Kulesa 1

Weather and Aviation:

How Does Weather Affect the Safety and
Operations of Airports and Aviation,
and How Does FAA Work to
Manage Weather-related Effects?

By Gloria Kulesa

Weather Impacts On Aviation In addition, weather continues to play a

significant role in a number of aviation
Introduction accidents and incidents. While National
Transportation Safety Board (NTSB) reports
ccording to FAA statistics, weather is most commonly find human error to be the
the cause of approximately 70 percent direct accident cause, weather is a primary
of the delays in the National Airspace contributing factor in 23 percent of all
System (NAS). Figure 1 illustrates aviation accidents. The total weather impact
that while weather delays declined with overall is an estimated national cost of $3 billion for
NAS delays after September 11th, 2001, delays accident damage and injuries, delays, and
have since returned to near-record levels. unexpected operating costs.

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Weather Delays Other Delays

Figure 1. Delay hours in the National Airspace System for January 2001 to July 2002. Delay hours peaked at
50,000 hours per month in August 2001, declined to less than 15,000 per month for the months following September
11, but exceeded 30,000 per month in the summer of 2002. Weather delays comprise the majority of delays in all
seasons.

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2 Weather and Aviation: How Does Weather Affect the Safety and Operations of Airports and Aviation, and How Does FAA
Work to Manage Weather-related Effects?

Thunderstorms and Other Convective In-Flight Icing. In the period 1989-early 1997,
Weather. Hazards associated with convective the NTSB indicated that in-flight icing was a
weather include thunderstorms with severe contributing or causal factor in approximately 11
turbulence, intense up- and downdrafts, percent of all weather-related accidents among
lightning, hail, heavy precipitation, icing, wind general aviation aircraft. Icing was cited in
shear, microbursts, strong low-level winds, and roughly 6 percent of all weather-related
tornadoes. According to National Aviation accidents among air taxi/commuter and
Safety Data Analysis Center (NASDAC) agricultural aircraft. The percentage was 3
analysis, between 1989 and early 1997, percent for commercial air carrier accidents. The
thunderstorms were listed as a contributing 1994 crash of an ATR-72 near Roselawn,
factor in 2-4 percent of weather-related Indiana, which claimed 68 lives, took place
accidents, depending on the category of aircraft during icing conditions.
involved. Precipitation was listed as a factor in
6 percent of commercial air carrier accidents, In-flight icing is not only dangerous, but
roughly 10 percent of general aviation accidents, also has a major impact on the efficiency of
and nearly 19 percent of commuter/air taxi flight operations. Rerouting and delays of
accidents. American Airlines has estimated that commercial carriers, especially regional carriers
55 percent of turbulence incidents are caused by and commuter airlines, to avoid icing conditions
convective weather. lead to late arrivals and result in a ripple effect
throughout the NAS. Diversions en route cause
In addition to safety, convective weather additional fuel and other costs for all classes of
poses a problem for the efficient operation of the aircraft.
NAS. Thunderstorms and related phenomena
can close airports, degrade airport capacities for Icing poses a danger to aircraft in several ways:
acceptance and departure, and hinder or stop
ground operations. Convective hazards en route Structural icing on wings and control
lead to rerouting and diversions that result in surfaces increases aircraft weight, degrades
excess operating costs and lost passenger time. lift, generates false instrument readings, and
Lightning and hail damage can remove aircraft compromises control of the aircraft. See
from operations and result in both lost revenues Figure 2.
and excess maintenance costs. In Figure 1, the
vast majority of the warm season delays are due Mechanical icing in carburetors, engine air
to convective weather. intakes, and fuel cells impairs engine
performance, leading to reduction of power.

Figure 2. Photo of structural icing on an aircrafts wing.

