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An asymmetric or split flap condition is one in which the flap(s) on one wing extends or retracts while the one(s) on the other wing remains in position. The situation can be caused by mechanical failure or jamming.
If the situation is allowed to progress unchecked, it will result in a pronounced roll towards the wing with the lessor amount of flap extended. In this case, it is possible that the induced roll could exceed the amount of aileron authority available and could result in a spin or other loss of control situation. In all cases of asymmetric flap, the wing with the greater amount of flap extended produces more lift. As a consequence, the wing with the lessor amount of flap extended will stall first.
Many aircraft have manufacturer installed provisions for detecting and, in some cases, minimizing an asymmetric flap condition. The more simplistic approach is to have two needles in the flap position indicator. In this situation, so long as the needles overlap, extension or retraction of the flaps is progressing normally. However, should the two needles separate, an asymmetric flap situation is occurring and pilot intervention to stop the flap movement is required. More sophisticated systems incorporate flap position sensors of some description and a flap brake system. If the position sensors detect that an asymmetric flap situation is occurring, the flap brakes are automatically activated to stop further movement of the flaps, thus preventing the situation from becoming worse.