March 18th 2010
The operator of a Canadian registered Cessna 401A reported that the R/H main landing gear (MLG) suddenly collapsed while taxiing out for takeoff. A maintenance investigation revealed that both the MLG adjusting screw, Part Number (P/N) 0841113-3, and the pivot bolt (P/N NAS 464P4-26) attaching the bellcrank assembly to the landing gear trunnion, had sheared. The operator of the Cessna 401A had not complied with all of the manufacturer’s applicable Supplemental Inspection Documents (SID) listed below. It is important to note that the follow-up investigation revealed that the adjusting screw and other MLG parts that were found broken are part of a SID inspection
A search of the Service Difficulty Report (SDR) database revealed an earlier event whereby the MLG inadvertently retracted during landing and caused the aeroplane to veer off the runway. In that case, a maintenance investigation revealed that the R/H MLG adjusting screw failed and caused the side brace to unlock.
A SDR service history review shows that a number of previous events of MLG collapse or extension/retraction issues can be attributed to improper rigging of the landing gear. The MLG is actuated by an electric motor and reduction gear located in the area of the central lower fuselage. The rotation of this motor is transmitted simultaneously to all 3 legs of the landing gear through a set of levers, bellcranks, pivot bolt, side link, adjusting screw and torque tube, thereby moving each respective push-pull tube. Therefore, it is imperative that maintenance personnel precisely follow each sequential step of the manufacturer’s instructions while rigging the landing gear system.
On 3 June 2002, the Cessna Aircraft Company issued SID Number 32-30-05 titled “Main/Nose Gear Retraction Systems Teardown and Inspection”. The primary purpose of the Cessna corrective action SID is to thoroughly inspect the MLG and adjacent structure in order to prevent gear extension and retraction malfunctions.
The SID also refers to Multi-engine Service Bulletin (SB) MEB88-5, Revision 2, which advises owners/operators to carry out initial and repeat inspections of the MLG trunnion lugs. Failure of the MLG trunnion lugs can result in substantial damage to the aeroplane and possible injury to occupants and/or ground personnel.
In addition, the Federal Aviation Administration (FAA) has recently issued Special Airworthiness Information Bulletin (SAIB) CE-09-16 advising operators of Cessna twin-engine aircraft to inspect for fatigue cracks on the MLG torque tubes. Please refer to Cessna SID 32-10-01 or 32-10-02 entitled “MLG Torque Tube Assembly” to determine affected models. Failure of a torque tube can cause damage to the main gear bellcrank assembly resulting in MLG indication and retraction/extension problems. The SAIB also emphasizes that proper rigging of the landing gear is critical for safe operation. For further information please refer to the Cessna SB MEB09-2 entitled “MLG Torque Tube Life Limit”.
Additionally, Transport Canada Civil Aviation (TCCA) has reviewed a previous (Spanish) foreign accident report on a Cessna 402B in which the L/H main landing gear collapsed while taxiing for take-off roll. The post accident investigation revealed that the bellcrank pivot bolt (NAS 464P4-26) had failed first in this sequence of events. Yet another Cessna 421B (Spanish) accident event reported landing gear collapse during landing. Significant damage occurred following skidding off the runway. The sheared bellcrank pivot bolt, once again, appeared to be the initial component in the sequence of failures. Spanish authorities are aware of a number of other main landing gear pivot bolt failures and concluded that improper rigging of the landing gear can result in overload failure of the pivot bolt and consequent overload failure of the bellcrank assembly. The complicated nature of the rigging procedures required for the main landing gear needs to be completed from start to finish. Even small adjustments can introduce a pre-load that exceeds the design criteria of the landing gear resulting in main gear collapse. To verify the integrity of the bellcrank pivot bolt, compliance with Cessna SID 32-10-03 is strongly advised.
Aircrew should be aware that any problems with slow gear retractions/extensions and/or gear unsafe indications, coupled with a decay of climb/cruise speed, may be a warning of an impending MLG failure.
TCCA strongly advises owners, operators and other responsible agencies to comply with Cessna SID Numbers 32-30-05, 32-10-01, 32-10-02, 32-10-03, SB MEB88-5 and recently issued MEB09-2.
TCCA also advises Cessna 300/400 owners, operators and other responsible agencies that close monitoring of the various landing gear mechanisms and warning systems is needed. In particular, we strongly emphasize strict adherence to the manufacturers’ maintenance instructions whenever rigging the landing gear system.
Malfunctions, defects and failures occurring on aeronautical products should be reported to Transport Canada, Continuing Airworthiness in accordance with CAR 521 mandatory Service Difficulty Reporting requirements.
For further information, contact a Transport Canada Centre, or Mr. Barry Caldwell at 613-952-4357 or e-mail CAWWebFeedback@tc.gc.ca or any Transport Canada Centre
For Director, National Aircraft Certification
ORIGINAL SIGNED BY
Chief, Continuing Airworthiness