ASSESSMENT OF THE BENEFITS FOR IMPROVED TERMINAL WEATHER INFORMATION*

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ASSESSMENT OF THE BENEFITS FOR
IMPROVED TERMINAL WEATHER INFORMATION*
James E. Evans
David A. Clark
Massachusetts Institute of Technology
Lincoln Laboratory
Lexington, Massachusetts 02173-9108
An important part of the FAA Aviation Weather
Development Program [Sankey and Hansen, 1993] is a
system, the Integrated Terminal Weather System
(ITWS), that will acquire data from the various FAA
and National Weather Service (NWS) sensors and
combine these with products from other systems (e.g.,
NWS Weather Forecast Offices and the FAA Aviation
Weather Products Generator) [Evans, 1991]. This wide
variety of input data and products will enable the I1WS
to provide a unified set of weather products for safety
and planning/capacity improvement for use in the
terminal area by pilots, controllers, terminal area traffic
managers, airlines, airports, and terminal automation
systems (e.g., Terminal Air Traffic Control Automation
(TATCA) Center Tracon Advisory System (CTAS)
[Andrews and Welch, 1989] and wake vortex advisory
systems[Evans and Welch, 1991)).
The assessment of benefits from the ITWS,
particularly in the area of reducing delay and other
aviation system operations costs, has been an important
element of the I1WS initial development phase. At the
last Aviation Weather Conference, initial results were
reported on delays associated with various types of
weather based on use of climatology and FAA National
Airspace Performance Reporting System (NAPRS)
statistics for O'Hare airport [Weber, et aI., 1991]. This
paper extends the earlier results to consider a broader
range of terminal weather impacts on aviation and
discuss how the ability of the ITWS to reduce the
impact will be quantified.
• This work was sponsored by the Federal Aviation
Administration. The views expressed are those of the authors
and do not reflect the official policy or position of the U.S.
Government
Corresponding author address: James E. Evans or David A.
Clark, Massachusetts Institute of Technology, Lincoln
Laboratory, P.O. Box 73, Lexington, Massachusetts 02173·
9108
Weather-related impact on airline operations has
been estimated using airline internal data on delays and
other impacts at several major hub airports. These
results show that the earlier results seriously
underestimated the delays that arise when weather
impacts an airport. This underestimation arose because
air traffic flow control procedures now attempt to hold
planes on the ground at the departure airport when the
destination airport is impacted by weather, and the
NAPRS statistics did not associate delay times resulting
from traffic management gate and runway holds at the
departure airport with the destination airport impacted
by weather.
However, by using airline internal data for
carefully chosen weather events, one can assess the
relative magnitudes of the delay associated with an
airport impacted by weather. Table 1 summarizes
internal airline delay data from several major airlines at
O'Hare (ORD), Minneapolis-St. Paul (MSP), and
Stapleton (DEN) international airports. We see that
typically the NAPRS data underestimated the actual
total direct delay by approximately 50-100% for
thunderstonns and 200-300% for heavy fog.
There are a number of other significant airline costs
such as fuel tankering, cancellation, diversions, and
costs associated with rescheduling aircraft and flight
crews that have not been considered in the aviation
weather cost/benefits analyses to date. Table 2 shows
the diversions and cancellations for major air carrier
0-1 in table 1 during a number of weather incidents at
O'Hare. These results should be compared to those
cited in [Hartman, 1993] in which it is argued that there
is a constant ratio between diversions, cancellations and
delays (1:3:100 hours of delays on flights to/from the
affected airport] for low-visibility events.
Another very important element of weather impact
assessment is the "delay ripple" effect If an aircraft is
delayed on one leg of a flight (e.g., due to adverse
weather at the airport), then there is a probability that
414
the delay will carry over, or "ripple," onto the next leg
(and subsequent legs) flown by that aircraft that day. In
cases where the subsequent leges) are not impacted by
weather, the delay on the subsequent legs may not be
attributed to terminal weather. DeArmon states that
"delay ripple is in general pretty strong" and persists
over a number of successive legs [DeArmon, 1992].
Hartman cites a case where the number of passengers
delayed (down-line impact) due to delay ripple was 27
times greater than the initial number delayed [Hartman,
1993a] and has suggested that typically the initial delay
at an airport impacted by low visibility is multiplied by
a factor of3-5 due to delay ripple [Hartman, 1993b].
Table 1.
Average Daily Airline Operations and Delay Minutes
on Sample Days of Varying Weather Type
Operations Avg. Daily Delay Minutes Delay Min. per Oper.
