Accident Summary
On June 2, 2016, about 0220 Pacific daylight time, a Piper PA 46-350P, N301DM, was substantially damaged during a gear-up landing at Boeing Field/King County International Airport (BFI), Seattle, Washington. The private pilot and passenger were not injured. The airplane was operated as a personal flight under Title 14 Code of Federal Regulations Part 91. The cross-country flight departed Reno, Nevada, about two and one-half hours prior to the accident and was conducted in night visual meteorological conditions.
Electrical Failure Sequence
The pilot reported that before being given a final vector for the instrument landing system (ILS) 13R approach to BFI, at an elevation of 2,200 ft mean sea level, he noticed that two tie bus generator circuit breakers had tripped with no noticeable failures. He pushed both circuit breakers in. Shortly after receiving the final vector, the entire panel went dark—post lights, backlit gauges, and engine instrument gauges failed. The G500, both GNS530s, two Garmin transponders, the GMX200, the audio panel, and the autopilot remained functional. The failure also affected the three green landing gear down and locked lights and the landing gear warning horn. The pilot proceeded with the approach to land as soon as possible. He lowered flaps and extended the landing gear sequentially. He felt the gear extend but could not determine if it was down and locked due to the absence of the green lights and warning horn. He subsequently made a gear-up landing on runway 13R, slid down the runway, and came to a stop upright, after which he shut down the engine.
Postaccident Examination
Later that morning, the pilot observed that the landing gear was extended by gravity free-fall. He noticed that one of the two tie bus main circuit breakers on the right side of the panel, an 80-amp breaker, was popped out. When he reset it, panel lights, gear warning horn, and gear cycle functioned normally, and the three green lights illuminated. An airframe and powerplant mechanic discovered that the second 80-amp main breaker on the left side was inoperative. During postaccident examination, the right main electrical bus circuit breaker on the tie bus panel was found open; when reset, functions returned. The left main electrical bus circuit breaker remained closed, but no conductivity to the main bus was found. A diode between the left circuit breaker and the main bus had a separated flag terminal, causing an open circuit. As a result, the right tie bus/main bus circuit carried the full load. When the right circuit breaker tripped, power was lost to the main bus, preventing landing gear deployment, gear warning lights, and unsafe warning horn.
Service Information Letter
On June 22, 2016, JetPROP LLC released Service Information Letter (SIL) 16-560-01 for aircraft converted prior to 2008 to replace the two 80-amp generator circuit breakers with 90-amp breakers. JetPROP had added Rev D to drawing 560.12.053 in April 2008 to upgrade these breakers to increase available current from 160 to 180 amps. JetPROP stated that the only submission regarding this change was the 2016 SIL.
The rectifier diode was examined by an NTSB Materials Engineer. A Systems Specialist report indicated that a solder joint between the diode and flag terminal failed in a tension/overstress condition along the bottom edge of the diode. The diode was functionally tested and found to be operational.