October 29, 2010
By Dan Namowitz
The FAA has proposed an airworthiness directive (AD) on some modified Cessna 402C and 414A twin-engine aircraft, requiring a complete inspection of the flap system, and modification of the flap control system, to avoid possible asymmetrical flap deployment and loss of control.
The proposed AD was prompted by a report of a modified Cessna 414A experiencing an asymmetrical flap condition that caused an uncommanded roll when the pilot set the flaps to the approach position. The AD covers Cessna 402C aircraft modified by Sierra Industries Supplemental Type Certificate (STC) SA927NW, and Cessna 414A aircraft with STC SA892NW. The STCs were formerly held by Robertson Aircraft Corp.
The modified installations required extra travel of the original production flap preselect cable, which connects to the arm assembly and provides the flap position to the flap selector. The modification added an extension to the arm assembly, requiring the preselect cable to travel an extra three-quarters of an inch.
“However, the original cable has internal mechanical stops that prevent it from traveling the additional distance,” explains the AD. “Since more linear displacement of the cable is required to obtain the same switch action, the internal mechanical stops of the cable are reached before the switches designed to stop the motion of the flaps activate.”
Under those circumstances, “the motor continues to run until either the motor drive shear pin fails, a cable breaks, the structural bracket breaks, or the secondary switches stop the motor before something breaks.” The FAA determined that the problem was likely to occur on other aircraft modified by the STCs.
The AD requires compliance with Sierra Industries Service Bulletin SI09-82 Series-1, Rev, 1R of Sept. 8, 2010, which describes procedures for inspecting the flap system, installing a new preselect cable, other modifications, and installing and rigging the flap control system. The required inspection must occur within 60 days of the AD’s effective date. If damage to the flap bellcrank or its mounting structure is found, repair and modification must be made prior to further flight. If damage was not found in the initial inspection, modifications must be made within 180 days after the effective date, in accordance with the service bulletin.
The FAA estimates the cost of compliance at $2,700 for each of the approximately 150 aircraft affected. Comments will be accepted on the proposed AD until Dec. 13, 2010.
Dan Namowitz is an aviation writer and flight instructor.
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