 I

1960 1965 1970 1975 1980 1985 1990 1995 2000

Source. Off Ice of Technology Assessment, from Federal Aviation Administration data.
Ch. 4—Aviation Growth Scenarios ● 4 7

Figure 10.— FAA En Route Workload, Actual and Forecast, 1960-2000

I

Difference between ~

actual and forecast ~

*

1973* *1975

I

I

I

I

i

I

I

NASP*

1982

*1961

*1980

I

1 I I

I

1 I I I

1960 1965 1970 1975 1980 1985 1990 1995 2000

Source Office of Technology Assessment, from Federal Aviation Admlnistratlon data

uncertain impacts of the PATCO walkout had

invalidated the short-term projections. Preliminary long-term figures only are used in the following discussion and accompanying graphics,

but these projections are somewhat higher than

those of 1980 despite a decline in overall activity

since 1979. Forecasting procedures, assumptions, and scenario specifications are based on

the last published forecast, that of September

1980.

Baseline Scenarios: Procedures

and Assumptions

As described in the 1980 “Aviation Forecasts, ”

FAA predictions are based on a combination of

econometric modeling, trend extrapolation, and

expert judgment. Forecasts of key economic indicators are prepared by Wharton Econometric

Forecasting Associates, Inc., using their longterm industry and economic forecasting model.

In the withdrawn 1981 forecasts, however, the

baseline scenario is based on economic projections supplied by the Office of Management and

Budget (OMB) rather than the Wharton model.

Aviation activity levels and ATC workloads are

derived from these economic indicators by

means of aviation submodels designed and run

by FM itself.

The baseline (or most probable) projections

are based on the general assumption of unconstrained growth—that past trends will continue

and that there will be no change in the relationships between economic activity and aviation

variables. Specific assumptions about the various user groups include the following:
48 • Airport and Air Traffic Control System

Figure 11.— FAA Flight Service Workload, Actual and Forecast, 1960-2000

150

140

F

130

r

120

110 [

Di

975

*1978

‘1977

979* *

NASP

1982

*

*1981

*1980

1960 1965 1970 1975 1980

Source. Of ftce of Technology Assessment, from Federal Aviation Admlnlstratlon data.

●

●

●

Federal policy—no change in Government

policy toward the aviation industry (i.e.,

airline deregulation goes forward, existing ●

noise and pollution standards are implemented, but no new environmental or policy constraints—such as higher user fees—

are imposed).

General aviation —continued rapid growth

of business and commercial GA (i. e., larger

turboprops and jets used as corporate air- ●

craft or air taxis) and continued availability

of aviation fuel, although prices rise more

rapidly than the consumer price index.

Air carriers—additional mergers, resulting

in route optimization and more efficient

fleet utilization, and continued replacement

1985 1990 1995 2000

of older equipment with larger, quieter,

more fuel-efficient aircraft.

Commuter carriers-a decrease in the number of carriers as competition leads to mergers, no loss of competitiveness with the

personal automobile, increases in average

aircraft size and stage length, and a relatively stable, mature industry after 1984.

FAA workloads—increases in the number

of FAA-towered airports and terminal control areas, which will tend to increase the

number of IFR operations and flight plan

filings, and greater utilization of flight services due to increased convenience and improved services.
Ch. 4—Aviation Growth Scenarios ● 4 9

Photo credit: Business and Commercial Aviation Magazine

Business and commercial aviation—a growing sector

Alternative Scenarios

Because of the uncertainties involved in trying

to predict the future, FAA forecasts include not

only a baseline scenario (the most likely foreseeable outcome) but also alternative scenarios that

reflect what might happen if there were major

changes in the driving economic, societal, or

political factors. Higher and lower economic

projections from the Wharton model are run

through FAA aviation submodels, and the formal techniques of trend-impact analysis and

cross-impact analysis are used to determine the

further effects of other events or changes.

Because FAA varies several factors at once,

however, it is difficult to assess the sensitivity of

the projections to changes in any specific variable. In some cases, moreover, the scenario specifications are so extreme that they undermine

the credibility of the resulting projections. Finally, the resulting range of possible outcomes over

an 12-year projection is so wide that the alternative scenarios may be of little value for longrange planning purposes. In the 1980 forecasts,

for example, the alternative projections of FAA

workloads in 1993 were as much as 40 percent

higher or 25 percent lower than the baseline.

This “range of uncertainty” has increased in recent forecasts (see below).

In 1978 and 1979 there were two alternatives,

“high prosperity/slow growth” and “rapid

growth/stagflation, ” respectively. In 1980 there

were three alternatives, with the following scenario specifications:

●

●

●

“Economic expansion’’—rapid economic

growth accompanied by a resurgence of the

work ethic, attempts to reestablish U.S.

military and economic preeminence in the

world, easing of Federal environmental restrictions and market intervention, “tremendous increases” in user fees (especially

GA) for airports and ATC services as Federal subsidy of system costs is eliminated,

but strong growth in corporate and personal flying due to continued business dispersal and mobile lifestyles.

“Energy conservation’’—aviation becomes

a “special target” of Federal efforts to

achieve energy independence through regulation and taxation, U.S. lifestyle shifts

toward that of “a more slow-paced culture, ” increasingly stringent environmental

standards and the closing of some metropolitan airports, reestablishment of Federal

control over airline routes and fares, and

severe constraints on GA (including higher

user fees, fuel rationing, and banning from

hub airports).

“Stagflation” -prolonged worldwide recession, strong Federal intervention through

nationalization and reorganization of aviation and other industries, severe rationing
50 ● Airport and Air Traffic Control System

and high prices to encourage energy conservation, increased defense spending and welfare costs, Federal aid keeps major hubs

open but many GA airports close and air

service to small communities deteriorates,

and both business and government make

more use of teleconferencing and other substitutes for personal travel.

Preliminary projections for the 1981 “Aviation Forecasts” also include three alternative scenarios: “economic expansion, ” “Wharton Econometric Model, ” ‘4

stagflation. ” The middle scenario reflects the baseline Wharton economic

indicators and would have been called the “baseline” scenario in past years; the 1981 baseline,

however, is based on OMB’s economic projections, which are closer to those of 1980 “economic expansion” scenario (3.6 and 3.9 percent

average real GNP growth per year, respectively). “Energy conservation” was dropped; the

specifications for the other scenarios remain the

same as for 1980.

FAA projections of ATC workloads from recent “Aviation Forecasts” are presented in figures 12 through 15. Several features of these projections are worth noting:

●

●

Figure 12.—Tower Operations, Actual

the spread between high and low projections has increased dramatically, suggesting

greater uncertainty about future trends;

the overall range of the projections is lower,

suggesting less-confidence ‘about the probability of rapid growth;

and Forecast, 1974-93

I

x

x

X

1981

50 I I 1 I 1 I I I I i I 1 1 I I I I I I I

1975 1978 1979 1980 1981 1985 1990 1991 1992 1993

SOURCE: Office of Technology Assessment, from Federal Aviation Administration data.
Ch. 4—Aviation Growth Scenarios ● 5 1

Figure 13.— Instrument Operations, Actual and Forecast, 1974-93

A = Historical data

* = Baseline scenarios

● = Alternative scenarios

x

x

x

x

I I I I I I I I I i I I I

20 I I I I I 1 1

1975 1978 19791980 1981 1985 1990 1991 19921993

SOURCE: Office of Technology Assessment, from Federal Aviation Administration data.

Figure 14.— IFR Aircraft Handled by En Route Centers, Actual and Forecast, 1974-93

.

.

J I I I 1 I I I I I 1 I I I I I I I I f

1975 1978 19791980 1981 1965 1990 1991 1992 1993

SOURCE: Office of Technology Assessment, from Federal Aviation Administration data.
52 • Airport and Air Traffic Control System

Figure 15.—Total Flight Service Station Activities, Actual and Forecast, 1974-93

I

● the baseline projections, on which FAA

bases its system plans, have nevertheless

moved from the middle of the overall range

toward the upper end; and

. the baseline projections are higher in 1981

than in 1980, despite changes in the historical data that would seemingly have caused

them to be lower.

The reason for the growing uncertainty in recent

“Aviation Forecasts” is not immediately clear.

However, in combination with FAA’s poor forecasting record in the past (see figs. 9, 10, and

11), it raises questions about the usefulness of

FAA forecasts as a guide to decisions about

long-term investments in system improvements

and expansion.

OTHER AVIATION FORECASTS

Long-range forecasts of aviation activity are

also made by a number of organizations other

than FAA, including airlines, aerospace manufacturers, investment firms, and private consultants. The scope and emphasis of these forecasts

differ according to the purposes and interests of

those who make them; understandably, only

FAA projects FAA workloads. Nevertheless,

they follow the same general approach and employ the same general techniques of analysis and

projection. In some cases, however, there are

significant differences in their assumptions

about the specific variables, trends, or events

relevant to the future growth of domestic aviation.

OTA reviewed several forecasts about which

the available documentation was sufficiently detailed to permit comparison with FAA projections:

• Boeing Commercial Aircraft Co, —These

forecasts aim primarily at identifying the
Ch. 4—Aviation Growth Scenarios ● 5 3

world market for aircraft in the commercial

fleet, rather than the level or patterns of airline operations. Two sets of projections

were reviewed: “Dimensions of Airline

Growth” (March 1980) and “Current Market Outlook” (November 1981); both are

based on economic projections from Case

Econometrics.

• Transportation Research Board (TRB). —

This is not a regularly published forecast

but rather a result of the ongoing activities

of the Aviation Forecasting Committee of

TRB, which is part of the National Research

Council of the National Academy of Sciences. Published in August 1981 as “Assumptions and Issues Influencing the Future

Growth of the Aviation Industry,” the forecast represents the consensus of forecasting

workshop participants representing most

segments of the aviation community.

● Office of Technology Assessment (OTA). —

These projections were commissioned by

OTA to provide different kinds of information than was provided by the other major

forecasts. In particular, its structure and assumptions are designed to project the distribution as well as the volume of future aviation activity, in order to determine its impact on airport congestion and ATC capacity (see below). It is thus a “conditional”

forecast, since its different assumptions require a change in current traffic patterns

and industry structure.

● Other Aviation Forecasts. —Recent updates

to the 1975 Air Transport Association

(ATA) forecast became available during the

course of this study, as did the most recent

edition of Lockheed-California Co. ’s regularly published “World Air Traffic Forecast.” The ATA forecast focuses on the financial performance and capital needs of

the airline industry, while the Lockheed report emphasizes international rather than

domestic traffic. However, neither report

presents its forecast on a level of detail consistent with the above forecasts, and as a result they are given only cursory treatment

in the discussion that follows. The judgments and informal forecasts of a number

of other sources have also been considered

in OTA’s analysis.

Forecast Structures and Assumptions

Table 4 presents the specific features and results of the six forecasts that have been studied

in detail. In each case, the forecast begins by assuming the macroeconomic indicators that are

believed to be the driving force behind air traffic

growth, and then uses these variables to generate the growth rates and absolute levels of aviation activity at the end of the forecast period.

Although disposable personal income (DPI) appears to be the most important driving variable

in most of the forecasts, the direct link between

macroeconomic forecasts and traffic forecasts is

seldom explictly given.

On the basis of their economic projections,

the forecasts then derive growth rates and actual

levels of commercial air traffic in terms of revenue passenger miles (RPMs). FAA and OTA

forecasts are the only ones that include explicit

reference to GA operations; given the increasing

importance of GA activity, its absence is a major

shortcoming in the other forecasts. Similarly,

only FAA’s “Aviation Forecasts” proceed from

traffic levels to FAA workloads; lacking this further analysis, the other major forecasts (including OTA’s) are useful only for purposes of comparison in evaluating the traffic growth and aircraft fleet mixes that the ATC system would

need to accommodate.

All of the projections include alternative scenarios that reflect different assumptions about

economic growth, typically referred to as low,

medium, and high. The most recent FAA forecasts contain four scenarios, but only the baseline scenario is described in detail. Beyond these

scenario specifications, none of the forecasts

postulates specific events that might affect traffic

growth of system evolution; all of them assume—explicitly or implicitly—that no “major

catastrophe” will occur. (The PATCO strike and

subsequent traffic restrictions may not constitute such a catastrophe, but they do affect the

short-term prospects of growth and may affect

long-term patterns. This has created sufficient
54 • Airport and Air Traffic Control System

Table 4.-Comparison of Selected Economic Assumptions and Aviation Growth Predictions

Real GNPa growth Real DPlb growth RPMc growth

(percent/year) (percent/year) (percent/ year) Load factor

RPMs 1991 1991

Forecast 1979-86 1986-91 1979-86 1986-91 1979-86 1986-91 (millions) (percent)

FAA 1978 . . . . . . . . . . high

med

low

FAA 1979 . . . . . . . . . . high

med

low

FAA 1980 . . . . . . . . . . high

med

alt

low

FAA 1981 . . . . . . . . . . high

OMB

med

low

Boeing 1980 . . . . . . . . high

low

Boeing 1981 . . . . . . . . high

low

TRB 1981 . . . . . . . . . . high

med

low

OTA 1981 . . . . . . . . . . high

med

low

Range of all

forecasts . . . . . . . . high

med

low

aGross national product.

bDlsposable personal income.

cRevenue passenger miles.

4.4

3.3

2.8

4.0

2.8

2.5

2.3

3.6

3.0

2.4

4.3

3.2

2.4

3.7

2.9

2.1

N/A

N/A

N/A

3.0

2.6

4.3

3.4

2.5

3.0-4.5

2.7-3.6

2.0-2.8

4.6

3.2

2.5

4.9

2.8

2.1

2.9

4.4

3.7

2.9

3.9

2.5

1.8

2.3

3.3

3.1

3.0

3.5

2.8

2.2

4.3

3.2

2.4

3.8

2.8

1.9

NIA

NIA

N/A

N/A

N/A

N/A

N/A

4.3

3.4

2.5

3.8-4.6

2.7-3.4

1.7-2.5

4.4

3.1

2.2

5.7

2.8

1.6

3.0

3.5

2.8

2.2

6.8

5.4

2.8

6.0

5.5

4.4

4.8

4.9

6.5

4.6

7.0

5.8

4.3

3.6

N/A

N/A

N/A

7.3

4.6

N/A

N/A

7.5

5.5

4.1

5.8-7.5

4.3-7.0

3.6-4.6

uncertainty that FAA has delayed publication of

the 1981 forecasts until the impacts can be assessed. )

Comparison and Critique of Forecasts

All of the major forecasts assume roughly similar economic growth rates. FAA’s projections

have tended to be lower than the others and had

become more so in recent years, although the

preliminary figures for the withdrawn 1981 forecast reflect OMB’s optimism about future economic growth. Nevertheless, given the range of

forecast growth rates, the differences between

the individual economic assumptions are probably not significant. In terms of aviation-specific

factors, there also seems to be general agreement

among the projections about variables such as

load factors, aircraft size, and stage length.

Not surprisingly, the resulting growth rates

for domestic RPMs are also quite similar. OTA’s

projections for RPMs tend to be at the upper end

of the range for all the forecasts. The 1980 FAA

4.6

4.5

4.4

6.7

4.2

4.0

3.7

5.5

3.9

7.0

406

369

308

426

365

336

405

341

342

314

N/A

346

N/A

N/A

434

354

358

336

N/A

450

N/A

443

360

311

405-600

341-460

311-450

60.0

60.0

60.0

62.0

62.0

62.0

63.3

63.3

63.3

63.3

N/A

N/A

N/A

N/A

66.2

66.2

N/A

N/A

N/A

63.0

N/A

60.0

60.0

60.0

60.0 -66.2

forecasts are slightly but not significantly lower

than the others. Despite the more optimistic economic assumptions, the 1981 FAA forecasts (if

and when published) will probably be somewhat

lower as well. Lockheed’s corresponding forecast, a single figure of 307 billion RPMs in 1990,

is somewhat lower than any of the forecasts included in table 4.

Only the FAA and OTA-commissioned forecasts break down these RPM figures into projections of air carrier operations by type. FAA’s

operations forecasts are considerably lower than

OTA’s, particularly in the 1980 forecast. Where

the OTA “low” scenario translates 4.1-percent

RPM growth into 1.5-percent annual growth in

air carrier operations, the 1980 FAA “baseline”

scenario shows 4.3-percent RPM growth but no

operations growth, and the FAA “stagflation”

scenario translates 3.6-percent RPM growth into

a 0.8-percent decline in operations. As a result,

OTA’s forecast range for air carrier operations

in 1991 is 12.1 million to 19.6 million, while the

FAA’s is 9.2 million to 15.5 million. The corre-
Ch. 4—Aviation Growth Scenarios ● 55

sponding projection from the Air Transport As- ●

sociation, reflecting the judgments of its airline

members, is for 10.4 million air carrier operations in 1990. The overlap between these projections is sufficiently wide that the differences are

probably not significant, particularly when

structural differences between the models are

considered. However, because the forecasts rely ●

on common assumptions, they produce similar

results all of which may be in error for the same

reasons. ●

The TRB Aviation Demand Forecasting Committee’s 2-day workshop on FAA aviation forecasts resulted in four principal recommendations, all of which also apply to the other fore- ●

casts considered here. In the opinion of the

workshop participants, the following features

are needed by planners and decisionmakers

alike:

high and low estimates of key assumptions

to measure the extent of uncertainty about

driving variables, and consequently an increase in the number of alternative scenarios (at present the FAA provides complete results only for its “baseline” scenario);

a variety of techniques rather than a single

technique, in order to produce better forecasts or competing scenarios;

in particular, less reliance on econometric

models and more on expert judgment (especially industry experts), taking account of

nonlinear economic relationships and noneconomic factors; and

forecasts of components rather than aggregates alone—regional and local activity

rather than national, for instance, and

point-to-point traffic levels rather than only

total volumes.

FACTORS AFFECTING TRAFFIC GROWTH

The future growth of aviation activity in the

United States will be affected by a number of

factors that are not or cannot be anticipated adequately or with certainty in the models used for

the forecasts discussed above. In some cases

these factors may constitute “levers” through

which the rate or pattern of growth might be influenced through appropriate policies or programs. In most cases, however, neither the direction nor the impact of these factors can be accurately foreseen. These factors include but are

not limited to those discussed below.

U.S. Economic and Regulatory Policy

The preliminary figures for FAA's 1981 forecasts reflect considerable optimism about the implementation and success of the present administration’s economic recovery plan. The growth

and structure of the aviation system will be influenced significantly by the speed and strength

with which the Nation recovers from the current

recession. The growth of aviation will also continue to be influenced by air safety and air traffic

regulations, by the way in which ATC system

costs are apportioned through user fees and aviation taxes, and by the constraints imposed by

present and future noise and environmental regulations. The potential impact of

and policy factors is uncertain

future changes.

Deregulation

these economic

and subject to

Airline deregulation has destabilized the industry’s price and market structures. Some analysts believe that the transition toward a free

marketplace is causing overcompetition, which

in turn is undermining major airline profitability

and reducing their ability to finance badly

needed new equipment. Termination of Section

406 and 419 subsidies in 1985 and 1988 will also

affect commuter airline profits and may affect

air service to as many as 100 small- and mediumsize cities. Some analysts feel that the demise of

some carriers may be a natural and indeed desirable result of complete deregulation, since the

elimination of financially ailing carriers would

relieve the overcapacity that currently hinders

healthier competitors. Some analysts predict the
56 ● Airport and Air Traffic Control System

bankruptcy of a major carrier by mid-1982, and

that by 1990 the industry will probably witness

considerable consolidation through mergers, acquisitions, and outright failures. The survivors,

however, may be in a far stronger financial and

competitive position.

Industry Maturity and Structure

Rolls Royce, a major aerospace manufacturer,

has suggested that even if positive steps are

taken to reduce costs and increase efficiency, the

U.S. airline industry has already reached about

60 percent of its mature size (see fig. 16). Others

put the figure at closer to 80 percent. If this is so,

then major air carrier passenger traffic may

begin to level off before the end of the century,

and tower operations might actually decline.

The continued growth of commuter carriers and

GA traffic might nevertheless result in a continued increase in the number of airport and

ATC operations beyond 2000, but FAA expects

commuters too to become a “stable, mature industry” after 1985 and GA may face growth constraints. It seems likely, in any case, that by 1990

there will be a smaller number of trunk carriers,

offering primarily long-haul service; a declining

number of specialized carriers, offering low-cost

service in major hubs and major markets; and a

large number of commuters of various sizes, including some that offer “regional” service.

Fuel and Labor Costs

The greatest uncertainty facing domestic aviation in both the short and the long term is the future price and availability of aviation fuels. This

factor is crucial to the continued profitability of

the airlines, which depends in a major way on

their ability to absorb any differences between

the increase in fuel prices and the increase in the

CPI. The future course of fuel prices can only be

Figure 16.—Projected U.S. Certificated Air Carrier Growth

400

350

300

–4 –3 –2 -1 0 1 2 3 4 5 6

SOURCE: Rolls Royce, Inc., U.S. Air/ines /ndlcators and Pro/ecflons, July 1981,
Ch. 4—Aviation Growth Scenarios ● 5 7

guessed at, particularly in view of uncertainty

about future OPEC policy and the inherent instability of the Middle East. However, the current “oil glut” and price decreases are probably a

transient event in the long-term price trend,

although it is less certain whether or how rapidly

the real price of fuel will rise in the future. No

long-term shortage is expected. There are indications, however, that aviation gasoline (used by

smaller piston-engined GA aircraft) may be increasingly difficult to obtain. GA activity is particularly sensitive to fuel prices, but rapid increases are more likely to reduce personal GA

traffic than business and commercial GA (corporate and air taxi users, who generate greater

demand for ATC services).

Labor costs are also a major factor in air carrier profitability, and airlines can be expected to

seek long-term wage and benefit concessions

from their unions during the 1982 round of contract negotiations. Financing costs may also become an increasingly important factor in the

future.

Technology

Considerable optimism remains about the future impact of advanced air transport technology, but such improvements are likely to be introduced more slowly in the future than over the

last 20 or 30 years. Recent improvements in

airline efficiency and productivity have come

through higher utilization and economies of

scale (aircraft size and seating density) rather

than technology (aircraft speed or fuel efficiency). Several promising new developments appear to be possible in the near future, but there is

a considerable amount of aviation technology

currently “on the shelf” that is only beginning to

appear in the U.S. fleet. Whether the aerospace

industry will continue to develop a new generation of advanced-technology aircraft will depend on the potential market, and this in turn

depends on the ability of the airlines to generate

profits and/or obtain financing. Several

manufacturers have announced plans for a new

150-passenger aircraft for the late 1980’s; several

new commuter aircraft will be available even

sooner. Some near-term increases in fleet efficiency could, however, be achieved by retrofitting engines and making other modifications to

existing aircraft.

Financing

Reports by various airline and banking

sources indicate that the equipment needs of the

U.S. airline industry will impose capital requirements of $50 billion to $100 billion by 1990,

compared to total capital additions of only $30

billion between 1960 and 1979 (current dollars).

This capital requirement would demand an average annual corporate return on investment

(ROI) of 13 to 15 percent for the entire decade.

Industry ROI averaged 6.4 percent during the

1970’s, and only once—in 1978—has it risen as

high as 13 percent. There are signs of increasing

reluctance on the part of insurance companies

and even banks to provide long-term debt, even

when secured by the leveraged-lease financing or

equipment trust certificates that were used in the

1970’s. Deregulation has further increased the

risks and uncertainties of airline financing, although a restructuring of the industry through

bankruptcies or mergers (see above) might alter

this situation in the future. Without a firm market, furthermore, aerospace manufacturers

might be less willing to develop and introduce

more advanced aircraft in the future.

Substitution for Air Transport

Very little can be said with any certainty

about the future impacts of developments in

either substitute transportation modes (such as

high-speed trains or, with higher speed limits

and gas mileage, the personal automobile) or alternatives to travel (such as advanced telecommunication technologies and corporate teleconferencing). Neither is likely to cut into aviation’s

long-haul markets, although the industry may

find it increasingly difficult to compete with the

automobile and train in short-haul markets

(under 200 or perhaps even 300 miles).

Strike Impacts

Ironically, the PATCO strike has in effect deregulated the industry by imposing traffic re-
58 ● Airport and Air Traffic Control System

striations on the 22 busiest hubs and by placing

severe constraints on GA traffic. Some observers feel that the strike may actually have helped

airline profits by removing overcapacity and

enabling major carriers to ground inefficient aircraft, lay off personnel, and reduce other costs.

On the other hand, these same restrictions impose constraints on GA traffic and on the expansion of commuter carriers and new entrants.

Strike-related traffic restrictions will probably

continue for at least 2 more years, and adjustments made by users during this period may permanently change aviation growth trends and

traffic distribution. As a result, there is little certainty about the long-term impact on the level of

operations: traffic might rebound rapidly, but

previously projected levels might not be reached

until later than anticipated, if at all (see fig. 17).

In addition, these traffic restrictions (particularly at major hubs) could be extended or reimposed in the future as a means of addressing airport congestion and encouraging redistribution

of operations to second-tier hubs (see the following section).

Figure 17.–Possible Long-Term Impacts of PATCO

Strike on ATC Workload Levels

1981 1984

A = Built-up demand causes rapid recovery and

workload quickly matches projected levels.

B = Steady recovery and projected rate of growth, but

workload matches projection later than anticipated.

C = Strike stunts demand growth and ATC workload

never achieves projected level.

NOTE For Illustrate purposes only, and not based on speclflc FAA forecasts

SOURCE Off Ice of Technology Assessment

IMPLICATIONS FOR AIRPORT CONGESTION

Despite the uncertainties involved in forecasting precise rates of growth, there is a general

consensus that air traffic and the demand for

ATC services will increase in the next 10 to 20

years. There is also a consensus that much of

this growth will come from the GA sector rather

than the airlines, and within the GA sector from

business and commercial aircraft rather than

personal flying. There is far less agreement on

how this growth will be distributed through the

system or how it will affect the problem of airport congestion and delay.

FAA forecasts indicate that continued rapid

growth of air traffic, if it occurs along existing

patterns at existing airports, will result in severe

airside congestion at 46 air carrier airports by

2000. FAA’s forecasts have consistently overestimated growth in the past, and a number of factors may constrain growth in the future (see

above). Nevertheless, airside capacity could become an increasingly serious problem at more of

the Nation’s airports by the end of the century

unless there are improvements in airport capacity or traffic management (see ch. 6).

An alternative to this prospect, however, is

the redistribution of air carrier operations across

more of the top 50 airports, in combination with

improved facilities at additional GA reliever airports. This alternative is discussed below; specific improvements in ATC technology and airport management that would complement it are

examined in chapters 5 and 6. The economic and

aviation growth rates on which the following

discussion is based are presented in table 5.

Continued Growth and

Airport Saturation

The primary measure of aviation activity as it

bears on airport and ATC decisions is “opera-
Ch. 4—Aviation Growth Scenarios • 59

Table 5.—Aviation Growth Assumptions for “Redistribution” Scenarios, Domestic Service, 48 States

Jets Propeller aircraft

1978:

Revenue passenger miles . . . . . . . . . . . . . . . 200 billion 1.7 billion

Operations at top 50 commercial

airports. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 million 1.8 million

Low Average High Low Average High

economic economic economic economic economic economic

growth growth growth growth growth growth

2000:

Revenue passenger miles: average annual

growth rate ... ... ... ... ... ..percent...4.1 5.5 7.5 4.1 5.4 6.9

Revenue passenger miles:

year 2000 . . . . . . . . . . . . . . . . . . . billions. . 450 600 900 - 4 - 5 - 7

Operations: average annual growth

rate . . . . . . . . . . . . . . . . . . . . . . percent. . 1.6* 2.2* 3.0* 2.4 1.6* 2.4*

Operations at top 50 commercial

airports ... ... ... ... ... ... .millions. . 10* 11 .2* 13’ 2.9 2.5* 2.9*

“Assuming effects of airport capacity constraints.

NOTE: Real GNP growth rates: Low 2.5

Average 3.4

High 4.3

tions’’—landings and takeoffs, or arrivals and

departures (each flight generates two operations). Figure 18 illustrates the 1978 mix of air

activity at the top 50 commercial airports,

ranked by air carrier operations and aggregated

into sets of 5 airports to simplify presentation.

Most of the operations at these airports are generated by scheduled passenger flights, but although there are few local operations at the top

15 airports, GA traffic (predominantly corporate aircraft and air taxis) is seldom less than

10 percent of operations.

Figure 18 also shows the estimated airside capacity of these airports, expressed in terms of the

“practical annual capacity” (PANCAP) that can

be handled safely, as estimated by FAA in 1978.

Actual airside capacity is variable, however,

changing with weather conditions or aircraft

mix; the balance is a delicate one, and at busy

hubs even a slight deterioration from optimum

conditions can cause long lines of delayed aircraft. PANCAP—the level of operations at

which 80 percent of aircraft encounter delays of

4 minutes or longer— thus represents an approximate figure based on assumed average utilization of the existing number and configuration of

runways, rather than an absolute or reliable

measure of capacity.

Saturation—the level at which delay is chronic—may not occur at a given airport until operations are as much as 100 percent abov e

PANCAP, so that small differences between actual operations and PANCAP are not necessarily significant. Large differences, on the other

hand, indicate a rising probability of encountering delays at the airport at least part of the time.

The discrepancy at most of the top 10 airports in

figure 18 represents a significant capacity shortage relative to demand (the desired level of operations), and in most cases this situation has existed since the late 1960’s. * It is assumed in the

following discussion that when operations are

more than 10 percent above PANCAP, the result will be airport saturation and chronic delay.

Figures 19 through 21 show the PANCAP, the

1978 level of operations, and the levels of operations in 2000 projected under three aviation

growth scenarios. These projections assume that

traffic growth will occur at the same rate across

existing airports, irrespective of capacity limita-

*The discrepancy between PANCAP and actual operations in

the sixth airport group (which includes Phoenix, Fort Lauderdale,

Orlando, San Diego, and Portland) does not indicate a significant

capacity problem. These airports handle a large volume of GA

traffic that is discretionary as to time of day and weather conditions, both of which increase actual capacity over a PANCAP

figure.
60 ● Airport and Air Traffic Control System

Figure 18.–Activity at Top 50 Commercial Airports, 48 States, 1978

500

400

Average

annual

operations 300

(thousands)

per

airport

200

100

Groups of 5 airports, ranked by air carrier operations

SOURCE: Office of Technology Assessment.

