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Wind that blows from the side of an aircraft is called crosswind. The term does not differentiate between wind coming from the left and one from the right.
Crosswinds cause the aircraft to drift. Unlike the headwind and tailwind scenario, crosswinds affect both aircraft speed and the flight direction. Moving the aircraft to the left or to the right from its original path is called drifting and the angle between the longitudinal axis and the direction of movement is called drift angle (see picture below).
The drift angle can take values between 0 and 90 degrees. It depends on both the windspeed and on the aircraft speed. Stronger winds will result in a larger deviation. On the other hand, faster aircraft will experience less drift than slower moving ones. Free baloons are the extreme case as their own speed is zero.
In the picture above, the resultant aircraft speed is greater that the original one. This may lead to the wrong assumption that crosswinds generally increase the speed which is not actually the case. In reality, the pilot would counter the drift by turning the aircraft into the wind. As a result, the wind will no longer come from the side but will be split into two components and one of them will be a headwind, i.e. it will reduce the aircraft speed (see picture below).
Therefore, aircraft tend to fly at lower groundspeeds in crosswind conditions.
Note: In reality, crosswind, headwind and tailwind are extreme cases. Normally the wind hits the aircraft at an angle that is neither 0, nor 90 degrees and can be split into two: a crosswind component and a headwind/tailwind component (see picture below). Changing the heading will change these components as well.
Impact on Operations
Crosswinds impact all phases of the flight:
- During the cruise, crosswind (or the crosswind component) needs to be compensated by turning the aircraft into the wind. This will reduce the groundspeed (unless there is a significant tailwind component as well) and increase fuel burn. If the drift remains undetected, it may move the aircraft away from the planned path and sometimes even cause loss of orientation.
- The effects during the climb and descent phases are somewhat similar to the cruising part. However, as the aircraft speed is usually lower, the effect of the crosswind will be greater if the windspeed is the same.
- During taxi, the aircraft shows a tendency to weathervane (i.e. yaw in the direction of the wind so that the impact of the wind is minimal). The pilot may need to use aircraft controls to counter this tendency, especially on lighter tailwheel aircraft (e.g. MUDRY CAP-230/231/232).
- During the take off roll, the yaw tendency continues but there is also a roll tendency caused by the increased lift of the upwind wing. This needs to be countered by aeleron deflection into the crosswind.
- Landing in crosswind conditions is particularly challenging, as it requires the nose of the aircraft to be at an angle to the runway (extended) centreline until touchdown and then to quickly align it. Also, the above mentioned roll tendency is present which often results in the downwind landing gear touching down first.
The primary risk with crosswind operations is runway excursion. Unlike the tailwind scenario, veer off (leaving the runway surface to the side) is a more probable scenario than overrun (being unable to stop before the runway end). The effects of the wind can be further aggravated by the surface conditions (e.g. presence of contaminants such as water, snow and ice). Touchdown outside the runway is another possibility due to the need to fly at an angle during the final approach. Even a slight error in the nose direction may result in the aircraft being (at least partly) to the side of the runway when reaching the touchdown zone. Also, final approach in crosswind conditions is more likely to become unstabilized, leading to the execution of a missed approach.
Accidents and Incidents
Crosswinds have contributed to a number of aviation occurrences during the take off and landing phase:
- A333, Montréal QC Canada, 2014 (On 7 October 2014, an Airbus A330-300 failed to maintain the runway centreline as it touched down at Montréal in suddenly reduced forward visibility and part of the left main gear departed the runway edge, paralleling it briefly before returning to it and regaining the centreline as the landing roll was completed. The Investigation attributed the excursion to a delay in corrective action when a sudden change in wind velocity occurred at the same time as degraded visual reference. It was found that the runway should not have been in use in such poor visibility without serviceable lighting.)
- AT45, Sienajoki Finland, 2006 (On 11 December 2006, a Finnish Commuter Airlines ATR 42-500 veered off the runway on landing at Seinäjoki, Finland.)
- AT72, Shannon Ireland, 2011 (On 17 July 2011, an Aer Arann ATR 72-200 made a bounced daylight landing at Shannon in gusty crosswind conditions aggravated by the known effects of a nearby large building. The nose landing gear struck the runway at 2.3g and collapsed with subsequent loss of directional control and departure from the runway. The aircraft was rendered a hull loss but there was no injury to the 25 occupants. The accident was attributed to an excessive approach speed and inadequate control of aircraft pitch during landing. Crew inexperience and incorrect power handling technique whilst landing were also found to have contributed.)
- AT72, Trollhättan Sweden, 2018 (On 9 October 2018, an ATR 72-200 left the runway during a night landing at Trollhättan before regaining it undamaged and taxiing in normally. The excursion was not reported or observed except by the flight crew. The subsequent discovery of tyre mark evidence led to an Investigation which concluded that the cause of the excursion had been failure of the left seat pilot to adequately deflect the ailerons into wind on routinely taking over control from the other pilot after landing because there was no steering tiller on the right. The non-reporting was considered indicative of the operator’s dysfunctional SMS.)
- ATP, Birmingham UK, 2020 (On 22 May 2020, a BAe ATP made a go around after the First Officer mishandled the landing flare at Birmingham and when the Captain took over for a second approach, his own mishandling of the touchdown led to a lateral runway excursion. The Investigation found that although the prevailing surface wind was well within the limiting crosswind component, that component was still beyond both their handling skill levels. It also found that they were both generally inexperienced on type, had not previously encountered more than modest crosswind landings and that their type training in this respect had been inadequate.)
- B733, Aqaba Jordan, 2017 (On 17 September 2017, a Boeing 737-300 requested and was approved for a visual approach to Aqaba which involved a significant tailwind component and, after approaching at excessive speed, it touched down late and overran the 3000 metre runway onto sandy ground. The Investigation found that despite EGPWS Alerts relating to both the high rate of descent and late configuration, the Captain had instructed the First Officer to continue what was clearly an unstabilised approach and when touchdown had still not occurred with around 1000 metres of runway left, the Captain took over but was unable to prevent an overrun.)