What happened
On July 13, 2017, a Beech 95-B55 operating a commercial flight from Diepholz to Sylt experienced a significant electrical malfunction during its approach to runway 32. While established on the ILS, the aircraft suffered a total loss of electrical power following the simultaneous failure of both alternators. The crew did not immediately report the failure to air traffic control, as they were focused on executing emergency checklists and reducing battery load to maintain sufficient voltage for the landing.
During the landing sequence, the crew extended the landing gear. Although they briefly observed a green gear indication, the battery voltage dropped rapidly thereafter, preventing further communication with ATC. The crew attempted to verify the gear position via a nose gear position indicator and a visual check in an engine nacelle mirror. Believing the gear was secure, they proceeded with the landing. Upon touchdown, the nose gear collapsed, causing the aircraft to come to a stop on the runway centerline. There were no injuries to the two pilots or two passengers, though the aircraft sustained light damage.
The investigation
The BFU investigation focused on the electrical system and the landing gear mechanism. Investigators found that the aircraft's two 12-volt batteries, connected in series, experienced a rapid voltage drop from 21 volts to approximately 15.7 volts within two minutes of being under load. Technical examination of the electrical wiring revealed that one cable (P41A18) had short-circuited internally to its shielding, which subsequently caused another cable (P55A18)—the sole supply for the alternator field excitation—to burn out. This failure prevented the alternators from providing power to the system.
Regarding the landing gear, the investigation determined that the gear had not reached its fully extended and locked position. The mechanical gear position indicator in the cockpit was found to be unreliable due to inherent design tendencies toward inaccuracy caused by friction and bending. Furthermore, the investigation noted that the manual emergency gear extension handle was located under a carpet, which had to be moved to access the mechanism.
Findings
- A short circuit in the electrical wiring caused the failure of the alternator field excitation cable, leading to a total loss of alternator power.
- The crew relied on a visual check in a mirror and a mechanical indicator that could not definitively confirm the gear was locked.
- The landing gear had not reached its end position, meaning the gear was not fully locked before touchdown.
- The high resistance encountered during the final turns of the manual emergency extension process can lead a pilot to mistakenly assume the gear is fully extended.