Figure 19.—Airport Airside Capacity Perspective—Low Economic Growth Scenario

(Jets plus propeller service plus 10 percent for general aviation)

(1.3 percent average growth rate in operations)

A B

Growth unconstrained by capacity

2000

1234567 6 9 10

Airport group

(5 airports per group)

800

600

400

Expanded hub structure

(operating 10 percent over PANCAP)

123456 7 8 9 10

Airport group

SOURCE: Office of Technology Assessment.
Ch. 4—Aviation Growth Scenarios • 61

Figure 20.—Airport Airside Capacity Perspective—Average Economic Growth Scenario

(Jets plus propeller service plus 10 percent for general aviation)

(2.3 percent average growth rate in operations)

800

600

400

200

A

Growth unconstrained by capacity

1234567 8 9 10

Airport group

(5 airports per group)

SOURCE: Office of Technology Assessment.

B

600

t

400

200

1234567 8 9 10

Airport group

Figure 21.—Airport Airside Capacity Perspective— High Economic Growth Scenario

(Jets plus propeller service plus 10 percent for general aviation)

A

800 With growth constrained assumptions:

2,36 percent average growth In air

carrier operations

600

400

200

1 23456 7 6 9 10

Airport group

(5 airports per group)

Airport group

SOURCE: Off Ice of Technology Assessment
62 ● Airport and Air Traffic Control System

tions. Under conditions of low economic

growth, desired operations exceed PANCAP

only at the top 10 airports; at the top 5 airports,

however, demand will be about 50 percent

above PANCAP (fig. 19A). Under conditions of

average economic growth, desired operations

would exceed PANCAP at the top 20 airports,

and traffic at the 5 busiest hubs would be almost

200 percent of PANCAP (fig. 20A). Under conditions of high economic growth, desired operations would be higher than PANCAP at over 30

airports, and the tops hubs would experience almost 250 percent of PANCAP (fig. 21A). To

avoid these conditions, the carriers would probably increase aircraft size and drop service

points, particularly in short-haul markets, in

order to reduce overall operations. This adjustment, also shown in figure 21A, could reduce

overall traffic levels by roughly 24 percent, but

there would still be serious congestion problems

at the top 10 or 15 hubs.

Redistribution of System Operations

In 1978 the level of scheduled commercial operations at the top 50 airports was about 52 percent of their combined PANCAP. However,

these operations were heavily concentrated

toward the five largest airports (where traffic

levels exceeded PANCAP by 20 percent), while

considerable excess capacity existed at the other

45 hubs. In addition, over half of the passengers

arriving at the five largest hubs did so only to

change planes.

OTA examined the effect of redistributing the

expected increases in operations to these less

crowded airports. In the following discussion it

will be assumed that 110 percent of PANCAP—

i.e., saturation—represents a desirable level of

operations (or an acceptable level of delay) at

any given airport. The results, shown on the

right side of figures 19 through 21, indicate that

the combined existing capacity of the top 50 airports could accommodate substantial increases

in commercial operations if they were redistributed.

Low economic growth would result in 20 airports at 110 percent of PANCAP, instead of 5

airports at 150 percent (fig. 19 B). Average economic growth would result in 38 airports at 110

percent of PANCAP, instead of 10 airports over

150 percent and the top 5 at almost 200 percent

(fig. 20B). High economic growth would result

in traffic levels of 113 percent of PANCAP at all

of the top 50 airports even if redistributed, instead of almost 15 airports at 150 percent and

the top 5 airports at almost 250 percent; but if

airlines respond to capacity constraints by increasing aircraft size and dropping some service

points, as well as redistributing operations, the

result would be levels of 110 percent of

PANCAP at only 38 of the top so air carrier airports (fig. 21 B).

Such a redistribution would be accomplished

primarily by “rehubbing” airline route structures-that is, by moving the interline function

(that of providing a transfer point) from congested airports to the “second tier” hubs where

excess capacity still exists. There are indications

that such changes in the airline network are

already taking place. United Airlines, for instance, has been shifting some of its operations

from Chicago-O’Hare to St. Louis over the past

5 years; in addition, Denver (the western hub)

has been growing in importance relative to

Chicago in United’s overall system. Similar

shifts by other carriers can be detected from

Chicago to Kansas City, from Atlanta to Birmingham, from Dallas-Fort Worth to Houston,

from Miami to Tampa, and from Memphis to

Nashville. FAA, for its part, has been trying

for years (with only limited success) to shift

airline operations from Washington-National to

Dunes International.

Market forces will continue to promote this

redistribution, as will the traffic restrictions imposed by FAA at the 22 largest hubs as a result

of the PATCO strike. Direct-service links already exist between most of these new transfer

hubs, but the frequency and aircraft size of traffic between them would increase. Nevertheless,

some hub airports will continue to experience

higher than desirable levels of traffic and delays

unless further measures are employed, such as

peak-hour landing fees, access quotas, or slot-allocation schemes. Commuter airlines would be

hardest hit by these restrictions, and even with

new hubs available they would be hard pressed
Ch. 4—Aviation Growth Scenarios ● 6 3

to improve service at existing points or add new

service points to their networks. In addition, it

would eventually be necessary to shift most GA

traffic out of the top 20 or more airports (down

to the supposedly “irreducible” 10 percent),

which implies the need for improved facilities at

reliever and other IFR-equipped airports if future GA growth is to be accommodated.

Expanded Capacity and

Improved Management

The above scenarios indicate that attempting

to accommodate expected aviation growth within the existing airport capacity will have mixed

effects on the air service network. Although the

adverse effects of growth, such as increasing

delays or reductions in service, might be tolerable, it would nevertheless seem both prudent

and desirable to increase capacity, where feasible, if this can be done at a reasonable cost and

to the benefit of system efficiency. However, it is

not feasible to supply the amount of new capacity required to eliminate or even appreciably

reduce airside delay, particularly in major urban

areas. In the short and long term, the alleviation

of delay will be best achieved through tighter

Figure 22.—Number of Commercial

control over the level and distribution of airport

operations, rather than the addition of new

capacity (see ch. 6).

However, both commuter access and overall

capacity constraints could be addressed by the

construction of short, independent “stub” runways for turboprop aircraft where feasible, and

especially at the most congested airports. This

alternative (discussed in detail in ch. 6) would

increase propeller capacity as an addition—

rather than a detriment—to jet capacity, thereby

reducing the severity of hub saturation and

allowing GA and commuter aircraft to compete

more effectively with jets for airport access. Figure 22 shows the effect of such runways in relieving saturation at commercial airports in

2000: by adding about 25 percent to the effective

capacity of an average hub, they would allow a

considerably higher level of traffic growth or,

alternatively, reduce the number of airports saturated by any given level of economic and traffic growth. However, the addition of stub runways would also result in more complex traffic

patterns, which might require new landing systems and improved traffic management in terminal areas.

Airports Over Capacity–Year 2000,

48 Contiguous States

(Jet plus propeller operations plus 10 percent allowance for general aviation)

Saturation at - 1.1 1.2 1.375 Times current

practical

annual

capacity

Average annual growth rate of operations—props plus jets

SOURCE Off Ice of Technology Assessment
Chapter 5

TECHNOLOGY AND THE FUTURE

EVOLUTION OF THE ATC SYSTEM

Photo credit: Federal Aviation Administration

Air traffic control
Contents

Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Goals and Services of the ATC System. . . . . 68

Major Components of the Existing

ATC System. . . . . . . . . . . . . . . . . . . . . . . 68

Surveillance Radar . . . . . . . . . . . . . . . . . . . . 70

Airborne Transponders. . . . . . . . . . . . . . . . 70

Navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Communication. . + . . . . . . . . . . . . . . . . . . . 73

Future Requirements, Opportunities,

and Constraints. . . . . . . . . . . . . . . . . . . . . 73

Future Requirements . . . . . . . . . . . . . . . . . . 73

Technological Opportunities. . . . . . . . . . . . 74

Constraints and Other Factors

Affecting Future Evolution . . . . . . . . . 76

Continuity of Service. . . . . . . . . . . . . . . . 76

Timing of Design Decisions and

System Implementation. . . . . . . . . . 76

User Costs . . . . . . . . . . . . . . . . . . . . . . . . . 76

Locus of Decisionmaking. . . . . . . . . . . . . 77

Freedom of Airspace and Equipage. . . . 77

International Requirements. . . . . . . . . . . 77

Military Requirements . . . . . . . . . . . . . . . 78

Technical Options. . . . . . . . . . . . . . . . . . . . . . . 78

En Route Computer Replacement. . . . . . . . 78

Total Replacement . . . . . . . . . . . . . . . . . . 79

Hardware-First Replacement

(“Rehosting”). . . . . . . . . . . . . . . . . . . 81

Software-First Replacement

(“Offloading”). . . . . . . . . . . . . . . . . . 81

Modularity and Other Concerns. . . . . . . 82

Automated En Route Air Traffic

Control . . . . . . . . . . . . . . . . . . . . . . . . . . 82

Potential Benefits . . . . . . . . . . . . . . . . . . . 84

Potential Implications and Issues . . . . . . 84

Data Link. . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Potential Benefits . . . . . . . . . . . . . . . . . . . 85

Page

Mode S. . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Modes B and D......... . . . . . . . . . . . 87

VHF Data Link. ........ . . . . . . . . . . . 87

Potential Implications and Issues . . . . . . 87

Collision Avoidance. . . . . . . . . . . . . . . . . . . 88

Beacon Collision Avoidance System.. . 89

Tri-Modal BCAS. . . . . . . . . . . . . . . . . . , 90

Traffic Alert and Collision

Avoidance System . . . . . . . . . . . . . . 90

Airborne Collision Avoidance System. . 92

Microwave Landing System . . . . . . . . . . . . 92

Instrument Landing System. . . . . . . . . . . 92

Microwave Landing System . . . . . . . . . . 94

Potential Implications and Issues . . . . . . 95

Alternative ATC Processes . . . . . . . . . . . . . . . 96

Role of the Human Operator. . . . . . . . . . . . 97

Tactical v. Strategic Control . . . . . . . . . . . . 97

Autonomy and Flexibility of

Operation . . . . . . . . . . . . . . . . . . . . . . . 97

Ground v. Satellite Basing, . . . . . . . . . . . . . 97

Levels of Service. . . . . . . . . . . . . . . . . . . . . . 98

LIST OF TABLES

Table No. Page

6. Perform ATC Automation Processes. . . . 80

7. Summary of Functional Characteristics

of Alternative Collision Avoidance

Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

LIST OF FIGURES

Figure No, Page

23. National Airspace System. . . . . . . . . . . . . 69

24. Major AERA Functions. . . . . . . . . . . . . . . 83

25. Comparison of Microwave Landing

System and Instrument Landing

System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Chapter 5

TECHNOLOGY AND THE FUTURE

EVOLUTION OF THE ATC SYSTEM

INTRODUCTION

The present air traffic control (ATC) system

has evolved over several decades from the one

that was first put in place in the 1930’s. The operational characteristics and organization of the

original system were determined largely by the

technologies then available—radio for navigation and air/ground communication, and telephone and teletype networks for distribution of

information among ATC ground facilities. New

technologies—such as surveillance radar,

Air Traffic Control Radar Beacon System

(ATCRBS) transponders, microwave relays, and

electronic data processing—were added as demand increased and the state of the art progressed after World War II, but they did not

change the essential characteristics of the earlier

generation of air traffic control—a groundbased, labor-intensive, and increasingly centralized system.

Advanced data-processing and communication technologies have been introduced to meet

the growing demand for ATC services* and to

provide the controller with the information

needed to make the decisions required for the

safe and efficient movement of aircraft. However, these technologies were applied largely to

improve the acquisition, integration, and display of information, or to speed its dissemination among ATC facilities. Recently, the automated transmission of certain types of information to pilots has also been introduced, e.g.,

weather and terminal area briefings. However,

the making of ATC decisions and transmission

of ATC messages have remained essentially a

human function.

As the air transportation network grows and

evolves in response to economic conditions,

● These technologies have also found use in the cockpit where

RNAV and other systems have provided capabilities that have indirectly affected the ATC system.

market forces, and changing Government regulation, the requirements for the ATC system will

be affected in turn. In addition, new technological developments will make possible new functions and modes of operation that would have

been impossible with older equipment and resources. The extent to which the system must

grow depends primarily on the rate at which the

level of air traffic and the demand for ATC services increase. There is considerable uncertainty

on this score. The direction in which the system

evolves will be influenced by what services are

offered, how they are delivered, and how they

are paid for. The answers to these questions,

too, are subject to great uncertainty. Budgetary

constraints and the continuing effects of the air

traffic controllers’ strike have introduced further

complications. In addition, the evolution of the

ATC system takes place slowly: some of the

modernization programs now reaching fruition

were first conceived a decade or more ago. During this period new technologies have become

available, and there has been continuing controversy regarding the technical choices that will

determine the character of the future ATC system.

This chapter presents an overview of some of

the technologies and technological issues that

are of concern in decisions that will soon be

made about the future development of the ATC

system. It is not a detailed treatment of the

technological and engineering complexities of

the subject, nor does it attempt to resolve any of

the related economic and funding controversies.

Instead, this discussion is intended to provide

decisionmakers and the public with useful information about the implications of some of the advances in technology that have occurred or

which are on the horizon. This information

forms a background against which to assess

FAA’s 1982 revision of the National Airspace

System (NAS) Plan.

67
68 . Airport and Air Traffic Control System

GOALS AND SERVICES OF THE ATC SYSTEM

In order to accomplish the goals of safety, efficiency, and cost-effective operation, the present

ATC system offers the following services to the

aviation community:

●

●

●

●

●

separation assurance—tracking aircraft in

flight, primarily with surveillance radars on

the ground and airborne transponders, in

order to ensure that adequate separation is

maintained and to detect and resolve conflicts as they arise;

navigation aids—maintaining a system of

defined airways and aids to navigation and

establishing procedures for their use;

weather and flight information— informing

users of the conditions that may be expected

along the intended route so they may plan a

safe and efficient flight;

traffic management-processing and comparing the flight plans, distributing flight

plans to allow controllers to keep track of

intended routes and anticipate potential

conflicts, and ensuring the smooth and efficient flow of traffic in order to minimize

costly congestion and delays; and

landing services-operating airport control

towers; instrument landing systems, and

other aids that facilitate the movement of

air traffic in the vicinity of airports and runways, particularly during peak periods or

bad weather that might affect safety or

capacity.

These services together comprise an integrated

program, no part of which can be fully effective

without the others. Flight plans must take into

account weather and traffic, for instance, and

traffic must be routed to destinations so that it

arrives on time and can be handled at the airport

with a minimum of delay. Similarly, clearances

MAJOR COMPONENTS OF

The present ATC system can be divided into

two major subsystems: en route and terminal

area (see fig. 23). The en route subsystem is primarily concerned with aircraft moving along the

have to be modified so that traffic can be routed

around severe weather or away from bottlenecks that develop in the system. In a practical

sense, the aircrew and ground controllers cooperate as a team using various human and electronic resources to maintain safety and to move

traffic expeditiously. While the ultimate responsibility for safety of flight rests with the pilot, he

remains dependent in many ways on data or decisions from the ground.

Photo credit: Federal Aviation Administration

One of the Nation’s first air controllers—1929

THE EXISTING ATC SYSTEM

airway network, generally cruising at higher altitudes. To an increasing degree, it is also concerned with traffic flying point-to-point without

following the airway network. The terminal
Ch. 5—Technology and the Evolution of the ATC System ● 6 9
70 • Airport and Air Traffic Control System

area subsystem handles aircraft flying at lower

speeds and altitudes as they arrive at and depart

from airports, but it must also control IFR traffic

that is passing through a terminal area without

landing. * The major equipment components

that support these ATC facilities are surveillance

radar, airborne transponders, navigation aids,

computers, and communication links.

Surveillance Radar

Two types of radar are used for the surveillance of aircraft. Primary surveillance radar

(PSR) uses the return from the aircraft structure

to determine range and bearing. Secondary surveillance radar (SSR), triggers a response from

aircraft equipped with an ATCRBS transponder

and is able to obtain, in addition to range and

bearing, the aircraft’s identity and altitude.**

Because the transponder enhances the return

from primary radar, it improves the controller’s

ability to track individual aircraft. SSR is the

principal aircraft surveillance tool of the ATC

system; PSR is used as a backup for SSR and for

long-range weather data.

Airborne Transponders

The returns to surveillance radar vary considerably with range, aircraft structure, background clutter, weather, and several other factors. In addition, the present radar system does

not permit aircraft altitude to be determined

from the ground on the basis of raw return from

primary radar. This makes it difficult to track

specific aircraft using a reflective return alone,

although computer processing can be used to

isolate a moving aircraft from background clutter. Transponders are radio transmitters designed to respond to ground interrogation with a

strong signal that can easily be distinguished

from a purely reflective return. The ground

equipment and airborne transponders constitute

the ATCRBS.

● In a terminal control area, all traffic is controlled by the ATC

system.

**Altitude data is available only from aircraft equipped with a

transponder having Mode C and an encoding altimeter. Only

about one-third of the transponder-equipped aircraft have altitude

reporting capability.

Photo credit: Federal Aviation Administration

Air control in the 1940’s using table top plots

Photo credit: Federal Aviation Administration

Air control using a modern console

Current procedures require that all aircraft

operating in the busiest terminal control areas

(TCA), or flying above 12,500 ft must be

equipped with a transponder capable of reporting both an aircraft identification code and altitude, Modes A and C, respectively. These devices respond to a Mode C inquiry from an
Ch. 5—Technology and the Evolution of the ATC System ● 71

ATCRBS interrogator by giving the altitude

of the aircraft, reported to the nearest 100 ft as

sensed by an onboard barometric altimeter.

Transponders also have the ability to transmit

one of 4,096 different identity codes in response

to a Mode A query from an ATCRBS interrogator. Under Instrument Flight Rules (IFR) the

code to be used is specified for each aircraft by

the ground controller; for all VFR aircraft

equipped with a transponder a common identifier code (1200) is used. Some blocks of

numbers within the 4,096 identity codes are

reserved for classifying traffic such as coast-tocoast flights. Other codes have been set aside for

emergency purposes—aircraft that have lost

radio communication, aircraft in distress, or hijacked flights.

Navigation

Navigation aids are another important element of the ATC system. Although they are not

traffic control devices per se, they do have an influence on the structure and the operation of the

system. * As described in chapter 3, the primary

radio navigation aid is the very high frequency

omnidirectional range (VOR) system that operates in the VHF band immediately below the frequencies used for voice communication. VOR

ground stations provide coverage of nearly all

the continental United States and adjacent offshore areas, and most aircraft that have communication transceivers also are equipped to use

VOR for navigation. VOR equipment enables

the aircrew to determine the bearing to the

ground station. Distance measuring equipment

(DME), colocated with VORS, emits signals that

allow the aircrew to determine the distance to

the station as well. A station where VOR and

TACAN, the military navigation system that is

functionally equivalent to VOR/DME but more

accurate, are colocated is called a VORTAC.

Other navigation systems that are available are

listed in chapter 3.

Many large commercial transports, military

aircraft, and a growing number of corporate,

*For example, aircraft will follow radials to or from VOR stations. This tends to add some order to the flow of traffic even if it

is not operating within the ATC system.

general aviation (GA) aircraft are equipped with

inertial navigation systems (INSs) that permit

them to navigate without primary reference to

ground-based radio transmitters. INS-equipped

aircraft are not completely independent of

ground aids since VOR/DME, LORAN-C, or

OMEGA navigation signals are used for periodic

crosschecks of INS accuracy and realinement of

inertial platforms.

A growing number of commercial and GA aircraft are being equipped with navigational computers that enable them to operate off VOR-defined airways along direct origin-to-destination

routes. This capability for area navigation

(RNAV) can be achieved either with an INS or

with equipment that uses VOR/DME, OMEGA

or other navigation aids as the primary reference. The ability to fly RNAV makes it possible

to achieve considerable savings in time and fuel

consumption, and also allows aircraft to avoid

the congestion that sometimes occurs at VOR

airway intersections. FAA has begun publishing

RNAV routes for use by suitably equipped aircraft. At present, however, controllers grant

direct clearances only to the extent that they do

not conflict with traffic along airways or affect

adequate separation. While FAA is making an

effort to accommodate the increasing demand

for RNAV clearances, there are still cases in

which the limitations imposed by the present

VOR airways system prevent users from realizing the full benefit of installed RNAV equipment.

Computers

Computers are used extensively throughout

the ATC system to process flight plans, to correlate radar and transponder returns, to filter out

extraneous signals that could obscure controlled

aircraft, and to generate displays on the controller’s console. All control decisions, however, are

made by human operators. In the busiest terminal areas, an ARTS II or ARTS III computer

system combines SSR data and flight plan information to create a display on an analog terminal

(see fig. 23). Displayed alongside the position indicator for each aircraft is a data block that includes the transponder code of the aircraft, its

altitude and groundspeed, and the aircraft regis-
72 • Airport and Air Traffic Control System

tration number or flight designation to the extent that they are available, For example, none

of these data will appear for an aircraft flying

VFR without an operating transponder unless

they are entered manually.

The principal computer used at the 20 en route

ATC centers is the IBM 9020, an assemblage of

IBM 360 components that have been modified

for ATC applications. The technology incorporated in these machines is of 1962 vintage, and

there have been considerable advances in the design and construction of computers since they

were first built and installed. The IBM 9020s are

tied to either IBM or Raytheon digital display

subsystems that present radar surveillance and

clearance information in a brighter, sharper image than the analog displays used in the terminal

control facilities. In addition to driving the controller displays, the IBM 9020s also handle communications with computers in other en route

centers and terminal area control facilities as

well as other tasks such as flight plan processing.

In case of an IBM 9020 failure, the controllers

have a backup system, called Direct Access Radar Channel (DARC), that digitizes the raw data

from the secondary surveillance radar to create a

comparatively clean image on the control consoles. However, to use DARC, the controllers

must manually shift their display screens from

the vertical to the horizontal position and make

plastic markers (“shrimp boats”) to identify the

targets on the screen, because the DARC system

cannot obtain the clearance data from which to

generate a display of the aircraft call sign or intended route. If the DARC system is inoperable,

controllers have a second backup, a broad-band

system that displays radar data without computer enhancement and thus provides no data

Photo credit: Mitre Corp.

Computers for air traffic control system for aircraft en route
Ch. 5—Technology and the Evolution of the ATC System • 73

block for individual targets. FAA has indicated

that it plans to remove the broad-band capability when sufficient operational experience with

DARC has been established.

FAA is considering the option of installing

compatible computer and display systems in the

en route and terminal area control facilities. If

this were done, much of the line of demarcation

between these classes of facilities could be removed.

Communication

Communication is a key element in the present ATC system, and advances in communication technology may open new options for configuring the system in the future. Historically,

voice radio has been the primary and almost exclusive means of communicating between aircraft and the ground. Digital communication—

the transmission of data in the form of machinereadable binary signals—has come into use for

linking ground stations (particularly for computer-to-computer interchanges), but it has not

yet been applied for air-ground messages, except

in the limited case of transmitting aircraft identity and altitude by means of ATCRBS transponders. In the future, it is expected that an airground digital data link will play an increasingly

important role as the automation of ATC functions requires more direct communication between airborne and ground-based computers.

Another important advantage of the digital

data link is that it permits messages to be transmitted selectively. The present voice-radio

method is broadcast—i. e., available to any and

all aircraft equipped with an appropriate receiver, regardless of the intended recipient. This

“party line” feature has certain advantages, since

it permits pilots to develop a sense of what is

happening in the surrounding airspace. Nevertheless, a “discrete address” technology that permits messages to be sent to a specific recipient

can be more effective than broadcast for processes that require computer-to-computer communication. This is the underlying principle of

the Mode S data link (formerly the Discrete Address Beacon System, or DABS), which is an important building block in FAA’s plans for future

system development.

In the future, with the introduction of a digital

data link capable of selective address, two distinct modes of communication can be expected.

Broadcast, the mode now used, will continue for

voice or digital transmissions of general interest,

such as weather, airport status, and traffic advisories. Other transmissions, pertinent only to

specific aircraft, will be sent by a discreteaddress digital data link that allows isolation of

specific receiving stations. However to the extent that communication relative to position and

intent uses a discrete address data link rather

than broadcast, the side benefits of the party line

would be diminished.

The application of a digital data link is not

limited to air-ground communication; it could

also be used for exchange of messages between

aircraft. For instance, most of the air-to-air communication in proposed collision avoidance systems would be digitized; and by allowing airborne computers to direct messages to specific

aircraft, maneuvers intended to resolve conflicts

could be coordinated between aircraft. Alternative plans for the implementation of a digital

data link are discussed later in this chapter.

FUTURE REQUIREMENTS, OPPORTUNITIES, AND CONSTRAINTS

Future Requirements ity of applying them to achieve greater effectiveness of the ATC system through higher levels of

The evolution of the ATC system will be in- automation. In many cases there are several

fluenced by changes in user demand, market ways of meeting specific needs, and the choice of

forces, and regulatory policy, as well as the which path to take will reflect a combination of

availability of new technologies and the possibil- technological, economic, and policy considera-
74 ● Airport and Air Traffic Control System

tions. In general, however, prospective changes

in the system will be dictated by three related

technical requirements:

●

●

●

replacement of obsolete equipment, which

will become increasingly difficult to maintain and repair, with more modern equipment that offers higher reliability and might

also provide greater flexibility, higher capacity, or lower costs;

increase of system capacity in order to accommodate growth when and where it occurs, by improving the management of existing resources where feasible and by adding new resources where necessary; and

addition of new capabilities in order to support improvements in efficiency and productivity by automating more functions

and by introducing features that make it

possible to take advantage of improvements

in avionics and other newly available technologies.

Advances in technology have increased the

number of options that could meet these requirements. Computers will probably assume roles of

increasing importance, both in the air and on the

ground, because they present opportunities to

increase efficiency, productivity, or capacity by

relieving human participants in the system of

routine tasks, by facilitating human decisions,

and by improving the timeliness and quality of

information. As a result, the human operator’s

role will become more that of a manager of system resources than that of a direct controller of

aircraft. Communications will also be a critical

element, and digital communication between

machines (computers and various avionic devices) will be at least as important as voice communications between humans. Future systems,

therefore, may have to provide for one or more

high-speed data links of sufficient capacity to

handle the large volumes of data and messages

that will be generated. Collision avoidance will

receive increasing attention as the volume of

traffic grows, and both navigation and landing

aids may need to be upgraded in order to maintain safety and improve the efficiency with

which airways and airports are utilized. Specific

technical options for each of these functions are

discussed in later sections of this chapter. The

more general opportunities created by advanced

technology are discussed below.

Technological Opportunities

The development of microelectronics has been

a primary source of expanded technological opportunities for the ATC system. Data-processing

capabilities can now be tailored to meet virtually

any computational requirement, hardware costs

have fallen significantly, and reliability continues to increase. The ATC system as presently

constituted is highly labor-intensive; and since

the PATCO walkout, the system has been kept

operating with a greatly reduced work force

only by administratively limiting traffic. Some

observers have suggested that the current situation presents an opportunity to review the basic

structure of the system and to apply new technology so as to make it less labor-intensive and

less dependent on (or vulnerable to) the actions

of any specific group within the work force.

Computer software figures prominently in the

present ATC system and will have an even more

significant role in the future as the need for new

capabilities expands. Many systems related to

the safety of flight, both ground based and airborne, will be “software driven, ” in that the

processing of sensor data and the generation of

displays will be more dependent on computer

programs than is now the case. Processes running on different computers will communicate

directly with one another. There will thus be a

need for systems with the ability to identify errors and to take compensator action automatically. * Present ATC software uses a combination of computer languages, but new high-level

languages that are now available (like those used

for military command and control) and those

that will be developed in the future may make it

easier and cheaper to implement, modify, and

maintain ATC software.

Commitment to a highly automated mode of

operation is not without risk. When there is a

computer failure in the present system, the controllers can revert to manual methods and keep

traffic flowing. However, experiments with

● Systems with this type of capability are within the state of the

art and some are available “of f-the-shelf.”
Ch. 5—Technology and the Evolution of the ATC System . 75

more highly automated systems have shown

that traffic levels can reach a point that, although well within the capabilities of the automated system, is beyond the point where they

can be handled manually. At these traffic levels,

controllers experience considerable difficulty in

reverting to manual operations during computer

outages. ’ This suggests that even though computer technology offers promise for the future,

there may be a point of no return beyond which

the commitment to automation is absolute—the

only backup system for a highly automated

ATC system is another highly automated

system.

Decreasing size and costs of computers also

mean, however, that data-processing capability

can be located anywhere in a system, and that

redundancy can be provided where exceptionally high degrees of reliability are required. Micro1Leonard Tobias and Paul J. O’Brian, “Real-Time Manned Simulation of Advanced Terminal Area Guidance Concepts for Short

Haul Operations, ” Ames Research Center, August 1977, NASATN-D-B499.

The experiments, conducted in 1971 at the Ames Research Center and jointly sponsored by FAA and NASA, were to determine the comparative utility of 3-D and 4-D RNAV as aids to flying landing approaches. Controllers and pilots were placed in a

simulated high traffic environment and required to control traffic

and fly approaches in STOL [short takeoff and landing] aircraft

equipped with the two onboard navigation aids. Results showed

that the performance of both controllers and pilots improved

although the controllers were only secondary beneficiaries of the

equipment installed in the aircraft.

Generally, when the effects of 4-D RNAV were compared with

those of 3-D RNAV, two types of effects were observed. The first

related to improvements in the effectiveness of both pilots and

controllers. Pilots were able to fly a better track when assigned a

route and a time to arrive at a checkpoint. The range of deviations

of arrival time at a checkpoint from the time assigned dropped

from about 4 minutes to about 30 seconds. Requirements for voice

communication between controllers and aircraft under their control were cut by more than half. Traffic flows were more orderly,

and the number of aircraft in the system increased by about 25 percent.

A second major conclusion was that there maybe a point in the

development of automated systems beyond which it is no longer

possible to return to a manual back-up. At higher levels of traffic,

it was more difficult for controllers to make the adjustments required to handle an emergency and restore traffic once the emergency had been resolved. Controllers expressed a definite need for

more automated support for handling emergencies and restoring

traffic afterwards.

From this it seems that automated systems may have to be built

so that they are self-diagnosing, self-correcting and/or backed up

with other automated systems. Such back-up systems may not offer all of the features of the primary system but would be adequate

for an interim period while repairs to the primary system are underway.

processors have become integral elements of aircraft instrumentation, and modern aircraft can

and do carry general-purpose computers that

can be used for a variety of applications, such as

flight management, processing digital communications with the ground or other aircraft, updating the navigation system, developing alternative flight plans, or driving multifunction

cockpit displays that replace several electromechanical instruments. The introduction of these

airborne capabilities means that ATC functions

need no longer be wholly resident in groundbased computers. As a result, it might be possible to improve system operation and safety by

redistributing these functions among the various

participants in the ATC process. Many of these

functions will be critical to the safety of flight

and, therefore, the computer based systems that

perform them will fall within the airworthiness

certification program of FAA.

