Bakersfield, CA, USA
N346MC
CESSNA 340A
The airline transport pilot was conducting a personal cross-country flight. The pilot reported that he configured the airplane to land at his destination airport. However, during his instrument panel scan, he observed a warning light indicating that the right main landing gear (MLG) was not locked in the extended position. He then departed the traffic pattern and attempted to troubleshoot the indication by cycling the landing gear without success. He subsequently returned to the airport and used the hand crank to manually deploy the gear but continued to receive the "gear unlocked" indication. The airplane touched down normally, but as the pilot attempted a right turn to exit the runway, the right MLG collapsed, which resulted in substantial damage to the right aileron. Postaccident examination of the landing gear revealed that the right MLG aft drive tube, which supplies overcenter tension to the aft bellcrank and side brace lock link when the MLG is in the extended position, failed in tensile overload. According to an airplane manufacturer representative, low overcenter tension, which is typically a result of improper rigging, will cause the MLG to collapse when it is under load in the extended position. If the MLG collapses while it is in the extended positon, the aft drive tube will fracture in tensile overload. According to the airplane service manual, the landing gear must be re-rigged following any adjustment to the gear down-lock tension; however, a representative of the pilot's maintenance provider reported that maintenance personnel had not complied with this service manual requirement after adjusting the MLG down-lock tension in 2011, 2012, and 2013. Therefore, it is likely that maintenance personnel's failure to re-rig the MLG in accordance with the manual requirements led to its being improperly rigged and to the subsequent collapse of the right MLG.
On January 27, 2016, about 0939 Pacific standard time, a Cessna 340A airplane, N346MC, was substantially damaged during a landing attempt at Meadows Field Airport (BFL), Bakersfield, California. The business flight was operated by Pacific FBO Properties, LLC under the provisions of Title 14 Code of Federal Regulations Part 91. The airline transport pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the cross-country flight that departed Santa Barbara Municipal Airport (SBA), Santa Barbara, California, at 0900. According to the pilot, at the conclusion of his cross country flight he entered the left base leg of the airport traffic pattern and deployed the landing gear. During his instrument panel scan the pilot observed a "gear unlocked" indication and the absence of a gear down light for the right main landing gear. He asked the air traffic controller to describe the position of the landing gear, and after a "flyover," the controller announced that the landing gear appeared to be extended. The pilot then departed the traffic pattern to the west to troubleshoot the indication. After re-cycling the landing gear the pilot returned to the airport and manually deployed the gear, but continued to receive an identical "gear unlocked" indication. The pilot completed an uneventful touchdown and landing roll, but the right main landing gear collapsed when he attempted a right turn to exit the runway. A review of accident photographs by a National Transportation Safety Board (NTSB) investigator revealed substantial damage to the right aileron. According to FAA records, the airplane, manufactured in 1977, was issued its most recent airworthiness certificate on September 16, 1986 and was registered to Pacific FBO Properties, LLC, the accident pilot's company, on February 28, 2005. The airplane was powered by two Continental Motors, Inc. TSIO-520-NB, turbocharged, 335 hp engines, each equipped with McCauley C515 Sabre Propellers. The pilot reported that he observed a similar unlocked indication for the right main landing gear during a flight a few weeks prior to the accident. The indication disappeared after the pilot cycled the landing gear. A review of the airplane's maintenance records revealed that the most recent annual inspection was completed on August 7, 2015, at 4,149 hours of total time in service. A work order entry dated July 4, 2011 stated that the accident pilot observed a "red unsafe light" indication when he attempted to deploy the landing gear and subsequently used the manual extension to complete the gear extension. After the landing roll, the red indication disappeared. The maintenance facility discovered that the downlock tension to the right main landing gear was too "tight." The tension was then adjusted to comply with the service manual requirement. Subsequent work orders from 2012 and 2013 also noted downlock tensions that were also outside the manufacturer's published limitations. The entry in 2013 stated that the downlock tensions to the right and left main landing gear were adjusted to 50 lbs. The airplane is equipped with a fully retractable tricycle landing gear system comprised of a main landing gear wheel beneath each wing and a nose landing gear in the forward fuselage. Each landing gear is mechanically connected to a gearbox located aft of the pilot's seat, driven by a motor, and actuated by a landing gear switch on the instrument panel. A manual extension hand crank, located on the right side of the pilot's seat, can be used to extend the landing gear in the event of an electrical system failure. Landing gear position switches located on the aft bell crank of each gear transmit signals to the instrument panel position indicators to indicate when the landing gear is down and locked. When the landing gear is not locked, the light that corresponds with the affected gear will remain unlit and a light labeled "Gear Unlocked" will illuminate. According to the Cessna 340 Service Manual, "anytime a landing gear retraction or extension system component has been removed, replaced or the tension on the downlocks adjusted, the entire landing gear system must be re-rigged." This rigging procedure involves disconnecting and reinstalling each section of the mechanical assembly to within the manufacturer's specifications. According to a representative of the pilot's maintenance facility, they did not re-rig the landing gear after the downlock tension was adjusted in 2011, 2012, or 2013. In addition, the airplane logbooks did not contain any entries to indicate that the landing gear rigging procedure had been completed in the airplane's previous 964 hours of operation. An aft drive tube is used within the landing gear assembly to supply overcenter tension to the aft bell crank when the landing gear is deployed. A follow-up examination by the NTSB revealed that the aft drive tube rod end fractured about mid-span and a NTSB metallurgist confirmed that the fracture signatures were consistent with tensile overload. The landing gear mechanical and electrical systems were tested multiple times during the follow-up examination. A landing gear cycle test revealed that the left main landing gear and nose gear reached their full extended and retracted positions, which were validated using the instrument panel position indicators. The broken aft drive tube precluded a cycle test of the right main landing gear; however, the "Gear Unlocked" warning light illuminated when the gear was in the extended position. A subsequent electrical continuity test of the right gear downlock switch did not reveal any anomalies. Further tests of the main landing gear downlock tension revealed a measurement of 46 lbs for the left main landing gear. The right main landing gear tension could not be obtained due to the broken aft drive tube. According to a representative of the airframe manufacturer, low overcenter tension can cause the aft bell crank and side brace lock link to retract and the landing gear to collapse when the landing gear is under load in the extended position. After the landing gear is deployed, a torque tube rotates to move the aft drive tube into the extended position, which supplies overcenter tension to the aft bell crank and side brace lock link. If the landing gear collapses while the gear is in the extended position, the rod end of the drive tube will fail in tensile overload. The manufacturer added that these failures are the common result of improperly rigged landing gear.
Maintenance personnel’s repeated failure to re-rig the main landing gear (MLG) in accordance with the airplane manufacturer's service manual requirements, which resulted in the collapse of the right MLG.
Source: NTSB Aviation Accident Database
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