Safer Skies with TCAS: Traffic Alert and

Collision Avoidance System—A Special

Report

February 1989

NTIS order #PB89-169221
Recommended Citation:

U.S. Congress, Office of Technology Assessment, Safer Skies With TCAS: Traffic

Alert and Collision Avoidance System–A Special Report, OTA-SET-431 (Washington, DC: U.S. Government Printing Office, February 1989).

Library of Congress Catalog Card Number 89-600725

For sale by the Superintendent of Documents

U.S. Government Printing Office, Washington, DC 20402-9325

(order form can be found in the back of this report)
prevent such tragedies. Over the past 8 years, these efforts have culminated in a

cooperative government-industry program to develop, evaluate, and implement a traffic

alert and collision avoidance system (TCAS II) for commercial aviation.

Eager to reap the benefits of this important safety technology, the 100th Congress

passed legislation requiring that most commercial passenger aircraft be equipped with

TCAS II by December 1991, or they would not be permitted to fly in U.S. airspace.

However, during the second half of 1988, questions arose about the safety implications of

the certification and implementation schedule for TCAS 11. The Subcommittee on

Aviation of the Senate Committee on Commerce, Science, and Transportation asked OTA

to assess these implications and report in early 1989. The study was endorsed by the

House Committee on Science, Space, and Technology, and the Subcommittee on

Investigations and Oversight of the Hous e Committee on Public Works and

Transportation.

This special report contains the results of OTA’s assessment. It also provides an

admirable example of cooperative effort on the part of all segments of the aviation

community in providing information to OTA and working to develop a common solution

for a number of difficult issues.

Throughout the study, the Federal Aviation Administration, the airlines, equipment

manufacturers, pilots, maintenance specialists, and airframe manufacturers played key

roles i n assisting OTA through interviews, site visits, and written comments. The

workshop participants and numerous reviewers and contributors provided a broad and

invaluable range of perspectives. OTA thanks all of them for their substantial

commitment of time and energy. Their participation does not necessarily represent

endorsement of the contents of the report, for which OTA bears sole responsibility.

JOHN H.- GIBBONS

Director

. . Ill
g

Robert Buley

Flight Standards

Northwest Airlines

Sherry Chappell

NASA Ames Research Center

Jim Deckert

Marketing Manager

Sperry Commercial Flight Systems Group

Honeywell, Inc.

Joe Fee

AC ADS Program Manager

Federal Aviation Administration

John Fredericksen

Executive Vice President

Regional Airline Association

John Graham

Senior Staff Engineer

Flight Guidance and Control Engineering

Douglas Aircraft Co.

Ulf Gustafsson

Staff Engineer

Engineering Program

United Airlines

Jack D. Howell

Chairman, Air Traffic

Control Committee

Air Line Pilots Association

Thomas McSweeny

Deputy Director

Aircraft Certification Service

Federal Aviation Administration

John O’Brien

Director, Engineering and

Air Safety Department

Air Line Pilots Association

Carl Olberg

Director for Engineering

America West Airlines

Richard A. Peal

Director

Avionics/Flight Systems

Boeing Commercial Airplane Co.

Ron Smith

Vice President, Operations

Tracer Aviation, Inc.

Ray Valeika

Senior Vice President

Technical Operations

Continental Airlines

Frank White

Honeywell

Joe Wilson

TCAS Program

Air Transport Avionics Division

Bendix/King

iv
, 

sss, O

Science, Information, and Natural Resources Division

Nancy Carson, Program Manager

Science, Education, and Transportation Program

Edith B. Page, Transportation Project Director

Kevin Depart, Principal Analyst

Karen Mathiasen, Research Assistant

Marsha Fenn, Administrative Assistant

Madeline Gross, Administrative Secretary

Kimberley Gilchrist, Secretary
Air Canada

Air France

Air Wisconsin

Air-India

Alaska Airlines

Alitalia

Allied-Signal Aerospace Co.

America West Airlines

American Airlines

Associated Air Center

Wayne J. Barlow,

Federal Aviation Administration

Boeing Commercial Airplane Co.

Braniff

British Airways

Anthony J. Broderick,

Federal Aviation Administration

Command Airways

Continental Airlines

Dalfort Aviation Services

Joseph M. Del Balzo,

Federal Aviation Administration

Delta Air Lines

Douglas Aircraft Co.

Eastern Air Lines

El Al Israel Airlines

Evergreen Air Center

Finnair

Roger Fleming, Air

Transport Association

L.R. Ganse, Trans World Airlines

Michel Guyard, French Embassy

Joseph Hawkins, Federal

Aviation Administration

Heli-Dyne Systems

Honeywell

Horizon Air

D.P. Huffman, American Airlines

Iberia

Larry James, Trans World Airlines

Japan Air Lines

KLM Royal Dutch Airlines

Keith H. King, international

Air Transport Association

Clyde Kizer, Air Transport Association

Hart A. Langer, Pan

American World Airways

Lufthansa German Airlines

Aviation Administration

Northwest Airlines

Pan Am Express

Pan American World Airways

Michael Parkes, British Embassy

Philippine Airlines

B. Keith Potts, Federal

Aviation Administration

Qantas Airways

Jack Raia, Pan American World Airways

Francis C. Rock, Federal

Aviation Administration

Rockwell International

Sabena Belgian World Airlines

Daniel P. Salvano, Federal

Aviation Administration

Saudi Arabian Airlines

Scandinavian Airlines System

Southwest Airlines

William J. Sullivan, Federal

Aviation Administration

Swissair

Tracer Aviation

Tramco

Trans World Airlines

USAir

United Airlines

vi
Part I: TCAS Development and the Federal Role . .00.00...000.....,0....0.0....00 . 3

BACKGROUND . . . . . . . . . . . . . . . . ● 3

TCAS is Chosen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ........5

TCAS CAPABILITIES AND COMPONENTS . . . . . ● . . ● . . ● . ● . . . . . . . . . . . . . ● . . ● .. ......6

TCAS II Components . . . . . . . . . . . . . . . . . . . . . ... . 7

THE FEDERAL ROLE . . . . . . . . . . . ● ● . . . . . . . . ● . ● . . . ● . ● . . . . ● . . . . ● ● . . . . . ● . ● . . ● ● .. .14

Setting Standards . .....0.0..000.0..0.0. 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Rule making .. *Oooo. e*OOO. *oO*. 17

Testing and Evaluation ... .. oooo. .o. .. o.oo*. oooOooe ... 19

REMAINING CONCERNS 22 2

DEVELOPMENT FINDINGS . ● . . . . . . . . . . . . . . . . . . . . . . ● . . . . . . . . . . . . . ● ● . . . ● . .......23

Part II: Implementing TCAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......28

INSTALLATION OVERVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......29

INSTALLATION ISSUES . . . . . . . . . . ● . . . . . . . . ● . . . . . . . . . . . . . . . . . . . . . . . . . . . . ● .. ....31

TCAS II Equipment Manufacturing and Initial Certification . . . . . . . . . . . . . . . . . . . 31

Follow-up Certification of Airliner Modifications . . . . . . . . . . . . . . . . . . . . . .......32

Installing TCAS II .o. oo. ..*. **o* *o. ... ..*. *0.0 *.. ..o*o*o *.** **** o*. .*. .* e 33

Technical Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......35

IMPLEMENTATION FINDINGS . . . ● . . . . . . . . . . . . . . . . . . . . ● . . . . . . . . . . . . . . . . . . . . .. ..36

Part 111: Conclusions and Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......40

THE ISSUES . . . . . . . . . . . . . . . . . . . . . . . . . . . ● . . ● . . . . . . . . . . . . . . . . . . . . . . . . . . . .......40

Safety and Technical Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .....40

Economics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......42

International Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

CONCLUSIONS AND OPTIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......44

List of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......51

vii
p

FIGURES

Figures

1 TCAS Protected Airspace ,.***** 8

2 Basic Components of a TCAS II System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 TCAS II Resolution Advisory Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Integrated Traffic and Resolution Advisory Display . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 TCAS II Traffic Display . 13

6 TCAS Implementation Flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

7 U.S. Fleet Equipped with TCAS and Possible Timing Options . . . . . . . . . . . . . . . . . . 48

TABLE

Table

1 Effects of TCAS Options ... **** ..*. .e*. ..** .**. *.** *.** *.. **. .*. .*m ***m* * 50

. . . VIII
The compliance deadline established in Public Law 100-223 for implementing the

Traffic Alert and Collision Avoidance System (TCAS II) has safety, economic, and

international consequences not fully foreseen at the time of enactment. The TCAS II

program is unique in the combination of technological complexity, rapid introduction, and

the number of aircraft affected. OTA finds that aviation safety will be best served by

introducing TCAS II on commercial aircraft as soon as possible, by requiring a phased

implementation schedule, and by providing for a structured evaluation program carried

out jointly by industry and the Federal Aviation Administration (FAA) to oversee the

first year of operation.

