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Aircraft controllability and excessive trim
A recent SDR was issued where: "shortly after takeoff, the flight crew reported that abnormal control inputs and trim were required during climb-out and landing with flaps extended".
Initial maintenance inspections of both the flap and aileron systems revealed nothing out of the ordinary even though a complete operational and visual inspection was performed. A flight test was conducted and it was observed that the left out-board mid flap panel was not correctly aligned in relation to its adjoining flap panel and wing.
Further maintenance investigations revealed that on the left wing out-board mid-flap, the in-board carriage spindle (number 2) had sheared.
Another similar event had previously occurred where the aircraft was reported to roll to the right with flaps beyond 10 degrees and 15 units of trim was required to hold level flight.
Once again the initial inspection revealed no obvious fault but a further investigation found the right wing outboard mid-flap in-board carriage spindle (number 7) had sheared.
Back in July 2002, Boeing issued Alert Service Bulletin 737-57A1277 to address this issue due to a high number of spindle reported failures from several operators. Later that year in October, the Federal Aviation Authorities (FAA) issued AD 2002-22-05, which mandated a repetitive inspection interval task of all flap carriage spindles including a corrective action procedure.
The following year FAA AD 2003-24-08 was issued, superseding the previous one, due to an operator that experienced a right wing mid flap dual carriage spindle failure (number 7 and 8) during approach. This event was defined as follows: "at approximately 500 feet above ground, the flight crew heard a loud bang followed by the aircraft rolling off hard to the right where significant aileron and rudder inputs were required to maintain level flight to complete the landing".
This new AD reduced the inspection interval and revised the overhaul procedure.
For these events above and in all other cases that have been reported, the aircrafts controllability was affected, abnormal control inputs and trim were required by the flight crew to maintain normal level flight.
As stated by an operator who experienced this scenario “the flap is very heavy and difficult to move, when trying to trouble-shoot for this type of failure", emphasizes the importance to recognize this type of fault and to be aware of the Boeing 737 Maintenance Manual procedure 05-51-191 when trouble-shooting this type of snag.
Due to the continuation of flap carriage failure incidents, Transport Canada Civil Aviation (TCCA) along with the FAA are presently reviewing all SDRs issued since the release of AD 2003-24-08.
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