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9
Air Traffic Congestion:
Problems and Prospects
THOMAS CRAIG
Each year, hundreds of millions of passengers are flown all over the
United States. They rely on air travel to conduct their businesses and
maintain ties with friends and family. Although the airline industry orig-
inated more than 50 years ago, it is still a growth industry, with a rate of
increase faster than the growth rate of the nation's gross national product
(GNP). Moreover, air travel is likely to develop its dominant position
because no projected mode of transportation could displace it as the quick-
est and most efficient way of moving people from city to city. Nor are
more exotic forms of communication, such as video conferencing, ex-
pected to displace it. By any yardstick, air travel is a success.
Yet this very success has created problems. Because so many people
are flying on so many planes, the airports and airways of the United States
are no longer adequate to move them smoothly to their preferred desti-
nations.
Airport congestion has many causes airline marketing policies, airport
layout, and noise considerations are examples but one principal effect:
delay. The dollar amount of the cost of annual air travel delay is difficult
to state with precision, but it is certainly large (Table 9-1~. This cost would
be even larger if it were possible to quantify the frustration of millions of
travelers who are experiencing more and more delays in an ever more
crowded system.
The problems of relieving congestion in the air and in airports do not
have simple solutions. The authority and responsibility for tackling them
clearly lie with the Federal Aviation Administration (FAA). But the au
222
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AIR TRAFFIC CONGESTION
TABLE 9-1 Estimated Annual Cost of Air Traffic Delay, 1984
~223
Cost of Delays
Amount
Aircraft
Average delay per operation (minutes)
Air camer operations (thousands)
Total delay (thousands of hours)
Average cost of delay per hour
Total cost of delay to aircraft (millions of dollars)
Passengers
Passenger hours lost (millions)
Value of passenger time lost (dollars per hour)
Total cost of delay to passengers (millions of dollars)
Total cost of delay (passengers plus aircraft) (millions of dollars)
.
SOURCE: Transportation Systems Center. Airport Capacity Enhancement Plan. 1986.
Cambridge, Mass.
6.8
10,839.5
1,228.5
1,647.0
2,023.0
1 17.9
22.3
2,629.3
4,652.3
thority for relieving congestion on the ground is divided among many
entities and is thus a controversial and highly politicized issue. Conse-
quently, constructing new airports and improving old ones are slow, pain-
ful processes. Under the circumstances, there is little hope that many, if
any, new airports will be built in the United States during the rest of this
century.
Although its authority is clear, the FAA still faces a difficult task in
tackling the problems of delay due to congestion in the air because so-
lutions require federal funding and the cooperation and agreement of
various parties. Congress, faced with extremely high deficits, is slow to
provide adequate funding to upgrade the air traffic control (ATC) system.
Moreover, most of the proposed solutions are technical, and controversy
and delay surround the implementation of each new proposed piece of
advanced technology. In many cases the parties involved cannot concur
because they genuinely disagree on the technical merits of alternative
systems. Improvements are also delayed because the various factions in-
volved take divergent stances on the merits of the National Airspace Plan,
which is the overall FAA blueprint for reaching multibillion-dollar partial
solutions to the congestion problem. Although the FAA is the sole des-
ignated authority for operating and improving the federal airways system,
it is still subject to the wishes of Congress and the public as expressed in
federal legislation. The FAA administers the law; it does not create it.
CAUSES OF AIRPORT CONGESTION
Airline marketing policies, which are driven by passengers' preference
for prime-time flying, contribute significantly to the problem of air travel
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224
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THOMAS CRAIG
Arrivals
0 2 4 6 8 10 Noon 2 4 6 8 10
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FIGURE 9-1 Scheduled operations at Atlanta's Hartsfield International Airport.
demand exceeding supply. As a result the number of planes scheduled to
arrive and depart almost simultaneously at an airport can overwhelm the
system (Figure 9-1 and Table 9-21. For example, as shown in Table 9-2,
41 aircraft operations are scheduled for the first 10 minutes of the hour
at Atlanta's Hartsfield Airport; unscheduled flights raise the number even
higher. Such situations can generate backups and cause delays that last
throughout the day. A New York area airport official, in summing up the
problem of schedule bunching, stated, "When there are delays, they (the
airlines) say, 'Others will blink first.' Well, nobody is going to blink."