While evaluations to date have indicated that TCAS II works quite well, no more

than two TCAS II equipped commercial aircraft have flown at any given time. The

reactions and interactions of pilots, controllers, and TCAS II within the air traffic system

cannot be understood until large numbers of aircraft equipped with TCAS are in

operation. OTA concludes that to ensure full safety benefits, a critical number of

aircraft must be outfitted with TCAS II at an early date – possibly 15 to 30 percent (600

to 1,200 aircraft) of the commercial fleet by December 1990. Industry and FAA will

need to cooperate in an evaluation that covers the spectrum of aircraft and airspace

types and allocates sufficient resources for collecting, analyzing, and disseminating data.

Public Law 100-223 requires that airlines meet a December 30, 1991, installation

deadline for TCAS II. This will strain the resources of virtually every participating

aviation organization. Manufacturers must produce and deliver equipment, airlines and

others must redesign and modify aircraft, and FAA must certify equipment and altered

aircraft. While many major airlines can probably meet the deadline, other critical

maintenance and modification programs are likely to suffer. Airlines will have to

remove about one-half of their fleets from scheduled service for at least a few days to

install TCAS II. Faced with limited numbers of skilled technicians and engineering and

maintenance resources, airlines plan to contract out TCAS II work, use overtime, cut

discretionary maintenance, and petition for exemptions from other maintenance

requirements, such as inspection and modification of aging aircraft. Development is still

1
of installing TCAS II. Airlines without extra aircraft are likely to lose passengers to

other airlines for a short time, whereas companies with more resources can avoid

canceling service. Airlines that fail to meet the deadline will be penalized severely if

unequipped aircraft are not permitted to fly in U.S. airspace in 1992. However, airlines

that complete TCAS II installations on time will face indirect cost penalties if their

competitors do not commit similar resources and are granted extensions.

Although OTA finds no reason to delay initial TCAS II implementation, sufficient

airline resource limitations, economic inequities, and international implications stem

from the present deadline for Congress to consider extending the installation schedule.

If an extension is enacted, specific requirements in the same law will be needed to ensure

that installation of TCAS II begins promptly after production equipment is available and

proceeds at a reasonable pace over the span of any extension. Prompt congressional

consideration of any change to Public Law 100-223 is also important. Indeed, the forcing

effect of legislation is likely to be necessary to ensure maximum safety benefits as early

as possible and to allow airlines to make appropriate plans for investments in personnel

and equipment. Although requiring and linking an operational evaluation program, a

phased compliance schedule, and an extended deadline places additional responsibilities

on each affected party, this approach spreads economic burdens more equitably than

other possible options and provides maximum safety benefits.

2
then walk, then jog, then run.

— J. Lynn Helms, former FAA Administrator in announcing TCAS

BACKGROUND

Pilots are and always have been the first and foremost collision avoidance system.

As pilots and aircraft became capable of flying “blind” by instruments in the 1930s, the

need for air traffic control (ATC) and coordination increased. In the early days of

commercial aviation (and to this day in oceanic airspace), air traffic was controlled

procedurely, through reserved sections of airspace and radio reports from pilots verifying

their positions. It was not until the development of radar during World War II that

surveillance technology became available to assist air traffic controllers. By 1955 radar

was in use at 2 of the 20 ATC enroute centers, and direct controller-pilot radio

communication facilities had been established at all of them. 1 Most military and airline

aircraft operated under visual flight rules, and the opportunities for collision multiplied

as air traffic increased. In the wake of a catastrophic midair collision between two

commercial transports over the Grand Canyon in 1956, the airline industry began the

first concerted effort to develop an airborne collision avoidance system

now, over 30 years later, are we on the verge of seeing the fruits

development that has continued amid industry and technical controversy.

(CAS).

2 Onl y

of technology

1. Nick Komons, Aviation’s Indispensable Partner Turns 50, (Washington, DC: U.S.

Department of Transportation, 1986), pp. 12 and 14.

2. Frank C. White, “Where Does Aircraft Separation Assurance (or CAS) Now Stand?:

An Historical Perspective

before the Subcommittee

Committee on Science and

24, 1979 (Washington, DC:

and Project ion, f

’ in Aircraft Collision Avoidance, hearings

on Transportation, Aviation and Communications of the

Technology, U.S. House of Representatives, June 27, 28; July

U.S. Government Printing Office, 1979), p. 360.

3
Aviation Administration (FAA) followed and supported early airborne CAS efforts, it did

not begin directly developing and evaluating collision avoidance technology until 1971,

after congressional hearings on aircraft collisions. The Agency conducted a

comprehensive evaluation of three different systems, collectively known as the Airborne

Collision Avoidance System, between 1971 and 1975. FAA concluded that while the

systems gave

density areas.

equipment be

good protection in some airspace, they had severe limitations in highAdditionally, these systems required that dedicated collision avoidance

installed on all aircraft. 4 Moreover, the establishment of ATC Terminal

Control Areas and expanding computer automation, including conflict alert, made the

ATC system much more versatile than existent airborne collision avoidance technology.

Every CAS devised for commercial

installed on each aircraft to be protected

common ATC transponders installed on all

aircraft requires compatible equipment

or avoided. Using the radar signals from

commercial and military aircraft and the

majority of the private

dedicated systems. First

system (BCAS) relied on

protection from all other

were invisible to BCAS.

fleet would eliminate the need to equip all aircraft with

demonstrated in 1974, the beacon-based collision avoidance

transponder replies for traffic data, immediately providing

transponder equipped aircraft. Aircraft without transponders

However, BCAS development ran into difficulties. Selfcontained airborne versions caused too much radio interference in high-density airspace,

and solving that problem required expensive ground coordination equipment. 5

3. Komons, op. cit., footnote 1, pp. 22.

4. Neal A. Blake, Acting Deputy Associate Administrator for Engineering and

Development, Federal Aviation Administration, in testimony, Aircraft Collision

Avoidance, op. cit., footnote 2, pp. 59-60.

5. U.S. Congress, Office of Technology Assessment, Airport and Air Traffic Control

(continued)

4
research, developing prototype equipment, demonstrating the operational and technical

feasibility of the TCAS concept, generating national standards for the equipment, and

certificating TCAS-equipped aircraft for normal operation. TCAS is designed to:

●

●

●

●

The

be compatible with the present ATC system and a logical extension of it;

be suitable for use in high-density traffic;

require no ground-based equipment;

offer a range of capabilities suitable to the needs of various classes of

airspace users. 6

1987 Airport and Airways Capacity Expansion and Improvement Act, Public

Law 100-223, established deadlines for completing development and installing the system

known as TCAS II on commercial transports. By June 30, 1989, FAA must approve and

validate the TCAS II performance standards. FAA finished its regulatory requirements

for development on time in October 1988. The remaining FAA responsibility for

establishing TCAS II is to test and evaluate TCAS II equipment that meets the latest

standards. This testing is now scheduled to begin at the FAA Technical Center in early

7 Each passenger-carrying aircraft with April 1989 and to be completed by Summer 1989.

more than 30 seats must be equipped with TCAS II to operate in U.S. airspace after

December 30, 1991.

System, OTA-STI-175 (Washington, DC: U.S. Government Printing Office, January 1982),

pp. 89-90.

6. Ibid., p. 91.

7. Joseph Fee, ACADS Program Manager, Federal Aviation Administration, in U.S.

Congress, Office of Technology Assessment, “Transcript of Proceedings — Getting

Collision Avoidance Airborne: TCAS Installation and Federal Deadlines,” unpublished

typescript, Jan. 12, 1989.

5
beag, ad wepossbe, attude oeaby acat ad aetg te cew to

conflicting traffic. To serve the varied needs of the aviation community, three versions

of TCAS — TCAS I, TCAS II, and TCAS III – are being developed, each with distinct

performance characteristics.

Prototypes of each version have been flight-tested. Designed for airline use and

farthest along in development, TCAS II is the system addressed in Public Law 100-223.

TCAS I, appropriate for general aviation and smaller commuter airlines, will be required

along with TCAS II under FAA rule making. TCAS III, the most complex and sophisticated

version, will probably not be fully specified until at least 1992. 8 In any case, TCAS III

may be subject to separate rulemaking procedures.

