Appendix

# Benchmark Values and Airline Schedules

## Introduction

Airport delay can be expected when too many aircraft want to use the
same runway or airspace at the same time. At the major airports,
scheduled traffic by air carriers and commuters is the main component of
total operations, and thus is an important factor in airport delays. On
the following pages, air carrier schedule data from the Official Airline
Guide (OAG) is compared to the capacity benchmarks at ten of the busiest
and most delayed airports in the country.

## Airline Schedule-Benchmark Charts

Charts comparing schedule and benchmark data were generated for ten
major airports:

-   Chicago O'Hare International Airport (ORD)

-   Dallas/Fort Worth International Airport (DFW)

-   Fort Lauderdale-Hollywood International Airport (FLL)

-   Hartsfield-Jackson Atlanta International Airport (ATL)

-   Houston George Bush Intercontinental Airport (IAH)

-   Minneapolis-St. Paul International Airport (MSP)

-   New York LaGuardia Airport (LGA)

-   Newark Liberty International Airport (EWR)

-   Philadelphia International Airport (PHL)

-   Washington Dulles International Airport (IAD)

Each chart depicts scheduled traffic (arrivals, departures, or total
operations) by 15-minute periods from 7 AM until 10 PM local time. The
schedules for each day of the week, Sunday through Saturday, were
averaged over three months for both a peak and an off-peak season at
each airport. Although traffic on weekdays is slightly higher than
traffic on the weekends, using the schedule for all seven days allowed
us to include the busy Sunday evening period.

Each chart also shows the Current Optimum and IFR benchmark values,
adjusted for the 15-minute period, as well as the *average called rates*
during the given period, by 15-minute period.

The arrival and departure benchmark rates indicate the number of flights
that the airport could be expected to handle during an hour, given a
typical operational configuration. The actual number of operations (or
*throughput*) during that period is a result of many factors such as
traffic schedules, weather, and the runway configuration in use. En
route airspace congestion and delays at other airports may also affect
throughput, especially if flow management measures such as ground delay
programs are implemented. The ATC facility at the airport constantly
advises the Air Traffic Control System Command Center (ATCSCC) on the
number of arrivals and departures that they expect to be able to handle
based on conditions at the airport, taking into account the weather and
runway configuration. These airport *called rates*, the Airport Arrival
Rate (AAR) and the Airport Departure Rate (ADR), reflect actual
conditions at the airport during the given time period. The called rate
may be as high as the Optimum rate or lower than the IFR rate; the
average usually lies in between, depending on the weather and runway
configurations in use during the charted period.

A sample chart, for arrivals at EWR during the period of May-July 2004,
is presented in Figure A‑1.

![](media/image1.wmf){width="5.081944444444445in"
height="3.3944444444444444in"}

**Figure A-1 -- Schedule vs. Benchmarks at EWR**

## Scheduled and Non-scheduled Traffic

Scheduled carrier operations constitute a significant part, but not all,
of an airport's traffic. Non-scheduled traffic includes air taxi
flights, military operations, general aviation (including charter
flights), and some cargo operations. Scheduled flights, including air
carriers and commuter carriers, accounted for approximately 78-98
percent of the total traffic at these ten airports during 2002 and 2003,
according to the FAA Terminal Area Forecast[^1] (see Table A-1).

**Table A-1**

**Selected Airports and Percentage of Scheduled Operations**

  ------------- ----------------------------------------- ---------------------
  **Airport**   **Airport Name**                          **Air Carrier,
                                                          Commuter and Air Taxi
                                                          Operations**

  ATL           Hartsfield-Jackson Atlanta International  98%
                Airport                                   

  DFW           Dallas/Fort Worth International Airport   97%

  EWR           Newark Liberty International Airport      97%

  FLL           Fort Lauderdale-Hollywood International   78%
                Airport                                   

  IAD           Washington Dulles International Airport   79%

  IAH           Houston George Bush Intercontinental      95%
                Airport                                   

  LGA           New York LaGuardia Airport                97%

  MSP           Minneapolis-St. Paul International        89%
                Airport                                   

  ORD           Chicago O'Hare International Airport      97%

  PHL           Philadelphia International Airport        86%
  ------------- ----------------------------------------- ---------------------

The following charts depict only airline and commuter schedules. The
effect of the non-scheduled traffic is uncertain. Some of the
non-scheduled flights may use a separate runway, as at FLL, or they may
try to avoid operating during the busy periods for the air carriers.
However, non-scheduled flights might contribute to delay during the busy
times of the day at some airports, even though air carrier operations
are the main component of operations at these ten airports

## Arrival and Departure Benchmarks

The overall benchmark rate for total arrivals and departures at an
airport generally assumes a balanced operation, with equal numbers of
arrivals and departures. In the long run, the number of aircraft that
land at an airport will equal the number of aircraft that take off. But
from hour to hour, the number of scheduled arrivals may be much more or
much less than the number of scheduled departures.