An ATC system that places more information

and functions in the cockpit will also require

changes in communication technology. As ATC

automation becomes more widespread and more

integrated into the system, digital data communication will come into greater use. Transmissions directed to a specific receiver—the principle underlying the proposed Mode S data link

described later—would facilitate communication between ground-based and airborne computers. They would also allow a computer to

continue with other functions once it determines

that it is not the intended recipient, a feature that

increases the effective capacity of a processor.

The capacity required for data links between

ground facilities would also need to increase.

While telephone and other ground links are used

at present, point-to-point satellite channels

might provide an alternative in the future.

Satellites could also be used for aeronautical

navigation and surveillance. Singly or in constellations, satellites with accurate sensors and

computing capabilities can be used to determine

aircraft position and relay the information to

other aircraft and ground stations. Satellitebased collision avoidance systems have been

suggested. Large satellites that support a number

of functions are also being considered for civil

aviation, notably in the Aerosat system of the
76 ● Airport and Air Traffic Control System

European Space Agency. The reliability and

longevity of satellites are high and likely to increase in the future. The space shuttle makes it

possible to recover, refurbish, and relaunch

satellites, or even make repairs while in orbit.

However, the leadtimes for scheduling shuttle

payloads will preclude its use in responding

rapidly to unforeseen emergencies. In addition,

frequencies and orbital slots are limited and

ATC applications must compete with other potential users of space technology, Both NASA

and FAA have spent considerable amounts on

R&D for ATC satellite applications, but no significant U.S. program is currently under way.

Much of the required technology is available,

but it has not yet reached the point of being a

cost-effective alternative to ground-based ATC

facilities and configurations. At some point in

the future, however, this may change and the

option of using satellites in ATC applications

may have to be reevaluated.

Constraints and Other Factors

Affecting Future Evolution

Continuity of Service

ATC is an ongoing activity that cannot be interrupted while a replacement system is put into

place. Any changes in the system must therefore

be implemented gradually, and new and old

equipment will have to be operated in parallel to

assure continuity of service throughout the transition period. FAA can reduce the length of this

transition period by mandating equipage by certain dates. If installation is voluntary, however,

some users will hold off replacing existing equipment until it wears out, and some users might

never make the change. At a minimum, parallel

operation will be needed for perhaps as long as a

decade while users install new equipment. In

some cases, it could be in the best interest of all

parties to establish a firm date on which existing

services will terminate and by which all users

will have to be equipped to use the new service.

Timing of Design Decisions

and System Implementation

Identifying future needs and installing the facilities to meet those needs take a considerable

amount of time. In periods of rapid technological progress, new equipment or facilities may

become obsolescent before the implementation

phase is completed. Redesigning the system to

incorporate newer technologies, however, may

take so long that a badly needed function remains unavailable, or that a deteriorating

system is kept in place long after it has become

inadequate. At some point, therefore, the decision to go ahead with system enhancements

must be made, despite the realization that the incorporation of newer technologies will have to

be deferred until a later cycle of system

modifications.

The design and development of some prospective ATC systems and facilities began over 10

years ago, and it will be late in the present decade or early in the next before implementation

can be completed. A substantial portion of the

needed ground facilities would have to be installed before users would begin to install the required equipment on their aircraft, since they

would see little benefit in spending money for

equipment before it is of practical value. The

rate of installation of airborne components

would also be limited by the rate at which they

can be produced, and the avionics industry

would be unlikely to commit to production until

it foresees a market of sufficient size to assure

profitability.

User Costs

FAA is responsible for the design, procurement, installation, operation, and maintenance

of equipment used in ATC installations and for

establishing standards for the equipment to be

carried on aircraft. However, the responsibility

for and costs of procuring, installing, operating,

and maintaining the airborne equipment rests

with the users. Any adverse impacts on aircraft

performance resulting from the installation of

airborne equipment also translates into increased user costs. Decisions about changes to

the ATC system must consider these user costs

and the effect that required equipment might

have on aircraft performance.

Large aircraft have the space to accommodate

new avionics, but in small GA aircraft or densely packed tactical military aircraft space is at a
Ch. 5—Technology and the Evolution of the ATC System . 77

premium, and room for additional equipment to

meet the needs of the ATC system may be hard

to find. Antenna location, in particular, often

involves a tradeoff between aerodynamic and

electromagnetic characteristics. For instance, the

small blade antenna used for a standard

ATCRBS transponder has little effect on aerodynamics, but the larger direction-finding antennas

required for some collision avoidance systems

may adversely affect aircraft performance or

even structural integrity when retrofitted into

existing aircraft.

Not all new functions require the replacement

of existing equipment. Some experts suggest that

the capabilities in existing equipment are ample

for future needs and that new or upgraded

equipment is not required. Some entrepreneurs

have been successful in adapting existing equipment to new purposes without making any fundamental changes. RNAV, as mentioned, uses

VOR/DME signals and existing receivers to obtain the data required for navigation outside the

defined system of airways. Tri-Modal BCAS, a

collision avoidance system, is designed to operate with the installed ATCRBS transponders and

interrogators.

In addition, the ATC system serves a broad

mix of users who operate aircraft having a wide

range of performance characteristics and who

use the airspace for a variety of purposes. Over

half of all air operations are not under the control of FAA terminal and en route facilities, but

the ATC system must recognize the existence of

these “off system” activities so that the available

airspace and airport facilities are used in a safe,

efficient, and equitable manner. The heterogeneity of the user mix complicates both the design

and the implementation of new systems, and the

GA community is particularly sensitive to the

issues of user costs and mandatory equipage.

Locus of Decisionmaking

Decisionmaking in the ATC system is distributed between ground controllers and aircrew.

Ultimate responsibility resides with the pilot,

but controller-supplied services are particularly

important in high-density traffic and at times of

poor visibility. Some pilots feel that the amount

of ground control is becoming excessive and that

they are burdened with the responsibility of

operating the aircraft safely without having

available the information required to meet that

responsibility. Technologies now available or

under development could make additional information available to both ground controllers and

aircrew and might permit redistribution of the

decisionmaking function. These alternative concepts have not yet been validated and tested; but

they could lead to an ATC system that is less dependent on ground-based equipment and control decisions.

Freedom of Airspace and Equipage

The passage of time has also brought increasing limitations on the amount of airspace available for VFR operations. The GA community

(traditionally vocal in this matter) has been

joined by the military services, who believe that

access to suitable training areas is becoming excessively restricted. The airlines, faced with high

fuel prices and low profitability, have also argued that they should be permitted to fly the

most fuel-efficient routes possible between

points served.

While FAA has not required all aircraft to be

equipped to participate in the ATC system, it

has imposed limitations on the operations of aircraft lacking specific pieces of equipment. While

the requirements are still minimal, freedom of

airspace is already directly affected by the

amount of avionics an operator is able and willing to install on an airplane. As more airspace

becomes congested, the areas in which unrestricted VFR flight is permitted may have to be

reduced, or some other method be found to assure separation and preserve safety of flight. It

may not be possible in the future to permit the

some degree of flexibility and freedom of airspace use that has been accorded in the past to

those operating outside of positive control by

the ATC system.

International Requirements

The United States is party to a number of international agreements that affect the operation

of the air transportation system. It is legally obli-
78 ● Airport and Air Traffic Control System

gated to provide ATC services that conform to

international standards at gateway facilities unless airspace users are notified that particular exceptions are taken to the applicable agreements.

Foreign-flag carriers enter U.S. airspace at gateway facilities with the understanding that they

will receive full services if they are equipped in

accordance with international standards. U.S.

aircraft similarly expect a full range of services

from foreign controllers. There is no legal

obligation to operate the domestic ATC system

in conformity with international standards,

although many nations (including the United

States) find it desirable to do so.

Two international bodies establish standards

that affect aeronautical operations. The International Civil Aviation Organization (ICAO) promulgates standards that establish flight procedures and aircraft equipment specifications. For

example, one ICAO standard governs the signal

format used by each mode of the ATCRBS

transponder. Mode S, the signal format of the

DABS data link, is currently being considered

for establishment as an ICAO standard, without

which it cannot be implemented for international operations.

The second organization, the International

Telecommunication Union (ITU), also establishes conventions that affect aeronautical operations, but the relationship is not as close as that

of ICAO. ITU assigns portions of the radio frequency spectrum to various applications

throughout the world. The spectrum is a finite

resource, and competitition among alternative

applications is intense. Aeronautical radio has

been assigned bands that are of sufficient capacity to meet present needs, but it may be difficult

to obtain additional spectrum allocations for

new aeronautical applications in the future.

However, it may be feasible to reduce the channel spacing in bands that are currently allocated

and thus increase total effective capacity. One

area where there is significant pressure is in the

allocation of spectrum to satellite applications;

and this may be a factor that could limit the development of ATC services that use satellites.2

Military Requirements

The ATC system will be constrained by national security considerations. In time of war the

system must meet the needs of the military without aiding an enemy in locating and hitting targets in the United States. In addition, ATC

equipment and facilities must not compromise

the operational integrity of military equipment.

The military is a full participant in the ATC system, and FAA is charged by law with ensuring

that the system meets both civil and military requirements. Some arrangements for coordinating the activities of FAA and the Department of

Defense (DOD) have been established, but these

have not been completely formalized.

‘For further information on this subject see OTA’S assessment,

Radio frequency Use and Management: impacts From the World

Administratiz]e Rudio Conference of 1979, OTA-CIT-163

(Washington, D. C.: U.S. Government Printing Office, January

1982).

TECHNICAL OPTIONS

En Route Computer Replacement

The computer now in use at en route ATC

centers is the IBM 9020, a designation given to a

derivative of the IBM 360 line that has been specially modified to perform ATC functions. Although the IBM 9020 was first commissioned by

FAA in 1974, it incorporates a technology that is

close to 20 years old. It has less speed and capacity, is less reliable, requires more energy and

floor space, and is not as easy to maintain as

more modern computers that could be used in

support of the ATC system.

Growth in the demand for ATC services has

exceeded the data-processing capability of the

IBM 9020. Some ARTCCs are already operating

at capacity, while others are expected to reach

capacity later in this decade. Alleviating capacity problems by acquiring additional IBM 9020
Ch. 5—Technology and the Evolution of the ATC System • 79

computers is not a practical alternative, since the

IBM 360 has been out of production for several

years. Buying used IBM 360s and modifying

them to make them IBM 9020s would be expensive in the short term and would provide, at

best, only a stopgap solution.

The reliability of the IBM 9020 hardware and

software has also been troublesome, giving rise

to concern that the cost of repairing and maintaining the system will become excessive. As

time passes and existing stocks of spare parts are

exhausted, maintenance of the computers could

become very expensive because spares would

have to be fabricated to order. Similarly, the

task of modifying and maintaining software to

meet evolving needs is likely to be increasingly

difficult to perform. In the future, it will be difficult to recruit and retain programmers capable

of maintaining the software because those who

are best able to do this job prefer to work on

more modern equipment. Further, there is ample

demand for their talents outside of FAA.

FAA is now in the process of planning the procurement of a replacement computer system that

will overcome present operational problems and

provide additional capacity to meet the needs of

the en route centers during the last decade of this

century and into the next. Plans are to use the increased capacity of the replacement computers

to provide a variety of new and improved services, as well as to satisfy the requirements generated by the anticipated increase in aviation activity. Table 6 indicates the range of services and

activities FAA expects to support with the replacement computer system. These applications

fall in three major areas: control of individual

aircraft, conflict alert and resolution, and management of traffic flow.

The basic technical issue is not whether the

9020 system needs to be replaced—there is wide

agreement that it does—but what replacement

strategy should be pursued.

There are many strategies for replacing the

IBM 9020s, but all can be placed in one of three

groups:

• replace all hardware and software simultaneously;

●

●

The

that

place initial emphasis

the hardware; or

place initial emphasis

the software.

first strategy—total

on the replacement of

on the replacement of

replacement— implies

the present system, with minor modifications needed to keep it operating, will be kept in

place until the replacement hardware and software are ready for commissioning. The latter

two strategies are incremental approaches that

provide for a transition to the new system in

comparatively small steps over an extended period. Some believe that either of these strategies, if

successful, could provide relief from the most

pressing problems within a period of 3 to 5

years, as opposed to the more than 8 years required for the total simultaneous replacement

option.

The en route computer replacement strategy

has been reviewed as part of the FAA effort to

produce a revised NASP. Implicit in past FAA

statements is the presumption that the replacement computer, like the IBM 9020s, would have

to be uniquely designed for ATC applications.

Critics of the full replacement strategy have put

forth options that would effect the replacement

of the computers incrementally.3 Generally,

these plans envision using off-the-shelf equipment to replace the IBM 9020s rather than obtaining a computer that has been designed or

modified specifically for ATC applications.

Total Replacement

The total replacement strategy has much to

recommend it. First, FAA has learned from its

experiences with the present system and, given

the opportunity to make a fresh start, would be

in a position to design a replacement that would

correct present weaknesses. Second, advances in

hardware, software, and communication technologies have created new options that were not

available when the present system was installed.

A complete replacement of the present system

‘See, for example, FAA Air Traffic Control Computer Modernization, Hearings before the Subcommittee on Transportation,

Aviation, and Materials of the Committee on Science and Technology, U.S. House of Representatives, June 16-18, 1981.
80 ● Airport and Air Traffic Control System

Table 6.—Perform ATC Automation Processes

Sustain ATC system operation . . . . . Assemble system information:

● Acquire or negotiate decisions

● Collect and analyze system status information

Calculate state of ATC system:

● Calculate system load

● Predict system state

Resolve management actions:

● Resolve differences in system state and decisions

. Translate resolutions into automation directives

Manage ATC automation processes performance:

● Formulate required processes actions

● Monitor processes status and performance

● Monitor plan status and performance

Perform ATC planning processes . . . Assemble planning information:

● Assemble trajectory information

● Assemble flow information

● Create multidimensional profile

Identify strategic planning problems:

● Predict strategic delays

● Predict long-term conflicts

Resolve strategic planning actions:

• Absorb strategic delays

● Resolve long-term conflicts

Issue strategic planning actions:

● Formulate clearance plan

Perform ATC controlling processes . Assemble control information:

● Assemble control information

● Convert to appropriate reference

● Apply control conditions

Identify control problems:

● Predict short-term AC/AC conflicts

● Predict environmental conflicts

● Detect track/trajectory deviations

Select control actions:

● Assess “accept/handoff” situations

● Resolve tactical situations

● Generate clearances

Control ATC system:

● Perform aircraft accept/handoff

● Deliver clearances

● Deliver advisories

SOURCE: ATC Computer Replacement Program System Level Specification (Preliminary). En Route ATC Automation System.

FAA-ER-130-003, May 1981 (draft).

offers the opportunity to explore all of these options fully and to select the one that best suits

ATC requirements in terms of both technical

characteristics and overall system productivity.

On the other hand, the total replacement option would do little or nothing to relieve the deficiencies of the present system in the short term.

If procurement were to start immediately, it is

unlikely that the first replacement computers

would be in operation before the end of the decade. In the interim, the IBM 9020s would have to

be kept in operation to meet the ongoing needs

for ATC services—a task that could become increasingly difficult and costly.

Critics of FAA have pointed out that the number of interruptions to service experienced with

the present computers constitutes a threat to the

safety of flight. ’ A more recent study by the Na-

*Air Traffic Control Computer Failures, Committee on Government Operations, U.S. House of Representatives, House Report

No. 97-137, June 11, 1981.
Ch. 5—Technology and the Evolution of the ATC System • 81

tional Transportation Safety Board5 indicates a

significant decrease in the number of computer

outages since the controller strike in the summer

of 1981 due in part to the subsequent reduction

in the level of traffic. Concern with the reliability of the ATC computers remains, however,

and FAA has pointed out that some of the en

route centers were approaching capacity limits

at the time of the strike. This last consideration

would favor a conversion strategy that will have

a positive short-term effect on en route traffic

capacity.

Hardware= First Replacement (“Rehosting”)

Either of the alternative strategies for the incremental replacement of the computers entails a

number of assumptions about the structure and

operational characteristics of the present system.

For example, a proposal to move some of the

functions from the IBM 9020s to an auxiliary

computer assumes that it is possible to isolate

the software elements that perform those functions from the rest of the IBM 9020 software. A

proposal to move the existing software to a new

processor assumes that interface problems arising from differences in the internal timing of the

machines can be overcome. Such assumptions

are critical both to the feasibility of incremental

replacement strategies and to the schedule and

budget to carry them out.

The second option—incremental replacement

with initial emphasis on substituting new hardware—would “rehost” or move the existing software to a new processor capable of supporting

the IBM 360 instruction set. Several manufacturers produce machines with this capability,

but in every case some modification of the existing software would be required. * At a minimum, some allowance would have to be made

for handling the instructions unique to the IBM

9020. Real-time applications, such as the ATC

software, are characteristically sensitive to the

timing of internal machine operations, and this

‘Air Traffic Control System, Special Investigative Report,

NTSB-SIR-81-7 (Washington, D. C.: National Transportation

Safety Board, December 1981).

*The ability to modify software rests on an understanding of the

existing structure and the procedure it executes in performing required functions.

●

too could cause severe problems in rehosting the

software. There could also be problems in meeting the requirements of the interface between the

main processor and the IBM or Raytheon systems that drive the displays used by the controllers. However, there are probably technical solutions to these problems given enough time and

resources to work them out.

Even though there may be problems with rehosting the existing software in a new processor,

there are several points that recommend this

strategy. Some suggest that this approach could

be implemented by 1985. Second, once the constraint of machine capacity has been relieved, it

would be possible to begin restructuring the software to improve its maintainability and reliability. Finally, the replacement computer could be

selected with a view toward providing enough

additional capacity to support the new functions

and services planned by FAA as part of longer

term improvements of the ATC system.

The “hardware-first” approach does not rest

on the assumption that the processor to which

the ATC software is moved would necessarily be

the long-term replacement for the IBM 9020. It

could be viewed as an interim replacement that

would serve while FAA proceeded with a procurement program for a totally new hardware

and software package, to be commissioned

around the turn of the century and intended to

serve well beyond the year 2000. On the other

hand, the procurement of an interim computer

replacement would involve a sizable investment

that might, for budgetary reasons, effectively

foreclose the option of initiating a second round

of computer replacement after the interim system was put in place.

Software-First Replacement (“offloading”)

The strategy emphasizing the replacement of

the software first would involve separating individual functions of the existing software. This of

itself would be beneficial, since it would make it

easier to maintain the existing software and provide an opportunity to increase overall operating efficiency. Weaknesses in the software that

are known to have contributed to service inter- “

ruptions could also be corrected during this ini-
82 • Airport and Air Traffic Control System

tial reworking of the existing software. Once this

initial phase had been completed, the software

could either be rehosted intact in a new computer, or some functions could be offloaded

from the IBM 9020 to another processor. The

offloading approach would free capacity on the

IBM 9020, allowing it to absorb increases in

workload due to higher traffic levels.

In the short run, this strategy makes no provision for adding the new functions envisioned by

FAA. However, as various functions are moved

from the IBM 9020s to other processors, there

would in effect be an incremental replacement of

the present computer. This would offer considerable latitude in specifying the replacement

processor. It could be a large main-frame processor to which elements of the ATC system could

slowly migrate. Alternatively, the migration

could be to several smaller processors, so that

the system would finally evolve into a network

of distributed, modular processors. Compared

to the hardware-first strategy, this one offers the

opportunity to migrate to a system that has been

selected specifically to meet the requirements of

the ATC application. Since the software would

be designed first, and then a computer configuration suited to supporting it selected, it

would be less likely that a second conversion

would be required or that the resulting system

would be less than optimal in terms of its ability

to meet the long-term needs of the ATC system.

A potential disadvantage of this strategy,

however, it that it depends on being able to separate specific functions in the existing software.

There are indications that the subroutines within

the present ATC programs are strongly interdependent, and that it might therefore be very difficult to modularize the present software system.

If this is true, then it might be necessary to essentially rebuild the existing software in order to

implement this strategy; and the cost of doing

this could be prohibitive relative to other available options.

Modularity and Other Concerns

The total system replacement strategy advocated by FAA in the past recognizes the need to

replace the controller displays and other peripherals, as well as the 9020 mainframe. ETABS, the

electronic display of flight strip information,

and other display features planned for the controller suite require replacing not only the main

computer but the computers that generate displays as well. In addition, FAA is contemplating

eventual replacement of the ARTS II and ARTS

111 computers now used in the terminal areas.

The ATC functions performed by computers

in the en route centers and those performed in

the terminal areas are similar. Therefore, one

might consider procuring a computer for the en

route centers that could also be used in the terminal areas. Most manufacturers produce lines

of compatible machines with a considerable

range of capacity. Thus, the concept of using a

smaller version of the en route computer in the

terminal areas could be attractive. In fact, such a

strategy could reduce the overall costs of software maintenance for the ATC system because

there would be fewer software packages in use.

At some point, FAA will incur the cost of replacing the IBM 9020s now installed in the en

route centers. Operational factors create considerable pressure to begin doing so in the near

term. However, once the initial conversion has

been completed, future steps to upgrade or to

modify the system could be accomplished at a

slower pace. Manufacturers of computers generally design them so as to provide paths by which

users can upgrade capabilities incrementally

without large-scale rebuilding of software. Such

avenues would be available to FAA in the future

so long as off-the-shelf hardware was selected to

replace the IBM 9020s. If, on the other hand, a

unique processor were to be selected, it is likely

that second conversion—of a magnitude similar

to the one now being undertaken—would be required at some point in the future to support

new ATC services and capabilities.

Automated En Route Air Traffic Control

Another factor influencing the selection of the

en route computer replacement is its compatibility with the long-term evolution of the ATC system. The future requirements and operational

characteristics of the en route portion of the
Ch. 5—Technology and the Evolution of the ATC System ● 83

ATC system are currently defined by FAA under

the concept of automated en route air traffic

control (AERA).

The essence of FAA’s AERA concept is to

automate the functions of maintaining aircraft

separation, metering traffic flow, delivering

clearances, and transmitting ATC messages.

These functions would be assigned to computers, thereby relieving the controller of many

routine tasks. The controller’s role would then

be primarily to handle exceptions and emergencies and oversee (manage) the operation of automated ATC equipment. Operationally, AERA

would perform four principal functions: 1) automatically produce a clearance for each aircraft

operating in positive control airspace that would

ensure a conflict-free, fuel-efficient flight path;

2) formulate messages to aircraft needed to execute the planned flight profile and to assure separation; 3) transmit those messages by data link

or VHF voice radio; 4) and monitor actual flight

movements relative to flight plans, revising

Figure 24.–Major

those plans and clearances as necessary to ensure

continued freedom from conflicts. Major AERA

functions are summarized in figure 24.

As currently envisioned, AERA would be a

continuation and extension of the present

ground-based ATC system. It could be implemented incrementally over an extended period

automating first those functions that are most

routine and repetitive for the human controller.

Instructions to ensure separation and coordinate

traffic flow would still come from ground facilities. However, these instructions would be formulated and issued by computers operating

under the supervision of human controllers, Further, the control instructions would be derived

from a more extensive data base (geographically

broader and covering a greater span of time)

than the present system. In effect, the AERA

system would operate strategically—planning

overall traffic flow as well as individual aircraft

movements so that conflicts do not arise—although some form of tactical control would also

AERA Functions

Adjacent

facilities

I I

Current Coordination Alerts Feedback Alerts

trajectories

I

to

i control I

—Surveillanc e

data

SOURCE: Federal Aviation Administration.
84 ● Airport and Air Traffic Control System

be provided in order to resolve potential conflicts before backup collision avoidance systems

would be activated. G

While AERA would entail extensive groundbased data-processing capability, detailed analysis of aircraft flight plans, and close surveillance

of actual flight paths, it would not necessarily

lead to undue restrictions on aircraft movements. As envisioned, AERA could in fact reduce or eliminate many of the procedural constraints now imposed on the use of airspace. It

would be a system of management by exception,

in which controller intervention would be limited to situations (or localities) where conflicts

could not be reliably resolved by computer routines. The controller would not have to visualize

or direct overall traffic patterns, as in the present

system, because the AERA concept envisions

automated planning, monitoring, and metering

of traffic flow in a four-dimensional region made

up of several airspace sectors over an extended

period of time. ’

Potential Benefits

Initial estimates of the benefits of AERA indicate important savings in two areas: fuel savings

due to more direct routings and reduced labor

costs. The fuel savings for domestic airlines

could be on the order of 3 percent; at present

fuel prices, this would amount to a $250 million

reduction in annual fuel costs.

The principal benefit to the Government

would come in the form of increased controller

productivity and the attendant reduction in operating costs: the volume of airspace assigned to

a control team could be greatly enlarged; it

might also be possible to reduce the size of the

control team by automating the routine tasks of

clearance coordination and flight data entry.

Preliminary estimates are that controller productivity could be doubled, i.e. that individual en

route controllers could handle perhaps twice as

many aircraft as with the present system. a This

‘R. A. Rucker, Automated En Route A TC (A ERA): Operational

conce~fs, MTR 79WO0167, The Mitre Corp., May 1979.

‘L. Goldmuntz, et al., The AERA Concept, Economic and Science Planning, Inc., for the Federal Aviation Administration,

December 1980.

‘Personal communication, S. B. Poritzky, Director, FAA Office

of Systems Engineering Management, Dec. 21, 1981.

in itself would not necessarily increase the capacity of the system, but it could significantly

reduce future operating costs. One recent estimate places these savings at $300 million annually (1979 dollars), g but these preliminary figures

would need to be refined as the AERA program

progresses and a more precise picture of its operational characteristics is obtained.

A third advantage of AERA—and a strong

part of the rationale for seeking a high level of

automation—is that it would help reduce system

errors. * In the present ATC system about 60

percent of these errors are attributable to mistakes on the part of controllers: improper coordination between controllers, inattention, forgetting, failure to communicate, poor judgment,

and the like. 1

0 The underlying causes of many of

these errors can be traced to the nature of ATC

as a work activity—routine, repetitive tasks requiring vigilance and close attention to detail,

and often conducted at a forced pace. Computers are ideally suited to this kind of activity; and

if the tasks to be automated are judiciously selected and the software carefully designed, an

automated system such as AERA could eliminate a major part of system errors, or at least

provide a backstop to the shortcomings of human operators. In this sense, AERA is expected

to be safer than the present system of traffic control.

Potential Implications and Issues

It must be emphasized that AERA is still in the

early stage of engineering development. Extensive effort, over perhaps 5 to 10 years, will be

needed to bring AERA to a precise and detailed

definition of requirements and equipment specifications. Installation, test, and full operational

deployment will take an additional 5 to 8 years.

‘Goldmuntz, op. cit. This benefit is calculated by taking the

$375 million annual expense (1979) to operate ARTCCS, increasing

it by a factor of 1.6 to account for traffic growth by the time AERA

would become operational taking so percent of that as the benefit

due to AERA productivity improvements.

● By FAA definition, a “system error” occurs whenever the actual horizontal or vertical separation between aircraft is less than

prescribed minima.

‘OGoldmuntz, op. cit.; and G. C. Kinney, M. J. Spahn, and R.

A. Amato, The Human Element in ATC: Observations and

Analyses of the Performance of Controllers and Supervisors in

Providing ATC Seruicest MTR-7655, The MITRE Corp. ,

December 1977.
Ch. 5—Technology and the Evolution of the ATC System • 85

Thus, AERA cannot be expected to replace the

present generation of en route ATC until sometime near the end of the century. Similarly, the

development costs and subsequent expenditures

for facilities and equipment (F&E) have not yet

been estimated, except in the most general

terms. The latest available projections of R&D

expenditures for en route control systems over

the coming 10 years, much of which would be

for AERA, show a total outlay of $170 million

(1980 dollars).11 As of the writing of this report,

detailed estimates of the required F&E investments and costs to users for avionics appropriate

to AERA have not been published.

Three major implications of AERA are already apparent, however. One is that AERA

would require computer capacity and software

far beyond what is now available in ATC applications, although not beyond the present or

foreseeable state of computer technology. Second, AERA will require a two-way data link capable of rapid and high-volume exchange of information between the air and the ground. FAA

now envisions that Mode S will provide this

data link, and plans for AERA are predicated on

the availability and widespread use of Mode S

by the early 1990’s. (See the discussion of “data

link” in the following section.) Third, AERA implies equally extensive automation in terminal

areas and in a central flow management facility

capable of coordinating traffic throughout the

ATC system.

This last point is particularly important both

for the immediate plans to replace en route computers and for the design of the entire ATC system over the long term. It implies a modular

computer architecture, in which en route and

terminal facilities utilize similar hardware and

software. This would make possible a flexible

system design, in which individual modules

would be capable of mutual support and backup

in the event of local equipment or software failure. Human controllers would have difficulty

operating the ATC system manually in the event

of a failure of AERA if adequate automated

backup were not provided.

‘‘National Aviation System Development and Capital Needs for

the Decade 1982-1991 (Washington, D. C.: Federal Aviation Administration, December 1980).

The development and implementation of

AERA is likely to raise several important issues.

Some are technical and concern the reliability

and safety of AERA, specifically its vulnerability to undetected software errors or hardware

failures, and the adequacy of current hardware

and software design techniques. The degree of

automation envisioned for AERA may also be

controversial, and this could give rise to issues

pertaining to the division of tasks between human operators and computers or the design of

the man/machine interface. The design will have

to include features that keep the controller’s attention and insure that he has enough information to deal promptly with anomalous situations

as they arise. Acceptance of the system by both

controllers and airspace users may prove to be

troublesome.

A third set of issues pertains to the costs and

benefits of AERA, especially the savings in operational costs ascribed to AERA in comparison

with the investments needed to implement the

system. A corollary question will be the costs

and benefits to various classes of airspace users,

especially if AERA entails mandatory equippage

with data link or other avionics in order to participate in the automated ATC environment.

Resolution of these issues, rather than the somewhat narrower questions of technical feasibility

or system design, may prove to be critical to the

acceptance and success of the AERA concept.