TCAS I. Primarily intended to assist the pilot in visually acquiring nearby traffic,

TCAS I is the simplest and least costly TCAS. TCAS I detects and displays range,

approximate bearing, and altitude of traffic that is equipped with a Mode C or S

transponder 9 within 4 nautical miles of the host aircraft. Traffic equipped with Mode A

transponders is displayed without altitude information. TCAS I alerts the crew with a

visual and aural traffic advisory to any intruding aircraft within about 40 seconds of

closest approach. TCAS I does not offer guidance to the pilot for maneuvering away

from potential collisions.

8. John M. Graham, Chairman, Special Committee 147 of the Radio Technical

Commission for Aeronautics, personal communication, Feb. 7, 1989.

9. Three versions of air traffic radar transponders — Modes A, C, and S — are used by

civilian aircraft. Mode A transponders reply to radar interrogations with a four-digit

identification code. Mode C equipment includes the aircraft’s altitude in the reply. The

signal format for the newest transponder type, Mode S, allows radar interrogations and

other information to be addressed to specific aircraft.

6
Mode C or S (altitudeencoding transponders) equipped and is typically less than 25

second s

10 from a potential collision. Figure 1 depicts TCAS II protected airspace.

TCAS II RAs are restricted to the vertical plane. Through Mode S air-to-air data links,

TCAS II coordinates with other TCAS II-equipped aircraft to fly complementary

avoidance maneuvers.

TCAS III. Not yet addressed directly in legislation or rulemaking, TCAS III will

have all the features of TCAS II and will offer horizontal resolution maneuvers as well.

To resolve conflicts with horizontal turns, TCAS III will measure the bearing of targets

more accurately than required for TCAS II.

TCAS II Components

Each TCAS II unit is effectively a small, but versatile ATC-type radar station,

consisting of a computer processor and software, a directional antenna system, a Mode S

transponder, and cockpit displays, indicators, and controls. (See figure 2.) Although

some TCAS II equipment options are still being developed, the principal features of the

components as presently defined are described below.

Processor. The heart of each TCAS II is its processor, which contains the hardware

and software for connecting all the components. The processor transmits and receives

radar signals through the antennas, measures range, bearing, and altitude of nearby

traffic, watches for conflicts, computes escape paths if necessary, and sends this

information to the cockpit indicators. During an RA, the processor coordinates the

maneuver through Mode S transponder datalink if the other aircraft is TCAS II equipped.

10. Resolution advisory sensitivity varies with own aircraft altitude: 20 seconds below

2,500 feet above the ground; 25 seconds between 2,500 feet and about 10,000 feet; and 30

seconds above 10,000 feet.

7
Figure 1.–TCAS Protected Airspace

SOURCE: Office of Technology Assessment.
Mode S

transponder

Top directional antenna
antenna. The TCAS II processor does not use bearing information in generating RAs, so

directional antennas are needed only for cockpit traffic displays and for reducing radar

interference.

Cockpit indicators. TCAS II provides two types of traffic-related information to

the cockpit: 1) a representation of nearby traffic and its status, and 2) resolution

advisories to prevent potential collisions. Each airline, depending on its aircraft types

and cockpit configurations, will have a number of options for displaying TCAS II

information to the flight crew.

TCAS II will provide aural and visual advisories for all aircraft configurations.

Visual RAs will be presented on modified instantaneous vertical speed indicators (IVSIs)

for most existing aircraft. Red and green arcs appear during an RA, indicating vertical

speeds to avoid (red) and to fly safely (green). (See figure 3 for an example. ) Still under

development are the RA indicator formats for “glass” cockpits, where many instruments

are displayed on cathode ray tube (CRT) systems.

Four basic display options to indicate traffic location and threat status will be

available. Airlines may install a dedicated TCAS II traffic display, modify weather radar

display or an electronic flight instrument system, or replace the IVSI with one that will

not only indicate RAs, but will also present traffic on a liquid crystal display in the

center of the IVSI dial. Figure 4 shows a combined traffic display and IVSI, and figure 5

represents a traffic display for a CRT system. Human factors consideration are of

crucial importance in the design of these links between TCAS II and the flight crew.

110 Federal Aviation Administration, “Airworthiness and Operational Approval of

Traffic Alert and Collision Avoidance Systems (TCAS II) and Mode S Transponders, ”

Advisory Circular 20-131, Oct. 3, 1988, p. 18.

10

TCAS II traffic display symbols

● Within 6 nautical miles and 1200 feet

relative altitude
13
THE FEDERAL ROLE

Much of the basic research and fundamental technology development used in the

FAA’s TCAS II program was completed in earlier collision avoidance projects. These set

the stage for current Federal efforts requiring TCAS II on some categories of aircraft.

In coordination with industry, three interrelated Federal activities to establish TCAS II

have proceeded in parallel: setting national standards defining TCAS II; mandating TCAS

II implementation through rulemaking; and testing and evaluating TCAS II technology.

Setting Standards

The characteristics of aircraft equipment covered under the Federal Aviation

Regulations are usually defined by national standards published in Technical Standard

Orders (TSOs), the “. . . minimum performance standard for a specified material, part,

,,13 process, or appliance. FAA has approval authority for standards governing aviation

system designs. The Agency works in consort with members of the aviation community

to establish the standards, often incorporating directly the findings of independent

committees such as the Society of Automotive Engineers or the Radio Technical

Commission for Aeronautics (RTCA). FAA does not specify design specifications in a

TSO, but states the minimum performance requirement for the equipment and grants

TSO “authorization” to manufacturers of articles that meet the TSO.

12. John O’Brien, Director, Engineering and Air Safety Department, Air Line Pilots

Association, in Office of Technology Assessment, op. cit., footnote 7.

13. 14 CFR 21.601 (Jan. 1, 1988)

14
epaatoote OS.) e buote OS

have been revised a number of times as the result

However, the latest revision of the MOPS, referred

deawtcoputeagots ad

of analyses, simulations, and tests.

to as “Change 6," has not yet been

approved by RTCA; consequently FAA's TSO is based also on an FAA report prepared by

MITRE Corporation, 15 which established Change 6.

Also in October 1988, FAA released an Advisory Circular (AC) 16 which provides

guidance for the airworthiness and operational approval of TCAS II. An AC is not

mandatory, but following its guidance ensures compliance with the Federal Aviation

Regulations.

BOX A: Minimum Operational Performance Standards for TCAS

RTCA, established in 1935 to solve aviation problems involving electronics and

telecommunications, is the joint government/industry advisory committee that is

developing and recommending MOPS for TCAS I, II, and III. RTCA recommendations are

usually incorporated directly into TSOs or otherwise accepted by FAA.

The MOPS for TCAS II are the most mature, first published in 1983 and then

followed by a series of changes. Since a large part of the MOPS deal with TCAS

computer instructions, such as resolving conflicts and coordinating maneuvers between

aircraft, software changes to fix problems or enhance performance are not unusual.

U.S. Department of Transportation, “Traffic Alert and Collision Avoidance System

~’;6AS) Airborne Equipment, TCAS II,” TSO-C119, Oct. 14, 1988.

15. Federal Aviation Administration, "Required Modifications to the Traffic Alert and

Collision Avoidance System (TCAS II) Minimum Operational Performance Standards

(MOPS)," DOT/FAA/SA-88/3, October 1988.

16. Federal Aviation Administration, op. cit., footnote 11.

15
qy y

p

include a Change 7 to enhance TCAS II.

Recently, two United and two Northwest jets were outfitted with pre-production

TCAS II units, incorporating MOPS Changes 1 to 5, for operational evaluation under a

limited installation program (LIP). At the same time, work continued on Change 6 to

incorporate some changes identified before these evaluation flights began. The main

issue was that once TCAS II issued an RA, if the other aircraft changed its path, TCAS II

would not be able to resolve the new conflict and would issue a TCAS “invalid” warning,

leaving the flight crew to fend for itself. Change 6 removes the invalid option and

permits TCAS II to calculate additional maneuvers if the initial RA is not sufficient.

Change 6 also biases against maneuvers that cross through (instead of staying above or

below) the other aircraft’s altitude and simplifies logic for air-to-air TCAS II

coordination.

Findings from the LIPs 17 suggest further TCAS enhancements, including reducing

the low-altitude traffic alert rate during approaches and in areas with many Mode A

targets. By November 1989, RTCA plans to finish Change 7 addressing the LIP results.