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 Kulesa 3

Small aircraft routinely operate at altitudes injury and temporary loss of aircraft control.
where temperatures and clouds are most Recently an air carrier en route from Japan to the
favorable for ice formation, making these U.S. encountered turbulence which caused the
aircraft vulnerable to icing for long periods of death of a passenger.
time. Larger aircraft are at risk primarily during
ascent from and descent into terminal areas. Clear-air turbulence is not only dangerous, it
also has a major impact on the efficiency of
Turbulence. Non-convective turbulence is a flight operations due to rerouting and delays of
major aviation hazard. All aircraft are aircraft.
vulnerable to turbulent motions. Non-convective
turbulence can be present at any altitude and in a Ceiling and Visibility. Low ceiling and reduced
wide range of weather conditions, often visibility are safety hazards for all types of
occurring in relatively clear skies as clear-air aviation. The NASDAC study of NTSB
turbulence. Any aircraft entering turbulent statistics indicated that ceiling and visibility
conditions is vulnerable to damage; smaller were cited as contributing factors in 24 percent
aircraft (both fixed- and rotary-wing) are of all general aviation accidents between 1989
susceptible at lower levels of turbulent intensity and early 1997. They were also cited as
than are large aircraft. See Figure 3. contributing factors in 37 percent of
commuter/air taxi accidents during the same
The effects of turbulence range from a period. Low ceiling and poor visibility accidents
jostling of the aircraft that is mildly occur when pilots who are not properly rated or
discomforting for passengers and crews to are flying an aircraft not equipped with the
sudden accelerations that can result in serious necessary instrumentation encounter such
conditions, resulting in loss of control, or
controlled flight into terrain.

Figure 3. Photo of an aircraft missing an engine which had been torn off by turbulence.

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4 Weather and Aviation: How Does Weather Affect the Safety and Operations of Airports and Aviation, and How Does FAA
Work to Manage Weather-related Effects?

The NTSB statistics also imply that air and runway surfaces in slush or standing water
carriers have the expertise, procedures, and at near-freezing conditions are also susceptible
equipment necessary to fly safely in reduced to surface contamination, even after
visibility conditions. Low ceiling and poor precipitation has stopped. Even a very small
visibility were cited as contributing factors in amount of ice on a wing surface can increase
less than 2 percent of the commercial air carrier drag and reduce airplane lift by 25 percent. This
(Part 121) accidents between 1989 and early type of ice accumulation has been a cause or a
1997. factor in 10 commercial aircraft takeoff
accidents between 1978 and 1997. Ice blockage
In 1991, the University of Illinois used of airspeed or altitude measurement
simulated weather conditions to test twenty instrumentation can cause loss of control or
Visual Flight Rule (VFR) rated pilots. When navigation errors.
deprived of visual contact, each pilot
experienced loss of control. On average, it took Ice and snow also have an impact on
approximately 178 seconds giving each pilot terminal operations. Boarding gates, taxiways,
less than 3 minutes to live after entering a cloud. and runways may become unusable. Airport
operational capacities may be sharply reduced.
Low ceiling and poor visibility are not just a See Figure 5.
safety issue. They can also severely degrade the
efficiency of commercial and military aviation.
Reduced ceiling and/or visibility can severely
reduce the capacity of an airport and lead to
airborne or ground delays that result in
diversions, cancellations, missed connections,
and extra operational costs. See Figure 4.

Figure 5. Photo of an aircraft being de-iced on the

ground.

Volcanic Ash. Volcanic ash is pulverized rock.

It is composed largely of materials with a
melting temperature below the operating
temperature of a jet engine at cruise altitude.
Volcanic ash in the atmosphere is usually
accompanied by gaseous solutions of sulphur
Figure 4. Photo from an aircraft on approach to San dioxide and chlorine. The combination of the
Francisco International Airport. Two parallel pulverized rock and acidic gases can
runways can be seen. Pilots of aircraft on parallel significantly affect the performance of jet
approaches must be able to see each other. engines at cruise altitudes. Ash clouds are often
invisible, particularly at night.
Ground De-Icing. Aircraft on the ground during
periods of freezing or frozen precipitation and To put this problem in perspective, the ash
other icing conditions are susceptible to the from the Mount Pinatubo eruption in 1991
buildup of ice on control surfaces, instrument circled the globe within a matter of days and
orifices, propellers, and engine inlets and affected a multitude of air traffic routes.
interiors. Aircraft that are moving along taxiway Consequently, aircraft that traversed this thin