Data Link

Potential Benefits

Communication is central to the ATC process, and at present voice communication is the

primary medium even for messages that involve

computers processes. For example, a controller

reads data from a computer-generated display,

transmits it by voice radio to an aircraft, and the

crew then enters the data manually into an onboard computer. This process wastes crew and

controller time and is prone to reading or transmission errors. As the ATC system changes to

incorporate higher levels of automation, therefore, great benefits could be gained from a digital data link that permits direct communication
86 • Airport and Air Traffic Control System

between automated components. Among these

potential benefits are the following:

● Digital messages can include special codes

to detect and correct transmission errors.

● Processes that are running on computers

can exchange data of little immediate interest to the human participants without human involvement.

● Digital transmissions can be addressed to a

specific recipient such as an aircraft without

diverting the attention of others to whom

the information is not of concern.

● Digital messages can be transmitted, stored

by the receiving terminal, and recalled on

demand by the recipient.

In the present ATC system, the ATCRBS

transponder provides limited data communication, Digital messages are sent by the transponder in reply to interrogations from the ground

that request aircraft identity (transponder code)

or altitude. Some observers, as discussed later in

this section, argue that the inherent capability of

the ATCRBS transponder is currently underutilized and that it is capable of meeting many of

the future requirements for a digital data link.

Others, including FAA and a significant segment

of the user community, question this conclusion.

While there is little dispute that a data link is

needed for the ATC system of the future, there is

considerable discussion on how it would best be

implemented. * FAA has suggested the addition

of a data link capability—Mode S—to the specifications for the standard ATCRBS transponder.

Others have suggested alternatives, and one organization, Aeronautical Radio, Inc. (ARINC),

is now operating a nationwide data link that is

used by the airlines for administrative communication. These alternatives are described in the

sections that follow.

● Data links are also used to connect computers at the various

ATC facilities operated by FAA. They use leased commercial telecommunication facilities at the present time; but in the future, satellites might be used to perform this function more efficiently. For

this discussion, which will focus on data links for air-to-ground

and air-to-air communication, the links between the ground-based

computers are not of direct interest.

Mode S

The operating characteristics of the ATCRBS

transponder conform to a standard established

by the International Civil Aviation Organization (ICAO). For civil aviation, four modes of

operation are defined, of which only two are in

actual use: Mode A for aircraft identity, and

Mode C for aircraft barometric altitude. Interrogation messages are formatted so that the transponder will recognize the mode of the query and

reply appropriately. Since the transponder is already the primary link between ATC computers

on the ground and aircraft in flight, it is logical

to argue that the data link function be incorporated in the transponder.

FAA has suggested adding a fifth mode, Mode

S, to the specification for the ATCRBS transponder. ** This mode would provide a generalpurpose data link designed to operate in a manner compatible with the existing ATCRBS

modes. Mode S was on the agenda at the April

1981 meeting of the ICAO Communications Division, and position papers relating to it have

been circulated among members. Great Britain

and the Soviet Union have independently developed data link specifications that are compatible

with Mode S. As of now, however, no member

of ICAO has formally proposed detailed specifications that could be adopted as a Mode S standard.

Mode S permits a digital message to be addressed to a specific recipient. Each aircraft

would have a permanently assigned code to

identify itself in all ATC-related communications using the data link. When a Mode S interrogation or message is sent, replies from all

transponders operating in Modes A and C are

suppressed. Thus, during any transition period,

interrogations cycles would have to be divided

between Mode S interrogations and those in the

existing Mode A and Mode C formats.

One of the applications of Mode S is for the

surveillance function of the ATC system. When

two aircraft are in proximity (i.e. in line or almost in line and differing in range from the inter-

● *Until recently, Mode S was referred to by FAA as DABS

(Discrete Address Beacon System).
Ch. 5—Technology and the Evolution of the ATC System ● 8 7

rogating ground station by 1.5 miles or less),

their replies to a Mode A or C interrogation will

interfere with one another, creating what is

called “synchronous garble. ” The ability to address the interrogation to a specific aircraft is

one method of resolving this difficulty. Other

methods, such as computer processing of returns

or the use of multiple sensors, can accomplish

much the same thing.

A second anticipated benefit from Mode S

would be the ability to deliver control messages,

such as clearances and en route weather information, to specific aircraft. The data needed to

generate onboard displays of traffic could also

be transmitted using this technique. Further, a

Mode S data link could be useful in an exchange

of data between aircraft, allowing them to coordinate conflict-resolution maneuvers (i.e., as an

element in a collision avoidance system). Again,

however, Mode S is not the only means by

which these needs could be met.

Modes B and D

Most of the cost of implementing Mode S

would be borne by the users, although some expenditures by FAA for the modification of its

computers and software would be required.

Some observers, however, consider the expense

required for the introduction of Mode S to be

unwarranted. They argue that the capability of

the present ATCRBS transponder is underutilized. Modes B and D, it is suggested, could be

used for some data link purposes, since they

have sufficient capacity to meet the needs of the

ATC system and would require no change in the

existing ICAO specification. In addition, the

message format for Modes B and D is shorter

than that suggested for Mode S, and therefore

less likely to result in the interference that might

occur between Mode S transponders replying to

simultaneous interrogations from different stations. However, in considering this alternative,

one should also note that existing transponders

do not include the components needed to process Mode B and D interrogations and would

have to be modified (at users’ expense) to do so.

VHF Data Link

A second alternative to Mode S is the use of a

part of the VHF radiofrequency band assigned to

aeronautical voice communication. ARINC, a

corporation organized and owned by the airlines

to provide communication services, already operates a data link of this type, know as ARINC

Communication Addressing Reporting System

(ACARS), which is being used by airlines for administrative messages. At present, small printers

in the cockpit are used to record ACARS messages. A future modification could be conversion of the onboard weather radar screen or one

of the multipurpose displays used by electronic

instrument systems found in some aircraft to

double as a display for ACARS messages.

Some critics suggest that ACARS would not

meet the requirements for an ATC data link,

pointing out that the VHF voice band is already

crowded and that the one frequency used by

ACARS (although currently underutilized)

would not have sufficient capacity to meet the

needs of the ATC system. This deficiency could

be overcome by assigning multiple frequencies

and scanning them automatically to detect incoming messages. There has also been a start

(for reasons having little to do with data link) at

reducing the current 50 kHz spacing in the VHF

band to 25 kHz, effectively doubling the number

of channels available. Some of these new channels could be allocated to the data link function.

Potential Implications and Issues

A data link is a primary resource that can be

applied in a number of ways, and the benefits

obtainable will be a function of the purposes to

which it is applied. If the data link is to be used

primarily for surveillance, then it would be advantageous to integrate it with the radar beacon

system. On the other hand, if it is used primarily

for nonsurveillance purposes such as delivering

clearances, reporting weather conditions, or

sending and receiving advisories, the need to associate it closely with the radar beacon system is

less compelling. The balance in traffic between

the uplink and downlink is also significant. If the
88 ● Airport and Air Traffic Control System

great majority of the message traffic is “up”-

from ground to air—the ground station could

assume responsibility for allocating time among

users. If there is a substantial flow of information in the opposite direction—air to ground,

with a large part of it initiated by aircraft—the

task of coordinating the activities of the users

would become much more difficult. The latter

situation would be complicated further by the

introduction of substantial amounts of air-to-air

traffic, as in the Traffic Alert and Collision

Avoidance System (TCAS) concept (described

later).

In considering the candidate forms of data

link, another important consideration to keep in

mind is that the data link is not an isolated subsystem of ATC, nor does it provide any unique

service. Some form of data link is indispensable

to the future scheme of operation and services

envisioned by FAA, such as AERA and the collateral improvements of terminal area control

and central flow management. The level of automation and the degree of strategic and tactical

control that AERA would bring about requires a

high-speed and high-volume flow of information, decisions, and replies between the air and

the ground. Thus, even though FAA is committed to Mode S, it is important that all questions about data link be promptly resolved and

that the necessary ground facilities and aircraft

avionics be put in place so as to keep pace with

the parallel computer replacement program.

Both of these resources will have to be available

within a decade if longer range improvements

are to be accomplished in the 1990’s.

It is also important to recognize that the data

link decision is not one where the United States

can act with complete independence. ATC requirements and development programs of other

nations must also be considered, and the direction chosen by FAA must be coordinated

through ICAO to ensure compatibility of signal

format, modes of operation, equipment characteristics, and the like. On balance, a data link

system that is compatible with the needs of other

ICAO member nations is preferable to one that

is unique to the United States.

Another important aspect of the data link decision concerns the avionics equipment that airspace users will have to install in order to take

advantage of the services that data link offers.

The data link is more than just a special kind of

high-speed receiver-transmitter: to make any

meaningful use of this capability, aircraft will

also have to be equipped with processors to encode and decode messages, and with some kind

of input-output device (displays and controls)

that presents information to the aircrew and allows them to interact with the onboard processors and ground stations. Such equipment is

costly to acquire (about $10,000 for a commercial aircraft, but somewhat less for GA) and

would require special maintenance. For commercial and corporate operators the expenses of

acquisition and maintenance could be absorbed

without great difficulty, and the costs would

probably be offset by operating benefits such as

fuel savings, avoidance of delay, and greater

flexibility of flight planning. For smaller GA operators, on the other hand, the cost-benefit

equation may not be as favorable, and they may

consequently conclude that the expense is not

justified by the improved services or operational

savings made available to them.

The matter could become particularly acute

for GA if equipage with data link avionics were

to be made mandatory for access to airspace or

for receipt of essential ATC services. FAA currently envisions a tiered program of services in

which users receive progressively more extensive

service in relation to the sophistication of the

avionics carried on the aircraft. The concern of

GA is that the areas in which they will be allowed to operate with only minimal equipment

(that is, without a two-way data link) will become so restricted that small GA aircraft will be

effectively excluded from the Nation’s airspace.

The extent to which these concerns are warranted will depend heavily on the type of data

link that is selected and how it is to be incorporated in various classes of aircraft.

Collision Avoidance

A primary function of the ATC system is providing separation assurance. Ground-based sur-
Ch. 5—Technology and the Evolution of the ATC System • 89

veillance equipment and computer software include features that will alert the controller to situations where separation standards have been

violated or are about to be violated. Nevertheless, a small number of midair collisions and

near misses continues to occur, most of them involving aircraft not under positive control. At

the present level of traffic, the probability of collision is very low, but as traffic density increases, so does the threat of collision. The few

accidents suffered by commercial carriers have

heightened public awareness of the consequences of a midair collision involving large passenger aircraft. This common concern has led to

significant public and private efforts to develop

collision avoidance systems that would give the

aircrew direct warning of the threat of collision.

A collision avoidance system is conceived as a

last-resort measure to protect against collisions;

it would come into play only after all other

means to ensure separation have failed. A collision avoidance system is not intended to be the

primary method of ensuring the separation of

aircraft. But the extra margin of safety provided

by a collision avoidance system could lead to

changes in ATC procedures for separation assurance. For example, a reliable collision avoidance

system could justify a reduction in separation

standards, thus effectively increasing the capacity of the airway and airport system. This section

discusses some of the alternative collision avoidance systems that have been proposed over the

years in order to give the reader an awareness of

their relative merits and implications.

In general, two major classes of collision

avoidance systems have been proposed: those

that depend on ground facilities; and those that

require only airborne equipment. Ground-based

collision avoidance systems characteristically require the expenditure of Government funds for

facilities and equipment, while airborne systems

do not. Some of the so-called airborne systems,

however, are in fact passive users of ATC equipment—that is, they “eavesdrop” on replies to

ATCRBS interrogations from ground surveillance stations in order to obtain the data needed

to locate nearby aircraft. Some systems would

be effective only when a large portion of the aircraft in the fleet are equipped, while others

would provide some protection regardless of the

number of users who install the equipment.

Beacon Collision Avoidance System

The Beacon Collision Avoidance System

(BCAS) is one that had been under development

by FAA for some time and was nearing the point

of implementation when FAA made the decision, in the summer of 1981, to adopt another

system that is a derivation of BCAS (see below).

The initial version of BCAS, known as Active

BCAS, would have been implemented first; and

Full BCAS, a more complex version designed to

operate in congested airspace, would have followed several years later.

In operation, Active BCAS on board aircraft

would emit interrogation pulses to which

ATCRBS and Mode S transponders on the other

aircraft would reply in the same manner as they

would reply to an interrogation from a ground

station. The BCAS concept offered immediate

protection against aircraft equipped with Mode

C ATCRBS transponders and altitude encoders

and promised more efficient performance and

broader protection against aircraft equipped

with Mode S Transponders. The BCAS system

used the elapsed time between interrogation and

reply to determine the range to other aircraft,

and by calculating the rate of closure it determined the potential for collision. If a collision

threat were detected, an indicator would advise

the pilot whether to climb or descend to resolve

the conflict. The DABS data link was to be used

to coordinate the maneuvers of two BCASequipped aircraft. Active BCAS did not, however, provide the pilot with the relative bearing

of the intruder aircraft. * Full BCAS, in addition

to originating interrogations, also gathered data

by listening to replies to interrogations from the

ground and correlated these replies to determine

bearing as well as range.

There was little question that BCAS would be

effective in low-density airspace, but there was

considerable concern that the system would become saturated in areas of high-traffic density

where a collision avoidance system is most

● A proposed follow-on version of Active BCAS would have

provided direction-finding capability.
90 ● Airport and Air Traffic Control System

needed. For this reason, FAA planned to install

ground equipment (an RBX transmitter) to suppress BCAS and prevent system saturation in

areas of high-traffic density where it planned to

rely instead on a ground-based system called the

Automatic Traffic Advisory and Resolution

Service (ATARS) to resolve conflicts. ATARS

would use ATCRBS and Mode S interrogations

and replies to gather traffic data and convey

traffic information to suitably equipped aircraft

by means of the Mode S data link; ATARS was

designed to provide a turning manuever as well

as the climb or descend maneuver of BCAS.

While ATARS would overcome the major

weakness of BCAS, however, it would also require considerable expenditure for both ground

and airborne equipment. Both BCAS and

ATARS planned to use the Mode S transponder

as a key element, and both therefore were

caught in the debate that surrounded the Mode S

data link concept. Some critics have claimed

that Full BCAS would be required to support a

cockpit display of traffic information (CDTI),

since the simpler Active BCAS provided no intruder bearing and thus could not provide the

aircrew with a picture of surrounding traffic

analogous to that available to ground controllers. In many cases it was difficult to separate the

arguments for and against DABS from those

pertaining to a collision avoidance system.

Tri-Modal BCAS

Tri-Modal BCAS was one proposed alternative to the BCAS program. It was similar to

BCAS in concept but based on the existing

ATCRBS transponder rather than the new Mode

S capability, and it would operate in three different modes. In areas of high traffic density, TriModal BCAS would operate passively, generating all of the required information by analyzing

standard ATCRBS transponder replies to interrogations from ground surveillance stations. In

areas without coverage by ground radar, it

would operate like Active BCAS. Where coverage was provided by only one ground radar station, it would operate in a semiactive mode to

generate its own interrogations while also listening to replies to interrogations from the ground

station. The logic used by Tri-Modal BCAS

would enable it to determine both range and

bearing in airspace adequately covered by

ground interrogators and, thus, to generate the

data needed to support a CDTI.

Advocates of Tri-Modal BCAS cited the following advantages of this system:

●

●

●

●

It does not require the Mode S transponder

and provides full protection from all aircraft equipped only with a standard

ATCRBS transponder.

In airspace where the geometry of the distribution of ground-based interrogators is appropriate, it provides bearing without requiring the directional antenna that is

needed for TCAS (discussed next) and Full

BCAS.

It requires no change to the ground facilities

except for the activation of the north pulse

on the secondary surveillance radars now

installed.

It can operate independently of all ground

facilities in the same manner as active

BCAS.

NASA, with the sponsorship of FAA, successfully tested Tri-Modal BCAS, but its report indicated that the tests were not exhaustive because

a working model that included all of the features

of the system was not available. However, the

developers of the system have continued their

work since the NASA tests and claim that their

system is ready for certification and operational

use.

Traffic Alert and Collision

Avoidance System

After supporting the development of BCAS

for several years, FAA announced in the summer of 1981 its decision to adopt an enhanced

air-to-air version of BCAS, the Traffic Alert and

Collision Avoidance System (TCAS). The abruptness of this change has led to controversy in

the aviation community, and various observers

have questioned both the suitability of TCAS

and its superiority to alternative systems.

TCAS is a direct derivative of BCAS and is

designed to meet the following criteria:
Ch. 5—Technology and the Evolution of the ATC System ● 91

● It does not require ground-based equipment.

● It is compatible with the present ATC system and a logical extension of it.

● It is more suitable for use in high-density

traffic than BCAS.

● It offers a range of capabilities suitable to

the needs of various classes of airspace

users.

To meet the last criterion, two versions of

TCAS have been specified; both include the

Mode S data link as an integral component.

TCAS I is designed for use by general aviation

and the basic system is estimated by FAA to cost

in the range of $2,500 to $3,500 per aircraft.

TCAS I would indicate to the pilot the presence

of a transponder-equipped aircraft without providing either range or bearing information; it

would be the responsibility of the pilot to locate

the intruder by visual means and to take the appropriate action. An upgraded version of this

basic system would provide the pilot with intruder range and bearing and with information

describing the maneuver that a TCAS IIequipped aircraft intended to execute. TCAS I

estimates range by the strength of the signal received from another aircraft, at best an imprecise measure, and in high-density airspace the

proximity-warning indicator tends to be triggered repeatedly, thus minimizing its value as a

warning device (if the false alarm rate is high, pilots might tend to ignore the warning). The addition of an altitude stratified in TCAS I, however,

appears effective in minimizing high alarm rates.

TCAS II is a more sophisticated version designed for use by air carriers and larger corporate GA aircraft. FAA estimates that the necessary avionics will cost on the order of $45,000 to

$50,000 per aircraft, slightly more than the projected cost of an Active BCAS unit. TCAS II operates in the same way as Active BCAS, but with

two major enhancements:

●

●

A directional send-receive antenna that will

provide both range and bearing without

creating the interference in areas of high

traffic density expected with Active BCAS.

The ability to transmit to TCAS I and other

TCAS II aircraft information regarding its

relative location and the intended maneuver

to resolve a conflict.

Initially, the TCAS 11 antenna will provide bearing information accurate to within 300, sufficient to provide the pilot with an “o’clock” indication of relative bearing and activate a climb or

descend indicator. In later versions, FAA plans

to specify an antenna with much higher angular

resolution (1° to 2o), which would permit the

system to generate a command for a horizontal

as well as a vertical maneuver. The improved

version would also support a CDTI.

FAA has issued a contract for the development of the high resolution antenna to determine

if or when an antenna with this degree of resolution, yet suitable for installation on commercial

aircraft, can be designed and tested. One early

version of the sector scan TCAS II antenna was

approximately 18 inches in diameter and extend

slightly above the fuselage contour. Mounting

such an antenna might require significant modifications of aircraft structure even on a large aircraft; the problem would be more severe in the

case of small GA or tactical military aircraft.

Further, if a large antenna were to result from

the development efforts, it could have detrimental effects on aerodynamics, aircraft performance, and fuel consumption.

The adoption of TCAS means that the DABS

transponder remains a key element in FAA

plans. However, the fact that TCAS is groundindependent and capable of operating in airspace with high-traffic density puts in question

the need for ATARS, one of the key applications

heretofore envisioned for DABS. There are

strong indications that FAA will drop ATARS

from its plans and that, as a result, the level of

expenditures on ground equipment will be significantly less than they would have been had

the ATARS program been implemented.

FAA has also made a point of leaving the way

open for entrepreneurial innovation in the development of TCAS, Thus, it is conceivable that

FAA might certify other collision avoidance systems if their capabilities were demonstrated and

if they would not interfere with TCAS or other

elements of the ATC system.
92 • Airport and Air Traffic Control System

For instance, TCAS and Tri-Modal BCAS

could operate in the same environment because

both depend primarily on responses from airborne equipment and neither requires the installation of equipment on the ground. However, as

noted above, the TCAS concept contains a provision for coordinating conflict-resolution

maneuvers of TCAS-equipped aircraft. Such

coordination would not be possible between an

aircraft equipped with TCAS and one equipped

with Tri-Modal BCAS as these systems are

presently designed. On the other hand, the

TCAS concept does not assume that it will

always be possible to coordinate the maneuvers

of aircraft in a conflict situation. Therefore, the

inability to coordinate the maneuvers of TCAS

and Tri-Modal BCAS aircraft does not present

an insurmountable barrier to operation of the

two systems in the same environment.

Airborne Collision Avoidance System

All of the alternative collision avoidance systems that have been discussed to this point are

capable of providing users some level of protection from aircraft that are not similarly

equipped. The Airborne Collision Avoidance

System (ACAS), which was developed and demonstrated in the 1970’s, was not based on the

ATCRBS transponder and could have been

made available for about $1,500 per aircraft

(1977 dollars), considerably less than the alternatives being considered at that time. A major

drawback of this system, however, was that it

would not be effective unless a substantial portion of the aircraft operating in a given area were

ACAS-equipped.

Conceptually, the operation of the ACAS system was simple. It generated interrogations to

which all aircraft within a specified altitude

band would respond. Range was determined

from the delay between interrogation and reply,

and when an aircraft was detected at close

range, subsequent interrogations narrowed the

altitude band from which a reply was requested

in order to determine whether a detected aircraft

presented a threat of collision.

ACAS is no longer being actively considered

as an alternative collision avoidance system, but

it is presented here to illustrate another group of

alternatives that have been explored in the past.

Microwave Landing System

Instrument Landing System

Providing precise and reliable guidance for

approach and landing in conditions of reduced

visibility is a prime consideration for safety of

flight, but it also has important implications for

the efficient use of terminal area airspace and

airport runways. Generally, the highest runway

utilization rates are achieved under VFR. When

restricted visibility or weather conditions dictate

increased separation and the use of instrument

approaches, one consequence is a reduction in

the number of aircraft that can be landed in a

given space of time.

In part, this reduction in airport capacity utilization is a result of the guidance system in use.

The present Instrument Landing System (ILS),

which has been the standard U.S. system since

1941, provides guidance along a straight path at

a fixed slope of 3° or less extending 5 to 7 miles

from the runway threshold. All aircraft approaching the airport must merge to follow this

path in single file, spaced at intervals dictated by

separation minima and the need to avoid wake

vortex. Aircraft flying at different speeds along

this single fixed path complicate the controllers

task in achieving a uniform rate of traffic flow

and diminish the capability to use the full capacity of the runway served by ILS.

The runway utilization rate under IFR could

come closer to that attainable under VFR if aircraft could be permitted to follow multiple approach paths, descend at different flight angles,

fly at different approach speeds, or aim at different touchdown points on the runway—none of

which can be done with ILS. If these variations

were possible, as they are under VFR, the IFR

capacity of the airport would be increased to a

limit determined almost solely by the rate at

which successive aircraft could touch down, decelerate, and clear the runway. *

● Wake vortex, for example, would remain a constraint on capacity even if MLS with curved and variable glide slope approaches were installed.
Ch. 5—Technology and the Evolution of the ATC System • 93
94 • Airport and Air Traffic Control System

Microwave Landing System (see fig. 25). This capability is useful in avoiding

A precision approach and landing system that

overcomes these inherent disadvantages of ILS is

the Microwave Landing System (MLS). Because

MLS uses a scanning beam, rather than a fixed

beam like ILS, it allows aircraft to fly any of

several approach angles (including two-step

glide slopes) and, in the lateral plane, to approach along complex paths that intersect the

alinement of the runway at any selected point

noise-sensitive areas on approach paths and reducing the impact of the wake vortex problem.

MLS offers other important advantages in

comparison with ILS. The reliability of the MLS

signal is not influenced by ground-plane effects

(snow buildup, soil moisture, tidal effects, etc.);

this permits MLS to be installed at sites where

ILS will not function properly. Fixed or moving

obstacles in the approach zone do not interfere

Figure 25.—Comparison of Microwave Landing System and Instrument Landing System

Middle

marker

SOURCE: Federal Aviation Administratlon.
Ch. 5—Technology and the Evolution of the ATC System ● 95

with MLS signals to the same degree as with ILS.

In addition, MLS also provides precision

guidance for departures and missed approaches,

a feature of particular importance when traffic

patterns of closely located airports are in conflict. MLS operates in a frequency band that provides 200 transmission channels; ILS has used

only 20 of the 40 channels theoretically available

to it, and these are very near saturation in large

hubs such as New York and Los Angeles. Finally, ILS does not meet the joint civil/military

operational requirement for precision approach,

since it does not afford the tactical flexibility

needed by military aircraft. MLS does.

For these reasons, FAA has designated MLS as

the precision approach guidance system to replace ILS. The MLS transition plan, published

by FAA in 1981,12 calls for 1,425 installations to

be carried out in three phases over the next 20

years. In the first phase, between 10 and 25 systems will be installed over a period of 2 years at

selected airports in order to develop a base of experience and reach an operational confirmation

of the benefits that MLS can provide. The second phase will see the installation of 900 additional MLS units at a rate of 100 to 150 per year

over a period of 6 to 9 years, with priority given

to large and medium hub airports. The third

phase involves installation of an additional 300

to 500 units to meet the growth in demand anticipated by the end of this century. FAA estimates

the cost of purchasing and installing 1,425 MLS

ground units to be $1.332 billion (1981 dollars);

the cost to users to equip their aircraft with MLS

is estimated to be an additional $895 million,

yielding a total cost of roughly $2.2 billion.l 3

In selecting the transition plan, FAA worked

in consultation with various user groups under

the auspices of Radio Technical Commission for

Aeronautics, and considered 10 deployment

strategies—9 submitted by FAA and 1 developed

by RTCA Special Committee 125. These strategies differed in terms of the order and rate of

deployment at various sites, the length of the period of duplicative operation with ILS, and as-

‘‘Microuumw LandirIg Syst~m Trnmsition Plan, APO-8 I-I

(Washington, D. C.: Federal Aviation Administration, May 1981).

“Ibid.

sumed rates of user equipage. Each strategy was

analyzed to estimate costs, benefits, and operational effects. All strategies yielded favorable net

benefits in the range of $2.4 billion to $2.7

billion. The costs of the 10 strategies varied narrowly ($1.20 billion to $1.35 billion for ground

units), as did the benefits ($3.65 billion to $4.05

billion). These results led FAA to conclude that

“there is no clear-cut economic rationale for

choosing among the MLS implementation strategies” and that “the choice should be based upon

operational considerations or on the special opportunities for improved precision guidance

service created by the installation of MLS equipment .“l 4 The strategy selected by FAA reflects

these considerations.

Potential Implications and Issues

There are two factors that may complicate the

MLS transition plan, both of them involving the

replacement of the existing ILS. As of March

1981 there were 653 ILS units in commission at

458 airports, and an additional 155 units were in

various stages of procurement or installation.

Thus, the MLS transition plan has to take into

account how these ILS sites, many of them recently commissioned and with many years of

service life remaining, are to be phased out. ILS

and MLS can be colocated and operated simultaneously without signal interference or procedural difficulty, but the length of the period of joint

operation and the timing of ILS decommission at

specific sites could create difficulties for some

classes of airport users. FAA transition plan stipulates that no ILS will be removed until all of the

network’s Ill-equipped airports have operational MLS and at least 60 percent of the equipped

aircraft routinely using the ILS/MLS runway are

MLS-equipped. When this occurs however, 40

percent of the regular users of a given airport

could lose the precision-landing service, even

though they continue to operate with functioning ILS equipment.

The second complication is that, by ICAO

agreement, the United States is committed to retain ILS service at international gateway airports

through 1995. There are 75 such airports at pres-

“Microuun~e La)ldit~g Systetn Transition Pla/1, op. cit
96 ● Airport and Air Traffic Control System

ent, and generally they are among the busiest

U.S. airports. The retention of ILS service at

these sites may cause some users to delay purchasing MLS equipment, since the installed ILS

equipment will still be usable for another 10

years or more.

Despite the overall favorable benefit-cost

ratio of MLS indicated by FAA analysis, the specific benefits and costs to various classes of

airspace users remains a subject of controversy.

FAA’s analysis showed high positive net benefits

to air carriers and commuters largely due to the

value attributed to passenger time saved. For

general aviation as a whole, the costs exceeded

the benefits for all 10 deployment strategies,

although some classes of GA (notably corporate

GA operating multiengined piston and jet aircraft) were shown to derive substantial benefits

from MLS. Thus, there is likely to be continued

resistance to MLS from some GA operators,

probably in the form of opposition to decommissioning ILS at specific sites and reluctance to

purchase MLS equipment (at a cost of $5,000 or

more) so long as ILS is available.

It is also likely that specific details of the MLS

transition plan will continue to arouse debate.

Comment received by the FAA during the

course of preparing the plan indicates that there

are several sensitive points. One potential issue

is the priority given to installation of MLS at different types of airports. For example, commuter

airlines favor early deployment at small community airports, while the Airline Pilots Association seeks to have MLS first installed at hub airports on runways not now Ill-equipped. Other

user groups, for example the Air Transport

Association, recommend an installation strategy

that would create a network connecting major

airports (including many now equipped with

ILS), in order to encourage users who fly these

routes frequently to install MLS equipment on

their aircraft. Another, slightly different, recommendation would involve establishment of a

major-city network but with priority also given

to installation at sites where it is not possible to

locate an ILS and at small community airports

that have commercial service but not an ILS.

AS a final point, the MLS transition plan proposed by FAA may encounter administrative

and budgetary difficulties. The plan, particularly Phase II, is highly ambitious in that it calls for

installation of 900 units at a rate of 100 to 150

per year. It may be technically and administratively difficult to sustain such a pace, and it

might be even more difficult to justify the required annual outlay of funds in a time of

budget austerity. Implementation of Phase II

would entail annual expenditures of $125 million

on a 6-year schedule, or $85 million on a 9-year

schedule. Stretching out Phase II, in order to

hold it within some imposed budgetary limit, is

an alternative that may have to be adopted,

even though it might increase overall program

costs and defer realization of the full benefits of

MLS.

ALTERNATIVE ATC PROCESSES

FAA is nearing the end of research and development of several major components of the

ATC system and is about to begin operational

deployment of these new technologies. Most of

the system improvements planned by FAA

would continue the present trend toward a

ground-based, centralized control system with

increasingly more extensive requirements for

avionics and more restricted forms of operation.

These plans would also entail a major commitment of funds by the Federal Government and

the aviation community. It is important that the

Congress be satisfied, not only as to the soundness and appropriateness of these prospective

system changes, but also as to whether FAA’s

plans take into account the new alternatives that

are being made available by emerging technologies.

There are five aspects of the future ATC

system on which new technologies might have

an especially important influence in creating

new options:

● the role of the human operator;
Ch. 5—Technology and the Evolution of the ATC System ● 9 7

●

●

●

●

tactical v. strategic control;

autonomy and flexibility of control;

ground v. satellite basing; and

levels of service.