This version of the MOPS will be used for equipment purchased by most airlines. RTCA

plans to make the MOPS compatible with the international collision avoidance standards

now being reviewed by the International Civil Aviation Organization (ICAO) by the end of

1990.

END BOX

17. United Airlines completed its limited installation program (LIP) in October 1988;

Northwest Airlines will report on its LIP in May 1989.

16
18 months and mandated that each aircraft capable of carrying more than 30 passengers

have TCAS II installed and operating in the subsequent 30 months. This implied a

December 30, 1991, deadline for TCAS II installation and implementation for domestic

and foreign aircraft operating in U.S. airspace.

Public Law 100-223 also required FAA to promulgate a final rule expanding

requirements for aircraft to be equipped with Mode C (altitude encoding) transponders.

In response, FAA adopted Amendment 91-203, “Transponder Automatic Altitude

Reporting Capability Requirement, ” in June 1988, 18 requiring Mode C transponder use

within and above each terminal control area (TCA) and airport radar service area; within

30 miles of a TCA, and above 10,000 feet above mean sea level. Additionally, Public

Law 100-223 requires that TCAS II be “upgradable” to the performance standards of the

future TCAS III, although these are still being developed. FAA’s final rule for TCAS

states that other than air-to-air coordination logic, TCAS II may have a variety of

designs, and TCAS III may be addressed through separate rulemaking. 19

Prior to enactment of Public Law 100-223, FAA had issued Notice of Proposed

Rulemaking (NPRM) 87-820 intending to require either TCAS I or TCAS 11 on various

classes of passenger aircraft. Public Law 100-223 was generally similar to the NPRM,

which proposed a 3-year deadline for TCAS II implementation on large passenger

transports. At the time Public Law 100-223 was passed, the final rule was expected to

be released in late 1988; it was actually issued in January 1989.

18. 53 Federal Register 23356-23374 (June 21, 1988).

19. 54 Federal Register 944 (Jan. 10, 1989).

20. 52 Federal Register 32268-32277 (Aug. 26, 1987).

17
passenger aircraft from TCAS II to TCAS I and extended the TCAS I compliance date

from 5 years to 6 years. 21

BOX B: Certification — A Complicated Process

Public Law 100-223 requires FAA to complete “. . . certification of the collision

avoidance system known as TCAS-II . . .“ by June 30, 1989. The law’s intent was to

ensure TCAS validation, authorization, and implementation i n a timely manner.

Although FAA certification results in authorization, FAA can approve equipment

standards, such as those for TCAS II, without certification. Additionally, FAA may

formally approve equipment performance standards, before those standards are tested

and evaluated on an aircraft in flight. Thus, FAA certification as required in Public Law

100-223 is open to interpretation.

FAA certificates the major components of the aviation system —

pilots and mechanics, aircraft, and organizations, such as airlines and

Through these categories of certification, FAA approves aircraft design

people, such as

repair stations.

and production,

operations, and airworthiness. For example, each specific design or make and model of

airframe, engine, and propellor is manufactured under a unique Type Certificate

(TC).

2 2 Altering an aircraft’s design in a way that could affect flight safety, such as

installing TCAS, requires obtaining an amended TC or a Supplemental Type Certificate

(STC).

23 Extensive design changes require a completely new TC.24

●

21. 54 Federal Register 941 (Jan. 10, 1988).

22. 14 CFR Part 21, Subpart B (Jan. 1, 1988).

23. 14 CFR 21.113 (Jan. 1, 1988).

(continued)

18
ggp

airline that receives an STC for one B727-200 will be able to use that approved procedure

for modifying the rest of its 727-200 fleet, provided all its 727-200s are the same.

However, additional engineering work and FAA approval are required to address

individual differences among aircraft within a single make and model category.

Moreover, because approved production equipment will not be available, none of the

numerous varieties of aircraft equipped with TCAS II can be certificated before July

1989 at the earliest. Complicating the process, most aircraft types in airline fleets are

slightly dissimilar.

STCs are proprietary, but could be shared or sold to other organizations, although

doing so would require time-consuming and costly coordination. New aircraft types will

likely have TCAS II installations covered by TCs.

END BOX

Testing and Evaluation

Much of the basic collision avoidance technology used for TCAS was developed

during the past two decades by FAA at its Technical Center and by its contractors, the

MITRE Corporation and the Lincoln Laboratory of the Massachusetts Institute of

Technology. As for TCAS itself, limited numbers of TCAS II systems have been operated

on scheduled airline flights since 1987, TCAS I will be evaluated in the operational

environment later this year, and development testing is ongoing for TCAS III at the

FAA Technical Center.

24. 14 CFR 21.19 (Jan. 1, 1988).

19
types of data were collected in each program — electronic output from the TCAS II

equipment and comments from pilots and other observers. These programs included:

Piedmont Phase I. Collision avoidance equipment developed under the BCAS

program was modified to incorporate some TCAS elements and installed on two Piedmont

Airlines B727 aircraft. These aircraft were flown in scheduled service from November

1981 to March 1982. The purpose of Phase I was to measure TCAS II performance; flight

crews could not see or use any TCAS II-generated information. 25

Piedmont Phase II. TCAS II prototype equipment was first operated in regular

airline service in this program, whose purpose was to assess the effects of TCAS II on

both the flight crew and the ATC system. However, the TCAS equipment, built by

Dalmo Victor/Singer prior to full development of the MOPS and Aeronautical Radio, Inc.

(A RI NC) characteristics, lacked many of the capabilities of present systems. While the

pilots had TCAS displays in the cockpit, they could use the information only in visual

flight conditions. Additionally, the equipment lacked Mode S capability and could not

coordinate with another TCAS-equipped aircraft. A single TCAS-equipped B727 operated

from March 1987 to January 1988. 26

Assisting flight crews in visually locating nearby aircraft was found to be a major

benefit of TCAS II, garnering positive acceptance by Piedmont's pilots. TCAS II had no

noticeable effect on ATC or on pilot workload. Higher than expected alert rates and

minor problems with aural and visual TCAS II information suggested numerous

25. 52 Federal Register 32271 (Aug. 26, 1987).

26. ARINC Research Corp., In-Service Evaluation of the Traffic Alert and Collision

Avoidance System (TCAS) Industry Prototype, prepared for the Federal Aviation

Administration, DOT/FAA/SA-88/2 (Springfield, VA: National Technical Information

Service, May 1988), pp. vii-x.

20
gp

pg 

(Change 5) and ARINC characteristics, the equipment used in the LIPs was intended to

match closely in performance and appearance the versions to be installed fleetwide. The

equipment operated on two United Airlines aircraft, a B737 and a DC8, from January

through July 1988, were built by the Bendix/King Air Transport Division of the AlliedSignal Aerospace Company. Honeywell teamed with Northwest Airlines for the ongoing

operational evaluation onboard two MD80s, which began in October 1988 and is expected

to be completed in March 1989.

The Bendix/United LIP found that TCAS II substantially enhanced air traffic safety,

and is highly desirable for routine airline operations, provided certain CAS logic changes

are made to prevent disruptive and unnecessary advisories. 28 Additionally, United

assessed TCAS II's readiness for full implementation. The final report raises concerns

about:

● integrating TCAS II into glass cockpit aircraft,

●

the lack of ramp test equipment for efficient installation testing,

●

the fact that no airline experience with CAS logic beyond Change 5

will precede certification,

● incorporating ICAO requirements into U.S. standards,

●

the engineering, mechanic, and facility resources required for full

fleet retrofit, and

27. Ibid., pp. x-xvi.

28. George K. Schwind et al., United Airlines, Inc., l~sum mary User Evaluation Report

on the Traffic Alert and Collision

Program, ” prepared for Bendix/King

vii.

Avoidance System (TCAS II) Limited Installation

Air Transport Avionics Division, October 1988, p.
an operational evaluation with a Bendix/King TCAS II with Change 6 in March 1989. 30

FAA is currently testing Change 6 in computer simulations and will conduct flight tests

in April 1989 at the Tech Center. The three TCAS II manufacturers will begin flight

tests and other certification procedures to obtain TSO and STC approval. These are now

scheduled for March 1989. FAA also expects to contract for a (31 to 60 seat) turboprop

commuter LIP by October 1989.3

1 The LIPs are analogous to walking for TCAS.

REMAINING CONCERNS

FAA and industry agree that closely monitoring the initial implementation of TCAS

II will help ensure adequate TCAS II, flight crew, and air traffic system performance.

FAA has established a TCAS II Transition Program to coordinate data collection and

analysis among industry and FAA certification, ATC, and the TCAS Program office. 32

However, the Agency has not yet clearly defined how the program will work, or what the

scope and timing of its efforts will be.

