There are some fantastic videos out there of large commercial airliners coming into land practically sideways then at the last minute the pilot straightens up and lands or even drops the rear tires down while looking down the runway through their side windows! Why do they not just land facing straight?
Airplanes approach the runway sideways when a strong crosswind would otherwise blow it off course. By facing the aircraft into wind the pilot is able to maintain a straight line to the runway and then straighten up the aircraft just at touch down by using the rudder & ailerons to keep it straight.
So it is the wind that causes the pilot to be landing sideways! So let’s take a look at what’s going on…
How Does The Wind Make an Aircraft Drift?
When an aircraft is flying, the wind will push the aircraft in the direction it is blowing. This is known as ‘Drift’. The stronger the wind, the more the drift will be.
If the wind is blowing directly towards the pilot when they want to land, that is perfect. Aircraft always try to land into wind as it helps with the aerodynamics of the aircraft and increases its performance for free.
The problems lie when pilots want to land at an airport that only has one runway and the wind is blowing across it and not down it.
This is when a crosswind landing has to be made.
When an airplane is on final approach to a runway, a crosswind will blow the airplane off course if no action is taken by the pilot. The stronger the wind is blowing, the more off course the airplane will be. As you can see in the diagram above, landing in the grass would lead to a serious amount of paperwork!
To prevent this drift pilots can use various techniques when on final approach to ensure the airplane lands on the runway and is not blown off course by the crosswind:
- A Crabbed Landing
- A De-Crab Landing
- A Slip Landing
What Is A Crabbed Airplane Landing?
When the pilot turns onto the final approach heading for the runway they must steer the airplane into the wind. The harder the wind is blowing the more the nose of the airplane must point into the wind and away from the runway.
By turning the nose into wind it looks like the aircraft would fly to the right of the runway in the case below, but because the wind is blowing it towards the runway the airplane’s shadow flies a direct line to the runway. Matching the amount of nose point or ‘Crab Angle’ as it’s known, to the wind speed is key.
As the airplanes main landing gear touch the runway surface the pilot puts in left pedal to bring the nose of the airplane to the left and line it up with the runway. They will also put in a little right aileron to prevent the airplane drifting across the runway then they plant the nosewheel.
In strong and gusty wind conditions this can be a very tricky maneuver to master as many aircraft cannot land with large sideways forces imparted on their main landing gear. This could result in a blown tire, or structural damage if the airplanes full weight is placed onto the landing gear before the aircraft has straightened up.
This video of the giant Airbus A380 shows how much work the pilots need to do to keep the airplane on the runway:
What Is A De-Crab Airplane Landing?
The De-Crab landing technique is very similar to the crabbed approach where the nose of the aircraft points into wind. However, just before touching down the pilot ‘de-crabs’ the airplane by bringing the nose left to line up with the runway centerline and then immediately touches down before the wind has the chance to blow the aircraft across the runway.
The first airplane in this compilation video shows a great example of a De-Crab landing:
This kind of approach places far less sideways load on the landing gear but it can be tricky to get the timing just right, especially in strong winds or gusty conditions.
What Is A Slip Airplane Landing?
The Slip or Side-Slip landing keeps the airplane’s shadow lined up with the runway centerline by banking the airplane into wind. The wing on the upwind side will be rolled towards the ground as if turning to the right.
To stop the aircraft from turning to the right the pilot balances the aircraft with the left rudder and the elevator to maintain the path over the ground for the given wind strength.
This maneuver can make the passengers feel quite uncomfy as the airplane will appear to be wanting to hit the wing on the ground first. If the wind is gusting, the pilot will also be increasing and decreasing the amount of wing down to maintain track. This can become very discomforting.
As the airplane approaches the runway the art is keeping the wingtip or the engine cowl from hitting the ground. The upwind main landing gear will touch down first, then the downwind main landing gear, followed by the nosewheel.
This is a maneuver not recommended by Airbus because excessive bank angles have led to contacted wingtips or engines when the wind gust happened at just the wrong moment!
Here is a flight simulation of how the slip approach works:
Do Airplanes Have Maximum Crosswind Landing Limitations?
Yes they do! Every aircraft manufacturer will issue maximum crosswind speeds that the airplane can land and takeoff in. Once the wind speed passes this limitation, the aircraft is not allowed to operate.
Crosswind limits also change depending on the runway surface conditions. Dry runways provide the maximum amount of grip when landing and so airplanes will have a higher crosswind limit.
When a runway is under heavy standing water or covered in snow and ice the maximum crosswind limitation begins to drop because the friction on the tires also reduces.
Here are the crosswind landing limitations & guidleines for Boeing 757/767 airplanes (Source – Boeing)
|Runway Condition||Crosswind – Knots|
|Snow – No Melting||35|
|Ice – No Melting||17|
Once an airport begins to see sustained winds reaching 35 knots they can start to delay takeoffs and possibly divert aircraft to other airports. Pilots are constantly trained for crosswind landings and although they can look spectacular on video, to the pilots it’s just another day in the office.
If a pilot feels the approach is not going just right or the technique being used is not working for the type of wind and approach they will initiate a ‘Go-Around’ where they climb back up and try the landing again.
When airplanes appear to be landing sideways it is to keep the aircraft lined up with the runway centerline during their final approach. Depending on the wind strength, the airport’s location, the type of aircraft, and the pilot’s skill will dictate which technique they will use.
Sometimes pilots may even use a mix of various techniques to try and keep the approach and landing as smooth and safe as possible. If it’s not going right, then the Go-Around will be initiated and another attempt will be made.
Crosswind landings are a daily occurrence for professional pilots but when your are first learning them, boy do they make you sweat and pucker!!
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