Aircraft weight classes and differences in speeds contribute to conges-
tion because these variations affect air traffic control processing. Within
the terminal area, radar separation that must be maintained between planes
is established at 3 nautical miles (NM). Arriving aircraft that are following
a heavy jet, however, must be separated by 4, 5, or 6 NM, depending on
the weight class of the subsequent flights. All departing planes following
the heavy jet are under the same constraints. Consequently, as the number
of heavy jet operations increases, an airport's actual arrival and departure
capacity can be significantly reduced, thus increasing congestion and delay
(Figure 9-21. Because of these variations in terminal area and approach
speeds, controllers must ensure that initial spacings exceed the required
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traffic control
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FIGURE 9-2 Influence of heavy jets on arrival throughput. The figure assumes
the airplane mix is composed of large and heavy jets.
distance when a slower aircraft is leading so that the distance between the
planes is not less than the allowed minimum when the leading aircraft
crosses the landing threshold. The resulting spacings can double the target
minimum separation as aircraft are vectored to the final approach course
at various distances on the landing runway. When the trailing aircraft is
slower, separation increases at the threshold to more than the minimum
and throughput is lost. The same is true when planes with different climb
speeds depart consecutively.
One obvious way of reducing congestion lies in increasing the size of
airplanes so that more passengers travel on each flight. This stratagem
can succeed, provided allowance is made for the increased wake vortex
separation required when the maximum gross weight of the airplane ex-
ceeds 300,000 pounds (see Figure 9-3~.
The layout of an airport can also affect congestion, either positively or
negatively. Runway spacings, the point of intersection of the runways,
the location and turnoff angle of exits, and runway length are all relevant
factors when considering an airport's capacity. Figures 9-4 and 9-5 show
typical diversity in airport configurations. The air traffic control procedures
developed for the variety of airport configurations must include safety
margins that can limit operations when necessary, particularly in the case
of bad weather Such safety margins are required because pilots and
controllers still rely in part on their ability to see approaching traffic. Thus,
AIR TRAFFIC CONGESTION
227
for example, at Chicago's O'Hare Airport and at Dallas-Ft. Worth, triple
arrival streams to a set of parallel runways and one intersecting runway
are used only in daylight with a ceiling of at least 1,000 feet (ft) and 3
miles' visibility and, in Chicago, when runways are dry.
Several procedures tailored to runway spacings have been developed
for use in any weather conditions at busy airports. Such procedures include
parallel approaches to runways separated by at least 2,500 ft. using reduced
spacings (2 NM) between arrivals on the adjacent instrument landing
system (ILS) localizer courses; and simultaneous ILS approaches to par-
allel runways separated by at least 4,300 ft.
At Newark International Airport, tests are being conducted to reduce
the in-trial spacing between successive arrivals to that required to accom-
modate minimum runway occupancy time (ROT). The tests are based on
the concept that when the average ROT is 50 seconds (s) or less, a
separation of 2.5 NM is adequate unless wake turbulence rules apply. The
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FIGURE 9-3 Impact of wake turbulence rules on seat throughput. Increasing
airplane size to increase seat throughput can produce a reduction in throughput
if airplane gross weight exceeds 300,000 pounds.
228
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THOMAS CRAIG
N
FIGURE 9-4 Runway operation patterns at Atlanta's Hartsfield International Air-
port. West flow operated 60 percent of the time; reverse or east flow operated
40 percent of the time.
procedure cannot be used at all airports, however, because the SO-s average
is not attainable at some locations.
Airport capacity can also be affected by congestion on the ground. The
O'Hare Airport runways and air traffic control system currently can process
more traffic per hour than the gates will accommodate. Many ground
delays are caused by this situation.
iIsplaced Threshold
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FIGURE 9-5 Runway operation patterns at Boston's Logan International Airport:
four configurations with the west flow configuration operated 42 percent of the
time.
AIR TRAFFIC CONGESTION
TABLE 9-3 U.s. Air Traffic Forecasts
229
Traffic Volume (billions of RPMa)
Size of U.S. Jet
Fleet
(number of units)
Year Boeing FAA
Beginning Domestic Int'l. Domestic Int'l. Boeing FAA
.
1986 293 77 279 68 3,248 3,031
1990 351 88 341 82 3,446 3,412
1997 480 119 479 119 3,821 3,976
aRPM = revenue passenger miles.
Environmental concerns and local noise abatement rules influence the
capacity of many, if not all, major airports. If noise problems limit the
airport to a single initial departure track when actually multiple runways
are available, the airport's departure capacity can be reduced by two-thirds
or more. Arrival track location requirements can also limit capacity. The
Charlotte-Douglas Airport offers a classic example of this situation: only
one of the three runways may be used during church hours, and only two
may be used during the school year.
Some airports experience particularly serious congestion or delay when
weather conditions do not permit visual approaches. Denver, San Fran-
cisco, St. Louis, and Minneapolis all fall into this category. Arrival ca-
pacity can be reduced by 50 percent or more when visual separation
procedures cannot be used.*
PROSPECTS FOR IMPROVEMENT
FAA forecasts offer no prospect of relief from airport congestion and
delay, at least as far as scheduled air carrier operations are concerned.