There is widespread agreement in the aviation community that cockpit human

factors and air traffic system effects need further attention. From the inception of

TCAS, pilots, airlines, and manufacturers have been concerned about possible human

factors implications of traffic displays, warnings, and maneuver advisories in the

29. Ibid., pp. 99-103.

30. Buzz Hefti, Allied Signal, personal communication, Feb. 9, 1989.

31. Joseph Fee, Federal Aviation Administration, in Office of Technology Assessment,

op. cit., footnote 7.

32. C.R. Melugin, Jr., Executive Director for Regulatory Standards, Federal Aviation

Administration, personal communication, Jan. 10, 1988.

22
p

safety and operation of the entire ATC system. While the air traffic system can be

modeled to include TCAS on a simple basis, the human dimension escapes prediction.

Using past and predicted traffic patterns and TCAS detection and avoidance algorithms,

the number and extent of TCAS alerts, warnings, and conflict resolution maneuvers can

be studied along with the potential for electromagnetic interference. However, pilot and

controller performance could change due to TCAS, ranging from complacency to

interference with normal duties. The following issues need to

1) changes in the amount of pilot/controller communications; 2)

to other duties due to workload or complacency; and 3) the

reacting to TCAS information outside design boundaries —

without an RA or over/underflying an RA.

be more fully addressed:

pilot/controller attention

effect of pilots using or

maneuvering in traffic

These issues cannot be resolved until TCAS is implemented widely; if a problem

requiring TCAS modification exists, it must be uncovered early if changes are to be

effected economically. An early implementation period and evaluation program

(equivalent to jogging in Administrator Helms’ statement) could accomplish this. The

present schedule for TCAS implementation is unusual in that new technology will be

introduced to the full air transport fleet over a short timespan.

DEVELOPMENT FINDINGS

TCAS II technology has been proven feasible and is sufficiently developed to justify

Federal actions requiring airline implementation. Pre-production TCAS II technology has

been successfully demonstrated, and airline evaluations to date have uncovered no

23
FAA has approved the minimum performance standards for TCAS II, and if all goes

well, will complete simulation and flight test validation by

“certifying” TCAS by the deadline set in Public Law 100-223.

TCAS software required by FAA, known as Change 6 to the

extensively in computer simulations and will be flown at the

June 30, 1989, thereby

The last revision of the

MOPS, is being tested

FAA

beginning in March 1989. No problems that would prohibit approval

anticipated.

OTA concludes that an evaluation program that includes early

Technical Center

and validation are

implementation of

TCAS in a substantial portion of the fleet would benefit safety. Without such a program,

the worst case scenario is that the airlines could completely outfit their fleets only to

learn that a technical glitch requires major modification of the current TCAS

equipment. A structured evaluation phase would allow problems to be identified early,

preventing further installation of flawed units and permitting modifications soon enough

in the installation program to minimize costs.

In the best case, TCAS II works perfectly

will not take delivery of TCAS II equipment

in all respects. However, most airlines

until 1991 (see page 35) unless early

implementation is required. A monitoring program requiring early implementation for

part of the fleet could provide added protection to a portion of the traveling public

earlier than it would otherwise receive. As part of the program, industry and FAA will

want to consider ways to incorporate modifications identified through the evaluation.

According to LIP findings, software modification is desirable; however, only some

of the changes will be addressed in the baseline TCAS II requirements established by

FAA. The FAA position is that Change 6 is sufficient for safety, and no information has

24
the necessary information to support TCAS II modification decisions for all parties

involved.

OTA

program is

concludes that a basic requirement for a successful operational evaluation

having a critical mass of aircraft outfitted with TCAS II at an early date. If

15 to 30 percent of the commercial fleet (about 600 to 1,200 aircraft) were equipped

with TCAS II during 1990, a reasonable operational evaluation of system effects would be

possible. Operations under the evaluation should cover the full spectrum of geographical

locations and aircraft and airspace types, including sufficient numbers at hubs to address

high-density issues. FAA and industry must cooperate to plan and allocate sufficient

resources for collecting, analyzing, and disseminating TCAS data. A wide range of

expertise is required, including certification, air traffic, aviation medicine, safety, and

TCAS program officials from FAA, airlines, TCAS and aircraft manufacturers, pilots’ and

controllers unions, and aviation human factors experts from NASA.

Although some TCAS II technology is still being developed, this need not prevent

introducing TCAS. The major technology concerns that remain unresolved include:

● Displays: Only two display option types, the dedicated display and modified

weather radar, have been flight tested. The combination traffic display/IVSI

incorporates liquid crystal/flat panel technology that is new to commercial

aviation. The small size

display/IVSI is opposed by

concerns. Some new glass

available — modifying the

and combination of functions in the traffic

ALPA, whose members cite human factors

cockpit aircraft have only one display option

CRT systems. The display modifications for

25
● Ground test equipment: While not required for TCAS, acceptable ground

test equipment can reduce or eliminate flight test requirements and

expedite installation check-out. Such equipment may prove necessary to

meet the installation deadline, and none is yet available.

● TCAS II for commuter aircraft: Initial production versions of TCAS II may

not fit in some Part 121 commuter aircraft (31 to 60 passengers) and

questions remain about the effect the propellers and high wings that

characterize most commuter aircraft will have on TCAS signals. The results

of FAA testing, scheduled for late 1989, may come too late to give

commuter airlines any reasonable chance of meeting the installation

deadline.

Public Law 100-223 requires TCAS II systems to be upgradable to the performance

standards for TCAS III. These performance standards give TCAS III a more accurate

surveillance capability and an alternative escape maneuver selection in the horizontal

plane. Even though these performance standards are currently under development, a

number of common elements between TCAS II and TCAS III have been identified. Two

manufacturers are advertising their TCAS II units as upgradable. Thus, it can be assumed

that there will be some hardware and software commonality between TCAS II and

33. Ulf Gustafsson, United Airlines, personal communication, Feb. 8, 1989.

26
no reason to expect one manufacturer's TCAS II components to be compatible with

another's, except for air-to-air coordination logic. Presently, there are no regulatory

requirements for TCAS III.

27
collision avoidance system implemented . . .

I would say tomorrow. . . . Dealing with what we

now have, 1 cannot vote yes for the ’91 deadline.

— Ulf Gustafsson, Staff Engineer, United Airlines, OTA Workshop

The commercial aviation industry fosters and adopts technological advances.

Nonetheless, the proposed TCAS II implementation is unique in the combination of

technological complexity, rapid introduction, and the number of aircraft affected. The

introduction of technology such as jet engines, radar, or electronic cockpits pale in

comparison since they arrived gradually over many years. Based on present airline plans,

the proportion of the fleet equipped with TCAS II may go from less than 10 percent to

over 80 percent in a 12-month period.

The closest analogy to this rapid introduction of a complex new technology is

probably the ground proximity warning system (GPWS) requirement. Following a series

of accidents in which airplanes flew into the ground — controlled flight into terrain or

(C FIT) accidents — and congressional pressure, FAA issued a rule in December 1974,

allowing U.S. airlines 1 year to outfit their fleets with electronic devices that warn of

impending collisions with the ground. GPWS technology was sufficiently mature, but the

program was initially plagued by technical problems, including excessive false alarms

that eroded pilot confidence in the equipment. FAA had to extend the deadline by 6

months, and some airlines still did not comply until the end of 1976. 34 However, t h e

safety benefits out weighed these problems — the C FIT rate plummeted. Subsequent

crashes were caused by pilots who ignored or turned off the GPWS.

34. Edmund Preston, Troubled Passage: The Federal Aviation Administration During

the Nixon-Ford Term (Washington, DC: U.S. Government Printing Office, 1987), pp. 156-

158.

28
ypp

y g y y 

TCAS II design will meet the basic technical performance standards. Attention is

increasingly focusing on the higher order or “system" effects of TCAS II, such as its

influence on pilots and air traffic controllers (human factors) and the air traffic system.

Most troublesome system effects could be identified within a few months under a

structured operational evaluation program.

INSTALLATION OVERVIEW

Adapting TCAS II to the complex and diverse U.S. and worldwide transport fleets

will require dedicated efforts by avionics and airframe manufacturers, FAA, NASA,

ICAO, industry/government advisory groups, and most importantly, the airlines. The

airlines and their contractors must redesign each aircraft presently in their fleets to

accept additional new antennas, wiring, computers, and cockpit instruments and displays

and complete the installations by December 30, 1991.35 Each aircraft

such as the 13727-200, will require about 1,000 hours of engineering

configurations of a given type and model will require additional retrofit

example, United has 6 aircraft types, but will need about 14 STCs).

type and model,

work .

