What happened
On May 19, 2020, a Cessna 501, registration N501KM, was on final approach to Houston Hobby Airport (HOU) under visual meteorological conditions. Upon extending the landing gear, the pilot observed only two of the three green position indicator lights. The left main landing gear light failed to illuminate, and a warning horn activated. After the pilot recycled the gear with no change in the indication, he contacted the Houston tower to request a visual confirmation of the gear status. Both the tower controller and a pilot of a taxiing commercial airliner reported that all gear appeared to be down.
Following the procedures in the pilot operating handbook, the pilot performed aggressive right and left banks to generate lateral G-forces, attempting to force the left main gear into a locked position. This maneuver did not resolve the indication issue. During the landing roll, the left main landing gear collapsed, causing the aircraft to veer left into a grassy area adjacent to the runway. The impact caused structural damage to the airplane's underbelly and left wing. The pilot was not injured.
The investigation
An FAA airworthiness inspector supervised an examination of the landing gear system using ground hydraulic and electrical power. The nose and right main landing gears functioned normally, as did the cockpit position indicator lights. However, the left main landing gear actuator failed to lock in the down position under any hydraulic pressure setting; the gear only achieved a locked state when manually pushed into place.
An evaluation of the left main landing gear actuator determined that its internal down-lock mechanism was malfunctioning. While these actuators are typically replaced at 37,000 cycles, the aircraft logbook indicated the left main actuator was original to the aircraft and had been last inspected on July 6, 2017, at 5,287 cycles. No recent logbook entries noted any anomalies regarding the left main landing gear.