Of all the weather factors pilots must contend with, adverse winds are among the most critical. Even limiting the focus to conditions close to the ground — particularly for takeoffs, approaches and landings — a few situations can be particularly threatening. In a recent study of business jet runway excursions (ASW, 7/13), as well as in other reported incidents and accidents, crosswinds and tailwinds were noted as contributing factors. Low-level turbulence often accompanies these and other wind conditions and events.
Basic pilot training covers application of the crosswind component, a method of addressing operational effects when the direction from which the wind is blowing is not aligned with the runway to be used; that is, other than a headwind or tailwind. When the wind is blowing perpendicularly to the runway, the crosswind component is simply the wind velocity. Awareness of the crosswind component whenever the wind crosses the runway at an angle requires performing a geometric calculation by entering the variables into a flight calculator or checking a pre-calculated table, slide rule–style computer or other tool. Pilots then compare the crosswind component — the resulting velocity expressed as an equivalent perpendicular wind — against, for example, the aircraft manufacturer’s published maximum crosswind component for takeoff or landing as derived from safe performance demonstrated by test pilots during aircraft certification.
One example of an air carrier accident involving strong and gusty crosswind during takeoff occurred at Denver (Colorado, U.S.) International Airport on Dec. 20, 2008. Among the issues investigated were the pilots’ actions, training and experience; how air traffic controllers obtained and disseminated wind information; runway selection and use; crosswind training; simulator modeling; crosswind guidelines and limitations; certification and inspection of crew seats; and galley latches.1
The Boeing 737-500 veered off the left side of Runway 34R, crossed uneven terrain, became airborne and crashed on the airport. The captain and five passengers were seriously injured. The first officer, two flight attendants and 38 passengers received minor injuries. A post-crash fire occurred, and the aircraft was substantially damaged.
While issuing the takeoff clearance at 1817:26 local time, air traffic control (ATC) advised the flight crew that the wind was from 270 degrees at 27 kt. This wind speed was 16 kt greater than the report of wind from 270 degrees at 11 kt that the crew had received approximately 20 minutes earlier from the current departure information on the automatic terminal information service recording. The flight crew was attempting to take off when they encountered a gust in the wind from the west. Investigators later found that the wind had increased from 27 kt — a crosswind component of 26.6 kt and within the airline’s published crosswind guideline of 33 kt for takeoff in that aircraft on a clear, dry runway like Runway 34 at the time — to 40 kt, based on the greatest wind speed recorded around this time by the airport’s low-level wind shear alert systems.2
The captain told investigators, in part, that the physical sensation he experienced during the takeoff roll was like “someone put their hand on the tail of the airplane and weathervaned it to the left” and that his rudder pedal steering inputs and his use of the nosewheel steering tiller were unsuccessful in correcting the deviation from the runway centerline. The report said, based on estimations of wind conditions, “Performance calculations indicated that the airplane’s rudder was capable of producing enough aerodynamic force to offset the weathervaning tendency created by the winds [including a peak gust of 45 kt that occurred at 1818:12] the airplane encountered during the accident takeoff roll.”
The final report for the accident said that the probable cause was “the captain’s cessation of rudder input, which was needed to maintain directional control of the airplane, about four seconds before the excursion, when the airplane encountered a strong and gusty crosswind that exceeded the captain’s training and experience.” Contributing factors were “an air traffic control system that did not require or facilitate the dissemination of key, available wind information to the air traffic controllers and pilots; and … inadequate crosswind training in the airline industry due to deficient simulator wind gust modeling.”
The airport’s official automated surface observing system’s five-minute weather observations nearest the time of the accident (essentially, the mean values computed for a five-minute period) were wind from 280 degrees at 11 kt at 1815:31 and at 24 kt with gusts to 32 kt at 1820:31. The maximum wind in a one-minute period was from 277 degrees at 36 kt about 1823. Moreover, “the wind conditions at [Denver] were more complex and variable than the crew realized … very localized, intermittent wind gusts as high as 45 kt … crossed the airplane’s path during the takeoff ground roll,” the report said. “Although the pilots’ [recorded] comments showed that they were aware of the crosswind, they only became aware of the high crosswinds as they approached Runway 34R.”