36 Othe r

engineering (for

Aircraft design

changes, such as those needed for TCAS II, must be approved by FAA under the STC

process. The first STC for each manufacturer's TCAS II will require extensive testing

and analysis. Other aircraft types must have any differences in aircraft configuration

35. The airframe manufacturers have taken responsibility for redesigning in-production

and future aircraft for TCAS.

36. Ulf Gustafsson, Staff

Assessment, op. cit., footnote

Engineer, United Airlines, in Office of Technology

7.

29
Airlines may begin installing some provisions for TCAS II in advance, and many are

planning to do so since the industry, through ARINC, has defined the form, fit, and

function specifications for TCAS II, standardizing the size of components, the

interwiring, and the location of plugs and connectors. Provisions include building

equipment racks, cutting holes for antennas, 37 running wire bundles, and reconfiguring

cockpits, as necessary. 38 Final provisions cannot be installed until STCs are granted

following the delivery of production Change 6 TCAS II equipment in late 1989. Change 7

equipment will not be available until early 1990.

Modifying each aircraft and installing the TCAS II equipment will take 500 to 1,000

hours of labor, depending on the skills and experience of the technicians and the aircraft

type and configuration. 39 The U.S. airline industry will need about 1,000 additional

technicians 40 to meet the TCAS II workload without overtime or cutting back on other

maintenance. TCAS II installation activities alone will require each aircraft to be

grounded for about 5 days, 41 although these will not necessarily be consecutive days.

Each installation must be tested to ensure proper operation. The airlines expect to

check out TCAS II on the ground. No test equipment is yet available, although two

manufacturers have said they can provide it in early-1990. 42 Each TCAS II-outfitted

37. Airlines are expecting engineering data from manufacturers within the next few

months establishing TCAS antenna locations for existing aircraft types.

38. At least three traffic display options are available, and few airlines have made

final decisions.

39. OTA data; and Office of Technology Assessment, op. cit., footnote 7.

40. Ibid.

41. Page Avjet has stated it can commit to accomplishing TCAS retrofit during four

overnight stays of the aircraft. Joe Wilson, Bendix/King, personal communication, Feb.

7, 1989.

42. Ibid.

30
INSTALLATION ISSUES

TCAS II implementation issues include a need to start installation procedures

before equipment is fully validated, a fast-paced installation rate, and a deadline for

installation completion that will require aircraft to be out of service. The established

timeframe will strain the resources of virtually every participating aviation

organization. TCAS II manufacturers must produce and deliver equipment, airlines and

others must redesign and modify aircraft, and FAA must certify equipment and altered

aircraft. Questions about the technical quality, safety effects, and economic

consequences accompany the introduction of any new and complex technology. However,

such concerns are amplified in the case of TCAS II by the time pressure and number and

variety of aircraft covered. TCAS II hardware and software, while successful to date in

limited operations, are still being developed and may encounter “intermix ,,43 obstacles.

More so than for most other aviation technologies, understanding cockpit human factors

and air traffic system effects is essential for TCAS II.

TCAS II Equipment Manufacturing and Initial Certification

In response to OTA inquiries, the three main TCAS II equipment manufacturers —

Bendix/King, Honeywell, and Rockwell/Collins — indicated that they will be able to meet

worldwide TCAS II needs during the next 3 years. These companies will begin an

43. While manufacturers will provide complete TCAS systems to their customers, some

airlines may intermix components from different companies. For example, the

communication link between the TCAS computer and the Mode S transponder is critical;

each different combination of a Mode S transponder from one company and

computer from another will require a separate certification from the Federal

Administration.

a TCAS

Aviation

31
version of the collision avoidance software known as MOPS Change 6. Change 6 and

production versions of TCAS II have yet to be flight tested. Although few surprises are

expected from the flight tests, airlines are expected to request further software changes

to address concerns raised in the LIPs and to meet international standards, which are still

being deliberated. FAA views software changes beyond Change 6 as enhancements, and

any changes must be compatible with FAA's baseline TCAS II for approval.

Follow-up Certification of Airliner Modifications

With each aircraft type requiring an STC, a heavy load of engineering changes for

review and approval will confront FAA Aircraft Certification Offices (ACOs). Moreover,

approval of most STCs will require flight testing. FAA has designated a TCAS II

certification team and has pledged to provide trained personnel to meet the

requirements. The agency informed OTA that its ACOs should have sufficient numbers

of engineers and inspectors to accomplish all TCAS II certifications, and “. . . does not

anticipate at this time that it would need to relocate personnel or resources for TCAS II

certification. "

44 However, the magnitude of the burden on FAA will be partly a function

of how many airlines independently pursue STCs instead of seeking a common source and

partly of the number of variations necessary to cover the Nation's civilian aircraft fleet.

FAA needs validated engineering and performance data to certificate a retrofit.

Once data have been certified for one aircraft type, only the data addressing the

differences in other aircraft require confirmation and review. Industry coordination and

cooperation to reduce redundant STC support work could lower the burden for FAA and

44. Melugin, op. cit., footnote 32.

32
delays. Three major issues remain to be addressed including: the effect of high wings

and propellers on TCAS II signals, necessary changes in the TCAS II algorithm to address

the low maneuvering performance of some commuter aircraft, and whether TCAS II

equipment designed for large jets will fit in the smaller commuters. FAA plans to

sponsor a LIP for commuter aircraft later this year to seek answers to some of these

questions.

Other special performance or limited production aircraft that operate now in U.S.

airspace face difficulties in installing TCAS II. For example, the TCAS II computer logic

and antenna design are incompatible with the supersonic Concorde.

installing TCAS II

The bulk of the airlines’ TCAS II installation workload will be in modifying

aircraft. Many preparations for installing TCAS II can and will be made before the TCAS

11 equipment is delivered. Installing the TCAS II equipment itself will not be an undue

burden, although system validation may prove cumbersome unless acceptable ground test

equipment is available. Many of the large U.S. airlines informed OTA they ". . . will

meet the deadline if (they) have to;" 45 other large and many small airlines could face

difficulties.

The ARINC Characteristic 735 and antenna location data will be available to the

airlines by June 1989, leaving about 2 1/2 years to complete all installations. To

complete installation by December 1991, best industry estimates indicate that airlines

and aircraft modification companies must add about 1,000 skilled technicians to their

45. Office of Technology Assessment, op. cit., footnote 7.

33
relatively inexperienced, they will require substantial extra time and supervision for

their work. Additionally, some airlines indicated that their own maintenance facilities

may be insufficient for the extra tasks. Faced with these shortages, airlines plan to

contract out some TCAS II work, use more overtime, cut back on other discretionary

maintenance, and petition for exemptions from other maintenance requirements, such as

modifications of aging aircraft. 46

Even if all testing and certification procedures proceed smoothly, uneventfully and

promptly, most airlines will have to pull aircraft out of normal scheduled service to meet

the deadline. Heavy maintenance periods (“D” checks) for large jets, which are long

enough to permit TCAS II installation without disrupting scheduled passenger service,

occur about once every 4 years. Since the deadline leaves roughly 2 years for

installations, about 50 percent of the U.S. fleet will have to be removed from service for

at least a few days to have TCAS II installed if routine procedures are used Other

installation scheduling options are being explored by some airlines. Some airlines have

suggested a phased approach using "C" checks, but none indicated to OTA that they have

firm plans for such a program. During 1990 and 1991, on average an additional 1 percent

of the U.S. fleet not previously scheduled for heavy maintenance will be on the ground

each day due to TCAS II.

Contractors perform heavy maintenance and modifications for many airlines.

These airlines, as well as those that will not have the capacity to handle the increased

workload, must turn to independent modification companies to perform TCAS II

46. Ibid.

34
installing windshear systems requires using the same technicians and will make it

difficult to accomplish other cockpit work concurrently. However, the airlines will find

it most efficient to do windshear and TCAS II cockpit work during the same out-ofservice period to minimize the number of times the sensitive cockpit instruments have to

be disturbed.