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 Kulesa 5

layer of ash required more maintenance. (PDTs). The primary laboratories performing
Statistics show that there are 575 active AWRP research include: the National Center for
volcanoes globally which normally contribute to Atmospheric Research (NCAR), Massachusetts
50 eruptions, resulting in 50-75 danger days Institute of Technology/Lincoln Laboratory,
per year. Volcanic ash exceeds 30,000 feet on National Oceanic and Atmospheric
active air routes 25-30 days per year. There Administration (NOAA) Forecast Systems
have been over 100 damaging encounters to Laboratory and National Severe Storms
aircraft in the last 20 years costing more than Laboratory, Naval Research Laboratory, and the
$250M in damages. National Weather Service's - Aviation Weather
and Environmental Modeling Centers.
Within the United States, a particular area of
concern is along the Aleutian Islands and the Convective Weather Product Development
Alaskan Peninsula. The density of active Team
volcanoes in this area, lying as it does adjacent
to the heavily-traveled North Pacific Air Traffic On September 27, 2001, the AWRP participated
Routes, makes the ash threat especially acute. in a landmark event when one of its weather
The generally westerly flow of winds in the research products, the National Convective
region means that ash can be transported easily Weather Forecast (NCWF) transitioned from an
into airspace over the Canadian and U.S. Pacific "experimental" stage, to an operational National
Northwest regions. Ash from volcanoes on the Weather Service (NWS) product, approved for
Kamchatka Peninsula of Russia also poses a use by both meteorologists and end users. The
threat because it tends to drift into the heavily NCWF is designed specifically to minimize
traveled North Pacific airways, which are within delays caused by convection as it provides
U.S. Flight Information Regions. locations of significant convection one hour in
the future, with updates every 5 minutes. A goal
AWRP Mitigation Initiatives of the Convective Weather PDT is to eventually
develop forecasts of convective weather out to
The FAA Aviation Weather Research Program six hours. The team also developed a 1-hour
(AWRP) has as a goal to relieve weather Terminal Convective Weather Forecast (TCWF)
impacts on NAS safety, capacity, and efficiency. product which is being tested at the Dallas-Ft.
To work towards this goal, the program Worth, Memphis, Orlando, and New York
conducts applied research organized around ten airports. See Figure 6. This product provides
meteorological product development teams more than an extrapolated position of storms; it

Figure 6. Succession of screenshots from the Terminal Convective Weather Forecast (TCWF), showing forecast
convective weather for a local area.

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6 Weather and Aviation: How Does Weather Affect the Safety and Operations of Airports and Aviation, and How Does FAA
Work to Manage Weather-related Effects?

also includes the effects of growth and decay. convective induced turbulence. From this
The Regional Convective Weather Forecast diagnostic data, and the addition of in-situ and
(RCWF) includes a 2-hour prediction, and it is remotely sensed data, an Integrated Turbulence
being tested in the Northeast U.S. Forecast Algorithm (ITFA) was produced and is
now being used experimentally (see Figure 8).
In-Flight Icing Product Development Team

To address the in-flight icing problem, the

AWRP has developed a Current Icing Potential
(CIP) product (shown in Figure 7). The first
generation CIP became an operational NWS
product on March 27, 2002. As improvements
to CIP are being made, a related product, the
Forecast Icing Potential (FIP), which provides a
forecast of icing conditions, is being developed.
FIP is presently approved by the FAA and the
NWS for experimental use. These products
enable users to better anticipate where icing
hazards are going to occur, and will allow air
traffic controllers to make more informed
decisions when assigning altitudes to aircraft.

Figure 8. Screenshot of the output from the

Integrated Turbulence Forecast Algorithm, showing
forecast turbulence overlaid on a map of the United
States.

Terminal Ceiling and Visibility Product

Development Team

San Francisco International Airport is adversely

affected by low clouds and poor visibility due to
its location along the coast. During periods of
poor weather, aircraft are assigned to holding
patterns, or are prevented from taking off en
route to San Francisco until the weather clears.
Figure 7. Screenshot of the Current Icing Potential The Terminal Ceiling and Visibility PDT is
(CIP) product, showing icing potential for a sample developing a 1-6 hour forecast of the time when
flightpath from Denver to Washington, DC. Hazard simultaneous parallel approaches can be
areas are shown both as an overlay on a map of the resumed so that the aircraft arrival rate at San
United States and as a vertical cross-section. Francisco matches the acceptance rate. This
would allow additional aircraft to arrive at the
Turbulence Product Development Team terminal as extra capacity becomes available.
An automated algorithm for predicting the time
The mission of the turbulence PDT is to produce when the airport could increase its capacity is
timelier and more accurate analyses and undergoing evaluation.
forecasts of turbulence, and develop user-
friendly turbulence products. An algorithm
designed to forecast turbulence models jet
stream, mountain induced turbulence, and

