What happened
On 3 May 2018, a British Aerospace ATP, registration SE-MHF, was operating a cargo flight from East Midlands Airport to Stansted Airport. While cruising at FL110, the crew received a master caution alert indicating that the No 1 Transformer Rectifier Unit (TRU) had failed to supply voltage to the No 1 essential DC busbar.
Following the initial failure, the crew attempted to reset the unit and successfully coupled the No 1 and No 2 DC essential busbars. However, the electrical situation deteriorated approximately 15 minutes later when the flight director failed and the busbar coupling function failed. This led to a significant loss of electrical services, including the corruption of primary flight display screens, the disconnection of the autopilot, and the failure of the flight management system. During the descent, the crew also experienced flickering flight deck lights and a pulsing sound on the radios. Despite these complications, the crew performed a normal landing at East Midlands Airport.
The investigation
The investigation focused on two distinct issues: the electrical system failures and an intermittent fault within the Flight Data Recorder (FDR).
Regarding the electrical system, investigators examined the TRUs, batteries, and associated wiring. While the failures were consistent with a failure of the No 1 TRU or its contactor, followed by a failure of the busbar tie contactors, subsequent testing of the aircraft's electrical system could not replicate the faults. The restoration of power upon landing suggested the failures may have been intermittent or triggered by airframe vibration.
Regarding the FDR, investigators discovered that the unit was recording intermittently and had failed to overwrite old data for several hours. Upon disassembly of the PV158 and FDR, evidence of moisture residue was found on the internal metal panel. The investigation determined that rainwater could enter the cargo bay during loading via the Large Freight Door (LFD) and migrate to the rear equipment bay where the FDR is located.
Findings
- The initial loss of DC power was caused by a failure of either the No 1 TRU or its contactor.
- A subsequent failure of the DC essential busbar coupling function occurred, likely due to a busbar tie contactor failing open.
- The intermittent recording fault in the FDR was caused by moisture ingress into the electronics module.
- The FDR's design lacked the waterproofness required to prevent damage from dripping liquids, a vulnerability exacerbated by the aircraft's LFD modification.