Accident Summary
On January 27, 2016, about 0939 Pacific standard time, a Cessna 340A airplane, registration 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 14 CFR 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) at 0900.
According to the pilot, during the approach he entered the left base leg and deployed the landing gear. He 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 observe the gear position; after a flyover, the controller reported the gear appeared extended. The pilot departed the pattern to troubleshoot. After recycling the gear and returning, he manually deployed the gear but still received the same indication. The pilot made an uneventful touchdown and landing roll, but when he attempted a right turn to exit the runway, the right main landing gear collapsed. A review of accident photographs by an NTSB investigator revealed substantial damage to the right aileron.
Aircraft and Maintenance History
The airplane, manufactured in 1977, was issued its most recent airworthiness certificate on September 16, 1986, and was registered to Pacific FBO Properties, LLC on February 28, 2005. It was powered by two Continental Motors, Inc. TSIO-520-NB turbocharged 335 hp engines with McCauley C515 Sabre Propellers. The pilot reported observing a similar unlocked indication for the right main landing gear a few weeks prior; it disappeared after cycling the gear.
The most recent annual inspection was completed on August 7, 2015, at 4,149 hours total time. A work order dated July 4, 2011, noted that the pilot had observed a red unsafe light and used manual extension; the red indication disappeared after landing. The maintenance facility found the downlock tension too tight and adjusted it. Work orders from 2012 and 2013 also recorded downlock tensions outside the manufacturer's limits; in 2013, left and right main gear downlock tensions were adjusted to 50 lbs.
Landing Gear System and Examination
The airplane has a fully retractable tricycle landing gear with a mechanical linkage to a gearbox, actuated by a switch. A manual extension hand crank is available. Position switches on each gear transmit signals to panel indicators. When the gear is not locked, the corresponding light remains unlit and a "Gear Unlocked" light illuminates.
The Cessna 340 Service Manual states that any time a landing gear system component is removed, replaced, or downlock tension adjusted, the entire system must be re-rigged. According to a representative of the pilot's maintenance facility, they did not re-rig the landing gear after the 2011, 2012, or 2013 adjustments. The airplane logbooks contained no entries indicating the rigging procedure had been completed in the previous 964 hours of operation.
A follow-up NTSB examination found that the aft drive tube rod end fractured about mid-span; a metallurgist confirmed tensile overload signatures. A landing gear cycle test showed left main and nose gear extended and retracted normally. The broken aft drive tube precluded a test of the right main gear, but the "Gear Unlocked" warning light illuminated when the gear was extended. An electrical continuity test of the right gear downlock switch revealed no anomalies. Left main gear downlock tension measured 46 lbs; the right could not be measured due to the broken tube.
According to an airframe manufacturer representative, low overcenter tension can cause the aft bell crank and side brace lock link to retract, leading to gear collapse under load. When the gear is deployed, a torque tube moves the aft drive tube into position to supply overcenter tension. If the gear collapses, the rod end of the drive tube fails in tensile overload. The manufacturer stated these failures are common results of improperly rigged landing gear.