Role of the Human Operator

The AERA concept implies that computers

will assume many of the controller’s routine

decisionmaking tasks and, by means of digital

data link, many of the communications tasks as

well. The immediate consequences would be

that fewer human operators would be needed to

handle a given volume of traffic and that the

human role would evolve toward that of a

manager of automated resources.

However, there would also be important consequences for the pilot. The increased level of

automation on the ground would bring a corresponding increase in opportunities to employ

automation in the cockpit. Aircrew dependency

on airborne data processors and displays would

increase as more information would be transmitted digitally and the relative importance of

the voice channel waned.

Another consequence of automation is that

the burden of responsibility for operational reliability would shift. Safety would be assured

more and more through the design process and

less through the compensatory actions of the human operator.

Tactical v. Strategic Control

A system supported by powerful data processors can collect, analyze, and distribute information on a much wider scale than the present ATC

system. This makes it possible to plan and coordinate the movement of traffic over a broader

area and a longer span of time. The basic mode

of control could therefore become more strategic

and anticipatory—relying more on prevention

of conflict through planning, and less on tactical

or reactive response to actual or imminent violations of separation minima.

For the ground controller, whether human or

computer, the principal task would be monitoring aircraft movements to ascertain conformance with a flight plan that, through planning,

had been determined to be conflict-free. For the

pilot (aided by a flight management computer

and onboard ATC systems), the principal task

would be to fly from origin to destination without deviating from that flight plan unless unforeseen circumstances (such as weather or deviations of other aircraft) forced rerouting. Tactical

control measures would still be available, but

they would be called into play only when

strategic measures proved inadequate to forestall conflict.

Autonomy and Flexibility of Operation

IFR control is now centralized on the ground

because only the ground controller has the information needed to assure separation and an orderly flow of traffic. However, improvements in

communication and processing technologies

have made it possible to redistribute information

among the various participants in the ATC system.

Given greater access to information, aircrew

could become more active participants in the

ATC process. As the quality and timeliness of

the information improves, interaction with

ground controllers could become infrequent.

However, there is a logical limit to their independence from ground control, because overall

strategic control of the flow of traffic will remain

a ground-based function.

Ground v. Satellite Basing

Navigation and surveillance functions in the

present system are ground-based, as are the facilities for relay of air/ground radio transmissions. The development of space technology

makes it possible to consider satellites as alternatives for all three purposes. Satellites could be

used in either an active or a passive mode. In the

passive mode, they could serve as relay stations

for communication between the air and the

ground or between ground sites where present

methods are limited to line of sight. Satellitemounted transponders could also provide position reference for airborne navigation systems.

In an active mode, data processing capabilities

could be installed in satellites to track aircraft

and report their location to ground-based con-
98 • Airport and Air Traffic Control System

trol facilities, either

surveillance radar.

replacing or supplementing mediate levels of ATC services between these

two extremes. The level of service could vary according to 1) the density of traffic; 2) the mix of

Levels of Service aircraft; 3) the avionics carried by those aircraft;

4) flight conditions;

Under the present ATC system there are only

and 5) the ground-based

capability for separation assurance and traffic

two forms of operation—controlled (corre- management. The result could be a more varied

spending roughly to IFR) and uncontrolled (cor- range of services, more closely tailored to the

responding roughly to VFR). In the future, im- needs and capabilities of the airspace users, than

provements in ground-based and airborne tech- is now the case.

nologies could make it possible to provide inter-
Chapter 6

AIRPORT CAPACITY

ALTERNATIVES

Photo credit: Federal Aviation Administration

Dunes International I
Contents

Page

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Airside Components. . . . . . . . . . . . . . . . . . . . 102

Limitations on Airside Capacity. . . . . . . . . . 103

Aircraft Performance Characteristics. . . . 103

Wake Vortex... . . . . . . . . . . . . . . . . . . . . . 103

Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Airfield/Airspace Configuration. . . . . . . 105

Aircraft Noise . . . . . . . . . . . . . . . . . . . . . . . 106

ATC Equipment and Procedures.. . . . . . 107

Demand Considerations. . . . . . . . . . . . . . 107

Delay and Delay Reduction. . . . . . . . . . . . . . 107

Demand-Related Alternatives . . . . . . . . . . . . 109

Peak-Hour Pricing. ......... . . . . . . . . . 109

Quotas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

Balanced Use of Metropolitan Area

Airports . . . . . . . . . . . . . . . . . . . . . . . . 110

Restructuring Airline Service Patterns. . . 111

Reliever Airports. . . . . . . . . . . . . . . . . . . . . 112

Airport Development Alternatives. . . . . . . . 113

Expanding Existing Airports. ....... . . 113

Development of Secondary Runway

Operations. . . . . . . . . . . . . . . . . . . . . . 113

Building New Airports. . . . . . . . . . . . . . . . 114

ATC Improvement Alternatives. . . . . . . . . . 116

Airfield/Airspace Configuration

Management . . . . . . . . . . . . . . . . . . . . 116

Wake Vortex Prediction. . . . . . . . . . . . . . . 116

Microwave Landing System . . . . . . . . . . . 117

Reducing Separation or Spacing

Minimums . . . . . . . . . . . . . . . . . . . . . . 117

Page

Automated Metering and Spacing . . . . . . 118

Cockpit Engineering. . . . . . . . . . . . . . . . . . 118

Summary of Alternatives. . . . . . . . . . . . . . . . 119

Future Research Needs. . . . . . . . . . . . . . . . . . 121

Wake Vortex Avoidance. . . . . . . . . . . . . . 121

Wake Vortex Alleviation. . . . . ........ 121

Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

Airport Design . . . . . . . . . . . . . . . . . . . . . . 121

Ground Access. . . . . . . . . . . . . . . . . . . . . . 122

LIST OF TABLES

Table No. Page

8. ’’Top’’ U.S. Airports, by Enplaned

Passengers, by Air Carrier Operations,

and by Reported Delays.. . . . . . . . . . . . 101

9. Arrival and Departure Separations. . . . 104

10. Operational Characteristics of Airports

With Potential Benefits From a

Separate General Aviation Runway. . . 115

11. Summary of Alternatives.. . . . . . . . . . . 119

LIST OF FIGURES

Figure No. Page

26. Airport Hourly Capacity Varies

Strongly With Weather. . . . . . . . . . . . . . 104

27. Runway Configuration. . . . . . . . . . . . . . 105

28. Typical Distributions of Delay. . . . . . . . 108
Chapter 6

AIRPORT CAPACITY ALTERNATIVES

INTRODUCTION

The ability of airports to accommodate traffic

can be expressed in terms of “airside” or “landside” capacity. “Airside” capacity is defined here

as the number of air operations—landings and

takeoffs—that the airport and the supporting air

traffic control (ATC) system can accommodate

in a unit of time, such as an hour. The capacity

of an airport is not a single number, but will

vary with the number of runways in use, the visual or electronic landing aids available, the types

of aircraft being accommodated, the distance

between aircraft in the approach pattern, and

the noise abatement procedures in effect. The

time each aircraft occupies the runway and the

facilities for handling aircraft on the ground, on

taxiways, or at gates also affect airside capacity.

All of these factors will vary depending on the

weather.

“Landside” considerations, such as the size

and number of lounges or the adequacy of baggage-handling equipment, affect the number of

passengers an airport terminal can accommodate. Ground access, including the adequacy of

transit connections, roadways, and parking

areas for passengers’ cars, is an important part of

an airport’s landside capacity, and in some cases

has become a limiting factor on an airport’s ability to handle passengers. Recent discussion

about putting a quota on operations at Los Angeles International Airport, for example, is related to growing ground access problems, not

lack of airside capacity.

This chapter discusses alternatives to increase

airport airside capacity. Landside problems will

only be treated here as they affect airside capacity.

When the traffic demand for an airport approaches or exceeds its capability, the result is

delay. Delay has been a major problem at the

Nation’s busiest airports, resulting in millions of

dollars of increased operating costs for air carriers and wasted time for travelers. Although several different methods of measuring delay exist

(as will be discussed later) it is generally agreed

that the six airports most affected by delay in

1980 were: O’Hare (Chicago), Stapleton (Denver), La Guardia and JFK (New York), Hartsfield (Atlanta), and Logan (Boston). As shown

in table 8, most of the airports which report

Table 8.— “Top” U.S. Airports, by Enplaned Passengers, by Air Carrier Operations,

and by Reported Delays

Passenger Air carrier Delays over

enplanements operations 30 minutes

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

Chicago O’Hare

Atlanta Hartsfield

Los Angeles International

New York J.F. Kennedy

San Francisco International

Dallas-Ft. Worth

Denver Stapleton

New York La Guardia

Miami International

Boston Logan

Honolulu International

Washington National

Detroit Metro

Houston Intercontinental

St. Louis Lambert

Chicago O’Hare

Atlanta Hartsfield

Los Angeles International

Dallas-Ft. Worth

Denver Stapleton

Miami International

San Francisco International

New York La Guardia

New York J.F. Kennedy

Boston Logan

Washington National

St. Louis Lambert

Detroit Metro

Houston Intercontinental

Honolulu

Chicago O’Hare

Denver Stapleton

New York La Guardia

New York Kennedy

Atlanta Hartsfield

Boston Logan

Los Angeles International

St. Louis Lambert

San Francisco International

Dallas-Ft. Worth

Philadelphia International

Newark

Washington National

Miami International

SOURCE: Federal Av/at/on Adm/n/strat/on, Term/na/ Area Forecasts, Fisca/ Years 1981-92, Washington, D.G. 1981 p 13; interview, FAA, Air Traff/c and A/rways Fac//dies, Aug. 20, 1981.

101
102 ● Airport and Air Traffic Control System

serious delay problems rank among the top 15

airports in terms of both enplaned passengers

and air carrier operations.

This chapter first describes the airside components in the operation of a typical airport. It

then reviews those major factors which influence

or limit airside capacity. Next the chapter discusses the problem of delay—how it comes

about and the methods for measuring it and estimating its costs. The next sections outline some

alternative methods for reducing delay or increasing the airside capacity. These include

changing the pattern of traffic demand, expanding the runway system, or modifying the terminal area air traffic control procedures and equipment. Finally, some suggestions for future research are made.

AIRSIDE COMPONENTS

The airside capacity of an airport is governed

by factors related to its runway system and the

airspace above and around the airport, as well

as the terminal area ATC and navigation equipment and procedures.

The number of runways, their layout, length,

and strength will in large measure determine the

kinds of aircraft that can use the airport and

how many aircraft can be accommodated in any

given time period. The layout depends on a

number of factors including the local terrain and

predominant direction of the wind. Federal Aviation Administration (FAA) safety regulations

dictate how close the runways may be to one another and to buildings, trees, or other obstructions.

In order to land on a runway, aircraft approach the runway in single file, with a safe distance between them. Air traffic may enter the

airspace around the airport (“terminal area”)

from many directions at a number of different

points (“entry fixes”), and in many metropolitan

areas the aircraft may be destined for one of several different airports. Thus, the task of delivering aircraft one by one to a particular runway at

a particular airport must begin many miles from

the airport itself, and controllers must orchestrate the orderly merging and diverging of many

different traffic streams until each aircraft

reaches the final approach to its destination runway. By the same token, departing aircraft must

be safely routed from the airport to the “departure fix” where they leave the terminal area and

join the en route ATC system.

Controllers use both

separation to maintain

vertical and horizontal

safe distances between

aircraft, a task that is complicated by their different performance characteristics. Jets flying at

a very slow (for a jet) 160 knots will nevertheless

overtake and pass slower aircraft. The controller

may assign different altitudes so that this can

take place safely, or he may vector the faster aircraft along a longer path so that it will safely

overtake and pass around the one ahead.

In good visibility conditions, tower controllers may clear aircraft, once they are in sight of

the airport, to make a visual landing under

tower control. The pilot assumes responsibility

for separating himself from other aircraft, with

the controller standing by to warn pilots to “go

around” in case of a potential conflict. During

times of poor visibility the ATC team retains responsibility for separating the aircraft on final

approach. In this case the Instrument Flight Rule

(IFR) radar minimum separation is observed, so

Photo credit Neal Callahan

The variety of airspace system users .
Ch. 6—Airport Capacity Alternatives ● 103

that distances between aircraft are greater than

in good weather. Under IFR conditions, pilots

are much more dependent on landing aids such

as the Instrument Landing System (ILS) to guide

them to the runway.

An aircraft is considered to be on the runway

from the moment it flies over the runway threshold until it turns off onto a taxiway. Angled

“high-speed” turnoffs can allow aircraft to leave

the runway at higher speeds than perpendicular

convenient to most of the aircraft using a runway is important for getting maximum capacity

from the runway system.

Departures from the airport may take place

on a separate runway or may be “interleaved”

between arrivals on the same runway. Aircraft

preparing to depart can wait beside the runway

on holding aprons until the runway is clear; then

they can then taxi onto the runway and take off

fairly quickly—the time spent on the runway for

ones. Placing the turnoffs where they will be departure is on the order

LIMITATIONS ON AIRSIDE CAPACITY

Among the major factors influencing airport

capacity are: aircraft performance characteristics, wake vortex turbulence, weather, airfield

and airspace configuration, aircraft noise, ATC

equipment and procedures, and demand considerations.

Aircraft Performance Characteristics

Characteristics of the aircraft—their size, aerodynamics, propulsion and braking performance, and avionics—will affect the capacity of

the runways they use. Pilot training, experience,

and skill will also influence performance, and

the capacity of a runway can vary greatly with

the types of aircraft using it. Runway capacity is

usually highest if the “traffic mix” is uniformly

small, slow, propeller-driven aircraft. The next

highest capacity would come with a uniform mix

of large jets. Where the traffic mix is highly diverse—with jet and propeller aircraft of widely

varying sizes and speeds—it is usually difficult

to maintain optimum spacing and optimum runway usage, and runway capacity is reduced. The

direction of traffic also affects runway system

capacity. When arrivals predominate, capacity

is lower then when departures predominate.

Wake Vortex

Related to aircraft performance characteristics

is the problem of wake vortexes. Aircraft passing through the air generate coherent energetic

air movements in their wakes, and under quiescent weather conditions

of 30 seconds.

the wake vortex can

persist for 2 minutes or even longer after an aircraft has passed. The strength of the vortex increases with the weight of the aircraft generating

it. As the use of wide-bodied jets (e.g., B-747

and DC-10) became more common in the early

1970’s, it became apparent that wake vortexes

behind these heavy aircraft were strong enough

to endanger the following aircraft, especially if it

was smaller. Until the potential danger of wake

vortex to transport sized aircraft was demonstrated (e.g., the 1972 crash of a DC-9 landing in

the wake of a DC-10) standard separations of 3

nautical miles (nmi) were required under IFR

conditions. In order to prevent accidents caused

by wake vortexes, FAA increased the separations for smaller aircraft behind larger ones during weather conditions when persistent vortexes

may be a danger. These minimums are shown on

the right side of table 9.

Weather

Heavy fog, snow, strong winds, or icy runway surfaces reduce an airport’s ability to

accommodate aircraft and may even close an

airport completely. For a given set of weather

conditions, several of the different runway configurations available at an airport may be suitable but only one will have the maximum value.

Using these maximum values, and plotting them

with the percentage of the year during which different weather conditions are likely to prevail, a
104 • Airport and Air Traffic Control System

Table 9.—Arrival and Departure Separations

Minimum Arrival

Visual Flight Rules*

Lead

s L H

s 1.9 1.9 1.9

L 2.7 1.9 1.9

H 4.5 3.6 2.7

Separations— Nautical Miles

Instrument Flight Rules

Lead

s L H

s 3 3 3

L 4 3 3

H 6 5 4

Minimum Departure

Visual Flight Rules*

Lead s L H

s 35 45 50

L 50 60 60

H 120 120 90

Separations— Seconds

Instrument Flight Rules

Lead

s L H

s 60 60 60

L 60 60 60

H 120 120 90

“VFR separations are not operational minima but rather reflect what field data show under saturated condition. Adapted from

Parameters of future ATC Systems Re/atirrgr to A/rport Capacify/De/ay (Washington, D. C.: Federal Aviation Administration,

June 1978), PP. 3.3, 3.5.

“capacity coverage curve” for any given airport

can be constructed.

An example of a capacity coverage curve is

shown in figure 26. The highest hourly capacity

of Boston Logan Airport is 126 operations per

hour in Visual Flight Rule (VFR) weather. This

combination of highest capacity runway use and

good weather is available 40 percent of the year.

Strong winds create crosswind components

which close some of the runways of that configuration, and hourly capacities continue to

decrease as marginal weather and finally bad

weather cause restrictions in safely operating the

runway system. There is a small percentage (2

percent) of the year when poor visibility, ceilings, and snow completely close the airport.

Notice that there is a wide variation in the hourly capacity from 126 operations per hour down

to 55 operations per hour before the airport

closes. This is typical of many major airports

where several runway combinations exist. This

wide variation in hourly capacity prevents the

establishment of a single capacity value for the

airport; instead, it will be variable depending on

weather conditions.

It is difficult to foresee any capital investment

in runways or technological improvements to

ATC facilities which can completely eliminate

Figure 26.—Airport Hourly Capacity Varies Strongly

With Weather

(There is a 3 to 1 or 2 to 1 ratio between good weather/bad

weather capacities)

Capacity Coverage Curve—Boston Logan Airport

20

t

0 10 20 30 40 50 60 70 80 90 100

Average percent of time

SOURCE: Robert W. Simpson, “Airside Capacity and Delay at Major U.S. Airports,” draft report prepared for the Office of Technology Assessment, U.S. Congress, Washington, D. C., October 1980.

this degradation of capacity with weather conditions. New runways can raise the overall level of

the capacity coverage curve, but they do not
Ch. 6—Airport Capacity Alternatives . 105

Photo credit: Federal Aviation Administration

Snow control at a terminal

prevent its degradation with weather. Some of

the ATC improvements discussed later in this

chapter attempt to improve overall capacity by

reducing the gap between IFR and VFR performances.

Airfield/Airspace Configuration

The capacity of an airport depends to a large

extent on the number of runways available and

their interactions. For example, for a given traffic mix a particular runway can handle 65 operations an hour in VFR conditions and 55 in IFR

weather. The VFR capacity of two parallel runways, 2,500 ft apart, might then be 125 operations per hour—twice the capacity of a single

runway. Yet the IFR capacity of this two-runway system would be more like 65 operations

per hour, because under IFR conditions runways

less than 4,300 ft apart are considered “dependent” for purposes of landings—that is, an operation on one prevents a simultaneous operation

on the other. Similar safety restrictions apply

where runways converge or intersect with one

another. Thus, not only is the capacity of each

runway reduced during bad weather, but the capacity of the airport is further reduced because

not all runways may be fully used.

In the illustration in figure 27, the three runways could be used in several different ways,

four of which are shown. Each of these combinations may have a different operating capacity,

and each might be suitable for a different set of

wind, visibility, and traffic conditions. A large

airport like O’Hare might have 40 or 50 possible

combinations of runway uses. The limitation

imposed by the available runway system varies

among the top air carrier airports. Chicago

O’Hare has seven runways, Kennedy has five,

and La Guardia has only two (La Guardia’s

additional short 2,000-ft runway can be used

only for departures during good weather conditions). Yet the capacity relationship is not linear:

La Guardia manages to handle 40 percent of

O’Hare’s total aircraft movements with less than

30 percent of its runways. An adequate taxiway/gate configuration is also needed in order

to support optimum runway usage. For instance, the La Guardia Airport capacity task

Figure 27.—Runway Configuration

SOURCE: Federal Awatlon Admlnlstration, Techniques for Determinmg Awport Alrside Capacity and Lle/ay, FAA-RD-74-124, June 1976,
106 ● Airport and Air Traffic Control System

force found that additional taxiways in one area

were critical to minimizing delays. This is because space at gates was limited, and the additional taxiways could be used to hold and sequence departing aircraft during periods of congestion.

Aircraft Noise

Aircraft noise, especially the noise of jet aircraft, has made airports unpopular with their

neighbors. The greatest noise impact is usually

in the areas just beyond the ends of the runways,

where arriving and departing aircraft fly at low

altitudes. If a high-noise area is occupied by a

factory or a highway cloverleaf there maybe little difficulty, but such land uses as residences,

hospitals, and schools are not compatible with

the amount of noise generated by an airport. In

some areas, ineffective or nonexistent zoning

and land use controls over the years have allowed these incompatible land uses to occupy

high noise impact areas near many airports. The

courts have generally found that the airport operator is responsible for injury due to reduced

property value, and owners of nearby property have been able to collect damages in some

cases. In Los Angeles, the courts have recently

awarded nuisance damages as well. In some

areas, including Atlanta, St. Louis, and Los Angeles, airport operators have been required to

purchase noise-impacted property and either use

it as a buffer zone or resell it for a more compatible use.

One method for reducing noise is to introduce

quieter aircraft or, as many air carriers have

begun doing, to re-equip old aircraft with

quieter engines. FAA has set standards for new

aircraft that are much quieter than in the past,

but noisy aircraft will remain in the fleet for

many years. The increasing sensitivity of the

public to noise may have offset much of the recent improvement.

FAA, at the request of individual airport operators, has also developed operational procedures that reduce noise impact. For example, use

of certain runways may be preferred, or pilots

may be required to make approaches over less

sensitive areas, weather permitting. However,

Photo credit: Federal Aviation Administration

Air use and land use

FAA has established very few mandatory noiseabatement procedures. Over the past few years

some operators have conducted airport noise

compatibility and land use studies for use as a

basis for their own noise planning. The new

Federal Aviation Regulation, Part 150, required

under the Aviation Safety and Noise Abatement

Act of 1979 (Public Law 96-193), provides

operators with guidelines for voluntary noiseabatement standards and establishes a standardized method for measuring noise exposure.

Many of these noise-control procedures have

a negative effect on capacity, and airports with

both capacity and noise problems have found

that the available solutions to one problem often

aggravate the other. The highest capacity runway configuration, for instance, may be one

which requires an unacceptable number of

flights over a residential area. Enforcing noiseabatement procedures may also cause an unacceptable level of delay at peak hours. Thus, airports must balance tradeoffs between usable capacity and environmental concerns.

The FAA Administrator recently reemphasized that the responsibility for establishing

proper land-use controls around airports rests

with local government. He also predicted that
Ch. 6—Airport Capacity Alternatives ● 107

more communities will be establishing local

noise limits by ordinance or statute. f

A local government, whether or not it is the

owner of the airport, can exercise some control

over noise, but must do so in a manner that is

nondiscriminatory and does not place an undue

burden on interstate commerce. For example, a

city may select a reasonable noise exposure limit

and exclude or fine aircraft exceeding that limit.

However, the total ban on jet aircraft in Santa

Monica, Calif., was overturned by the courts as

unduly discriminatory against one class of aircraft (some new jets are quieter than propellerdriven aircraft).

ATC Equipment and Procedures

Improvements in aircraft surveillance, navigation, and communication equipment over the

past decade have greatly increased the ability of

pilots and controllers to maintain high capacity

during all weather conditions (see ch. 5). However, there are still ATC-related limits on airport

capacity. Clearances used in the en route airways and the terminal airspace are frequently

circuitous, routing aircraft through intermediate

“fixes” or control points rather than allowing

them to travel directly from origin to destination. While this places aircraft in an orderly pattern so that controllers can better handle them, it

also reduces capacity and consumes time and

fuel.

“’Helms Places Airport Noise Problems on Operators, Communities, ” Alliatiou Daily, Sept. 29, 1981, p. 154.

The limitations in the accuracy of surveillance

equipment also can influence how airports are

constructed and how they may be used. For example, the spacing requirement between independent IFR runways was developed based on

the limitations of surveillance, navigation, and

communications equipment. Improvements in

equipment and procedures have allowed this

minimum to be reduced over the years.

Constraints on capacity can arise when

airspace near one airport must be reserved to

protect operations at another airport. This is an

especially pressing problem in some busy areas.

There is such an airspace conflict between La

Guardia and Kennedy in certain weather conditions, for example.

Demand Considerations

The daily pattern of demand is characteristic

of the airport and the travel markets it serves.

Air travelers prefer to travel at certain times of

the day—midmorning and late afternoon, for

example—and air carriers wish to accommodate

them. Heavy scheduling at peak hours makes it

easier for passengers to transfer to other planes

or other airlines, yet (as will be discussed shortly) peaks in demand can be major causes of delay. Even at airports with a high percentage of

scheduled traffic it is not possible to predict the

actual number of aircraft which will appear at a

particular hour of a given day, as nonscheduled

traffic volume can vary substantially. At quota

airports, the quota is set at a value between the

VFR and IFR capacity, resulting in a built-in

delay situation whenever weather conditions deteriorate.

DELAY AND DELAY REDUCTION

Airport delays received a great deal of public- peaks, there may be delays even when the numity during the late 1960’s and they continue to be ber of aircraft using the airport is less than the

a major waste of time, money, and fuel. Delay capacity for that peak time period. Some

can be expected whenever instantaneous traffic amount of delay arises every time two aircraft

demand approaches or exceeds the airport’s are scheduled to use a runway at the same time.

capacity. When traffic occurs in bunches or The probability of simultaneous arrivals in-
108 . Airport and Air Traffic Control System

creases rapidly with traffic density, so that average delay per aircraft increases exponentially

well before traffic levels reach capacity levels.

A typical variation of delay with operation

rates is shown in figure 28. When the traffic level

is above capacity, the accumulation of aircraft

awaiting service is directly proportional to the

excess of traffic over capacity. For example, if

the capacity of a runway system is 60 operations

per hour and traffic rates are averaging 70 operations per hour, then every hour will add an average of 10 aircraft to the queues for service, and

10 minutes to the delay for any subsequent arrival or departure. Even if the traffic level drops

to 40 operations per hour, delays will persist for

some period since the queues will be depleted at

a rate of only 20 aircraft per hour.

The principal delay-reporting systems of FAA

currently measure only the occurrence of large

delays. The National Airspace Communications

System (NASCOM) delay reports record instances of delays of 30 minutes or more at 46

participating airports. The Performance Measuring System (PMS) records delays of 15 minutes

or more at 15 major airports. The PMS also attempts to estimate “average delay per aircraft

delayed.” Both NASCOM and PMS rely on controller’s manual recording of instances and

causes of delays during periods when he is already busy. Weather is listed as the primary

cause for these delays, ranging from 76 percent

Figure 28.—Typical Distributions of Delay

50 percent 100 percent

Traffic rate capacity

SOURCE: Office of Technology Assessment.

of the 30-minute delays in 1976 to 84 percent in

1979 in the NASCOM system. The total number

of delays reported also increased, from approximately 36,200 in 1976 to approximately 61,600

in 1979. It must be emphasized that while

weather may indeed be the primary cause, the

ability of the system to anticipate, adjust to, and

recover from weather-related problems is dependent on a number of the other determinants

of airside capacity.

Another major delay-reporting system is

sponsored by FAA and three airlines—Eastern,

United, and American—which have been pooling their operational flight-time data since 1976.

This Standard Air Carrier Delay Reporting

System (SACDRS) covers 36 airports and measures taxi times, gate holds, and flight times

against standard values in an attempt to determine delay. Unfortunately, an error in this

method causes an overestimation of delay: for

example, the standard times used for taxi in and

out are based on the average over all runways at

a given airport, but at some airports there is

wide variation in taxi times for different runways and terminals; some percentage of these

longer taxi times are always counted as delay

under the SACDRS. FAA recognizes the deficiency in this system, but no correction has yet

been devised. Estimates of the annual cost of

delay based on SACDRS have ranged as high as

237 million gallons of fuel and $273 million of

additional operating costs to the three airlines

involved, although these costs too are overestimated. 2 The PMS and NASCOM systems, on

the other hand, because they only count long delays, probably underestimate delay. The true

value of delay lies somewhere in between and

has not been determined with accuracy. Thus,

estimates of the cost of delay based on any of

these reporting systems have to be viewed with

some caution. However, all observers agree that

delay is a serious and expensive problem at some

airports, especially in light of the high cost of

fuel in recent years.

One method of dealing with delay is to constrain traffic to manageable levels. This is the

‘Virginia C. Lopez (cd.), Airport and Airway Congestion, A Sen“ous Threat to Safety and the Growth of Air Transportation

(Washington, D. C.: Aerospace Research Center, July 1980).
Ch. 6—Airport Capacity Alternatives ● 109

origin of the quota systems which have been imposed at a few major airports. Each carrier has

representatives on the “scheduling committees”

to negotiate the carrier’s share of allowed peak

hour operations. FAA, through its flow control

center, also works to ameliorate the costs of

delays by forewarning air carriers when delay

conditions develop at major airports. For example, when weather deteriorates and capacity

goes down in Chicago, FAA may advise aircraft

scheduled into Chicago to delay their arrival

there by waiting on the ground at other cities.

Waiting on the ground is much less wasteful of

fuel than waiting in holding patterns in the air.

Although the lengthy delays of the late 1960’s

are no longer typical, delay remains a major

problem at many airports. Further, the number

of operations will increase as air traffic grows,

and additional airports may experience this

problem. Some possible approaches to dealing

with delay are discussed below.

DEMAND= RELATED ALTERNATIVES

Delay problems tend to be concentrated at the

Nation’s major airports, and even at these locations the problem is most acute during certain

hours of the day (usually midmorning and late

afternoon). If operations could be shifted from

these peak hours to less busy times, delay could

be reduced and the overall capacity of the airport better utilized. Variable user fees or quotas

during peak hours are tools which have been

suggested, and tried at some locations, to reduce

peak demand and increase operations in nonpeak hours. All these mechanisms,

duce the ease of transferring from

another at hub airports, making

achieve ideal airline economics.

however, reone flight to

it harder to

Peak-Hour Pricing

Most airports now charge a landing fee based

on the weight of the aircraft. This fee schedule is

designed to recover construction and operating

costs of the airport, not to ration capacity. However, when the use of an airport is nearing capacity it could be more economically efficient to

base landing fees on the marginal costs imposed

by each additional aircraft served. This means

that the user should pay not only for use of the

airport, but for the delay caused other users who

want to use it at the same time. This method

allows users who value access to the airport at

peak times to pay for their preference; those

who do not wish to pay the higher fee would use

the airport at other times, or perhaps use

another airport.