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 Kulesa 7

National Ceiling and Visibility Product Winter Weather Research Product

Development Team Development Team

Since low ceilings and visibilities impact all The FAAs Winter Weather PDT began
airports to some degree, the AWRP began a supporting ground deicing research in 1991.
national ceiling and visibility research program The research resulted in development of an
in March 2001. The primary beneficiaries of integrated display system that depicts accurate,
this PDTs work are expected to be operators of real time determinations of snowfall rate,
general aviation, who are often involved in temperature, humidity, wind speed and
controlled flight into terrain accidents. A direction, called the Weather Support to
preliminary version of such a product is shown Deicing Decision Making (WSDDM) system
in Figure 9. (Figure 10). The sources of weather data used
by WSDDM include Doppler radars, surface
weather stations, and snow gauges located near
the airport, which accurately measure the
amount of water in the snow. Research indicates
that the icing hazard for aircraft directly
corresponds to the amount of water in the snow.
Additionally, WSDDMs accuracy is enhanced
by using current and site specific weather
information that results in a more accurate
decision making tool for a particular airport.
The system requires little meteorological
knowledge and minimal training to operate, and
enables decision makers to obtain valuable
information in seconds. A new snowgauge has
recently been developed which is smaller than
the traditional snowgauge and requires very little
maintenance. This new hotplate snowgauge
determines the liquid water equivalent of
precipitation by measuring the amount of
electrical current required to maintain the top
Figure 9. Screenshot of preliminary National Ceiling
and bottom sides of a horizontal plate at the
and Visibility product, showing current ceilings
overlaid onto a map of the United States.

Figure 10. Screenshot of the Weather Support to De-Icing Decision Making (WSDDM) display, showing sample
data for the LaGuardia airport area.

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8 Weather and Aviation: How Does Weather Affect the Safety and Operations of Airports and Aviation, and How Does FAA
Work to Manage Weather-related Effects?

same temperature. The Winter Weather PDT is

also involved in the Northeast Corridor project
and has started to investigate the formation of
winter fog by deploying numerous fog sensors at
Rutgers Airport.

Oceanic Weather Product Development

Team

At present, aircrews for long-range oceanic

flights receive a general weather briefing before
departure, including a summary of flight level
winds and expected en route weather conditions.
While the current weather products do provide
valuable information for strategic planning, the
information is already hours old by the time the
aircraft depart and only the most general weather
updates are provided during the flight.

The Oceanic Weather PDT was established

to conduct applied research leading to the
phased introduction of advanced weather Figure 11. Screenshot from the Oceanic Weather
products for oceanic areas including convection, Cloud Top Height product.
turbulence, in-flight icing, high resolution
winds, and volcanic ash dispersion. The Aviation Forecasts and Quality Assessment
observation of weather phenomena over the Product Development Team. This PDT
ocean is more difficult than over the continental produced and maintains the Aviation Digital
United States due to the relative scarcity of Data Service (ADDS). ADDS is a tool which is
weather data. Within the convective weather available on the Internet and which allows
domain, research is being done on a diagnosis, pilots, airline dispatchers, and other users easy
nowcast and a forecast out to 6 hours. The first access to weather data. An important
product that has recently entered the testing component of ADDS, the Flight Path Tool, is
phase is the Cloud Top Height product. a relatively new innovation which employs user-
friendly graphics to display vertical cross-
The Cloud Top Height product, which is sections of turbulence, icing, thunderstorms, and
available on a cockpit printer or graphic display other aviation weather hazards for specific flight
system, portrays areas along the aircrafts flight altitudes and flight paths selected by the user.
path where cloud tops are expected to be By allowing pilots and dispatchers to have easy
between 30,000 to 40,000 feet. Another access to forecast weather conditions, ADDS
graphical designation shows clouds with tops facilitates flight planning and minimizes the
above 40,000 feet (Figure 11). time required to change the route of flight when
required. The convective weather, in-flight
Support Product Development Teams icing, and turbulence products mentioned above
have been incorporated into the ADDS web site.
The three PDTs discussed below do not address ADDS is a winner of the Government
individual weather phenomena directly, but Technology Leadership Award. See a sample
rather provide capabilities needed by the direct ADDS screen in Figure 12.
impact PDTs.