Tables 9-3 through 9-6 show both FAA and Boeing projections for certain
classes of commercial air traffic. Table 9-3 shows, for instance, that both
the FAA and Boeing forecast continued growth in the volume of air traffic
and modest increases in the size of the U.S. jet fleet. Both sets of pro-
jections convey the same message: air taxi/commuter operations are mak-
ing, and will continue to make, heavy demands on airport and airway
facilities without moving a commensurate share of the traffic.
Defining this situation is not meant as an indictment of the air taxi/
commuter airlines. They are, in fact, doing precisely what the architects
*The author is indebted to Boeing Air Traffic Control Systems Analysis for this discussion
of conditions contributing to airport congestion.
230
THOMAS CRAIG
TABLE 9-4 Boeing and FAA Projections of Commercial Air Carrier
Operations Including Air Taxi/Commuter Operations (in millions of
tower operations)
Air Taxi/Commuter
Air Carrier Operations
Operations
Year Boeing FAA Boeinga FAA
1986 1 1.8 1 1.9 7.3
1990 13.1 13.4 - 8.7
1997 14.2 15.2 1 1.2
aBoeing has no forecast of air taxi/commuter operations.
Of deregulation envisioned providing service to smaller communities that
want the speed and convenience of air travel. Nevertheless, the impact
on major facilities is unacceptable. The system cannot afford to have 42
percent of commercial air carrier movements account for less than 8
percent of the passenger enplanements.*
One proposed way of easing the growing congestion at major hubs is
to bypass, or overfly, these cities. Unfortunately, most air travelers in the
United States either come from a large hub or want to get to one. In fact,
about 92 percent of all passengers begin and end their journeys at a large
hub, regardless of the route flown in between. This limits the amount of
relief that might be achieved by overflying.
In short, airport congestion will continue to be a major air travel problem
throughout this century. Solutions require cooperation among the many
interests involved, each of whom approaches the problem from a slightly
different point of view. The FAA must be involved, along with the local
airport authority; the community; special interest groups that represent
environmental concerns, the airlines, and the airframe and engine man-
ufacturers; Congress; the financial community, which must float bonds;
and finally, the passenger, who pays the penalty for delay in lost time
and frazzled nerves. The chances of an agreement among these various
entities on a set of actions designed to relieve airport congestion seem
remote.
The FAA now has in place a plan to meet its responsibilities for relieving
airside congestion. First released in December 1981 the plan includes four
main air traffic control programs. The cornerstone of the plan is a new
*Each time a passenger actually boards an airplane is a passenger enplanement. For
example, one person traveling from Carmel, California, to Buffalo, New York, would
represent four enplanements if routed over San Francisco, Chicago, and New York City
on four different carriers.
AIR TRAFFIC CONGESTION:
TABLE 9-5 Projected Commuter Air Traffic in Relation to Total
Passenger Enplanements (in millions)
Scheduled Passenger Enplanements
231
Year Total Commuter Percentage
1986 413.1 25.9 6.3
1990 5003 35.0 7.0
1997 693.1 54.5 7.9
SOURCE: Federal Aviation Administration. February 1986. FAA Aviation Forecasts,
Fiscal Years 1986-1997. Washington, D.C.: U.S. Department of Transportation.
state-of-the-art computer system, which provides the increased capacity
needed to handle growth in traffic. The new system will also integrate
and automate control services both in the terminal and en route. The first
installation was completed in Seattle in late 1986; the system is expected
to go on-line in June 1988. Initially, the computers will use the existing
air traffic control computer program. All computers are expected to be in
operation 6 months after delivery.
An advanced automation system is the second component of the plan.
The advanced system uses minicomputers to improve controllers' pro-
ductivity through modernized software and controller hardware. Third,
the voice-switching and control system will automatically switch any in-
trafacility voice communication between controller positions. Fourth, the
consolidation of 200 terminals and en route radar facilities into 23 area
control facilities will bring significant savings in manpower and costs.
When the fully modernized system is in place by the late 1990s, it will
be able to handle 25 percent more operations than was possible in 1980,
with a controller work force of fewer than 10,000.
The plan to modernize the nation's air traffic control system is now
being put into operation, but budgetary problems are jeopardizing its timely
completion. Congress must reauthorize the funding for the plan each year.
TABLE 9-6 Projected Commuter Operations as a Share of Total Air
Carrier Operations (in millions of tower operations)
Total (including Commuter Share
Year Air Carrier commuters) (percentage)
1986 1 1.9 19.2 38.0
1990 13.4 22.1 39.4
1997 15.2 26.4 42.4
SOURCE: Federal Aviation Administration. February 1986. FAA Aviation Forecasts,
Fiscal Years 1986-1997. Washington, D.C.: U.S. Department of Transportation.
