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. 60000 50000 40000 30000 20000 10000 0 1 01 01 0 01 02 02 ul an 01 J ep an 02 J Mar May S Nov 01 J Mar May 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. The Potential Impacts of Climate Change on Transportation 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 S 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 S 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 aircraft’s wing. The Potential Impacts of Climate Change on Transportation 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. The Potential Impacts of Climate Change on Transportation 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 The Potential