Some airports will change their operations to handle such unbalanced
demand, and this affects their arrival and departure benchmark rates.
For example, IAD can handle 90 arrivals per hour *or* 75 departures per
hour -- but not both in the same hour. The benchmark for IAD is 135
operations per hour in Optimum weather conditions. It is possible to
schedule fewer than 135 total operations in a single hour, yet still
exceed the departure benchmark rate of 75. Conversely, it is possible to
schedule more than 135 operations per hour without exceeding either the
arrival or the departure benchmark rate.

To recognize such cases, three charts were prepared for each airport:

-   Scheduled arrivals versus the arrival benchmark rate

-   Scheduled departures versus the departure benchmark rate

-   Scheduled total traffic, arrivals and departures, versus the
    benchmark rate for total traffic.

This allows us to identify whether the scheduled traffic exceeds the
arrival capacity of the airport, its departure capacity, or its combined
capacity.

## Schedule, Capacity, and Delay

In the following charts, the airline schedule lines exhibit several
peaks where the number of scheduled operations exceeds the Optimum
benchmark value, and even more instances where the schedule exceeds the
IFR benchmark. If the scheduled operations are greater than the
benchmark value for a given time period, some of those scheduled
operations may be delayed until the next time period. Airline
"overscheduling" -- scheduling more flights than the airport can be
expected to handle during that time period -- is thus one factor in
airport delay.

A small amount of overscheduling is not necessarily undesirable, and may
actually enhance the efficiency of the airport. Flights do not always
operate on schedule: they may arrive early due to favorable winds, or
they may leave the gate late due to mechanical problems. Some
overscheduling during busy periods helps to ensure that there is always
an aircraft waiting to use the runway despite such schedule deviations.
An airport runway is a limited resource; if it goes unused because no
aircraft is available to use it, the opportunity is lost and cannot be
reclaimed.

The airlines may also have business reasons for overscheduling. Each
airline would like to maximize their market share, and to do so they
will schedule flights during the preferred morning and evening travel
times. An airline with a hub operation at the airport may schedule a
bank of arrivals, followed by a bank of departures. The airline may
overschedule these banks knowing that some flights will arrive early and
some will be late, with the net effect of having continuous demand at
the runway end.

The amount of delay caused by overscheduling depends on many factors,
but one of the main factors is the availability of compensating
"underscheduled" periods during the day. If a schedule peak is followed
by an equivalent or greater "valley", then the scheduled traffic can be
handled in the next time period and delays will be short. If the peak
extends over several time periods, however, it will take longer to
eliminate the backlog of waiting flights, and delays will increase
accordingly.

## Other Factors Affecting Actual Operations and Delay

Delay occurs when there is more demand than the airport can accommodate.
Persistent, severe levels of delay are primarily due to excess levels of
traffic, which at these airports is mainly scheduled airline traffic.

The following charts of scheduled airline traffic versus the benchmarks
are intended to illustrate the general situation at each airport. The
actual delay at each airport, however, is governed by many factors, some
of which do not appear on these charts. These factors include:

Actual Flight Times: Airline schedules are generally based on block
times between airports, and may include an additional time margin to
compensate for congestion or other factors. Actual arrival times may
therefore be earlier or later than the scheduled time on a given day due
to non-airport factors. The scheduled arrival and departure times may
not therefore represent the actual pattern of operations at the airport.

Schedule Distribution: The delay experienced by flights in a given time
period is also affected by the distribution of flights within that time
period. Clustering of flights within the time period will lead to more
delay than if the flights were evenly distributed. For example, suppose
that a runway can accommodate one departure each minute. If the schedule
provides one departure per minute, delays will be minimal. However, if
15 departures leave the gate at the same time, one will be delayed by a
minute, another by two minutes, and so forth, with the last departure
delayed by 14 minutes.

