History of the Flight
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 with the No 2 autopilot engaged, the aircraft experienced a loss of DC electrical power about 8 nm west of Milton Keynes. The master caution aural alert sounded, and the TRU 1 and dc lo volts central warning panel lights illuminated, indicating the No 1 Transformer Rectifier Unit (TRU) was no longer supplying 28 VDC to the No 1 essential DC busbar.
The crew performed Emergency and Abnormal Checklist Card 49. The commander attempted to reset TRU 1 without success. He then switched off the No 1 non-essential DC busbar and selected the DC essential busbar couple to connect the No 1 and No 2 essential busbars. After these actions, the low voltage warning extinguished, and the No 1 battery voltage indicator showed normal range. The commander noted a possible busbar relay fault rather than a TRU fault.
The crew decided to return to East Midlands Airport. Approximately 15 minutes later, the commander's flight director failed, and the low voltage warning re-illuminated. The No 1 battery voltage dropped to 12V. Subsequently, the commander's electronic flight instrumentation system (EFIS) primary flight display and navigation display became corrupted, the autopilot disconnected, and the quick access recorder (QAR) stopped. The commander declared a PAN.
During descent, the No 1 engine control frozen indication and standby controls fail warnings appeared, flight deck lights flickered, and a pulsing sound was heard on the radios. The aircraft landed uneventfully on Runway 27 at East Midlands Airport, flown by the co-pilot whose displays functioned normally. Upon landing, the pulsing sound briefly returned. As the aircraft vacated the runway, the dc lo volts and tru 1 warnings extinguished, and power was restored to the commander's instruments.
Aircraft Electrical System
The BAe ATP electrical system comprises two engine-driven AC generators supplying 200/115 VAC. TRUs convert AC to 28 VDC. Two nickel-cadmium batteries provide emergency power. Two solid-state inverters supply 200/115 VAC 400 Hz. The DC essential busbars are supplied by TRUs and batteries. Non-essential busbars are fed from essential busbars. The No 1 inverter is supplied from the No 1 essential busbar; the No 2 inverter from the No 2 non-essential busbar. The DC LO VOLTS warning illuminates when a DC essential busbar voltage falls below 24.5 V after a 5-second delay.
Examination
Post-incident, the operator's maintenance organisation conducted visual inspections and continuity tests of the TRUs, DC essential busbars, and wiring, finding no abnormalities. High-load testing up to 230 A from a single TRU also showed no issues. The No 1 and No 2 batteries, No 1 TRU, and TRU contactors were removed and functionally tested with no defects identified. The aircraft was returned to service without further DC electrical failures.
Recorded Information
The aircraft was equipped with a two-hour cockpit voice recorder (CVR), a 25-hour flight data recorder (FDR), and a QAR. The CVR recorded the 49-minute flight. Both FDR and QAR recordings stopped before landing due to an intermittent recording fault with the FDR. The FDR (model PV1584) was installed in the rear equipment bay below the cabin floor, beneath a large freight door (LFD) opening. The FDR recorded onto magnetic tape and was not hermetically sealed, allowing moisture ingress. Conformal coating was applied to circuit boards but not connectors, which could cause intermittent failures when exposed to moisture. The FDR was not tested for waterproofness; such testing was introduced for later solid-state recorders.
Safety Recommendations
The investigation issued two Safety Recommendations: one to prevent moisture entering the FDR on BAe ATP aircraft with the Large Freight Door modification, and another to replace flight recorders using magnetic tape.