In general, peak-hour pricing would have little effect on air carrier operations unless the

price changes are very large. Airlines schedule

flights when they think passengers will want to

fly, and they would probably be willing to absorb moderate increases in user fees in order to

use the airport at those times. Even a landing fee

of several hundred dollars would be small compared to the total operating costs of a large

jetliner, and such an expense could be passed on

to the passenger by a relatively small increase in

fares. Commuter air carriers, with their smaller

number of passengers, would be unable to pay

landing fees quite as high as the larger carriers.

General aviation (GA) users on the other

hand, especially student and personal flyers, are

more sensitive to increases in landing fees. The

Port Authority of New York and New Jersey’s

1968 decision to increase minimum landing fees

from $5 to $25 during peak hours brought about

an immediate decline of about 30 percent in GA

operations during peak hours at its three air carrier airports (JFK, La Guardia, and Newark) and

a noticeable decline in aircraft delays of 30 minutes or more.3 In 1979, a $50 surcharge added to

peak-hour landing fees at Kennedy and La Guardia resulted in a further decrease in GA traffic at

those airports.4 The remaining GA users were

3Airport Quotas and Peak Hour Pricing: Theory and Practice

(Washington, D. C.: Federal Aviation Administration, 1976), pp.

54-60.

“Port Authority of New York and New Jersey, Aviation Department, interview, Oct. 23, 1981.
110 . Airport and Air Traffic Control System

primarily high-performance turboprop aircraft

used for corporate travel; corporations, like the

airlines, may be willing to absorb a fairly large

increase in fees in order to use specific airports

during peak hours.

One problem with a peak-hour pricing system

is that it is difficult in practice to determine

precisely what the marginal cost of an airport

operation is; several years of trial and error

would be necessary to settle on a pricing scheme

which both controlled delay and allowed the airport to cover its costs. However, if the same fee

were charged to air carrier, commuter, and GA

aircraft, peak-hour pricing might be strongly

resisted. Proportionately different fees for different categories of users might therefore be

necessary.

Quotas

An alternative method for managing demand

is to set a quota on the number of operations

which can take place during a peak hour. The

quota can be placed on total operations, or a

certain number of operations can be allocated to

different classes of users. The quota levels are

usually set between the IFR and VFR capacity of

the airport; thus, in VFR conditions, additional

aircraft could easily be accommodated. When

capacity is reduced, users without reservations

have to use the airport at another time or use

another airport.

Although reservations (slots) for GA or even

air taxis might be allocated on a first-come, firstserved basis, slots for scheduled carriers present

a more complex problem. At major airports

where quotas have been in effect for some time

(O’Hare, JFK, La Guardia, and Washington National) representatives of the air carriers are allowed (with antitrust immunity) to meet as

scheduling committees to negotiate how many

slots will be allocated to each carrier. Although

new entrants are able to participate in these negotiations, quota systems do tend to favor the

status quo. Since the air traffic controllers strike

in August, 22 airports have been brought under

a quota system designed principally to ease

peaks of demand on the en route ATC system.

The methods for assigning slots to new entrants

or allowing existing carriers to exchange slots are

still under development.

One objection to quota systems is that the allocations are made without any price signals to

show that the capacity is being used efficiently.

Thus, although the quota may provide some

stop-gap congestion relief, it does not provide

any long-run guide for allocating resources as

the system grows or changes. It has been suggested that this problem could be overcome by

auctioning the reservation slots among the carriers or by combining the quota system with some

sort of peak-hour pricing scheme.

Balanced Use of Metropolitan

Area Airports

Many major metropolitan areas are served by

two or more large airports. Where one or more

of these airports is underutilized, possibilities exist for increasing airside capacity through a more

balanced use of the region’s airports. Examples

include: Newark Airport, which is underutilized

compared to Kennedy and La Guardia; Oakland

Airport, which could relieve San Francisco;

Midway Airport, which is practically empty

while Chicago-O’Hare has delay problems; and

Baltimore-Washington and Dunes Airports,

which might relieve Washington National. The

problem of balancing use of metropolitan airports presents a chicken/egg dilemma: airlines

won’t serve the underutilized airport because

there are so few passengers, and passengers

don’t go there because there is so little service. It

is difficult to foresee when congestion in itself

will become great enough to cause redistribution, or to what extent the process can or should

be managed by local or even Federal authorities.

In some cases, better transportation between airports might make it easier to transfer between

flights and to attract passengers to underutilized

airports.

The Washington, D. C., area is illustrative of

the problems of imbalance airport use. Washington National Airport, operating since the

mid-1940’s, is convenient to the downtown area.

National has three runways (all under 7,000 ft)

and does not accept wide-body jets. Both its air-
Ch. 6—Airport Capacity Alternatives . 111

side and landside capacity are severely limited

and a quota system and airline scheduling committee are used to ration peak-hour operations.

Expansion is difficult due to surrounding development and the Potomac River. Complaints

about the airport’s noise have led to a 10 p.m.

curfew among other noise abatement policies.

From time to time some groups even call for the

airport to be closed.

Many of these problems could be alleviated if

some operations were transferred to Dunes International Airport, 26 miles from Washington.

Dunes, opened in 1962, has two 11,500-ft runways, one 10,000-ft runway and capacity to

spare. FAA (which operates both airports) has

repeatedly attempted to induce carriers to use

Dunes more; for example, only Dunes can

receive international and long-range domestic

flights. Despite the constraints of the quota

system, the curfew, and the restrictions on widebody and long-range flights, however, National

handled nearly 4 times the operations and 41⁄2

times the passengers that Dunes did in 1980. Further, National generated a net profit of $10 million that year, while Dunes incurred a net loss of

$3 million. ’ The principal problem is ground access; it is more convenient to fly from National

than from Dunes.

Some new airlines beginning service since deregulation have sometimes deliberately chosen

to operate out of underutilized airports to avoid

congestion and delay. One example is Midway

Airlines, which uses the nearly abandoned Midway Airport for its Chicago service. Midway’s

problem is also related to ground access: congested highways make trips to the airport long

even though Midway is closer to downtown

Chicago than O’Hare. Another example is People Express, which serves the New York area

from Newark. The Port Authority of New York

and New Jersey has been offering incentives to

passengers as well as airlines to increase the use

of Newark Airport: improved ground access by

train and express bus allows New York City passengers to get to Newark without paying high interstate taxi fares, and new airlines are offered

51nterviews, FAA, Metropolitan Washington Airports, July 6,

1981.

more and better space for future growth at

Newark. In addition to People Express, New

York Air has located part of its operation at

Newark. Now that permission has been gained

to use Newark as a international airport, several

established airlines are also bidding to offer

transatlantic service from there.

Restructuring Airline Service Patterns

When delay becomes intolerable at busy hub

airports, users themselves may voluntarily move

their operations to another facility. This movement might be to an underutilized airport nearby (e.g., Newark), but it could also be to a medium or small hub located at some distance from

the congested hub. This is especially likely for

transfer traffic. (See ch. 4 for a discussion of the

growth and capacity impacts of this redistribution scenario. )

Many major airports currently serve as hubs

for a large amount of transfer traffic. Threefourths of the arriving passengers at Atlanta and

about one-half the passengers at O’Hare, DallasFort Worth and Denver pass through these airports only to change planes for somewhere else.

Carriers choose to establish their hubs at these

busy airports so that passengers can choose from

many transfer flights. However, when the transfer airport becomes too congested the disadvantages of delay may begin to outweigh the advantages of convenience, for airlines as well as

passengers. Hence carriers may decide to locate

their new transfer operations, and even move

their existing hubbing activities, to other cities

that have more room for growth.

Redistribution of operations appears to be occurring under the new routing freedom available

under the Airline Deregulation Act of 1978. Carriers are finding it easier to change their routes

and establish new “second-tier” hubs at less congested airports. Between 1978 and 1980 the number of large hubs (handling more than 1 percent

of total U.S. passenger traffic) fell from 26 to 24,

while the number of medium hubs (handling

0,25 to 0.99 percent) increased from 33 to 36—a

market shift reflecting the distribution of operations over more airports. This trend may accelerate as regional carriers modify their patterns of
112 . Airport and Air Traffic Control System

service, and even the busiest airports such as

Atlanta and O’Hare, may see actual declines in

both enplaned passengers and operations in the

next 10 years. A similar decline in operations occurred at Kennedy Airport when international

flights were allowed to enter the United States at

other gateway cities.

Reliever Airports

In metropolitan areas where there is congestion at the main airport and excess capacity at

surrounding airports, diversion of GA traffic

would be effective in improving the use of airside capacity in the whole region. It would allow

a higher level of service for both air carrier and

general aviation, and in most metropolitan areas

there are smaller airports which might potentially attract some GA traffic away from the main

airport. For example, FAA lists 27 airports in the

Chicago area, 51 around Los Angeles, and 52 in

the Dallas-Fort Worth metropolitan area. However, most of these airports are quite small, and

only a few have runways long enough to accommodate business jets or instrument landing

equipment for bad-weather operations.

The FAA’s National Airport Systems Plan

(NASP) designates 155 airports as “satellites” or

“relievers” to major airports, and NASP provides for separate Airport Development Aid

Program (ADAP) funding to be set aside for relievers. Publicly owned reliever airports may use

ADAP funds for construction, installation of

safety equipment, and other eligible expenditures. The 25 or so privately owned reliever airports, although they presumably provide the

same benefit in terms of diverting traffic from

congested air carrier airports, are not eligible for

aid. Local and State governments may, however, use ADAP funds to help purchase privately owned reliever airports, and at least five

reliever airports have changed from private to

public ownership since 1973. One privately

owned reliever, Chicagoland (a reliever for

O’Hare) closed in 1978. Although the FAA reliever program was initiated largely to segregate

training activities from major commercial airports in the interests of safety, it also provides

additional airport capacity for a certain type of

traffic—namely, personal GA aircraft with origins or destinations in the local region; business

and commercial GA (i.e., corporate aircraft and

air taxis) delivering or picking up airline passengers will probably continue to use the major

commercial airport.

The process of diverting the personal GA traffic has already occurred at the Nation’s largest

major commercial airports. The fraction of GA

activity at Atlanta, O’Hare, Kennedy, Los Angeles International, etc., is very small (about 10

percent) because these regions have good alternate secondary airports with high levels of traffic. In fact, some of the large relievers such as

Van Nuys and Long Beach, Calif., Opa Locka,

Fla., and Teterboro, N. Y., are among the busiest

airports in the country in terms of annual operations. This trend toward establishing a system of

reliever airports is underway and has been endorsed by many user groups and observers,

most recently the President’s Task Force on Aircraft Crew Complement. e

To be of maximum benefit the reliever airport

should be located so that approach and airspace

conflicts between the reliever and the commercial airport do not place capacity limits on both.

In the New York area, for example, instrument

operations at Linden and Teterboro reliever airports must alternate with operations at the Newark Airport. In addition, the noise consequences

of increasing operations at the reliever airport

must be considered. Most reliever airports have,

or will soon have, IFR landing aids and runway

systems capable of handling sophisticated GA

aircraft. To be most attractive to users, airports

should also have commercial services for aircraft

servicing, repair and maintenance, ground

transportation, and flight crew amenities. With

sufficient amenities, such an airport might even

attract some commuter airline service, although

transfers and interlining would be difficult unless the airport is served by several carriers or

has excellent ground access to a major hub. In

some cases, however, the provision of better facilities may not be sufficient to divert additional

GA traffic away from major hub airports. Increased landing fees at the major airport can

‘Report of the President’s Task Force on Aircraft Creu) Complement (Washington, D. C.: July 2, 1981).
Ch. 6—Airport Capacity Alternatives ● 113

provide additional incentives for this shift, looked upon as a complement to the Federal proand such pricing policies—the domain of local gram of investment in satellite airports.

government and airport authorities—could be

AIRPORT DEVELOPMENT ALTERNATIVES

Expanding Existing Airports

Because runway availability is the major constraint on airside capacity, one way to increase

capacity is to add more runways. A new long

runway, properly equipped for independent IFR

operations can increase an airport’s capacity by

20 to 50 percent depending on the original runway configuration.

Adding another runway, however, requires a

large amount of land. One 11,000-ft runway for

large jet operations with its basic safety areas

covers 130 acres, and when other necessary

“clear zones” are considered, an area three to

four times that size would be directly affected.

Further, the additional operations enabled by

the new runway would probably require landside additions such as new gates, terminal space,

and parking for more passengers. Few airports

have the necessary land for this kind of expansion, which could add approximately 10 percent

to their present area, and for some airports like

Washington National and La Guardia, the prospect is especially bleak. Even for larger airports,

obtaining proper spacing from other runways

would be extremely difficult.

A 1977 report by the Department of Transportation (DOT) studied the possibility of major

expansion at 24 airports to meet projected needs

for 1985-2000. Expansion was found to be “feasible” in only four of these cases, and none of

these four airports (Detroit, Houston, Minneapolis, and Pittsburgh) are among those which

are experiencing the greatest capacity problems.

In 9 other cities the DOT study found expansion

“feasible within major constraints,” and in 11

cases it was considered “not feasible.” Both

economic and environmental reasons were cited

for preventing the land acquisition. ’ Airport de-

‘Establishment of New Major Public Airports in the United

States (Washington, D. C.: Federal Aviation Administration,

August 1977), p. 6-5.

signers foresaw the need for growth and most

major airports were built where land was plentiful, but sites that were on the edge of town in

1925 or 1948 are now in the middle of urban development. In some cases the airport itself attracted businesses; in other cases development

simply resulted from good highways, suburbanization, and all the other forces which have

caused urban areas to expand over the years.

Developed land tends to be expensive to buy: a

recent study of the cost to acquire and clear land

around some major air carrier airports estimated

these costs at between $100,000 and $200,000

per acre.8 Noise is among the largest environmental obstacles to airport expansion. ChicagoO’Hare has sufficient land for an additional runway, but the runway has not been built in part

because it would cause unacceptable noise exposure in nearby neighborhoods. JFK Airport in

New York is surrounded by intensive development on one side and a National Park and Wildlife Sanctuary on the other, making expansion

unlikely. Dallas-Fort Worth, on the other hand,

is planning an additional major new runway

that is expected to ease some of the capacity limitations imposed by noise abatement procedures

and airspace conflicts with nearby Love Field.

Development of Secondary

Runway Operations

At some airports where major expansion is

unlikely it may still be possible to add one short

runway for smaller, slow-moving commuter and

GA aircraft. This could improve airport capacity by diverting traffic from the longer runways

and may also provide a partial solution to the

wake vortex problem (previously discussed).

Many airports routinely use short runways, or

sections of long runways, for small aircraft durt

‘Louis H. Mayo, Jr., “Noise Compatible Land Uses in Airport

Environments, ” Environmental Comment, March 1979, p. 9.
114 ● Airport and Air Traffic Control System

ing good weather, but because of inadequate

landing aids or spacing these runways cannot be

used during bad weather; all-weather operations

would require additional navigational and approach guidance equipment.

One study found that the use of short IFR runways for small aircraft was feasible at 11 of 30

major airports. Of these 11, suitable runways already existed at 3 airports, existing runways

could be extended for use at 2 others, and at 6

airports space was available for short runways

to be constructed. The study estimated that the

value of reduced delays brought about by the

addition of such runways might be $450 million

to $810 million in current dollars between 1980

and 1990 at the airports shown in table 10. The

benefits would be unevenly distributed: Chicago, Atlanta, Philadelphia, and Denver would

receive 80 to 85 percent of the estimated savings;

among the users, 86 to 89 percent of the savings

from reduced delays would accrue to the air carriers. 9

A detailed study of the airfield and airspace at

each airport would be needed to see if the short

runway could really be constructed. Such studies done at Denver revealed two possible locations for a short GA runway. Construction of

either one could lead to a 35 to 70 percent increase in hourly operations, depending on

weather conditions. Total cost was estimated at

about $10.8 million. 0

Building New Airports

Another way to increase airport capacity is to

build a completely new airport to replace or supplement the existing one, an alternative that is

especially attractive where landside facilities

(terminals, baggage equipment, parking) are

also outmoded or inadequate. A new site would

provide the opportunity to design and build runways, terminals, and parking space to meet fu-

‘John D. Gardner, Feasibility of a Separate Short Runway for

Commuter and GeneraZ Aviation Traffic at Denz~er, prepared for

the Federal Aviation Administration by The Mitre Corporation,

McLean, Va., May 1980, pp. 1-1.

‘“John D. Gardner, Extensions to the Feasibility Study of a Separate Short Runway for Commuter and General Auiation Traffic

at Denuer, prepared for the Federal Aviation Administration by

The Mitre Corp., McLean, Va., September 1980, pp. 4-3 and 7-1.

ture needs, rather than making do with what has

evolved over time. Sufficient land could be purchased to allow for future growth and proper

land-use controls could be applied so that noise

compatibility problems do not arise again. In

some recent airport relocations, however, this

did not work as well as hoped. For example, at

both Dallas-Fort Worth Regional Airport and

Kansas City International Airport, built in the

mid-1970’s, encroachment by other land uses is

again leading to complaints about airport noise.

On the other hand, Montreal’s new Mirabel Airport seems to have little problem with noise

incompatibility; the airport itself covers 17,000

acres, and is surrounded by an additional 21,000

acres controlled by a specially created municipal

authority. However, its distance from the city

makes access a problem.

Building a new airport also provides an opportunity to add a large amount of new airside

capacity to a region. The opening of Kansas City

International, for example, more than doubled

the available capacity in that hub from the estimated 195,000 operations at the old municipal

airport to about 445,000 with the new airport.

Love Field in Dallas handled 410,000 operations

in 1972; in 1977, after air carrier operations were

transferred to Dallas-Fort Worth Regional Airport, Love Field still had 310,000 operations

(mostly GA), while the new airport had 385,000.

A 1977 investigation by DOT found that anywhere from 2 to 19 new airports might be needed

in the United States by the year 2000, depending

on the growth rate assumed. When the study

examined the feasibility of new airport construction for 10 hub areas, it found it to be “feasible”

in four instances, “doubtful” in four, and “not

feasible” in two. The reasons for the “doubtful”

and “not feasible” findings are related primarily

to site location, land acquisition, funding problems, and the difficulty of providing adequate

ground access to a remote location. The FAA’s

1980 NASP foresees the possibility of a new airport opening at Palmdale, Calif. (near Los Angeles), within the next 10 years; some initial work

on new airports at Atlanta and San Diego might

also be expected within the next decade. ’ 2

‘ ‘Establishment of New Major Public Airports, op. cit., p. 7-16.

“National Airport System Plan, Revised Statistics 1980-1989

(Washington, D. C., Federal Aviation Administration, 1980) p. vi.
Ch. 6—Airport Capacity Alternatives • 115

Table 10.—Operational Characteristics of Airports With Potential Benefits From a

Separate General Aviation Runway

Parallel Parallel Nonparallel

independent dependent dependent

Modification operations operations operations

New runway . . . . . . . . . . . Chicagoc

, Atlantac, Philadelphia,

Dallas-Ft. Worthc

, Denver Pittsburgh ab

Existing runway Portland,

or taxiway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Detroit a

,

b St. Louis

Extension of New York (JFK)a

,

Existing runway. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Indianapolis

aThe ~enera] aviation runway is Independent of 1 of 2 air Carrier runways for departures.

bGeneral aviation runway handles departures OnlY

c_friple parallel runways.

SOURCE: J. D. Gardner, “Feasibility of a Separate Short Runway For Commuter and General Awatlon Traffic at Denver, ”

prepared for the Federal Aviation Administration by Mltre Corp., McLean, Va., May 1980.

Building a new airport is a huge undertaking.

A new air carrier airport can represent an investment of $5 billion, to be shared among the airport sponsor (and local taxpayers), airport concessionaires, the airlines (through their landing

fees), and the Federal Government. Even a modest-sized GA airport would cost several hundred

million dollars. The length of time required for

planning and construction of a large airport—

up to 10 years—can also add substantially to

costs. Political and institutional factors can also

pose substantial difficulties. Building an airport

requires agreement from existing air carriers to

move to the new facilities, but while a new airport can reduce delays it will also increase airline

costs, and they must be convinced that the benefits will outweigh the costs. Further, approval

and support of a number of State, county, and

municipal governments, not to mention highway districts, zoning commissions, and various

citizens’ interest groups, must also be secured.

In some cases the divergent interests of different governments and constituencies can snarl

the process. In St. Louis, for example, a site for a

new airport was selected across the Mississippi

River in Illinois. The Illinois State government

was a major supporter of the project, as were the

St. Louis city government and FAA. The opponents included citizens groups of the county

where the new airport would be located (who

objected on environmental grounds), the State

of Missouri (which did not want the airport

moved out of the State), and groups in St. Louis

(which did not want the city to give up the closein Lambert Airport). The project was debated

for several years, but it was shelved after a

change in the St. Louis city government.

Photo credit: Federal Aviation Adrninistration

The design of a modern airport: Dallas-Fort Worth
116 • Airport and Air Traffic Control System

ATC IMPROVEMENT ALTERNATIVES

As mentioned earlier, existing ATC procedures and equipment can represent constraints

on the airside capacity. Improvements in these

areas can increase the number of aircraft operations.

Airfield/Airspace Configuration

Management

The ATC team at an airport decides how the

runway and ATC equipment should be used

based on wind, visibility, traffic mix, ratio of arrivals to departures, noise-abatement procedures, and the status of the airport (which run

ways or landing aids are under repair, etc.). In a

large air carrier airport like O’Hare, there may

be 40 or 50 ways in which the runways can be

used, so deciding which one offers maximum capacity for any particular set of conditions is a

complex task. The problem is compounded by

the interdependence of runway use and the configuration of the surrounding airspace. For example, changing which runway is used for landings may change the route that approaching aircraft must take through the terminal airspace,

which may in turn affect or be affected by activity at other airports in the vicinity.

One FAA analysis of capacity and delay problems in Chicago suggested that proper management of airfield and airspace could have a large

payoff:

Optimized management of the air traffic control system . . . could achieve now, at minimum investment cost, savings comparable to

those that will be achieved much later at much

higher cost when third generation ATC hardware is deployed. This highlights the importance of FAA management exploration of opportunities for improved system efficiency by

placing emphasis on optimization of operations

at least equal to that given development of ATC

hardware. 13

After study of the runway system of O’Hare airport, the task force found that a computerized

“Delay Task Force Study, Volume 1: Executive Summary,

O’Hare International Airport (Chicago: Federal Aviation Administration Great Lakes Region: July, 1976), p. 4.

airspace/airfield management system could be

used to assist the controller team in selecting the

highest capacity and most energy-efficient runway use for each set of circumstances.

Such a system could have several levels of

complexity. In its basic form it would aid in selecting the preferred runway configuration for a

given set of conditions; this basic system is

under development by FAA. The intermediate

form would update this assessment as changes in

weather or traffic conditions arise, and then

select the most efficient means of making the

transition from the one configuration to another. (This is important because the transition

period is often a time when airspace and airport

capcity are wasted. ) The advanced version

would have the ability to make longer term strategic decisions. The 1978 Chicago task force suggested that savings of $11 million to $16 million

annually in reduced delay costs might be expected from the basic system alone.14

Wake Vortex Prediction

Alleviation of the wake vortex problem offers

the possibility of a substantial potential payoff

in increased capacity without large capital expenditures for new runways. Research over the

past decade has shown some possible ways of

doing this. For example, it has been found that

certain wind conditions can quickly dissipate a

vortex or remove it from the path of oncoming

traffic. If wind conditions can be accurately

monitored and quickly analyzed, then the likelihood of wake vortex danger can be known on a

minute-by-minute basis.

FAA has been testing such a system at O’Hare

Airport since 1977. Wind sensors are located on

50-ft towers near the runway ends. A computer

analyzes wind conditions and when persistent

vortexes are unlikely it gives the controller team

a “green light” to permit reduced separations on

final approach. To have maximum effect (e.g.,

to allow all separations to be reduced to 3 nmi),

an advisory system would have to be able to

————

“Ibid.
Ch. 6—Airport Capacity Alternatives . 117

predict the likelihood of wake vortexes at

greater distances and higher altitudes than the

Chicago system now does. However even this

prototype system has been credited with allowing reduced average separations, and thus more

operations per hour, at O’Hare. There are no

current FAA plans for implementing full scale

wake vortex advisory systems at other airports.

Microwave Landing System (MLS)

As discussed in chapter 5, MLS allows airspace to be used more efficiently than the current ILS, since aircraft would be able to approach the airport on curved paths, as they do

under visual conditions, and turn onto their

final approach much closer to the runway.”

Variable MLS glide slope angles could also provide a partial solution to the long separations

required to avoid wake vortex; with MLS, the

trailing aircraft could avoid the vortex by approaching the runway at a steeper angle than the

lead aircraft.

Models suggest that where the traffic mix contains a variety of fast and slow aircraft, the use

of variable glide slopes could allow some capacity improvements—perhaps around 10 to 15 percent. However, where aircraft have similar performance characteristics, MLS landing procedures would offer about the same capacity as

current ILS procedures. MLS would also allow

the restructuring of airspace at some airports, so

that small aircraft can approach the airport in a

separate arrival stream from jets and make use

of a separate short runway. The Dash-7 aircraft

in Ransome Airlines’ Washington-Philadelphia

service use MLS equipment to land on short runways.

MLS equipment has been developed, tested,

and accepted for international use. Field evaluation is taking place at such airports as Washington National, and FAA has published a plan for

full-scale implementation beginning in the

mid-1980’s and to continuing into the next cen-

“An Analysis of the Requirements For and the Benefits and

Costs of the National Microwave Landing System, Volume 1

(Washington, D. C.: Federal Aviation Administration, June 1980),

p. 2-3.

tury. One reason for this delayed schedule may

be the international agreement to maintain ILS

until 1995; and another reason is the reluctance

of users, principally the airlines, to install MLS

avionics in aircraft already equipped with ILS

avionics.

Reducing Separation or

Spacing Minimums

Several studies have suggested that where

wake vortex is not a problem (for example,

where aircraft have similar performance characteristics) it maybe possible to reduce separations

from 3 nmi to as little as 2.5 or 2 nmi. The

amount of time each aircraft spends on the runway is another constraint in reducing separations, and depends on such factors as the number and spacing of the exits, visibility, runway

surface conditions, and the performance characteristics of the aircraft. In general, small, light

aircraft spend less time on the runway than

large, heavy ones. According to surveys, most

airports have an average runway occupancy

time of between 41 and 63 seconds for landing,

although these figures do not include the rare

snowy or icy days when separations might have

to be extended to allow time for aircraft to brake

safely and exit the runway. Where the average

runway occupancy time is so seconds or less, it

has been suggested that the minimum separation

could safely be reduced to 2.5 nmi instead of 3

nmi. Greater reductions might be possible

through automated metering and spacing.

Another way of increasing airfield capacity is

to reduce the required spacing between runways. For example, runways must be 4,300 ft

apart for simultaneous IFR operations to take

place. Reduction of this minimum to 3,500 or

3,000 ft would enable some airports to make use

of more of their runways during IFR conditions.

Minimum spacing standards have been reduced

before (e.g., from 5,000 to 4,300 ft for independent parallel IFR runways in the early 1960’s) as a

result of improvements in surveillance equipment and procedures.

“William J. Swedish, Evaluation of the Potential for Reduced

Longitudinal Sparing on Final Approach, prepared for the Federal

Aviation Administration by The Mitre Corp., McLean, Va., p.

4-1.
118 ● Airport and Air Traffic Control System

FAA is also investigating the possibility of allowing instrument approaches to triple parallel

runways during poor visibility. Currently triple

parallels can be used only during good visibility.

One of these three runways might be a short

runway for commuter or GA aircraft. Efficient

use of triple parallels would require redesign of

the airspace and approach patterns, a higher

degree of coordination between approach controllers than is currently the case, and possible

modifications to the ILS. MLS, with its greater

flexibility and navigational precision, might be

useful in bringing this procedure into practical

use. Use of triple parallels could make it possible

to make use of more existing runways during

poor weather, as at O’Hare, or even to allow

construction of new runways which are infeasible under current procedures. Capacity improvements would depend on traffic mix and on

whether the runways had sufficient spacing to

allow independent operations. Models indicate

that triple parallel runway systems might handle

up to 50 percent more IFR operations than double parallels with traffic mixes typical of today’s

major airports.l 7

A number of airports have been identified

which might benefit from either reduced spacing

standards or from use of triple parallel appreaches.18 However, site-specific analyses of

the airfield and airspace of each candidate airport are needed to measure the capacity benefits,

costs, and safety effects of these proposed

changes.

Automated Metering and Spacing

The controller’s ability to meter aircraft—to

deliver them to a specific point at a specific

time—is based on aircraft speed and position as

shown on the radar screen and the controller’s

‘ ‘T. N. Shimi, W. J. Swedish, and L. C. Newman, Requirements

for lnstrurnent Approaches to Triple Parallel Runways, prepared

for the Federal Aviation Administration by The Mitre Corp.,

McLean, Va., 1981, p, E-7.

‘“L. C. Newman, T. N. Shimi, and W. J. Swedish, Sur-ocy of 101

U.S. Airports for New Multipfe Approach Concepts, prepared for

the Federal Aviation Administration by The Mitre Corp., McLean,

Va., 1981, p. xxiv, 5-4, 6-2; and A. L, Haines and W. J. Swedish,

Requirements for Independent and Dependent Parallel instrument

Approaches at Reduced Runway Spacing, prepared for the Federal

Aviation Administration by The Mitre Corp., McLean, Va., 1981,

passim.

instructions to change speed or direction in

order to arrive at the runway threshold at the

proper time. Using this manual system the controller’s training and experience allow him to

deliver aircraft to the runway threshold with an

error (standard deviation) of about 18 seconds.l9

It has been suggested that an automated system could provide more accurate metering and

spacing. In such a system, the ATC computer

could analyze radar and transponder data directly and compute future aircraft location with

great accuracy, then generate commands designed to deliver each aircraft at a specific time

and thereby optimize the use of the runway’s

capacity. It has been suggested that an automated system could reduce the delivery error to

about 11 seconds.20 The automated concept has

been under development at FAA for about 10

years but has not yet been approved for implementation. FAA states that the computerized

methods developed so far are not as reliable as a

human controller. In addition, FAA believes

automated terminal metering and spacing will

not be of much value unless it can be tied in with

en route metering and other aspects of ATC

automation now under development (see ch. 5).

Cockpit Engineering

Advances in technology are in fact changing

the basic character of the cockpit. Electromechanical instruments are being replaced with

electronic displays that present full-color images

with a very high degree of resolution. Computers are also expanding the range of functions

that can be performed by aircrew. Advanced

navigation aids such as area naviation (RNAV)

make it possible to navigate from point to point

without following established airways. The FAA

has suggested the use of a data link to improve

the quality of the information available in the

cockpit. A cockpit display of traffic information

(CDTI), currently under investigation at the Na-

“New Engineering and Development initiatives—Polic y and

Technology Choices, coordinated by Economic and Science Planning, Inc. (Washington, D. C.: Federal Aviation Administration,

March 1979) p. 107.