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 Kulesa 9

Figure 12. Screenshot of the Aviation Digital Data Service (ADDS) web page.

Model Development and Enhancement years participated with the NWS and the
Product Development Team. The cornerstone Department of Defense in developing the
of modern weather forecasts is numerical present national weather radar system. AWRP
weather prediction. It is so critical to most of operates a special PDT for improving the radar
the weather PDTs that a special, separate PDT needed by the FAA or by the other PDTs.
was created solely for this purpose. The Model
Development and Enhancement PDT has AWRP Benefits Studies
produced a special aviation-oriented model
called the Rapid Update Cycle (RUC). RUC is The following benefits analysis information was
now an operational NWS product which runs obtained from benefits studies that were
once per hour. The latest version of RUC conducted by MCR Federal Inc. The results of
became operational on April 17, 2002. This the studies and the dates of the reports are
PDT is also involved with the NWS Eta model provided below:
and the futuristic Weather Research and
Forecasting (WRF) Model now being created in Current Icing Potential (CIP) Safety Benefits
the broader weather modeling community. Analysis Case Studies (Aug. 10, 2001): A
potential safety benefit of $30M annually is
NEXRAD Enhancements Product estimated for general aviation, air taxi, and
Development Team. All weather prediction commuter in-flight icing accidents.
begins with knowing the present state of the
atmosphere. Radar is a very important way of
obtaining this knowledge, and the FAA has for

The Potential Impacts of Climate Change on Transportation

10 Weather and Aviation: How Does Weather Affect the Safety and Operations of Airports and Aviation, and How Does FAA
Work to Manage Weather-related Effects?

Ceiling and Visibility (C&V) at San Francisco, research. The most significant of these is a
Marine Stratus Forecast Benefits Estimate (July project jointly funded by AWRP and the NOAA
18, 2002): A marine stratus forecast can Office of Global Programs (OGP). The project
potentially provide a benefit of $5.45M annually is referred to as the Water Vapor Sensing
in arrival and departure delay savings. System (WVSS). Until now, global
measurements of water vapor in the atmosphere
Terminal Convective Weather Forecast consisted of surface observations and of
(TCWF) Benefits Analysis at DFW and soundings taken by instrument carried aloft by
Orlando Airports (June 2, 2000): The total balloons twice per day. WVSS uses commercial
benefit point estimates are $18.4M for DFW and aircraft as data gathering platforms. AWRP-
$6.0M for Orlando, annually. produced sensors are flown on commercial
carriers (30 UPS aircraft presently for test
Terminal Convective Weather Forecast purposes), and the readings from the sensors are
(TCWF) Benefits Analysis at New York downlinked to the ground every few minutes in
Airports (Kennedy, LaGuardia and Newark) flight. The result is that measurements of water
(Nov. 21, 2000): The total benefit point vapor, which is the single most important
estimate is $80M annually for the New greenhouse gas, are being increased by a factor
York airports. in the hundreds.

Terminal Convective Weather Forecast In addition to WVSS, AWRP has produced

(TCWF) Benefits Analysis (national) estimated a climatology of in-flight icing conditions over
for a national system deployed at Integrated the US, and has also collected new, high-quality
Terminal Weather System installed airports data on turbulence, radar-measured parameters,
(Feb. 12, 2001): The estimated total benefit is and air chemistry. Any of these could be useful
$524M annually. to those studying various aspects of climate
change.
Weather Support to Deicing Decision Making
(WSDDM) Benefits Analysis Update (Oct. 2, More Information
2000): Annual benefits of $12.7M for Kennedy,
LaGuardia and Newark Airports combined. An overview of AWRP products can be found
on the AWRP web page at http://www.faa.gov/
Climate Change Research aua/awr/.

AWRP has no mission and no program for Live AWRP products are available on the
climate change research. However, several ADDS web page at http://adds.aviationweather.
projects undertaken by AWRP for aviation gov/.
purposes have applications for climate change

Gloria Kulesa is the Team Leader for the FAAs Aviation Weather Research Program.

The Potential Impacts of Climate Change on Transportation