Aerovodochody L-29 DELFIN N29AD accident near Roswell, New Mexico

Probable cause

The malfunction of a diode in the airplanes electrical system that resulted in low batter power and subsequent landing gear collapse.

NTSB analysis

The pilot of the retractable-landing-gear airplane reported that, following takeoff, the landing gear failed to retract when commanded. Multiple attempts to cycle the landing gear were unsuccessful, leading the pilot to suspect a malfunction within the hydraulic system, possibly involving the hydraulic pump. The pilot elected to leave the landing gear extended and return for landing.

Prior to landing, all three landing gear indicated down and locked through both the primary electrical indication system and the secondary mechanical indicators (barber pole indicators located on the wings and nose). In addition, a visual inspection conducted by a pilot in a nearby airplane confirmed that all three landing gear appeared to be in the down-and-locked position.

The landing and subsequent rollout were uneventful until the aircraft decelerated to approximately 60 knots and began moving toward the left side of the runway in accordance with racing procedures. At that time, the right main landing gear collapsed, the right wing contacting the runway surface and the airplane subsequently departed the runway. The airplane sustained substantial damage to the right wing.

Post-accident examination of the airplane’s hydraulic system revealed no mechanical malfunctions or failures that would have precluded normal operation. Examination of the airplane’s electrical system revealed that the battery was in a low-charge condition. Testing showed that battery voltage decreased from about 19 volts to 11 volts when the electrically driven hydraulic pump was activated. Further inspection identified a failed reverse-current diode that prevented the generator from recharging the battery during operation. The airplane was routinely started using a ground power unit and the battery was not being fully recharged between flights. As a result, insufficient system voltage was available to adequately power the hydraulic pump, which prevented normal landing gear retraction and extension.

Text from the NTSB. Open the NTSB docket for WPR26LA343 for the full record.