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How to Identify and Mitigate Asymmetric Flaps

Cessna 172 flap. Photography courtesy of Nicole Lund. Throughout primary pilot training, students are commonly taught how to land an aircraft if flaps do not deploy. However, few pilots are trained in mitigating a split flap scenario. Split flaps, also commonly known as asymmetric flaps, refer to a flight condition where the flaps are not at the same setting. Although rare, this condition can happen and lead to loss of control. Asymmetric flaps cause a violent roll toward the side with less flap deployment. The side with a higher degree of flaps causes the wing to rise with the increased lift associated with the flap deployment. Split flaps can occur in several situations. In my case, my split flaps were caused by metal fatigue of the flap rod. Asymmetric flaps cause a violent roll due to the increased lift on the side with the most flap deployment. Graphic courtesy of Boldmethod. In some instances, a broken flap rod would make the flap move freely with the relative wind. In my case, the flap rod broke in flight and caused the right flap to be jammed in the full down position. There are two ways to regain control. If a flap is jammed in the full position, the pilot can attempt to put the operating flap down too. If the flap is not jammed and is "flapping" around in the wind, the pilot can try to match the flap setting, however, the broken flap will constantly change the degrees of deployment. Rudder can be used to prevent the roll caused by the split flaps. The rudder would essentially put the airplane in a slip. It is crucial that the pilot keep airspeed up during this configuration since airspeed doesn't indicate accurately when in a slip. One wing travels faster than the other which causes inaccurate readings on the airspeed indicator due to the placement of the pitot tube. In general, it is critical to always keep airspeed up with split flaps because a stall would induce a spin due to the roll associated with the asymmetry in flaps. It is crucial to inspect flaps during the pre-flight inspection. Pilots should lower flaps and inspect rods, linkages, flap rails, and the electric motor if equipped. Extending flaps makes the rails and rods much more visible and easier to inspect. Another safety precaution is to refrain from adding flaps in a turn. It is much easier to identify the roll associated with asymmetric flaps if in straight and level flight. Flaps are considered a secondary flight control. They increase the camber of the wing which creates more lift while inadvertently creating induced drag. This assists in lowering stall speed which permits pilots to fly approaches at slower speeds. Flaps are not used as a primary control for the pitch, roll, and yaw of the airplane. Elevators, ailerons, and rudders are the three primary flight controls. Flaps and other secondary controls are used to enhance performance. Nevertheless, flaps are still a flight control. CFR 830.5 requires all flight control malfunctions or failures to immediately be reported to the National Transportation Safety Board (NTSB). It is important for the pilot in command to make the report even if the airplane makes it back to the runway without any damage to the occupants or aircraft. The Federal Aviation Administration (FAA) and NTSB can use the report to see if there is a common theme of the part breaking. If needed, an airworthiness directive (AD) may be issued. An AD notifies aircraft owners of a deficiency with a model of aircraft, part, engine, or system that needs to be fixed. Many pilots will go their entire careers without having an asymmetric flap emergency. It is crucial to be proficient with the recovery of this flight condition. Several accidents have resulted from a loss of control caused by asymmetric flaps. Although a rare occurrence, proper training can ensure that you will walk away from a split flap situation.
Created 334 days ago
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