In aviation we all strive to make sure that all situations that might be encountered are meticulously planned for and all possible eventualities accounted for – we don’t like surprises. There is, however, one issue that impacts every airborne operation over which we have no control; the weather.
We have all seen footage of aircraft coming into land at incredible angles, some of which land and some of which the pilots decide that the best course of action is to climb away to have another go, or perhaps divert to a less windy location. But what is actually going on in the cockpit when all this is happening?
Firstly let’s look at the take off. Aircraft are significantly affected by the wind because not only do they have a large surface area over which the wind forces can act but they also have a huge fin at the back that wants to act like a weather vane and cock the aircraft into wind. Because of this pilots have to be specifically trained to fly the aircraft in all the situations in which they might be expected to operate. This is normally done during initial training in the simulator and is refreshed on a regular basis. The aircraft will have maximum wind limits in which it can operate and these will be laid down in the appropriate operations manuals; the pilots will know these and only under very extreme circumstances will they exceed them.
As the wind very rarely blows directly down the runway the characteristics that are of interest to the pilots are the angle off the runway (the “crosswind component”), the average wind speed and the strength of the gusts. For a given wind speed the crosswind component will increase the further the wind is blowing off the runway direction. So, for example, a 30knot wind straight down the runway will have no crosswind component, but if the wind direction is from 30 degrees off the runway direction then the crosswind component will be half (15knots) and if it is 60 degrees or more off then the pilots will work on having all 30knots trying to blow them off course. The pilots will work out what the crosswind component is before take-off to ensure that it is within limits and to ascertain what control inputs they might need during the take-off run.
The technique for taking off in windy condition varies from aircraft to aircraft but the aim is to predict how the aircraft will want to react to the wind as the aircraft speed increases and apply control forces to keep it straight with the wings level – the crosswind will try to lift one wing and will try and turn the aircraft away from the runway heading, both of which need to be avoided. Typically this will be done using rudder input to keep straight and aileron to stop any roll, which will be gradually reduced during the initial climb.
Landing can be more of a challenge and a perfect landing in windy conditions is a very satisfying achievement for any pilot. Again, before starting the approach to land the wind strength and direction will be checked by the pilots to make sure it is within both their and the aircraft limits. Autopilots often can’t cope with strong crosswinds so manual landings in strong winds are the norm. If it is outside limits then the pilots will need to select another airport where they are able to land but this is obviously not ideal as it incurs delays for the passengers, but safety is always the top consideration so if it has to be done, it has to be done.
In gusty conditions the airspeed tends to move around a lot so quite often the pilots will fly slightly faster in order to ensure that a safe flying speed is maintained throughout the approach and landing. Again there are a couple of techniques that may be employed, depending on aircraft type, but the aim is to stop the wind blowing the aircraft off the runway centreline. One of these techniques is called the “one wing low” method. If the wind is from the right, say, then it will try and blow the aircraft to the left of the runway centreline. To stop this happening the pilot can apply some bank angle and effectively put the aircraft into a turn to the right that cancels out the wind-induced turn to the left. This is a useful technique as it means the aircraft is always pointing straight down the runway which makes any adjustments easier. It will mean landing on one wheel first but that is not a problem for most aircraft. The other method is the “crab” method. On some aircraft you have to land with the wings near to level to avoid, for example, the outboard engines striking the ground. To achieve this the pilots have to point the aircraft into wind and fly the entire approach pointing in a slightly different direction to the runway.
In very strong crosswind conditions you can actually end up looking at the runway through the side window and these tend to be the images that are popular on video sharing sites. It also means that in a large aircraft, because of the long distance between the wheels and the cockpit, when the wheels are over the centre of the runway (which is where they need to be) the cockpit can be directly above the edge of the runway. Whilst it is possible to land like this it puts a lot of strain on the undercarriage and tyres so ideally the aircraft is straightened up just before touchdown. In order for the pilots to do this they have to perform an ariel ballet which involves applying the rudder to straighten the aircraft, applying aileron to keep the wings level (as the aircraft turns owing to rudder input, one wing will go faster than the other and will therefore generate more lift which needs to be countered) and raise the nose to reduce the rate of descent. And all this has to be done with split second precision because doing it too early will mean the aircraft will start to drift (potentially off the runway) and do it too late and you end up with everything half done and a rather firm arrival. This can be a real challenge.
And that’s why pilots find it so satisfying to get a landing spot on in strong crosswind conditions.
Steve Landells has been a pilot for over 27 years, including 17 for commercial airlines including British Airways. He now works as a Flight Safety Specialist at the British Airline Pilots Association.
Posted on 24 February 2017