Resource availability and

indirect safety and economic

the implementation deadline may have both direct and

consequences. One direct effect of economics will

manifest itself in the rate the airlines outfit their fleets with TCAS II. While the airlines

must begin installing TCAS II wiring and other provisions as soon as possible in 1989, they

can postpone the TCAS II equipment delivery (and therefore payment) until late in 1991,

since installing the equipment is a much simpler task than installing provisions. 48 By

delaying delivery, airlines also can minimize other costs if the TCAS II design should

require early modifications. The effect of these circumstances is that over a few months

between 1991 and early 1992, the commercial fleet and U.S. airspace may go from

limited TCAS II exposure to almost total coverage. This would effectively eliminate the

possibility of benefits from an operational evaluation program for TCAS II. It also

postpones sales income for TCAS II

creating possible cash flow problems

manufacturers until the end of the demand period,

during the time of heaviest production.

Technical Issues

The two technical issues facing TCAS II implementation, meeting the equipment

47. For some older aircraft, windshear warning and guidance system installation may

take twice as long as the TCAS work.

48. Office of Technology Assessment, op. cit., footnote 7.

35
will intermix different Mode S transponders,

locations, and other equipment characteristics.

further evaluation and time for certification.

Everyone agrees that system or secondary

TCAS II computers, displays, antenna

This raises questions about the need for

effects of TCAS II on the traffic system

will remain unknown until implementation of TCAS II in a substantial portion of the

operating fleet. The complexity added by the human factor in the system prohibit

suitable pre-implementation analysis and make realistic simulation extremely difficult.

IMPLEMENTATION FINDINGS

If TCAS II production rate is sufficient, FAA certification resources are available,

and no technical barriers develop, TCAS II could be installed in most of the U.S. airline

fleet by December 30, 1991. However, OTA concludes that delays, especially those

facing commuter and special configuration aircraft will probably prevent 100 percent

compliance. Moreover, some airlines will endure greater economic hardship than others

in meeting the deadline. Figure 6 shows the conditions that must be met if installation is

to be completed by the current deadline.

Installing TCAS II on an airliner is a complex process requiring substantial aircraft

modification and FAA certification of the design changes. Airline fleets are diverse,

making the FAA certification process potentially both time consuming and difficult and

requiring more FAA personnel than the Agency has planned. FAA states that it has

sufficient resources to meet demand; however, airlines may not be able to obtain

certification quickly and move ahead with modifying their aircraft in a timely manner.

36
YES

\l/

Each airline has

sufficient labor an d

facilities to outfit

its aircraft b y

------------------- NO ----------------------> ALL INSTALLATIONS WILL NO T

BE COMPLETED BY 12/30/91

December 30, 1991.

I

YES

FAA certification resources

and process provide enough

time for the airlines , ------------------- N O ---------------------> ALL INSTALLATIONS WILL NOT

given their resources , BE COMPLETED BY 12/30/91

to meet the deadline .

I

YES

\l/

Operational and

technological probl

minimal and do not

or lengthe n

ems are ----_____-.--—————- NO ----------------------> ALL INSTALLATIONS WILL NOT

delay BE COMPLETED BY 12/30/91

installations.

I

YES

The safety, technical , and,

economic consequences of ---------------——-- NO ----------------------> ALL INSTALLATIONS WILL NOT

the current schedule are BE COMPLETED BY 12/30/91

acceptable to Congress .

I

YES

\l/

INSTALLATION FOR ThE ENTIRE FLEET

CAN BE COMPLETED BY DECEMBER 30, 1991

37
yg

airlines, domestic and foreign, view the deadline as difficult at best and unachievable at

worst, since installing TCAS II will double the rate at which airlines ground their aircraft

for heavy maintenance. The major U.S. airlines should be able to meet the deadline if

required, although other maintenance and modifications may suffer. However, those

airlines late in planning or those with limited facilities and financial resources are likely

to be unable to meet the deadline for the following reasons. Additional technicians will

be needed for the installation work force, and the supply of trained technicians will

probably not be adequate to meet all the needs for every airline. Limited ramp and

hangar space and other maintenance requirements may compound the labor shortage.

Additionally, support equipment that could help speed installation, such as ground testing

equipment, is still being developed.

Depending on start-up and learning curve rates and equipment delivery dates, the

aviation system may encounter high TCAS II installation rates in 1991 — with more than

two-thirds of the fleet being equipped in less than 1 year. Most aviation experts familiar

with TCAS II believe such a high installation rate is not a sufficiently prudent course for

49 implementing such a complex safety technology. While the fundamental technological

concepts of TCAS II have been tested extensively, certain difficulties with complex

aircraft systems often develop only in an operational setting. Thus an initial evaluation

program for TCAS II has gained widespread industry and FAA support.

OTA finds that the cost consequences of out-of-service time for outfitting their

fleets will not affect all airlines equally. The airline industry as a whole will suffer

financially from out-of-service time only if some potential airline passengers decide not

49. OTA data; and Office of Technology Assessment, op. cit., footnote 7.

38
be much greater for some airlines during certain periods. This makes economic equity a

major concern.

Airlines that plan and structure their programs to complete TCAS II installation by

December 1991 will incur substantial costs to do so, although those airlines with the

ability and schedule flexibility to minimize their passenger losses while capturing

passengers turned away by other airlines may come out ahead in the long run. Airlines

with financial or cash flow constraints may lose substantial revenue, especially if they

are unable to obtain adequate financial, personnel, or facility resources to outfit their

entire fleets by 1992 when unequipped aircraft will not be permitted to fly in U.S.

airspace. While the effect on major transportation centers will be virtually invisible to

the traveling public, a few smaller communities may find themselves with fewer and less

convenient flights.

The airlines must install windshear warning systems and undertake major

maintenance on older aircraft during the same period as they install TCAS II. The same

technicians will be used to install windshear systems, and accomplishing other cockpit

work will be difficult because of limited space. Maintenance of aging aircraft will also

draw on ramp and hangar space. OTA finds that out-of--service time and economic

penalties due to TCAS II will be compounded by the windshear and aging aircraft

requirements.

39
THE ISSUES

The basic

having safety legislated is not the ideal way to go, the fact

● * . that Congress set a deadline has been a very significant

piece of this program, and . . . for that,

I think we owe the Congress a thanks.

— Robert Buley, Northwest Airlines, OTA Workshop

ssues associated with TCAS II legislation include the safety, technical,

and economic consequences of rapid installation — by 1992 — in the Nation% commercial

fleet. All three are interrelated; however the safety and technical issues are so closely

connected that they will be summarized together. A fourth issue is the impact on

international relations.

Safety and Technical Issues

The safety concerns related to TCAS II are twofold: 1) the as-yet-unknown effect

of full TCAS II implementation on system safety, and 2) the possibility of reduced safety

because resources are strained or diverted from other maintenance needs in order to

implement TCAS II by December 30, 1991. FAA has not developed a well-defined plan

for evaluating operational performance in a setting that includes a substantial portion of

the Nation’s fleet equipped with TCAS II. The limited installation programs undertaken

to date have involved no more than two commercial aircraft and did not provide an

adequate assessment of the effects of a fully-equipped fleet on the air traffic system.

TCAS II technology is now well developed, and a more widespread operational evaluation

is justified to determine whether software or hardware modifications are called for or

whether pilot or air traffic control procedures must be changed. Without such a

40
modifications to be made early in the installation program.

A successful operational evaluation would require a critical mass of aircraft to be

outfitted with TCAS II at an early date. Operations under the evaluation should cover

the full spectrum of geographical locations and aircraft and airspace types, including

sufficient numbers at hubs to address high-density issues and assess the impact on air

traffic controllers in heavily used airspace. The effects of TCAS II on pilot and

controller performance are still unknown and must be carefully analyzed during the

evaluation.

Regardless of the results of an evaluation period, airlines are aware that some

upgrades to TCAS II will probably occur over the next 30 months, and they may postpone

taking delivery of TCAS II equipment until near the installation deadline. This would

permit airlines to avoid possible costs associated with removing TCAS and installing a

modified version, but it would confront equipment manufacturers with serious cash flow

problems. Moreover, this also means the air traffic system could suddenly be saturated

with new equipment, and the resulting effects on the system are virtually impossible to

predict.

Supplies of skilled labor, as well as ramp and hangar space, are limited. Thus,

airlines may need to divert maintenance attention from other programs to install TCAS II

on time. These strains on resources could degrade the quality of both TCAS II

installations and other important maintenance and safety activities, such as those

associated with aging aircraft.
for each aircraft. However, the December 1991 deadline leaves the airlines only 2 years

for fleetwide implementation, because production equipment of TCAS II will not be

available until December 1989. To complete the work in 2 rather than 4 years will

require airlines to hire additional labor, schedule more overtime, and take more aircraft

out-of-service time. The increased workload may cause sequencing problems with other

programs, such as aging aircraft requirements and windshear equipment installation.

