YAK 3UPW N153U accident near Half Moon Bay, California

Probable cause

The failure of left main landing gear actuator to lock in the down position due to low down-lock pressure as a result of a contaminated down-lock track and corroded down-lock balls.

NTSB analysis

Prior to landing the pilot observed 3 green lights indicating that the landing gear was down and locked. During the landing roll the left main landing gear collapsed, which resulted in substantial damage to the left wing and aileron. A Federal Aviation Administration inspector reported that a postaccident examination of the left main landing gear's pneumatic actuator revealed that the actuator's down-lock ball track was contaminated with dirt and that the steel down-lock balls were corroded. The combination of dirt in the actuator's down-lock track, the corroded down-lock balls, and the low down-lock pressure likely caused the gear to collapse.

Text from the NTSB. Open the NTSB docket for WPR10LA173 for the full record.