Aircraft: Piper Twin Comanche Injuries: None. Location: Roanoke, Texas. Aircraft damage: Substantial.
What reportedly happened: The pilot was on a cross-country flight when he noticed that the alternators were producing just 9 volts. He tried switching the alternators off and on several times, but was unable to get them to indicate more than 8 to 9 volts. He said he heard a “squeal” coming through the headset, and when the right alternator was switched off, the noise would stop.
He eventually lost radio communication capability while en route to his destination, so he diverted to an alternate airport. He lowered the landing gear on the downwind leg. He said he saw the nose landing gear was extended by its reflection in the spinner. The landing roll was normal until the main landing gear collapsed.
The post-accident examination of the landing gear did not disclose any evidence of a mechanical malfunction. The pilot told investigators that he lost all electrical power due to a loss of one or both alternators.
Investigators determined that is likely that the right alternator failed, so when the pilot turned off the right failed alternator, the noise stopped, however, the left alternator could not put out enough voltage to power the radios and other electrical demands.
By the time the gear was lowered, there was not enough power to lock the left and right main landing gears in place, so they both collapsed during the landing roll. Investigators determined that had the pilot realized that he had lost electrical power and followed the emergency checklist, he could have verified the locked status of the landing gear and followed the emergency landing gear procedures to extend the gear.
Probable cause: The pilot’s failure to follow the published emergency procedures to lower the landing gear following a loss of electrical power.
NTSB Identification: CEN12LA204
This March 2012 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.