The local controller had provided the flight crew, per common practice, the Runway 34R departure [runway end] wind data in conjunction with takeoff clearance, although adjacent data on the controller’s display terminal simultaneously showed that the airport wind was from 280 degrees at 35 kt gusting to 40 kt, the report said.
“Because an airplane can be adversely affected by strong and gusty crosswinds at any point during the takeoff roll and liftoff, the wind information provided to departing pilots should reflect the most adverse wind conditions they are likely to encounter at any point along the runway so that they can make the safest takeoff decision, such as delaying the takeoff or requesting a different runway,” the report said.
Tailwinds pose a different set of problems for aircraft taking off and aircraft landing. For landing, the problem is often significantly increased groundspeed, which can lead to runway overruns and excursions, especially if the runway is wet or contaminated. On June 10, 2008, an Airbus A310-300 overran the runway after landing at the Khartoum Airport in Sudan (ASW, 6/13). The ensuing crash and fire killed 29 passengers and one crewmember. A tailwind of 15 kt, more than twice what the pilots had been told to expect, was a contributing factor. During takeoff, tailwinds can lessen airflow over the wings, reducing lift.3
Whenever winds are strong, pilots must also deal with turbulence. These sudden vertical movements of the air can affect an aircraft’s attitude and/or altitude. At Shannon (Ireland) Airport on July 17, 2011, the flight crew of an ATR 72-212 was attempting to land on Runway 24. The tower had advised the crew of a strong crosswind from 300 degrees at 20 kt with gusts to 32 kt accompanied by moderate turbulence. The aircraft bounced hard on the first attempted landing and the pilot initiated a go-around. On the second landing, the plane hit the ground in a nose-down attitude, collapsing the nose gear. The plane skidded to a stop, and occupants evacuated. No one was injured but the airplane was destroyed.
Wind — air in motion — is the most variable weather element. Changes in wind speed and even wind direction can be almost instantaneous. Why is the wind so capricious?
Air, of course, is very light so, relatively speaking, it takes little force to move the air. Therefore, the wind is very responsive to small changes in conditions. In other words, many different factors easily can affect the wind conditions.
A complex flow pattern makes up the wind. As with water in a stream or river, there are places where the flow is faster and places where it is slower. In eddies, the water spins. The ocean has tides, waves and currents. In the atmosphere, similar patterns exist, but cannot be seen.
Airflow is almost never smooth (or laminar, in physics terms). Differences in density and friction cause the flow to be turbulent or inconsistent. Vortices or eddies both horizontal and vertical are common. Sometimes, swirls of dust or leaves are visible in the air. Even when they cannot be seen, those swirls are always there.
This inconsistency of airflow results in two characteristics of wind that are of great importance to aviation. Wind is never consistent and is gusty by nature. The magnitude of these rapid changes in wind speed increase as the overall speed of the wind increases. With friction slowing the wind near the Earth’s surface, almost always the wind speed increases at low levels with height. Any type of vertical mixing in the lower atmosphere can bring stronger winds down to the ground. This is typical during the day when solar heating of the surface can induce convective currents, even when the convection is far weaker than for showers and thunderstorms. When the sun goes down, surface winds often decrease as the ground cools. However, winds aloft may still be strong, perhaps becoming low-level jet streams (ASW, 11/12). These rapidly changing wind conditions especially can be challenging to pilots taking off or flying approaches.
The other concern for aircraft operators is that the turbulence inherent in airflow also increases as the wind velocity increases. This is due to the vertical eddies in the flow. An additional source of turbulence occurs when the wind blows around or over an object. Most pilots are aware of “mountain waves,” which occur when winds blow across steep terrain. With strong winds, mountain-generated turbulence can reach near ground level on the mountain’s lee side (the side sheltered from the wind). Even structures can cause turbulence if wind conditions are favorable. In the Shannon accident, turbulence involving an upwind hangar was noted.
The wind’s speed is a critical factor. Meteorologists, airports and others measure wind speed with anemometers, which typically have cups or a propeller that spins with the wind flow, or are the ultrasonic (sonic) type, which uses sound waves. The key question is, “How do you represent the highly variable element of wind speed?”
