Well, it sometimes does happen! It’s called a tail-strike where the rear end of an aircraft touches the runway. It can happen during take-off if the pilot pulls up too quickly. It can also happen during landing if the pilot raises the nose aggressively.
Why are the wheels arranged like that?
Almost all the modern passenger planes have a landing gear arrangement called tricycle undercarriage in which there are two or more main wheels under the wings and a third smaller wheel under the nose. This means almost half of the plane is protruding behind the main wheels.
The illustration shows that even 10 to 20 degrees rotation can cause a tail-strike. If you have ever been on a see-saw, and the other person suddenly jumped out while you are up, you will suddenly fall and hit the ground. This is somewhat similar to that.
However, these angles are enough for a plane to take off from the runway. It can increase the angle once it is in the air. Check out the recommended rotation angles for some Boeing planes.
In earlier days, a different type of wheel arrangement called conventional/taildragger landing gear were popular. It consisted of two main landing wheels in the front and one small wheel under the tail.
This type of wheel arrangement has some advantages such as the ability to land on rough runways, low cost of tail gear maintenance and you don’t need to worry about tail-strike. However there are some disadvantages, such as low forward visibility and susceptibility to serious ‘nose-over’ accidents if the pilot brakes aggressively during landing.
Every design change on an airplane or any other system is a tradeoff between some elements such as cost, technology, development difficulty, maintenance issues, safety, efficiency, etc. At present, tricycle undercarriage is the best suited design for a big commercial airliner.
Here is an illustration showing the body angle of both types of planes.
What causes tail-strike?
Boeing has done extensive research on this and the main reasons according to them are as follows.
- Mis-trimmed stabiliser. (Due to wrong weights, or an incorrect centre of gravity.)
- Rotation at improper speed. (Usually due to wrong calculation of weight or flap settings.)
- Excessive rotation rate. Usually when an inexperienced pilot is transitioning from a differentplane.)
- Improper use of the flight director.
- Unstabilised approach. (Excessive airspeed, excessive altitude, sudden changes, etc.)
- Holding off in the flare. (Often happens when the pilot tries to achieve an extremely smooth landing.)
- Mishandling of crosswinds. (Landing in gusty winds.)
- Over-rotation during go-around. (Sudden change from landing to take-off can cause mishandling of thrust and angle.)
Tail-strikes during landing are more severe than the ones during take-off.
Incorrect weight distribution during cargo loading can cause a tail-strike even before the plane starts.
Flight crews operating an airplane model that is new to them, especially when transitioning from unpowered flight controls to ones with hydraulic assistance, are most vulnerable to using excessive rotation rate and causing tail-strikes.
How are tail-strikes prevented?
All planes are equipped with a gyro horizon indicator which is often part of the Primary flight display. This gives the pilot crucial information about the angle, speed and altitude of the plane. If the plane takes off at the right speed at right angle, then it won’t cause tail-strike.
Some modern passenger planes have a tail-strike sensor which alerts the crew when the tail is too near the ground.
Some planes, which require a high angle of attack on take-off, are fitted with small tail wheels to prevent tail-strikes.
Tail-strike incidents usually do not cause significant damage and are not dangerous to passengers. But, it may cause financial losses due to the repair, maintenance and inspection costs. However, improper repair after a tail-strike causing structural failure have led to two major accidents.
Japan Airlines Flight 123 suffered explosive decompression 12 minutes into the flight and, 32 minutes later, crashed into a mountain near Tokyo. All 15 crew members and 505 of the 509 passengers on board died. It is the deadliest single-aircraft accident in history. Improper repair of the damages caused by a tail-strike which happened seven years earlier was the cause of this accident.
China Airlines Flight 611 disintegrated in mid-air and crashed into the Taiwan Strait 20 minutes after takeoff, killing all 225 people on board. This accident was caused by improper repairs to the damages caused by a tail-strike 22 years earlier.
Tenerife airport disaster was a fatal runway collision between two Boeing 747s which killed 583 people, making it the deadliest accident in aviation history. Although tail-strike was not the main cause of the accident; when the KLM 747’s pilots saw Pan AM 747 on the runway while they were trying to take-off, in desperation they prematurely rotated the aircraft and attempted to clear the Pan Am by climbing away, causing a severe tail-strike and ultimately the collision.