20 Parameters of Future A TC Systems Relating to Airport Capacity/Delay (Washington, D. C.: Federal Aviation Administration,

June 1978).
Ch. 6—Airport Capacity Alternatives ● 119

tional Aeronautics and Space Administration airport and airway facilities are used. There

could show pilots the locations of nearby air- have been suggestions that the distribution of

craft, thus reducing their dependence on ground the decisionmaking function in the ATC system

surveillance. Both RNAV and CDTI offer pilots must or should be changed to take advantage of

significant independence from controllers, and the capabilities these technological advances

this could increase the effectiveness with which have made possible (see ch. 5).

SUMMARY OF ALTERNATIVES

The alternatives discussed above all make use the other hand, rely on the application and enof some combination of economic, regulatory, forcement of regulatory measures to deal with

or technological tools to reduce delay or increase the delay problem. Automated metering and

airside capacity. For example, peak-hour pricing spacing is a technological tool, but its use will reis an economic alternative—allowing the market quire changes in existing rules and standards.

to allocate scarce airport capacity. Quotas, on Table 11 summarizes the alternatives discussed

Table 11 .—Summary of Alternatives

Economic

Alternative incentives Regulation Technology Comments

Change of airline

service patterns

Reliever airports

●

●

●

Demand-related

Peak hour pricing ● Could be implemented by local airport authority.

Devising and managing the pricing scheme may

be complex, but it could provide a substantial

long-term payoff in reduced delay.

Quotas Could be implemented by local authority or FAA.

Would provide some short-term relief for congestion and delay problems but is an inefficient

long-term solution. FAA has already imposed

quotas at 4 airports since 1969.

Balanced use of ● Could be implemented by local authority which

metropolitan airports might use economic incentives, improved access, and better facilities to encourage use of

underutilized airports; or could use regulation to

impose it.

Airlines may voluntarily shift some of their

hubbing activities to less congested airports to

save delay. (This trend seems to already be

underway.) The FAA might also be able to

achieve this redistribution by regulation. This

would make better use of airport capacity nationwide, but might do little to reduce delays at

congested airports.

FAA has already designated reliever airports.

Many are well used by GA traffic. Local

authorities encourage this trend with pricing

strategies, better facilities, or regulations requiring use of relievers by certain classes of users.

Relievers have been and will continue to be successful in providing capacity for GA operations

away from congested commercial airports.

Airport development

Airport expansion ● Responsibility of local authorities, possibly with

Federal aid. Could greatly increase capacity,

but is unlikely in many locations because of surrounding development or environmental problems.
120 ● Airport and Air Traffic Control System

Table Il.–Summary of Alternatives (Continued)

Economic

Alternative incentives Regulation Technology Comments

Addition of short ●

runway

New airport construction

ATC alternatives

Airfield/space

management

Wake vortex

prediction

Microwave landing

system

Reduced separation

or spacing standards

Automated metering

and spacing

Cockpit engineering

●

●

●

●

●

●

●

●

●

●

●

●

Possible in several airports to provide a separate

traffic stream for GA and commuter aircraft.

Increases capacity for both small and large aircraft. Responsibility of local authority with

possible Federal aid. Cost estimate for Denver

was $10 million to $11 million.

Responsibility of local authorities with Federal

assistance. Could have a major impact on local

airside capacity, but is unlikely in many areas

due to expense, lack of close in suitable land.

Good high-speed ground access might make

more distant airports likely in long range.

Allows modest capacity gains by making better

use of the runways available. Computerized

system has been tested in Chicago. Similar

system could be developed and implemented in

other areas by local authorities and FAA.

FAA would be responsible for installing vortex

detection or advisory equipment. FAA has

tested one wake vortex advisory system which

provides some capacity benefits, but is still in

the experimental stage.

Benefits are more efficient use of airspace and

availability of variable glide slopes which,

among other things, can allow aircraft to avoid

wake vortexes. Fairly substantial increases in

capacity available where traffic mix is diverse.

The technology now exists and FAA will probably install ground equipment in the 1985-2000

period. FAA’s installation costs are estimated to

be $300,000 to $500,000 per airport. Users costs

for avionics will range from $1,500 to $30,000

per aircraft.

Responsibility of FAA. Reduction of these

standards could offer large capacity increases,

but FAA’s first priority is safety of the system.

Reduction of standards is unlikely without some

technological change—elimination of wake

vortex problem or improved navigation or

surveillance.

increased accuracy of metering could optimize

runway use, offering modest capacity increases.

FAA has not yet developed a program which it

feels ready to implement. FAA wants to integrate terminal automated metering and spacing with the automated en route system, implementation might not be possible until after

the replacement of the en route computer

system.

RNAV technology is already available. Users must

buy the avionics, FAA is responsible for

developing RNAV procedures which might

reduce delays somewhat. Cockpit displays of

traffic information are being developed and

tested by the FAA but will not be available in

the near future.

SOURCE: Office of Technology Assessment.
Ch. 6—Airport Capacity Alternatives ● 121

above and indicates generally what types of

tools—economic, regulatory, or technological—

would be required to implement them. The comments in table 11 touch on several points—who

can implement the change, whether it would

make a large or small change in capacity, and

how likely it is to take place in the short or long

term.

In general, the demand-related alternatives do

not increase capacity; rather, they reduce delay

by molding traffic activity to fit existing capacity. Modest capacity gains are available through

ATC improvements that increase the efficiency

with which airfield and airspace are used, especially under IFR conditions, but the benefit

available to each airport is heavily dependent on

local conditions of runway configuration and

traffic mix. The addition of new runways is

clearly effective in increasing capacity, but this

option is available to only a limited number of

airports. In a few cases, short runways could be

constructed to increase capacity by separating

jet and propellor traffic. New airport construction also offers large capacity gains, but they

would likely be further from cities and therefore

face the problem of ground access. Reliever or

satellite airports to move GA out of air carrier

airports are necessary unless the growth of both

user groups is to be severely limited, but reliever

airports will also be constrained by land prices,

noise impacts, and community acceptance.

FUTURE RESEARCH NEEDS

Several areas offer possibly fertile ground for

future research on means to increase airport airside capacity.

Wake Vortex Avoidance

The FAA’s wake vortex advisory system has

been discussed, but more research is needed to

develop operational versions of this system

which can predict vortex problems at greater

distances from the runway ends—say, back to

the ILS middle marker or outer marker. FAA has

also studied the use of acoustical radar and lasers

to detect actual vortexes. Although some progress has been make in understanding the nature

of vortexes, these techniques are far from operational. However, with further research this line

of inquiry may be the basis for a ground-based

or airborne wake vortex detection system.

Wake Vortex Alleviation

Also important is the possibility of modifying

or minimizing vortexes at the source. NASA research has shown that certain combinations of

flaps, spoilers, or protrusions on the wings of

aircraft can cause the wake vortex to be unstable

and therefore to dissipate more quickly. Trailing

aircraft can then follow closer in safety. These

methods, however, also tend to increase the

noise level and decrease the energy efficiency of

the aircraft. More work needs to be done to develop a system which minimizes the vortex with

an acceptable price in terms of noise and fuel.

Noise

Many current noise abatement procedures require a tradeoff in terms of reduced airspace and

airport capacity. As long as aircraft remain

noisy, however, there is little alternative to routing them away from noise-sensitive areas. Some

new and re-engined jet aircraft are much less

noisy than their predecessors, but it has been

suggested that technology may have gone as far

as it can, and that administrative solutions are

the only alternative. In any case, a great deal of

further research is needed to develop creative solutions to the noise problem.

Airport Design

The scarcity of suitable land for expanding existing airports or building new ones means that

new research is needed on basic concepts of how

an airport and its access system should be designed. For example, it may be possible to redesign the runway-taxiway system in a manner
122 . Airport and Air Traffic Control System

that is less profligate of land. Research is needed

into the safety and capacity questions raised by

this type of design. In some locations where little

land is available for a new airport, it may be

possible to locate an airport on a nearby lake or

bay. Such an airport would be expensive to

build, even when the necessary technology has

been developed, but in some cases it might be

the most cost-effective alternative.

Ground Access

Airport access is a major area of concern.

Research is needed not only to alleviate the access problems plaguing some of today’s major

airports, but also on cost-effective means to get

passengers out to new airports which may have

to be constructed at distances of 30 to 50 miles

from the city center.
Chapter 7

POLICY IMPLICATIONS
Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Air System Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Discussion . . . . . . . . . . . . ......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Airport Capacity Alternatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ATC System Improvements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Computer Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Automated En Route Air Traffic Control (AERA). . . . . . . . . . . . . . . . . . . . . . . . . .

Mode S Data Link. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Collision Avoidance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Microwave Landing System (MLS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Funding and Cost Allocation Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Findings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

General Fund.... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Trust Fund. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Operating Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pending Legislation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

System Modernization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Airport Development Aid. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Trust Fund Usage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

User Taxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

FIGURE

Figure No.

29. Airport and AirwaysTrust Fund Expenditures, 1971-80. . . . . . . . . . . . . . . . . . . .

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Chapter 7

POLICY IMPLICATIONS

INTRODUCTION

The letter from the House Committee on Appropriations requesting this assessment indicated

the

●

●

●

●

following areas of concern:

scenarios of future air transportation

growth;

alternative ways to increase airport and terminal capacity;

proposed modifications of air traffic control

(ATC) system technology; and

alternatives to the present ATC process.

OTA’s analysis of these subjects is presented in

chapters 4, 5, and 6; this chapter summarizes the

major points emerging from those analyses and

examines their implications in terms of congressional interests. The intent is to highlight those

aspects of air system evolution that may be of

particular concern to the Congress in evaluating

the Federal Aviation Administration’s (FAA)

1982 National Airspace System (NAS) Plan .

The following discussion is organized under

three major headings. Under each heading is a

brief statement of findings followed by a discussion of specific problems and implications. A

fourth section deals briefly with the related questions of funding and cost allocation, which must

also be addressed in the years ahead. The final

section reviews recent congressional reports on

these subjects and identifies the relevant legislation now pending before Congress.

AIR SYSTEM GROWTH

Findings

Chapter 4 compares recent FAA Aviation

Forecasts and those of several other sources. The

following major points emerge from that comparison:

● FAA projections of future demand have

consistently been too high in the past, in

part because of the way they are made: they

assume that past trends will continue, that

there will be no constraints on continued

rapid growth, and that proposed ATC improvements will in fact be made when and

where needed to accommodate that growth.

However, other sources (including Rolls

Royce and the Air Transport Association)

feel that the airline industry is already approaching its mature size; this could lead to

a leveling off or even a decline in air carrier

operations. There is also considerable uncertainty about a number of other factors

that might affect future aviation activity,

such as changes in U.S. economic or regulatory policy, the long-term impacts of airline

deregulation and the PATCO strike, and

the ability of airlines to finance new equipment, Given these uncertainties and the

questionable economic assumptions underlying the 1981 baseline projection on which

the 1982 NAS Plan will be based, Congress

may wish to reexamine the deployment

schedule proposed by FAA for major ATC

system improvements.

There will be some growth in the system,

but the rate of growth will be slower than

was experienced in the past and may be

slower than has been anticipated even in recent forecasts. The various scenarios suggest that a 2- or 3-percent annual growth

rate for total operations at FAA-towered

airports would be a reasonable expectation,

although the rate might be as low as —1

percent or as high as +5 percent, depending

on a variety of economic, regulatory, and

operational factors that cannot be reliably

predicted. En route and flight service workloads are likely to increase as fast or faster

than tower operations.

125
126 ● Airport and Air Traffic Control System

● There is disagreement about the exact distribution of this future growth among user

groups, but the forecasts generally agree

that general aviation (GA), and especially

air taxis and corporate aircraft equipped for

IFR operations, will be the fastest growing

category. GA may account for as much as

75 percent of the increase in tower workload, particularly if FAA (as planned) increases the number of towered GA and reliever airports. Commuter operations will

increase moderately, on the other hand,

and air carrier operations (not passenger

traffic) may actually decline at some hubs.

• The relatively rapid growth of GA demand,

combined with the slower growth of commuter and air carrier operations, could

have several effects on the U.S. airport and

ATC system:

—Unconstrained growth of operations at

major hubs would lead to saturation at 15

to 20 airports by 2000, compared with 5

to 10 airports today. Growth rates above

4 percent annually, which are possible

but unlikely, might result in saturation at

—all 50 of the top air carrier airports by the

end of the century.

—In the absence of capacity improvements

at saturated hubs, increasing congestion

and delay will probably result in further

redistribution of air carrier operations

(especially transfer functions) away from

saturated major hubs to “second tier”

hubs where surplus capacity still exists.

–Similarly, GA traffic is likely to be

shifted out of more and more air carrier

hubs to reliever and other GA airports.

This will create a demand for improved

facilities at those airports.

—As a result, the principal opportunities

for capacity expansion will come not at

the major hub airports but rather at the

second-tier hubs and at GA and reliever

airports, as well as at the air route traffic

control centers and flight service stations.

If these increases in ATC system capacity

are to be provided without greatly increasing FAA’s operation and maintenance (O&M) expenditures, expanded

use of automated and

will be required.

Discussion

remote facilities

Forecasts of aviation activity are subject to

three principal kinds of uncertainties, all of

which affect the accuracy and usefulness of the

resulting projections of airport and ATC system

demands:

●

●

●

There is no common purpose or focus—

airline forecasts concentrate primarily on

measures of carrier profitability, aerospace

forecasts on potential aircraft markets, and

FAA forecasts on ATC workloads.

All of the projections nevertheless employ a

similar methodology and rely on similar

demographic and economic expectations.

Specifically, the forecasts assume a continuation of the past relationship between gross

national product growth and increased demand for air travel. As a result, commonmode failure is possible—the forecasts

could all be wrong for the same reason.

All of the forecasts are subject to factors

whose future influence can only be guessed

at, including the price and availability of

fuel, the effects of airline deregulation, the

resulting changes in industry structure, the

long-term impacts of the air controllers

strike, the uncertain availability of financing for reequipping airline fleets, and future

changes in Federal aviation policy or cost

allocation.

As a result, there is general agreement on the

likelihood of future growth, but little certainty

about its magnitude, and still less about the

more important questions of when and where

growth will occur or what its impact will be on

the Nation’s airport and ATC system.

Continued growth along historic patterns

would exacerbate congestion and delay at hubs

that are already saturated and would probably

spread these problems to additional airports.

This would present two possible courses of response:

. accommodate the growth wherever it occurs (as FAA has done in the past) by at-
Ch. 7—Policy /replications . 127

●

tempting to expand the capacity of affected

hubs; or

channel the growth, either actively or passively, so that it can be accommodated at

other hubs.

Neither of these courses will be applicable in all

situations, and in most cases the solution will involve some combination of the two; finding the

proper balance will require a case-by-case analysis of their relative costs and benefits.

Adding new capacity at congested hubs—in

the form of new runways or entirely new airports—could be extremely expensive in relation

to the number of additional operations that can

be accommodated. There are, however, a number of traffic management techniques that could

increase the efficiency with which existing capacity is utilized at airports and terminal areas

that are already saturated.

There are clear indications that market forces

have already begun to alter the historical patterns of demand distribution. Some airlines,

faced with high delay costs and strike-related restrictions at congested hubs, are finding it attractive to move some of their “hubbing” or transfer

operations to well-equipped second-tier hubs

where available capacity exists and delay costs

can be avoided. Local service airlines, with the

new route and entry freedom of deregulation,

are beginning to increase the number of directservice flights, and, consequently, to decrease

the number of transfer operations. New entrants

and low-cost carriers, unencumbered by large

investments in facilities at congested hubs, are

basing their operations at second-tier hubs. Although this trend may involve a small decrease

in the operational efficiency of system users, it

would greatly increase the efficiency with which

the airport and ATC system’s aggregate capacity

is utilized.

Growing congestion could have serious implications for commuter and GA users, who would

beat a considerable disadvantage in any competition for access to congested hubs. Neither user

group is likely to be completely priced or regulated out of major hubs, but growing congestion

may nevertheless prove to be a significant constraint on their future growth. Additional GA

operations might be accommodated at reliever

and other GA airports; this would make more

capacity available at existing hubs, but it could

also lead to additional FAA investments and

operating costs for new towers at lightly used

GA airports. (FAA plans have called for as

many as 50 new towers by 1993, but its experience in closing over 60 low-volume towers since

the PATCO walkout justifies a review of these

plans. ) Commuter carriers, on the other hand,

will continue to require access to hub airports,

since most of their passengers transfer to other

flights, Rehubbing by major airlines will not

change this requirement and might even create

additional complications in commuter routes

and operations, although it might also create

new market opportunities for commuter airlines. In addition, commuter and GA users will

generate most of the new demand for en route

and flight services.

AIRPORT CAPACITY ALTERNATIVES

Findings

The committee asked OTA to examine the

“relative merits of alternative ways of increasing

airport and terminal capacity to meet future demands and reduce safety hazards. ” The tools

that can be used to increase capacity or reduce

delay are examined in chapter 6, where the major findings and implications are:

● Changes in ATC equipment or procedures

can produce small increases in airside capacity by helping aircraft use available airspace and runways more efficiently. However, large capacity improvements, such as

would result from greatly reducing the

distance between aircraft on landing and

takeoff, must await technological breakthroughs like improved prediction of wake

vortices.

. Where ATC improvements are made, they
128 . Airport and Air Traffic Control System

would not necessarily eliminate the problem of delay: latent demand at a popular

airport could quickly consume new capacity, and the length of delay would remain

the same.

. Major increases in the physical capacity of a

hub would require building new runways or

entire new airports. Such major improvements are unlikely to be made in the near

future because of the unavailability or high

price of land, costs of construction, and

noise and other environmental constraints.

• If growth continues, however, some new

major airports may have to be built. Since

they are likely to be some distance from the

center city, the success of these airports will

depend upon suitable high-speed ground access. (Dunes International Airport demonstrates the need for such access. )

● Congestion at large hub airports may induce use of a variety of techniques to maximize effective capacity, including hourly

quotas and peak-hour pricing. GA users are

likely to be the major losers in competition

for slots at congested airports, although

these restrictions might also constrain the

growth of commuter carrier operations.

● If air carriers continue to redistribute their

transfer operations to second-tier hubs,

some added investment will be required at

these airports.

● In the near term, two forms of capacity expansion can be helpful: 1) construction at

congested airports of separate, short runways, equipped for instrument operations,

for use by small aircraft; and 2) construction or improvement of reliever airports to

accommodate GA traffic diverted from

congested commercial airports.

Discussion

Some improvements can be expected from

changes in ATC equipment or procedures in

congested terminal areas; but the net effect on

delay would be quite small. For instance, computerized airfield/airspace management might

allow better utilization of existing physical capacity, so that actual operations would approach

the theoretical maximum for each combination

of weather and traffic conditions. The Microwave Landing System (MLS) might also allow a

small increase in the number of Instrument

Flight Rules (IFR) operations under certain conditions of traffic mix. In general, mostly because

of the separation required by the danger of wake

vortex, there will be no significant ATC-related

increase in the number of aircraft operations

that can be handled by a given runway, airport,

or terminal area.

Past Federal, State, and local airport policy

has been to provide new capacity where demand

seemed to warrant it, if at all possible. Most of

today’s congested airports have gone through

periods of major expansion, only to become saturated by subsequent growth. As urban transportation planners have discovered, additional

capacity is not always the solution to the problem of delay. Building a new lane does not appreciably ease traffic jams on a busy freeway,

for instance, because new traffic is attracted by

the improved link and delays quickly reach the

previous level. The same principle applies to

many hub airports: the busier an airport is, the

more demand there is for access to it, simply because it is busy and thus offers a wide choice of

connections and services. Adding new capacity

may merely tap this latent demand—the airport

can accommodate those it couldn’t handle before, but the new traffic quickly saturates the additional capacity and delay soon rises to previous levels. This doesn’t mean that expansion is

futile, but it should be evaluated in terms of its

benefits and the available alternatives.

If expansion proves impractical, the 15 to 20

airports that will become saturated by the end of

the century will probably have to make wider

use of demand-managing alternatives—peakhour pricing, quotas, or access restrictions—to

deal with the problems of congestion and delay.

These tools do not increase peak capacity; they

shift traffic to a time or place where it can be better handled, thus increasing effective capacity.

Pricing schemes to ration scarce landing slots

place the greatest burden on operators of small

aircraft, since they have a smaller base of passengers over which to spread cost. Administrative quotas may also tend to favor larger aircraft, which serve more passengers and generate
Ch. 7—Policy Implications . 129

higher landing fees. In either case, commuters

and GA users will have the greatest difficulty in

competing for slots at crowded airports. Not all

GA activity could be displaced, since some GA

flights must use the main airport to deliver

passengers connecting with commercial flights.

Even at the busiest airports GA operations currently tend to average about 10 percent of total

operations.

The separation of fast and slow (or jet and

prop) traffic is one ATC procedure that could

benefit both types of traffic. Most GA and commuter aircraft can use shorter runways than

those required for large jet liners, and at some

busy commercial airports the construction of

short runways equipped for instrument operations could allow continued accommodation of

commuter and GA aircraft, and at the same

time, could also allow some secondary increase

in jet aircraft operations. These separate, short

runways would be especially important for commuter carriers

able to land at

whose business depends on being

major airports, and in many cases

they would add more capacity relative to cost

than a new mixed-traffic runway.

Another means of separating traffic that will

become increasingly important is the diversion

of some GA traffic from commercial airports to

reliever airports. This technique has some drawbacks. For example, users may resist going to a

“second best” airport which may not offer the

same services or ground access as the commercial airport. On the other hand, a properly

equipped GA reliever can often provide better

service to nonscheduled private traffic than the

main airport could. Constructing, improving, or

upgrading these airports would be largely the responsibility of local authorities, but Federal assistance (in the form of the Airport Development

Aid Program (ADAP) or other grants) is currently available for the 155 reliever airports included in the NAS Plan. The level of funding for

relievers in the recent past has been a little under

25 percent of all grants for GA airports, or 4 to 6

percent of all airport grants.

ATC SYSTEM IMPROVEMENTS

Future improvements in the ATC svstem will

be directed toward three general objectives:

●

●

●

replacing obsolete equipment with improved technology that is more effective

and reliable and less costly to operate and

maintain;

expanding system capacity to accommodate

expected growth; and

adding new capabilities to increase the productivity of the system and the efficiency of

its users.

Two improvements are basic to this process:

1) achieving higher levels of automation on the

ground, and 2) taking advantage of the capabilities of flight-management avionics that are appearing in the user fleets. In the 1980’s, the major

effort will be devoted to replacing the computers

in the en route centers, modernizing the flight

service stations, and beginning the deployment

of the MLS, the Discrete Address Beacon System

(DABS, now Mode S), and the Traffic Alert and

Collision Avoidance System (TCAS). For the

1990’s, the FAA’s plans included further implementation of the Mode S data link and MLS and

the start of a long-range program of automation

in en route and terminal area ATC centers. The

FAA plans are undergoing a major review,

however, and there are indications that the

FAA’s 1982 NAS Plan will include changes in

both technology and timing.

In general, OTA finds that the ATC system

improvements previously proposed by FAA in

the areas studied are technologically feasible. In

four of the five major areas addressed by OTA,

however, detailed cost and benefit information

is not yet available. This information will be

needed on all major programs before final judgment can be made on FAA proposals. The specific findings and potential issues in the five program areas studied by OTA are set forth under

separate headings below.
130 ● Airport and Air Traffic Control System

Computer Replacement

The computers used in en route ATC centers

will need to be replaced within the next 10 years

because the present IBM 9020 computers do not

have the computing speed or storage capacity

needed to accommodate the expected growth in

air traffic at the most heavily used en route

centers. These computers also lack the capacity

to support more automated modes of operation

that FAA estimates will be needed to assure

future system safety or to increase ATC system

productivity. There is also concern that the cost

of repairing and maintaining the present computers will become excessive, largely because the

IBM 360 series computers used in the 9020 are no

longer in production and replacement parts

would ultimately have to be specially made.

An important issue in the computer replacement program is the procurement strategy to be

followed. The program previously recommended by FAA was a total replacement strategy which would require about 10 years to complete and would entail specially designed ATC

hardware and software to meet near-term needs

and serve as the foundation for more advanced

automation in the 1990’s and beyond. The

schedule called for the first operational contract

to be let in 1988, with installation of production

systems starting in the 1990’s. The costs of this

program were at one time estimated at nearly

$1.7 billion (1980 dollars), over the 1982 to 1991

period.

Alternatives to this total replacement strategy

include incremental approaches which could

provide relief to computer capacity problems in

a shorter time—perhaps 3 to 4 years as compared to 10 years for total replacement. For example, a “software first” approach would focus

on rewriting ATC software to reflect modern

modular programing techniques. Then software

for particular ATC functions could be gradually

transferred to new computers which would at

first supplement and finally replace the 9020s. A

“hardware first” strategy would involve transferring (rehosting) the existing software package

to a new computer. Later this software could be

modified along more modern lines or totally replaced to support new functions and services.

There are technical difficulties to be overcome

in each of these incremental strategies, but they

have the advantages of allowing the replacement

process to begin quickly. The use of off-the-shelf

hardware would appear to offer some cost savings over specially designed equipment. Further

it would ensure that compatible hardware is

available to upgrade or expand the system at a

future date.

Automated En Route Air

Traffic Control (AERA)

Part of the rationale for en route computer replacement is to satisfy the long-term evolutionary requirements that are now defined in a general way under the concept of AERA. The essence of this concept is to transfer from controllers to computers some routine activities, such as

separating and metering aircraft or formulating

and delivering clearances. Relieved of these routine tasks, the controller’s role would be primarily to handle exceptions and emergencies and to

oversee (manage) the operation of automated

ATC equipment. Automation could achieve several benefits: increasing controller productivity

and reducing FAA personnel costs; reducing

user costs by permitting wider use of fuel-efficient flight profiles; accommodating more operations; and reducing system errors.

The AERA concept requires a great amount of

ground-based data processing to perform extensive and detailed management of aircraft flight

paths. It could also reduce many of the procedural constraints now imposed on the use of

airspace. In effect, it would be a system of

management by exception: intervention by a

controller would be limited to circumstances or

localities where conflicts could not be reliably

resolved by computer algorithms.

The major advantage claimed for AERA,

aside from more comprehensive management of

traffic, would be a substantial increase in controller productivity. It is contemplated that

AERA control sectors would be staffed by one

or perhaps two (rather than the present three)

controllers and that the volume of airspace controlled would be several times the size of present

en route sectors. A substantially greater number
Ch. 7—Policy Implications ● 131

of aircraft could thus be handled by a controller

team. On the other hand, this load would almost

certainly be heavier than human operators could

handle in the event of computer failure. As a

result, the AERA concept includes provisions for

automated backup for automated functions, as

well as a computer design that will allow the

system to “coast” safely while backup procedures are being initiated.

It must be emphasized that at present AERA is

only in an early stage of development. Extensive

efforts over perhaps 5 to 10 years will be needed

to bring AERA to a precise and detailed definition of requirements and equipment specifications.

Three major features of AERA are already apparent. First, AERA would require computer

capacity and software substantially beyond that

now available in ATC applications, although

not beyond the present or readily foreseeable

state of technology. Second, AERA will require

a two-way data link capable of rapid and extensive exchange of information between the air

and the ground. FAA now envisions that Mode

S will provide this data link, but other possibilities could be considered. Third, AERA implies a

like degree of automation in the terminal areas

and in a central flow management facility

capable of coordinating traffic throughout the

ATC system. This last point is particularly important for both short-term computer replacement and long-term system design, since it implies the advisability of procuring a computer

having a modular architecture. This would

make it possible for en route and terminal facilities to utilize similar hardware and software; it

would also encourage a flexible system design,

in which individual modules would be capable

of mutual support and backup in the event of

partial equipment failure.

Close scrutiny by Congress will be needed as

FAA’s plans mature. One major issue is likely to

be the acceptability to the users and controllers

of an ATC system automated to the degree envisioned in the AERA concept, especially its safety

and operational reliability. A second major issue

will be evaluation of the savings in operation

and maintenance ascribed to AERA, compared

to the needed investments in facilities and equipment to implement the system. A corollary issue

will be the costs and benefits to various classes

of airspace users. The information to support

judgments on these matters is not now available,

and OTA can reach no conclusion beyond the

general observation that resolving these issues is

likely to be far more important than seeking answers to the rather narrow question of technical

feasibility.

Mode S Data Link

Another key element in the FAA’s overall plan

for improving the ATC system is the Mode S

data link, an improvement to the secondary surveillance radar that allows properly equipped

aircraft to be interrogated selectively by ground

stations. Mode S provides greater surveillance

accuracy than the present Air Traffic Control

Radar Beacon System (ATCRBS) equipment and

avoids the problem of “synchronous garble” that

occurs when more than one aircraft respond simultaneously to interrogation. The discrete

address capability also provides a two-way

ground-to-air data link to transmit clearances,

weather information, traffic advisories, control

instructions, and flight data automatically in a

digital format without using VHF voice channels. The Mode S data link feature provides the

basis for automation of ATC functions and

other system improvements in the years beyond

1990.

Mode S has been under development by FAA

for nearly 10 years at an estimated cost to date

of $58 million. The first prototype unit was delivered for test and evaluation in 1978, and a

contract for initial production will be awarded

in 1982. FAA has not yet issued a formal implementation plan, but the preliminary plan calls

for a multiyear procurement and deployment

starting in 1986, at 197 sites—97 in terminal

areas and 36 in the en route system, plus 60 for

low-altitude coverage and 4 at support facilities.

Deployment at these 197 sites would not constitute full implementation of Mode S. Additional installations, which would not be completed until early in the next century, might be
732 ● Airport and Air Traffic Control System

needed at another 100 sites to provide coverage

down to 6,000 ft for the continental United

States and perhaps portions of Hawaii and

Alaska.

An issue that will need to be addressed during

examination of the plans for Mode S has to do

with the extent to which a Mode S transponder

would be required before permitting an aircraft

to enter airspace or receive services (e.g. access

to and operation in a terminal control area

[TCA]). Mode S and ATCRBS Mode C are compatible, so that in the short run either system

would qualify users to operate in TCA. GA operators, however, have expressed concern that

the Mode S format would eventually supplant

ATCRBS Modes A and C and that they would

be required to reequip their aircraft with Mode S

transponders. This concern would be reduced by

assurances that ATCRBS could be utilized for an

extended period following the initial implementation of Mode S.