Airlines that are financially healthy are much better able to meet these demands than

airlines with cash flow or labor problems.

Out-of-service time for aircraft raises equity issues for the various airlines. Some

of these equity issues are separate from the direct costs of installing TCAS II. Airlines

without any extra aircraft may have to eliminate some flights for short periods. These

airlines are likely to lose passengers to other airlines, whereas companies that have extra

aircraft in their fleets or the resources to lease them can avoid canceling any service. In

addition, some communities served by airlines without extra aircraft may be

inconvenienced during down time for the aircraft that normally serve them. Finally, the

costs associated with the multiple maintenance requirements now in place may cause

some airlines with older fleets to retire aircraft rather than complete the programs,

further compounding out-of-service problems.

Airlines that do not meet the deadline for any reason will be penalized severely if

unequipped aircraft are not permitted to fly in U.S. airspace in 1992. However, airlines

that complete TCAS II installations on time will face substantial indirect cost penalties

if their competitors do not commit similar resources and are granted extensions. If

42
pg gy

50 Under the current the end of 1991, these companies have invested accordingly.

schedule, airlines may postpone taking delivery of equipment until late in 1991 to allow

mod if i cat ions to be made before their purchases are effective . Equipment

manufacturers that were not early supporters of TCAS II development may reap benefits

from such postponements, while those that invested heavily in development and testing

programs will face cash flow problems as they gear up for production. A simple

extension of the deadline could heighten cash flow problems by further postponing

purchases.

International Issues

Foreign airlines contend that the installation of collision avoidance systems in

international air transports should occur on the basis of agreed international standards

and recoin mended practices. Such standards are under consideration in the International

Civil Aviation Organization and

question the appropriateness of

within the scope of its TCAS

are currently scheduled for adoption in 1990. Many

the United States action in including foreign carriers

II requirements, claiming this undermines the basic

objectives of ICAO in producing international standards. Moreover, critics claim the

United States may have abrogated Article 37 of the Convention on International Civil

Aviation by its actions. 51

An extended TCAS II implementation schedule could help ensure that U.S. and

ICAO standards are compatible. If international airborne collision avoidance standards

50. Joe Wilson, Bendix/King, personal communication, Feb. 7, 1989.

51. Office of Technology Assessment, op. cit., footnote 7.

43
OTA concludes that the TCAS II compliance deadline established in Public Law

100-223 (TCAS section) has some safety, economic, and international consequences not

fully foreseen at the time of enactment. The TCAS II requirement is unique in the

combination of technological complexity, rapid implementation, and the number of

aircraft affected. Moreover, the extensive maintenance requirements associated with

the aging of the national fleet were not anticipated when the legislation was enacted.

Maintenance for aging aircraft will place severe demands on airline personnel and

facilities resources concurrently with those needed for TCAS II.

Different groups concerned with aviation matters recognized some or many of the

issues summarized in this report several months ago. Three possible congressional

approaches were being discussed as OTA began this study in October 1988. The tradeoffs

associated with each option are discussed below and summarized in Table 1 at the end of

this section.

The first option was a signal from the congressional leadership that Public Law

100-223 would remain unchanged, thus ensuring rapid implementation of TCAS Il. The

signal would be necessary because uncertainty over whether Congress will make any

change has led some airlines to postpone planning and committing resources to TCAS II.

These airlines are likely to find that facilities and personnel are unavailable once they do

begin to prepare for TCAS II, since both types of resources have already been

committed. An early decision by Congress not to reconsider the legislation can

minimize, but not eliminate economic penalties for some of the industry groups. If the

52. M. Parkes, Civil Air Attache, British Embassy, in ibid.

44
appear to allow adequate time for this, given the constraints on certification, production,

and installation capabilities.

The second option included explicit indication by congressional leaders that the

deadline in the law was not going to be changed and that Congress encourages FAA’s

vague plans for a TCAS II transition program. However, even if FAA and industry tried

to undertake a more well-defined transition program during late-l 990 and early-1991, the

deadline under this option does not allow time for adequate evaluation. Delays are likely

during the certification process, which is complicated enough that it is likely to strain

both industry and FAA resources, limiting the capabilities for evaluation. Further, since

some airlines may fail to meet the current deadline, FAA would have to exercise its

authority to grant exemptions under this option as well as under option one.

Moreover, under any circumstances, OTA finds that the input of air traffic control

personnel to the TCAS II program must be stepped up considerably from its level to

date. Full participation by FAA air traffic control personnel is an essential component

of any evaluation. The effects on the air traffic system must be assessed, including the

reactions of pilots and controllers to TCAS II, and any initial problems resolved,

the full benefits can be realized.

Furthermore, and regardless of any decision on the deadline, Congress may

ensure that FAA has adequate resources for the installation period. These

so that

wish to

will be

necessary for the Agency to evaluate TCAS and maintain adequate oversight of other

airline maintenance and modification programs to prevent possible safety diminution

indirectly related to TCAS II requirements.

45
extended with no other specific, required actions, most TCAS II safety benefits will be

delayed, and equipment manufacturers will be severely penalized.

Yet, the present TCAS II implementation requirement is extremely difficult for

some segments of the industry. Aircraft that cannot easily be fitted with TCAS II

because of technical problems, such as some older commuter turboprops, would probably

require extensions without a longer installation period. Moreover, the unilateral U.S.

action in requiring TCAS II equipment of foreign airlines has created ill feelings around

the world. Many foreign carriers are likely to install TCAS II voluntarily for safety and

competitive reasons, once ICAO standards are established. Extending the deadline could

ease international concerns and help synchronize ICAO’s activities with U.S.

requirements.

As OTA's study neared completion, a fourth option emerged — amending Public

Law 100-223 to require a phased implementation schedule beginning in 1990 (to ensure

early equipage of a substantial portion of the fleet) and a structured operational

evaluation program, as well as extending the deadline. This would allow manufacturers

to incorporate any necessary modifications before airlines took delivery of the balance of

their orders. Sufficient airline resource limitations, economic inequities, and

international implications stem from the present deadline for Congress to consider

extending the installation schedule. OTA finds that the fourth option is the best choice

and that aviation safety will be best served by introducing TCAS II on commercial

aircraft as soon as possible, by requiring a phased implementation schedule, and by

providing for a structured evaluation program carried out jointly by industry and FAA to

oversee the first year of operation.

46
ppppp

equipment. Requiring and linking an operational evaluation program, a phased

compliance schedule, and an extended deadline (the fourth option) will place additional

responsibilities on each affected party, but it spreads the burdens more equitably than

the other options.

OTA concludes that introducing TCAS early into a substantial portion of the

commercial fleet and requiring a structured evaluation program conducted jointly

between FAA and industry could ensure early safety benefits. Industry officials at OTA’s

workshop indicated that such an evaluation program might include a requirement that

airlines purchase and install TCAS II equipment in 15 to 30 percent of the national fleet

(600 to 1,200 aircraft) as soon as possible after production equipment is available

(probably over the period from late spring to the end of 1990). The operational

evaluation would be conducted over this time period, with the specific details and

responsibilities to be worked out jointly under industry leadership. Congress could

further require that 50 to 60 percent of the national fleet be equipped by the end of 1991

and fully equipped by the end of 1992 or sometime 1993. For an indication of the percent

of the U.S. commercial fleet equipped with TCAS II over different time period options,

see figure

This

for early

7.

combination of requirements and an extended deadline offers the opportunity

identification of any technical and human factors problems during the

operational evaluation. It also addresses economic and international issues more

completely than simply extending the deadline. A phased compliance schedule balances

greater safety from more TCAS II-equipped aircraft early in the program against a more

lengthy period for full implementation. It minimizes the downside of a more flexible

47
Figure 7.– U.S. Fleet Equipped with TCAS and Possible Timing Options

,

,

. ’

1 I I

1 2/89 12/9 0 1 2/91 12/9 2 1 2/9 3

Month/yea r

a

Time constraints with a 12/91 implementation deadline do not permit

an installation level off during the operational evaluation and analysis.

b This curve suggests only one of several possibilities.

SOURCE: Office of Technology Assessment.

Current deadlin e

Two year extensio n

Current deadline wit h

phased implementatio n

schedule and operationa l

evaIuatio n

a

Two year extension wit h

phased implementatio n

schedule and operationa l

evaIuatio n

b

4 8
the fleet and analytical support, the industry and public gain peace of mind if TCAS II

works well, and avert financial and safety penalties if TCAS II should need to be

modified. Regardless of how well TCAS II works, early implementation and evaluation

bring the most TCAS safety quickly to the public.

49
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51