Cessna 210 N7736K accident near Marianna, Florida

Probable cause

The failure of the alternator control unit housing plug connection, which resulted in insufficient battery power to fully extend the landing gear. Contributing to the accident was the pilot’s decision to not use the emergency gear extension handle to secure the gear prior to landing.

NTSB analysis

The pilot was diverting to an airport to refuel when the airplane’s turbine inlet temperature (TIT) warning light illuminated. Shortly after, the autopilot “fail” light came on and the autopilot disengaged, followed by other electrical components losing power or ability to operate. The pilot said that he was able to extend the landing gear, but the light that confirmed that the gear was down and locked was not working. He said that the gear sounded and appeared to be fully extended, so he chose not to use the emergency gear extension handle to make sure the gear was fully extended. On the subsequent landing rollout, both main landing gear collapsed, resulting in substantial damage to the left horizontal stabilizer. Postaccident examination of the airplane revealed that the housing plug for the alternator control unit (ACU) was loosely fitted and not properly mated. It is likely the housing plug came loose during turbulence, and the battery drained to a point that it could not provide adequate electricity to fully extend and lock the landing gear. The pilot reported no previous issues with the electrical system and no maintenance had been done to the ACU after a recent annual inspection.

Text from the NTSB. Open the NTSB docket for ERA21LA159 for the full record.