Collision Avoidance

The primary function of air traffic control is

to assure the safe separation of aircraft. In the

present system, this is accomplished by controllers on the ground using surveillance radar and

computer aids; when conflict is detected, the

controllers use voice radio to advise pilots of

traffic or instruct them to perform appropriate

avoidance maneuvers. At present, the pilot has

no instrument or display in the cockpit to identify potential threats or to indicate a maneuver

that would resolve a conflict.

For many years, FAA (in cooperation with the

aviation community) has investigated a number

of collision avoidance systems that would provide a backup (rather than a substitute) for the

current ATC procedures and ground-based separation assurance service. During the summer of

1981, FAA selected a system known as TCAS.

FAA plans for TCAS to be operational by the

end of 1984, a goal that is considered by some to

be optimistic. FAA has justified the choice of

TCAS on the following grounds:

. it does not require ground-based equipment;

●

●

●

it is compatible with the present ATC system and is a logical extension of it;

it offers a range of capabilities suitable to

the needs of the various classes of airspace

users; and

it is more suitable for use in high-density

traffic than the Beacon Collision Avoidance

System (BCAS), the system that was favored by the FAA prior to the TCAS decision.

TCAS provides the user with protection from

other aircraft regardless of whether they are

equipped with TCAS or the standard ATCRBS

transponder. In the active mode, TCAS interrogates other aircraft to determine whether they

are threats. TCAS also identifies potential

threats from ATCRBS-equipped aircraft by

monitoring their replies to interrogations from

the ground. A central feature of TCAS is the use

of the Mode S transponder for the communication of data between aircraft. TCAS 1, the system intended for use by general aviation, provides general Mode S capability and would cost

$2,500 to $3,500 per aircraft. TCAS II, the version intended for use by commercial aircraft,

would cost between $40,000 and $50,000 per set,

plus the cost of antennas and installation. Some

believe these estimates to be low. TCAS requires

essentially no expenditures by FAA, except for

development and certification costs; but since it

will require Mode S for identification and data

link, aircraft equipped with TCAS will be prepared to take advantage of any new services requiring data link that may be offered by FAA.

Although FAA has decided that it will certify

TCAS as the collision avoidance system to be

used in the United States, not all features of the

system have been developed and demonstrated.

The TCAS II direction-finding antenna is of critical importance: there is some question regarding the aerodynamic effects of the antenna on

aircraft performance, particularly the performance of tactical military aircraft. TCAS I, on the

other hand, has been demonstrated; but it is not

clear how useful this more basic form will be

since it only indicates the proximity of another

aircraft without providing either bearing or

range.
Ch. 7—Policy Implications . 133

Prior to selecting TCAS, FAA was pursuing

development of active BCAS. Because there was

concern that omnidirectional BCAS might interfere with the surveillance system in congested

areas by saturating ATCRBS transponders, FAA

was also planning to base conflict resolution in

areas of high traffic density on DABS/Automatic Traffic Advisory and Resolution Service

(ATARS), a ground-based system that would require expenditures of $518 million to equip terminal and en route facilities. The decision to

adopt TCAS has led FAA to reevaluate the need

for DABS/ATARS.

Microwave Landing System

Another important component in the FAA’s

development plans is MLS, a precision landing

aid designed as a replacement for the Instrument

Landing System (ILS) that has been in use since

the early 1940’s. MLS is less sensitive to interference and distortions than ILS and will work at

sites where it is difficult or impossible to install

ILS. It is also anticipated that MLS equipment

will be more reliable than ILS. The chief operational advantage of MLS is that it permits variable glide slopes, curved and segmented approaches, and precision missed approaches,

where ILS does not. This would allow traffic to

be routed around noise-sensitive areas and

would also allow greater flexibility in handling

traffic in crowded TCAS. MLS can operate on

200 channels (compared to 20 for ILS) making it

possible to provide precision landing aid in areas

where closely spaced airports limit the availability of ILS channels.

FAA has announced plans to implement MLS

in three phases over the coming 11 to 16 years,

with 1,200 to 1,400 systems eventually installed.

In the first phase, between 10 and 25 systems

will be installed at selected airports in order to

develop a base of experience and reach an empirical determination of the benefits that can be

realized. The second phase would be the installation of 900 MLS units at the rate of 100 to 150

per year for a period of 6 to 9 years, with priority given to large and medium hub airports and

those where ILS siting problems exist. The third

phase would consist of installing of an additional 300 to 500 systems to meet the growth in demand anticipated during the remainder of this

century. FAA estimates the cost of 1,425 MLS

ground systems to be $1.332 billion (1981 dollars), and users will be required to spend an additional $895 million for avionics if they wish to

take advantage of this service.

OTA finds that the FAA’s analysis of MLS

benefits and costs does not establish a clear and

universal case for MLS as opposed to ILS, and

that for this reason the FAA plan for a first

phase to gain the operational experience before

the full deployment of MLS is entirely reasonable. However, at the end of the initial phase, it

would be appropriate to conduct a comprehensive review of the MLS program before proceeding with further implementation. A part of this

review should be development of additional increments or intermediate steps between the 25

sites planned for Phase I and the 900 planned for

Phase II. Another part of this review should be

more specific benefit-cost analyses that differentiate and specify the benefits at various airports

in terms of levels of traffic, the types of users

served, and the resulting reductions in noise,

delay, or fuel consumption.

FUNDING AND COST ALLOCATION ISSUES

Findings

The program of airport development and

ATC system improvement through 1991 previously proposed by FAA would require an expenditure of $1.6 billion to $1.9 billion per year,

or about 50 to 75 percent above the spending

level of recent years in real terms. Implicit in

these figures is a commitment to spend roughly

equal sums annually from 1992 to 2000 in order

to complete programs already initiated and to

undertake further improvements of the airport

and airways system. These figures may change,

however, as a result of changes in the forthcoming NAS Plan.
134 • Airport and Air Traffic Control System

Historically, such expenditures have been financed from the Airport and Airways Trust

Fund, which lapsed in October 1980 but had an

uncommitted balance of about $3 billion at the

end of fiscal year 1981. This sum would cover

less than 20 percent of the 1982-91 programs

contemplated by FAA.

Congress has two basic options to provide

funding for the developing airports and airways

over the coming years. One would be to cover

these expenditures wholly by appropriations

from general funds. The other involves funding

through user charges by reestablishment of the

trust fund in some form, including:

● Reestablishment of the trust fund with a

revenue and user charges similar to those

which existed prior to October 1980. This

would not cover the 1982-91 program of

capital spending if—as in the past—some

trust fund revenues were also spent for

O&M.

● Reestablishment of the trust fund, retaining

the present forms of funding but increasing

user charges to make revenues match projected expenditures. Rates could be raised

either uniformly (across the board) or selectively (to alter the mix of contributions

from various user classes).

● Reestablishment of the trust fund, but with

a different form of user charges. Existing excise taxes might be replaced with user levies

that would reflect more accurately the magnitude of the benefits received by various

classes of users, or by a system that would

charge individual users in relation to the

costs they impose on the airport and airways system.

All of these options would be controversial

and would exacerbate many long-standing issues

pertaining to access to the system, user cost allocation, and subsidies to aviation. The search for

a solution is further complicated by the fact that

the cost of operating the airport and airways

system would also be rising at the same time.

The disagreements over funding airport and

airways improvements are so wide, and the

sums so large, that the debate could conceivably

extend over a number of years. To the degree

that such a stalemate delays the funding of the

FAA’s proposed programs, some of the following courses of action might have to be considered:

●

●

●

●

keep the existing equipment running as well

as possible, with administrative restrictions

on traffic levels as needed to keep demand

within capacity;

cut back on the proposed plans, dispensing

with some improvements and funding only

those for which there is the greatest or most

immediate need;

stretch out the procurement process over a

longer period of time, in order to hold expenditures within the available revenues; or

consider alternative technologies or funding

mechanisms that shift more of the cost of

the system to airspace users.

Discussion

Capital expenditures for airport capacity improvements and new ATC technology planned

for the coming decade would result in a sharp increase in the FAA budget compared to the funding levels of the past 10 years. The combined

expenditures for airport grants-in-aid, for ATC

facilities and equipment (F&E), and for associated research, engineering, and development

(RE&D) were in the range of $0.95 billion to

$1.35 billion per year (in constant 1980 dollars)

between 1971 and 1980 (see fig. 29). * Capital expenditures for fiscal year 1982 to fiscal year 1991

could total between $16 billion and $19 billion

(1980 dollars), with $4.5 billion to $6 billion

allocated to airport grants in aid, $10 billion to

$11 billion for F&E, and $1.5 billion to $2 billion

for RE&D. The combined outlay in these categories would amount to $1.6 billion to $1.9 billion per year, a real increase of 50 to 75 percent

over the 1971-80 average.

A large part of airport expenditures throughout the 1982-91 period would be allocated to capacity increases at congested hub airports and

development of GA reliever airports to take

some of the pressure off large and medium hubs.

*In fiscal year 1980, the total in these three categories was $950

million; in fiscal year 1981, $885 million.
Ch. 7—Policy Implications ● 135

Figure 29.—Airport and Airways Trust Fund

Expenditure 1971-80*

1971 1972 1973 1974 1975 1976 1977 1978 1979 1980

Fiscal year

*Appropriations in various years for operating and maintenance expenses,

totaling $3,370 million, are not shown. They amount to about 30 percent of all

trust fund expenditures.

SOURCE: FAA Monthly Management Report, March 1981.

In the near term, the bulk of the F&E expenditures would be for replacement of en route computers, the first stages of MLS and Mode S implementation, and modernization of flight service stations. Beyond 1990, the major F&E expenditures would be for completion of en route

automation, initiation of terminal area automation, and further deployment of MLS. Programs

such as MLS, Mode S, and terminal and en route

automation would not be completed by 1991;

there would be a foIlow-on requirement for an

additional funding in the 1990’s to carry these

programs to completion and to initiate further

ATC technology improvements.

The FAA’s justification for these planned expenditures is that they will be needed to relieve

airport congestion, to enable the ATC system to

handle higher traffic levels without compromise

of safety, and to improve the efficiency (productivity) of the ATC system. Increasing productivity is especially important in view of the projected increase in aircraft operations and the resulting rise in ATC costs that would occur over

the next 10 years if automated en route, terminal, and flight service station equipment were

not installed.

Since establishment of the Airport and Airways Development Program in 1970, expenditures for airport improvements and ATC facilities and equipment, including the associated

RE&D, have been financed by the Airport and

Airways Trust Fund. Between fiscal year 1971

and fiscal year 1980, the trust fund provided $4

billion in airport grants, $2.6 billion for F&E,

and $0.7 billion for RE&D. During the same period, the trust fund also provided almost $2.2

billion for O&M expenses of the ATC system.

Expenditures from the trust fund have never exceeded revenues, and as of the end of fiscal year

1981 the trust fund had an uncommitted balance

of about $3 billion.

The principal source of revenue for the trust

fund through fiscal year 1980 was an 8-percent

tax on domestic airline tickets. Other taxes contributing to a lesser extent were a 5-percent waybill tax on air cargo, a 7 cents per gallon tax on

jet fuel and gasoline used by GA, a $3 international departure tax, an aircraft use tax for propeller aircraft, and taxes on airplane tires and

tubes. In fiscal year 1980, these taxes contributed $1.87 billion to the trust fund, with 85 percent coming from the domestic airline passenger

ticket tax.

On October 1, 1980, the legislative authorization of ADAP and the trust fund expired and

Congress declined to pass reauthorizing legislation. Since then, receipts from the passenger

ticket tax (reduced to 5 percent) have been remitted to the general fund. The air cargo waybill, international departure, and aircraft use

taxes have been abolished. Revenues from the

tax on aviation gasoline (4 cents per gallon) and

tube and tire taxes have been remitted to the

Highway Trust Fund.

There are now several bills before Congress

that would restore the trust fund. These proposals include provision for airline passenger

ticket taxes between 4 and 6.5 percent, taxes on

GA fuel, an air cargo waybill tax of 2 to 5 percent, and a $1 to $5 international departure tax. *

● Generally, the Administration’s proposal provides for higher

tax rates than any of the House or Senate bills. The tax rate for GA

jet fuel under the Administration’s proposal would be 20 cents pergallon initially, rising to 65 cents per gallon by fiscal year 1986.

The tax on aviation gasoline would rise from 12 cents per gallon in

fiscal year 1982 to 36 cents per gallon in fiscal year 1986. In congressional proposals, the tax on fuel ranges from 4 cents to 8.5

cents per gallon.
136 ● Airport and Air Traffic Control System

Many Members of Congress have voiced strong

opposition to reestablishing the trust fund or increasing the present user taxes so long as there is

a large uncommitted balance in the trust fund.

Sponsors of the various bills have pointed out

that reauthorization of trust fund taxes in some

form will be necessary to provide revenue for

projected airport and ATC capital improvements. They also point out that the trust fund is

consistent with the position of the present Administration that, e.g., whenever the Federal

Government provides a service directly to a particular industry, those who receive the benefit

should bear the cost.

Regardless of the action taken on these proposals, the Administration and Congress will, in

the long run, have to grapple with the question

of how to finance planned airport and ATC capital expenses. The balance in the trust fund now

would cover less than 20 percent of the outlays

by FAA for 1982-91. If these funds were to be expended at the fiscal year 1981 rate of $1.6 billion

annually and no new taxes were authorized, the

trust fund would be exhausted by the end of

1983. Even if the most ambitious of the current

tax proposals were to be enacted and if trust

fund moneys were also used to defray about

one-quarter of O&M expenses (as they were in

fiscal year 1980), trust fund revenues would

probably be insufficient to meet planned capital

expenditure and O&M costs beyond 1987 or

1988.

Some of the implications of providing funding

for FAA airport and airways programs by appropriations from the general fund or, alternatively, by reauthorization of the trust fund are

discussed below.

General Fund

Capital expenditures for airports and airways

could be financed from general revenues

through annual appropriations. There are

numerous precedents for this in other areas

although it runs counter to the 10-year Federal

policy of financing airport and airways improvements through a dedicated trust fund supported by user charges. Funding from general

revenues has the basic advantage of giving the

Congress close control of FAA capital programs

through the annual appropriations process. On

the other hand, financing from general revenues

has several major disadvantages: it introduces

additional uncertainty in to the funding process

and might make it difficult to plan and implement long-range programs, which might be canceled or delayed during periods of budget austerity, perhaps to the detriment of the national

airspace system. A corollary disadvantage is

that the FAA’s capital programs might have to

compete with operational expenses for a share of

the FAA budget and (if a choice had to be made)

operational expenditures would probably receive first consideration since they cannot be deferred or curtailed as easily as capital expenditures.

Perhaps the greatest objection to general fund

financing, however, has been that it would constitute a subsidy of aviation by the public, many

of whom would receive no direct benefit: onethird of the adult population in the United States

has never flown, and fewer than 10 percent use

commercial or general aviation on a regular basis. Such an approach, it is argued, would also

contradict the economic precept that the users of

a special service should bear the cost of that

service—a view that the present Administration

has advocated strongly. It is argued by some,

however, that the general public also benefits in

many indirect ways from services provided to

the aviation community, including mail service

and air freight as well as use of the system by

military aircraft.

Trust Fund

Financing airport and airways improvements

from a trust fund, either like that which existed

prior to October 1980 or in a modified form, is

an approach favored by many observers. It provides a continuing and stable source of funds

earmarked for capital programs, and it secures

those funds directly from users of the system.

On the other hand, it has the general disadvantage of any sort of trust fund: the statutory

restrictions on the purposes for which moneys

may be used might limit Congress’ flexibility in

meeting other, perhaps more pressing, needs.

The long-standing controversy over use of Airport and Airways Trust Fund monies for meet-
Ch. 7—Policy Implications ● 137

ing annual O&M expenses of FAA is a clear illustration of this.

If Congress elects to continue the trust fund

approach, as most of the pending bills pertaining

to funding FAA’s capital programs now propose, there are several options open:

● Reauthorize the Airport and Airways Trust

Fund as it existed before October 1980, This

fund, supported by various user excise

taxes, would provide for some or all of

FAA’s capital expenditures over the coming

decade. Whether it could also meet some

portion of operating expenses would depend on the rates established for the various

user taxes. Much of the current debate in

Congress is on this specific point: i.e., the

appropriate amount of taxation to be imposed on each class of airspace user.

• Retain the tax mechanisms of the former

trust fund but substantially alter the scheme

of taxation, so that each category of users

would pay a share more nearly proportionate to the benefits they received. In the trust

fund as constituted before October 1980,

commercial aviation (domestic and international air carriers and air cargo airlines)

contributed 93 percent of the revenues but,

according to cost allocation studies by DOT

and FAA, received a smaller share of the

benefits—in effect, cross-subsidizing GA.

Since nearly all of the revenues from commercial aviation were derived from the tax

on airline tickets, the subsidy to GA was actually provided by airline passengers, not

airlines. The Administration’s recent proposal would redress this imbalance somewhat by greatly increasing the tax on fuel

for GA aircraft, but it would probably still

fall short of levying charges on GA commensurate with the benefits received, especially by business aircraft operating in and

out of hub airports.

Private GA operators and the makers of

GA aircraft have vigorously opposed such

tax schemes, on the grounds that Visual

Flight Rules (VFR) and IFR users impose

greatly different costs on the ATC system,

and that high fuel taxes would reduce aircraft utilization in the short run and reduce

sales of GA aircraft in the long run. They

also state that the ATC system was designed to meet the needs of air carriers, and

a few hub airports, with facilities and services that GA users neither asked for, nor

want, nor need. In this sense, some GA

users claim that they subsidize commercial

air traffic. A third, and perhaps more fundamental, objection raised by GA is that

there is no accurate method of determining

the value of the benefits received by GA or

any other class of airspace user, and hence

no sound basis for establishing an appropriate level of taxation.

Levy charges on users, either based on the

actual use they make of the airport and airways system or based on the burden they

place on the system to provide various

types of services. The United States maybe

the only major nation that does not routinely charge for the use of its airspace; many

countries in Europe and elsewhere in the

world levy charges for the use of terminal

and en route airspace (based on distance,

time, and type of service provided), in addition to landing fees like those collected in

this country to defray the costs of airport

construction, maintenance, and operation.

The chief conceptual problem is how to

quantify user benefits or determine the cost

of a service. Two major attempts by FAA

and the Department of Transportation

(DOT) to develop such a methodology, the

cost allocation studies of 1973 and 1978, ’ 2

met with major objections from various

aviation groups on the grounds that costs

could not be determined with sufficient accuracy and that an equitable formula for allocating costs had not been developed.

Assuming that the methodological problems

could be overcome, there would still remain

practical problems of how to assess user charges.

The simplest and most direct method would be a

‘Airport and Airway Cost Allocation Study; Determi~?ation,

Allocation, and Recovery of System Costs (Washington, D. C.,

U.S. Department of Transportation, September 1973).

‘Financing the Airport and Air-may System: Cost Allocation and

Recovery, FAA-AVP-78-14 (Washington, D. C.: Federal Aviation

Administration, November 1978).
138 ● Airport and Air Traffic Control System

charge for service at the time a flight plan is

filed. While this would capture fees from IFR

users, it might encourage some GA operators to

fly “off the system” (i.e. VFR to smaller airports)

in order to avoid airport and airway charges,

perhaps to the detriment of safety. It would also

create a bookkeeping and administrative task

for FAA in levying charges for use of the system.

A second possibility would be to require all

aircraft to have a transponder and to use surveillance data to compute charges based on the time

in the system and the type of service received.

While this would free users from financial transactions when they file flight plans, it would still

impose on the ATC system a requirement for

recording and billing user charges. In addition,

the universal requirement for a transponder

would be viewed by many owners of small GA

aircraft as an extreme form of regimentation. A

third possibility involves approximation of user

costs through a combination of fixed and variable assessments on aircraft owners: fixed

charges could be collected in the form of annual

taxes based on aircraft occupants (including

flight crew) according to aircraft characteristics

or type of use.

Operating Costs

A corollary problem that Congress will have

to deal with is how to meet the operating costs of

the system. (Many of the planned capital improvements are intended to reduce these costs in

the long term. ) If these costs are covered primarily by appropriations from general revenues (the

practice of many years), the taxpayers would be

subsidizing special services for a mode of transportation that only a few use directly, although

they may receive some indirect benefit. If paid

wholly or largely by disbursements from the

trust fund, as the Administration proposes and

many Members of Congress oppose, the pressures on the trust fund would be greatly intensified. Over two-thirds of the FAA’s annual budget goes to meet operating costs, but disbursements from the trust fund have covered only

about 15 percent of these expenses in the past.

To take a more substantial portion of operational expenses from the trust fund, as it is presently structured, would exhaust the current surplus in a very short time. To prevent this, and at

the same time provide for needed capital investments, the taxes supporting the trust fund would

have to be increased to yield significantly more

revenue than contemplated by any of the legislative proposals before the Congress at this time.

A tax increase of this magnitude would raise all

of the issues cited earlier in connection with capital funding options and greatly exacerbate the

conflict among the various stakeholders in the

aviation community.

PENDING LEGISLATION

Areas of congressional interest in the airport

and air traffic control system include system

modernization (especially system automation

and the replacement of the en route computers),

airport development, trust fund usage, and user

charges. This section briefly reviews congressional activities in the past 2 years, outlines the

positions taken by various congressional committees on key issues, and identifies the major

legislation now before Congress.

System Modernization

Major capital expenditures like the en route

computer replacement have been the subject of

several congressional hearings and investigations. A recurring question has been the FAA’s

ability to plan and manage such a complex procurement.

In October 1980, the investigations staff of the

Senate Committee on Appropriations released a

report criticizing the FAA’s management of the

existing ATC computer system. The report cited

weaknesses in the reporting of equipment outages, a lack of planning, and the absence of a

well-defined approach to managing system operations and software changes. The investigators

recommended the Congress withhold funding

for computer replacement until the FAA had

demonstrated a better understanding of the capabilities and limitations of the existing system.
Ch. 7—Policy Implications ● 139

The report outlined specific actions FAA should

take to improve its performance and evaluation

methods. 3

After two sets of hearings on the safety aspects of computer outages, the House Committee on Government Operations raised many of

the same questions in October 1981. Their report found that the FAA’s management information system did not provide accurate data on

which to base important decisions about the reliability of the computer. The committee also

questioned the FAA’s ability to plan and manage

the development and procurement of a new

computer system. The report directed the

General Accounting Office (GAO) to initiate a

“comprehensive investigation of the FAA’s planning, management, and acquisition of automated information systems. ”4 The GAO final

report, due in October 1982, will cover FAA

planning and management for acquisitions in

three areas: ATC system automation, management information systems, and peripheral

equipment.

The Subcommittee on Transportation of the

House Committee on Science and Technology,

which has shown a continuing interest in the

ATC computer question, has stated that the current computer system needs to be replaced and

that unnecessary delay in doing so would pose

safety risks and increase the chances of further

breakdowns. in reviewing the alternatives for

replacing the system, the subcommittee’s report

of August 1981 favored a full modernization of

the computer system, as opposed to an interim

replacement followed by a long-range procurement. The full committee recommended that

FAA publish a management plan detailing the

costs, schedules, milestones, and funding plans

for the computer replacement.’

‘U.S. Congress, Senate Investigations Staff, FAA ErI Route Air

Traffic Co)~trol Computer System, submitted to the Subcommittee

on Transportation and Related Agencies, Committee on Commerce, Science and Transportation, Rpt. No. 80-5, October 1980.

‘U.S. Congress, House, Committee on Government Operations,

Air Traffic Cot~trol Computer Failures, Rpt. No. 97-137, June 11,

1981.

‘U.S. Congress, House, Committee on Science and Technology.

Subcommittee on Transportation, Aviation and Materials, Air

~ra~fic Co)ltrol EtI Route Cot)~pt/trr h~(~cft~rtli:(ltlc)tl, Rpt. No .

97-12, August 1981.

To give further emphasis to these findings and

recommendations subcommittee chairman, Representative Dan Glickman introduced H. Res.

202 in October 1981, which expressed the sense

of the House that FAA should consult with the

Committee on Science and Technology as it

develops plans for the future ATC system. It

also directed FAA to make regular reports to the

committee, commencing with a system description in December 1981 and a preliminary subsystem description in June 1982. This resolution

was passed by the House on October 19, 1981.

Airport Development Aid

The Federal role in airport development was

previously governed by the Airport and Airways Development Act of 1970, which expired

in October 1980 when the Congress could not

agree to new authorizing legislation. Projects extending into fiscal year 1981 were funded, but no

authorizations have been made for future years.

In writing new authorizing legislation in 1981,

the question of “defederalization” has been a

major issue. Defederalization would remove

large and medium hub airports from eligibility

for ADAP funding, on the grounds that these

airports generate enough revenues to be self-supporting without Federal aid.

The Senate version of the authorizing legislation, S.508, would make the top 69 air carrier

airports ineligible for airport development and

planning grants. The Administration position,

as contained in H.R. 2930 called for a more

modest defederalization measure, making the

top 42 airports ineligible for aid. These airports

would be permitted to impose a limited passenger facility charge (head tax) to make up lost

revenues (head taxes are currently forbidden at

all airports that have received Federal aid). The

report on S.508 by the Senate Committee on

Commerce, Science, and Transportation supports the defederalization concept and notes that

ADAP funds make up a fairly small proportion

of the total capital and operating budgets of

larger airports. If they were made ineligible, the

report points out, more Federal funds would be

available for small airports unable to generate

their own funds. Because the Senate bill limits
140 ● Airport and Air Traffic Control System

the total authorization to $450 million annually

for 5 years (1981-86), it is necessary to make

those funds available to those who need them

most.6

The House version of the authorizing legislation, H.R. 2643, contains no provision for defederalization, and members of the Committee

on Public Works who sponsored the House version have expressed opposition to the concept.

Questions of equity are involved: opponents of

defederalization are concerned that passengers

using major airports would have to bear a double tax—the Federal ticket tax in addition to any

local passenger facility charge. Further, the ticket tax on passengers at large airports already

generates the bulk of revenues in the Airport and

Airways Trust Fund, and it seems unfair to forbid these airports the use of those funds. The

House bill proposes a $450 million annual authorization for 3 fiscal years. ’

Trust Fund Usage

The uncommitted balance in the trust fund

(about $3 billion at the end of fiscal year 1981)

has long been a cause of controversy in Congress

and among users. The Senate Committee on

Commerce, Science, and Transportation attributes this balance to the fact that the OMB under

previous administrations has sought to keep

trust fund revenues high and expenditures low.

The current administration has proposed drawing down the balance significantly by funding 85

to 100 percent of the FAA’s operations and

maintenance costs out of the trust fund, in addition to capital costs. For example, the administration budget recommended financing expenditures such as aviation security and aircraft inspection from the trust fund. Both Senate and

House Committees on Appropriations,

however, have continued to allow these

regulatory and police functions to be funded

from general funds.8

‘U.S. Congress, Senate, Committee on Commerce, Science, and

Transportation, Report to Accompany S.508, Airport and Airway

System Development Act of 1981, S. Rpt. 97-97, May 15, 1981.

‘U.S. Congress, House, Committee on Public Works and Transportation, Report to Accompany H.R. 2643, Airport and Airway

Improvement Act of 1981, H. Rpt. 97-24 (Part II), May 19, 1981.

‘U.S. Congress, House, Committee of Conference for the Department of Transportation and Related Agencies for the fiscal

year ending Sept. 30, 1982, Conference Report to Accompany

H.R. 4209, H. Rpt. 97-331, NOV. 13, 1981.

The Senate Committee on Commerce, Science, and Transportation stated that the airport

and airway system provides benefit to the general public and therefore the general fund should

continue to contribute to its operation. g Although many in Congress agree that something

should be done to reduce the balance, some

Members feel that taking operating costs out of

the trust fund constitutes “raiding” the users’

funds, which were collected for the purpose of

improving the airways system, to subsidize activities that should be paid for out of general

revenues. The DOT appropriations bill for fiscal

year 1981, in both House and Senate versions,

appropriated funds from the trust fund to cover

about one-third of operating costs, about double

the average share of the past 10 years. H.R.

2643, as reported by the Committee on Public

Works, authorizes a ceiling of 50 percent on operating costs to be taken from the trust fund in

future years; S.508 authorizes a ceiling of about

one-third on operating costs to be taken from

the trust fund.

User Taxes

Current proposals for reestablishing the trust

fund call for no major changes in the user tax

structure. In general, the House, Senate, and

administration positions on user charges have

simply been differences in the level of tax in the

traditional categories:

● The administration proposal, embodied in

S. 1047, calls for the greatest increase in

user taxes. It differentiates between GA gas

taxes and GA jet fuel taxes, taxing gas at 12

cents per gallon (rising to 36 cents in fiscal

year 1986) and jet fuel taxes at 20 cents (rising to 65 cents). The passenger ticket tax

would be set at 6.5 percent, the waybill tax

at 5 percent, and an international facilities

charge of $3 per passenger would be authorized.

● Another bill, S. 1272, cosponsored by several members of the Senate Committee on

Commerce, Science, and Transportation,

calls for an 8.5 cent tax for all GA fuels, a 3

percent ticket tax, a 2 percent waybill tax,

and a $1 international facilities charge.

‘Senate Commerce, Science and Transportation, S. Rpt. 97-97

op. cit.
Ch. 7—Policy Implications ● 141

● A House bill, H.R. 4800, calls for a 12.5 sharp increases in user charges until some use is

cents tax on all GA fuels, a 5 percent ticket made of the existing balance.10

tax and a $5 international facilities tax. .

These measures are still under consideration

by the Senate Committee on Finance and House

Committee on Ways and Means, and it is uncertain how they will appear after committee markup. Part of the difficulty in reaching a decision

on the tax level is the current uncommitted balance in the trust fund and the unwillingness of

both past and present administrations to spend

the money for its specified purposes. Members

of the Senate Commerce Subcommittee on Aviation and the House Science and Technology

Subcommittee

and Materials

The uncertainty about the costs and timing of

future capital expenditures also clouds the

discussion of tax levels. The options appear to

be: 1) increase taxes to maintain a substantial

balance in the trust fund in anticipation of large

future expenditures, recognizing that the current

balance could not cover the proposed program

of system modernization; or 2) allow the trust

fund to be depleted, knowing that revenues will

have to be greatly increased later if these future

expenditures are to be paid for by user taxes.

on Transportation, Aviation,

have stated they do not favor ‘OAL,~utjOtl 13aily,

o

Nov. 19, 1981, p. 102.