Beech C90A N513JM accident near Helena, Montana

Probable cause

The failure of the left main landing gear to fully extend and lock as a result of broken downlock switch wires and the failure of the pilots to complete the manual gear extension checklist.

NTSB analysis

The two pilots were performing a simulated instrument approach when they noted the nose landing gear did not indicate it had extended and locked. The pilots performed a missed approach and began to troubleshoot the issue by raising and lowering the landing gear several times, which eventually resulted in the nose and right main landing gear locking; however, the left main landing gear no longer provided a down-and-locked indication. Noting that the gear in-transit lights and warning horn were not activated, the pilots believed the failure of the left main landing gear down light to illuminate was erroneous and did not complete the landing gear manual extension checklist. During the subsequent landing, the left main landing gear collapsed and the left wing impacted the runway, resulting in substantial damage.

Postaccident inspection of the airplane revealed several wires in the left main landing gear downlock switch were broken, likely as a result of snagging on the up position switch bracket while the gear was in transit. According to the airplane’s maintenance manual, when the landing gear is selected down, the airplane senses all three landing gears have fully extended and locked using downlock switches on each landing gear. Actuation of the downlock switches opens the circuits powering the landing gear hydraulic pump and in-transit lights, and closes the circuit to the green gear down lights. Labels on the broken downlock switch wires corresponded to the circuits that provide electrical power to the landing gear hydraulic pump and red in-transit lights.

The airplane was lifted on jacks and the landing gear was cycled several times. Each time the landing gear was selected down, the hydraulic pump stopped operating before the left main gear fully extended and locked and the left green landing-gear-down light failed to illuminate. When the landing gear manual extension pump handle was used, the left landing gear locked and the green gear-down light illuminated. Other than the damage to the up position and downlock switches, no other defects with the landing gear were observed that would have precluded normal operation.

Based on the available information, it is likely that when the landing gear was selected down, the hydraulic pump ceased operating before the left main landing gear had fully extended and locked as a result of the broken wire in the left main landing gear downlock switch. It is also likely that, had the pilots completed the manual gear extension checklist, which included the use of a hydraulic hand pump in the cockpit, they would have succeeded in fully extending and locking the left main landing gear. There was insufficient evidence to determine why the downlock switch wires became entangled with the up position switch bracket during the accident flight.

Text from the NTSB. Open the NTSB docket for WPR25LA122 for the full record.