Arrival/Departure Priority: At many airports, more arrivals can be
handled if fewer departures are scheduled for that time period, and vice
versa. Such airports (like IAD) may use different runway configurations
for an arrival peak versus a departure peak, or arrival separation may
be increased to allow one or more departures between each arrival pair.
The benchmark rate, however, reflects just a single mode of operation,
usually balanced operations. The benchmark rate may therefore
underestimate the ability of the airport to accommodate schedule peaks.

Weather: One weather effect is apparent in the following charts: the IFR
benchmark is lower than the Optimum benchmark. As ceiling and visibility
decrease, the number of operations per hour that the airport can handle
also decreases. Wind speed and direction can also affect airport
capacity by forcing the use of less efficient runway configurations.
Even with the highest ceilings and visibilities, an airport may not be
able to achieve the Optimum benchmark rate if it is forced to operate in
a different runway configuration.

Environmental Constraints: The runway configuration in use may also be
affected by local environmental restrictions, such as a requirement to
"rotate" runway usage over the day. Such restrictions usually require
the use of runway configurations that are less efficient but which
reduce noise exposure in sensitive areas.

Non-Runway Factors at the Airport: Delays can also be caused by factors
besides the runway. Congestion in the ramp area or on taxiways can keep
aircraft from getting to the runway or to the gate in a timely manner.
An arrival might find its gate still occupied by an earlier flight, and
be delayed waiting for that gate or another to become free. The aircraft
at the gate might be delayed on departure if it was late arriving, since
a minimum amount of time is still required to unload arriving passengers
and baggage, clean the aircraft, and load departing passengers and
baggage.

Non-Airport Factors: Lastly, events away from the individual airport can
lead to flight delays. En route weather is one example, particularly
thunderstorms that close one or more airways. Flights may be held in the
air or on the ground as needed to avoid exceeding the capacity of the
remaining available airways. Aircraft may also be held on the ground
because of congestion at the destination airport, to avoid airborne
holding en route or in the terminal area.

## Examples of Schedules and Delays

Certainly the airline schedule is one factor affecting delay, but actual
delays are the product of many different factors, particularly weather.
The same schedule can be in effect on two different days with very
different delay results. For example, the airline schedules at ORD on
12 July and 16 July 2004 were essentially identical. However, according
to OPSNET data, 23 percent of all operations on 16 July were delayed
more than 15 minutes, while only 3 percent were delayed on 12 July.

Figures A-2 and A-3 depict operations on these days, from 7 AM to
midnight local time. In each figure, the schedule of operations is shown
by a solid line, while actual operations appear as vertical bars. The
called arrival rate, the AAR, is also shown as a line.

Clearly, operations on 16 July were affected by a reduction in the AAR
between 1:30 PM and 6:30 PM, first to 80 per hour and then to 40 per
hour (10 per 15 minute period). This may have been due to thunderstorm
activity. A reduction in departures is also apparent.

The largest difference in the schedule for the two days was only three
arrivals, and only two departures, over any 15 minute period. However,
there were as many as 17 fewer arrivals in a 15-minute period on 16 July
(during four different 15-minute periods between 4 PM and 7 PM), and as
many as 19 more at 10 PM. Apparently flights were restricted in the
afternoon and the backlog lasted until late in the evening. Similarly,
there were fewer departures early in the day and more departures after
10 PM.

Airline scheduling practices can lead to delays on the best weather
days, as overscheduling can eliminate the ability of the airport to
recover from other operational problems. Overscheduling will exacerbate
the effect of bad weather, leading to long delays and cancellations.
However, it should be kept in mind that the schedule is only one factor
affecting delays.

## Schedule-Benchmark Charts for Ten Airports

Charts showing airline schedules and capacity benchmarks at ten of the
busiest and most delayed airports in the country were prepared for two
time periods: January-March 2004 and May-July 2004. Separate charts were
prepared for arrivals, departures, and total traffic (arrivals plus
departures). These charts appear on the following pages.

![](media/image2.wmf){width="5.997916666666667in" height="3.55in"}

**Figure A-2 -- 12 July 2004 (Monday) at ORD**

![](media/image3.wmf){width="5.997916666666667in" height="3.55in"}

**Figure A-3 -- 16 July 2004 (Friday) at ORD**

## 

[^1]: The Terminal Area Forecast provides data on Commuter (scheduled)
    and Air Taxi (unscheduled) operations combined. The percentages
    shown in Table A-1 include some unscheduled Air Taxi operations, and
    therefore overstate the actual percentage of scheduled operations.
    However, the number of Air Taxi operations is generally small
    (estimated to be 2-5 percent of total traffic).