Summary
A single-engine airplane experienced a left main landing gear collapse during the landing roll, causing it to collide with a taxiway sign. The event occurred after the pilot encountered an electrical system failure while en route to the destination and attempted to manage battery power.
Narrative
While en route, the battery warning light illuminated, indicating the electrical system was not charging. The pilot performed emergency checklist items but could not reset the alternator. He reversed course to return to the departure airport, turning off all nonessential electrical equipment to conserve battery power. The pilot extended the landing gear and 10 degrees of flaps, then turned the master switch off. He later told responding airport police officers that he did not wait for the gear enunciator lights to turn green (indicating the landing gear was down-and-locked) before shutting off the master switch.
As the airplane approached the airport, the pilot turned the master switch back on, but the battery had fully drained. Uncertain whether the gear were down and locked, he pumped the emergency gear extension handle on final approach. During the landing roll, the left main gear collapsed, and the airplane hit a taxiway sign.
Findings
Post-accident examination of the electrical system revealed that the main alternator field wire and power wire terminals were broken due to vibration of the alternator. The alternator was not secure on its mounting bracket; fretting on the forward portion of the alternator bracket housing had enlarged the mounting holes into an oval shape. When touched, the entire alternator and bracket assembly would move.