The official wind speed for an entire airport is defined as the two-minute average of the speed, although several types of measurements were involved in the Denver accident as noted. A wind gust is defined as a rapid fluctuation in wind speed, with the peak wind being at least 10 kt higher than the lowest speed over a measured interval of 10 minutes. The speed of the gust is the maximum instantaneous wind speed noted. Gusts may only last for a few seconds, but speeds can be 50 percent higher than the average wind speed or even more. This is critical because the force or power of the wind increases exponentially with its speed.
Besides inducing turbulence, topography can have other major effects on the wind. The winds tend to follow the flow of the land. They tend to blow parallel to mountain ranges, especially at low levels. Some of the worst situations develop with orographically (mountain-related) enhanced winds. Wind can be channeled and accelerated through valleys. Although the lee of a mountain range is often sheltered from strong winds, enhanced downslope flow occurs. In these cases, the air is sinking and compressing, thus accelerating the winds. The famed Santa Anas of southern California are a good example. The Denver accident involved this type of wind.
So, because aviation professionals are concerned with strong winds and/or rapid changes in wind direction, what type of weather situations should the aviation community look for?
Strong winds often accompany low-pressure areas and cyclones. Major winter storms (ASW, 2/12) and hurricanes (ASW, 7/12) are threats. Lows have a counterclockwise flow in the Northern Hemisphere and a clockwise flow in the Southern Hemisphere. Knowledge of typical storm tracks will give a good idea of the wind direction to be expected with these storms. For example, along the East Coast of the United States, lows often move northeastward just offshore. With the center of the low to the east and south, strong easterly winds can be expected. It is only when the center of the low passes north of a location that the winds shift to the west.
Intense cyclones are often the cause of crosswind situations. The strong winds can cover a large area and typically come from directions out of the norm. A deep low over Scotland produced the strong winds involved in the Shannon accident. Although the wind direction changes as the low passes, wind direction shifts tend to be more gradual than abrupt.
Frontal passages are associated with wind shifts and very often with strong winds. Therefore, fronts are capable of producing all three wind threats. Usually, cold fronts are the worst. Ahead of the front, the winds are from the south or southwest. They come about quickly following frontal passage, typically from the west, northwest or even due north. Strong winds can occur ahead of the front and behind it.
Coastal areas in the summer have to deal with the phenomenon called the sea breeze front. The push of cooler air off the ocean represents a small-scale cold front. It is accompanied by a wind shift, sometimes of 180 degrees. Gusty winds can also occur with frontal passage. Depending on the overall weather situation, sea breeze fronts can travel well inland. They may lose the cool air behind them but still be associated with a wind shift.
Winds generated by convection — showers and thunderstorms — have been a particular hazard since man first took to the skies. Although the problem is well documented (ASW, 6/11), the winds and turbulence associated with convection continue to cause accidents. Close to the ground, the convective downdraft can cause problems for aircraft crews taking off or landing. When the downdraft hits the ground, it spreads in all directions. Any location within this outflow can experience an almost immediate change in wind direction accompanied by wind speeds that can approach 100 mph (161 kph) in the worst-case scenario. Pilots taking off or landing may have to deal with an unexpected crosswind or tailwind. And these wind conditions can be very localized, only on part of the runway, and may not be reported.
Besides looking for the situations described above, what do actual aviation weather forecasters say about forecasting winds? Dan Miller and Jonathan Lamb are forecasters with the U.S. National Weather Service. Both say they use specialized local forecasting models, as well as current observations and Doppler wind data. But that is not enough. Miller stresses the importance of local knowledge and experience. Lamb says, “Climatology for the particular airport weighs heavily in the forecaster’s mind.” Miller adds, “Focus on the most important items for safety purposes, and be aware of impacts to users.”
Edward Brotak, Ph.D., retired in 2007 after 25 years as a professor and program director in the Department of Atmospheric Sciences at the University of North Carolina, Asheville.
- U.S. National Transportation Safety Board. Runway Side Excursion During Attempted Takfeoff in Strong and Gusty Crosswind Conditions, Continental Airlines Flight 1404, Boeing 757-500, N18611, Denver, Colorado, December 20, 2008. Aviation Accident Report NTSB/AAR-10/04. Adopted July 13, 2010.
- Runway 34R has a magnetic heading of 350 degrees.
- FAA Pilot’s Handbook of Aeronautical Knowledge. Chapter 10, “Aircraft Performance.”