What happened
On 3 May 2018, a BAe ATP, registration SE-MHF, was conducting a commercial cargo flight from East Midlands Airport to StanHD Airport. While cruising at FL110, the crew received a master caution alert indicating that the No 1 Transformer Rectifier Unit (TRU) was no longer supplying voltage to the No 1 essential DC busbar. The crew followed emergency checklists to couple the DC essential busbars, which temporarily restored voltage levels.
However, the electrical situation deteriorated during the descent. The flight director failed, and the crew observed the re-illumination of the low voltage warning. Shortly thereafter, a series of cascading failures occurred: the primary flight display and navigation screens became unreadable, the autopilot disconnected, and the aircraft's engine control and standby controls experienced issues. The crew also noted a pulsing sound on the radios and flickering flight deck lights. Despite these significant instrument losses, the crew successfully navigated a visual approach and performed a normal landing at East Midlands Airport. Upon landing and vacating the runway, electrical power to the flight deck instruments and lights was spontaneously restored.
The investigation
Investigators examined the aircraft's electrical system and the flight data recorders. While subsequent testing of the TRUs, batteries, and busbar contactors failed to replicate the electrical failures, the investigation focused on the intermittent nature of the fault.
Regarding the flight recorders, the investigation found that the Flight Data Recorder (FDR) had been recording intermittently, resulting in several hours of missing data and a failure to overwrite old data. Upon disassembly of the FDR, investigators discovered moisture residue and corrosion on the circuit boards and connectors. The investigation also looked into the aircraft's Large Freight Door (LFD) modification, noting that rainwater could enter the cargo area during loading and potentially migrate to the rear equipment bay where the FDR is located.