Cessna 421C N805DC accident near Amarillo, Texas

Probable cause

Maintenance personnel's failure to detect the misadjustment of the left landing gear actuator's switch after recent maintenance, which prevented the landing gear from locking in the extended position and resulted in the landing gear collapsing during landing.

NTSB analysis

The airplane's left landing gear collapsed during landing. Postaccident examination of the landing gear system revealed that the down-lock indicator switch on the left landing gear actuator had been turned about 1/2 turn farther inward than specified in the actuator manufacturer's assembly instructions. This incorrect adjustment of the left landing gear indicator switch resulted in the switch's plunger interfering with the actuator's internal locking mechanism, which prevented the actuator from locking in the down position. It also resulted in the left landing gear position indicator showing a down-and-locked position regardless of the gear's actual position. Maintenance records indicated that both main landing gear actuators were resealed at the annual inspection about 1 month before the accident. The maintenance manual included procedures for electrically testing the landing gear actuator switch for proper indication of the unlocked and locked conditions. If maintenance personnel had conducted the testing during the recent maintenance, the misadjusted switch likely would have been discovered.

Text from the NTSB. Open the NTSB docket for CEN13LA208 for the full record.