Airline Airport Weather Day Type Arriv.lDepart. Arrival Departure Arrival Departure
[# Days]
0-1 ORD Baseline Clear [11" 381/393 229 (94%) 15 (6%) 0.6 <0.1
0-2 ORD Baseline Clear [11" 315/316 1035 (43%) 1398 (57%) 3.3 4.4
D-1 DEN Baseline Clear [21 194/195 88 (51%) 86 (49%) 0.5 0.4
M-1 MSP Baseline Clear [11 257/259 1195 (50%) 1187 (50%) 4.6 4.6
0-1 ORD Thunderstorm [41" 392/387 3395 (63%) 1958 (37%) 8.7 5.1
0-2 ORD Thunderstorm [41" 311/314 2570(470/0) 2915(530/0) 8.3 9.3
D-1 DEN Thunderstorm [51 193/193 1180(76%) 380(240/0) 6.1 2.0
M-1 MSP Thunderstorm [5] 273/272 4591 (51 %) 4430 (49%) 16.8 16.3
0-1 ORD Heavy Fog [4]" 359/356 4676 (80%) 1196 (20%) 13.0 3.4
0-2 ORD Heavy Foa r41" 296/297 3970(540/0) 3345(460/0) 13.4 11.3
* These data subsets from two separate airlines represent operations and delays for a common set of
weather days at ORO.
Table 2.
Cancellations and Diversions on Weather-Impacted Days
fora Ma·Jor A'rrrme at O'Hare AIrport
Weather Type
Total Operations Diverted Canceled
Secondary
(Number of Days) Cancellation*
Clear (1) 779 0 0 0
Thunderstorms (4) 3116 41 19 67
FOQ (4) 2830 25 89 82
* Secondary cancellations are flights that were canceled because equipment was not available due to a
weather cancellation.
415
Delay ripple is clearly a very important issue in
weather benefits assessment that has not been
considered in recent weather system studies. One
useful tool for assessing the delay ripple effect as a
function of airport weather impact severity and duration
will be the National Airspace System Performance
Analysis Capability (NASPAC) [Frolow, et al., 1989].
Experimental validation of delay ripple estimates by
following specific aircraft throughout a day is also
desirable.
The ITWS can reduce the adverse impact of
weather by three methods:
1. Increasing the effective capacity of the
terminal routes and airport runways during
adverse weather (e.g., by providing
information for planning routes around
hazardous cells, winds information for
terminal automation and wake vortex advisory
services) [Evans, 1991, Evans and Welch,
1991J.
2. Avoiding unnecessary changes in terminal and
airport configuration (e.g., by short-term
forecasts of runway winds, ceiling and
visibility), and
3. Anticipating weather events which will
increase or decrease airport capacity so that air
traffic management systems can optimize the
flow of traffic to the terminal area.
Estimation of the reductions in delay and other
impacts of terminal weather that would be achieved by
the ITWS is being accomplished by a combination of
airport-specific weather studies, analysis of ITWS
operational demonstrations at major airports, analytical
studies, and discussions with aviation system experts.
Results to date and near-term plans for ITWS benefits
estimation will be presented in the full paper.
REFERENCES
Andrews, J. W. and J. D. Welch, 1989: "Challenge of
ATC automation," Proc. of 34th Annual
Conference of Air Traffic Control Association,
Boston, MA, Air Traffic Control Association.
DeArmon, J. S., 1992: "Analysis and research for traffic
flow management," Proc. of 37th Annual
Conference ofAir Traffic Control Assoc.. Atlantic
City, NJ, Air Traffic Control Association, 423429.
Evans, J. E., 1991, "The integrated terminal weather
system (ITWS)," Third International Conference
on Aviation Weather Systems, Am. Meteor.
Society, Paris, France.
Evans, J. E. and J. D. Welch, 1991: "Role of FAA/NWS
terminal weather sensors and terminal air traffic
automation in providing a vortex Advisory
Service," Proc. of the FAA Inti. Wake Vortex
Symposium, Wash. DC, Volpe National
Transportation Systems Center, pp. 24-1 to 24-22.
Frolow, I, J. Sinott, and A. Wong, 1989: "National
airspace system analysis capability: a status report
after one year," Proc. of 34th Annual Conference
of Air Traffic Control Association, Boston, MA,
Air Traffic Control Association.
Hartman, B., 1993a: "The future of head-up guidance,"
IEEE Aerospace .and Electronic Systems
Magazine, 8, 31-33.
Hartman, B., 1993b: Personal communication,
22 March 1993.
Sankey, D. and Hansen, A., 1993: "FAA's work in
improving aviation weather," 9th Inti. Con! on
Interactive Information and Processing Systems
for Meteorology, Oceanography, and Hydrology,
Anaheim, CA, Am. Meteor. Soc.
Weber, M. E., Wolfson, M., Clark, D., Troxel, S.,
Madiwale, A. and Andrews, J., 1991: "Weather
information requirements for terminal air traffic
control automation," Proc. Fourth fretl. Con! on
Aviation Weather Systems, Paris, France, June 2428,1991,
Am. Meteor. Soc., Boston.

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