Ask Captain Lim

Dear Captain Lim,

I just recently flew on a JAL 747 to Manila. I enjoyed the flight because it was smooth and relaxing. I was seated on a window seat near the wings. During the landing, I saw that the wing flaps were fully extended, and it increased its area by almost 50%.

My question is: What happens if, during the landing sequence, one of the flaps suddenly breaks and fall off? I saw the flaps vibrating and the connections seems not so sturdy. Also, the air brakes on the wings looks very flimsy - just thin rectangular pieces of aluminum.

If the flaps fall off during landing, will the plane suddenly roll over or become very unstable as to cause a crash?

Bernard Reyes

Hi Benard,

Firstly, you must understand the main purpose of flaps on airplanes. Basically, they have two main functions - to produce lift to get airborne and increase drag to partly decelerate and land. It adds lift because it increases the wing camber. Drag is also increased when the flaps moved further down in stages to add further resistance to the relative airflow - akin to stretching your palm out against the airflow when someone is driving a car at 120 kph!

Depending on the type of aircraft, the positions of the flaps are normally at Zero (or "Clean"), 1, 2, 3 or Full (or "Dirty") (5 positions). Full is usually the landing flap position as it allows the aircraft to fly more slowly and to steepen the approach towards the runway.

What happens if one of the flaps suddenly breaks loose or off during the landing sequence? It all depends at which stage and at what speed the flaps broke off. The effect is more pronounced at higher speed (but not higher than its limiting speed at Full flaps - 170 knots on a Boeing 777). In the air, flaps breaking off on one side may cause some asymmetrical problem in control but if it were detached during or after the landing, the plane would certainly be controllable. It would not cause it to roll over nor would it be uncontrollable or cause the plane to crash! The pilot only needs to worry about the slight directional control that is likely when a flap breaks off on one side during the landing. This is easily achieved by use of the rudder controls during the landing sequence.

Although your question is a hypothetical one, it is very rare for flaps on a Boeing plane to break loose or off during the landing sequence. Unlike your car, the requirement on maintenance of airplane is very stringent. The engineers perform very extensive inspection on the airplane structure at regular intervals and any rivets that may weaken from repeated use are replaced.

Yes, the "air brakes" (we call them ground spoilers) on the wings may look flimsy, but they are very strong indeed. It is natural for flaps to vibrate a bit (especially landing on a runway that is not very smooth - my experience in Bangkok recently) And, remember, I mentioned somewhere in my FAQ that the wing tips of a Boeing 777 can even bend as much as 24 feet and still flyable!