What happened
On 4 July 2008, a Piper PA-32R-300, registration N101DW, was conducting a private flight from Panshanger, Hertfordshire, to Oostend, Belgium. While crossing the English coast at approximately 5,000 feet, the aircraft experienced a sudden failure of its electrical generation system. This failure led to the loss of the autopilot and the primary GPS receiver, followed by a total loss of all electrical services.
As the electrical systems failed, the pilot diverted to Southend Airport. Due to the loss of primary communications, the pilot used a handheld radio to broadcast blind transmissions to air traffic control. Upon approaching Runway 2 and attempting landing checks, the pilot selected the landing gear to the down position but failed to observe that the gear had not deployed. The aircraft touched down and slid along the runway centerline before veering toward the left edge of the runway. There were no injuries to the pilot or the three passengers, though the aircraft sustained damage to the propeller, flaps, engine cowlings, and the underside of the fuselage.
The investigation
The AAIB examined the aircraft's electrical system and found that while the battery remained connected, the voltage dropped rapidly from 11.6 volts to 6.5 volts once the master switch was engaged. The investigation revealed that the alternator's main power output cable had poor-quality terminations, with the main feed cable having failed through much of its cross-sectional area at the connection point to the alternator.
Additionally, the investigation noted that the ammeter was displaying a significant zero error of approximately 10 amps, and the alternator failure warning light was too dim to be easily noticed. The aircraft's landing gear was found to be functional via the manual override lever, which allowed it to deploy under gravity during recovery.
Findings
- The primary cause of the electrical failure was the deteriorated condition of the electrical connections to the alternator, specifically a failure in the main power output cable.
- The battery was likely initially charged but was unable to sustain the electrical load as the alternator failed to provide sufficient output.
- The pilot's ability to detect the developing electrical issue was hindered by a faulty ammeter and an insufficiently bright alternator warning light.
- The landing gear was not extended because the pilot did not observe that the gear had failed to deploy during the landing checks.