Monday, 22 June 2020

Partial electrical failure on a 737 bound for Brussels caused mayday.

The UK's Air Accidents Investigation Branch based at Farnborough has released its report into a partial electrical failure on a Boeing 737-4Q8 of West Atlantic UK, registration G-JMCR on 4 June 2019.

The cargo flight was being flown by two pilots, the captain aged 34 had some 2525 hours worth of flying experience with around 2325 on a 737. The aircraft was en route from Oslo Gardermoen Airport, Norway to Brussels National Airport, Belgium with the commander, a company line training captain, in the right seat as the pilot monitoring and the co-pilot, who was completing his command upgrade line training, in the left seat as the pilot flying.
The weather was forecast to be thundery in the Brussels area and the pilots heard ATC directing other aircraft around active thunderstorms as they approached the airport.  While descending to land at Brussels National Airport, at 1846 the pilots heard a noise which they described as a “large electrical clunk”.   This was accompanied by the loss of the primary Electronic Flight Instrument System -EFIS on the left side of the cockpit and the disconnection of the autopilot and autothrottle. The commander immediately took control as PF and flew the remainder of the flight manually, with the co‑pilot assuming the role of the pilot monitoring. In addition to the loss of the EFIS screens, both control display units for the Flight Management Computer (FMC) were inoperative and several caution and advisory warnings had illuminated, including for the aircraft pressurisation and aft fuel pump. 


The pilot declared a MAYDAY and aware that a thunderstorm was approaching the airfield, assessed that the weather reported by Air Traffic Control (ATC) would allow him to continue and land at Brussels. However, visual references were lost at a late stage of the approach when the aircraft entered a heavy rain shower. A go-around was initiated during which the pilots estimated the amount of thrust required; the aircraft initially appeared to be slow to accelerate and establish a positive rate of climb. At around this time, the captain noticed that the Enhanced Ground Proximity Warning System (EGPWS) was also not working. The aircraft entered an orbit and subsequently landed successfully from a second approach.


The electrical failure was caused by a fault in the transfer relay which resulted in the loss of power to a number of electrical buses. Following the electrical failure, the commander’s assessment was that the aircraft was in a stable condition so continued the approach to land at Brussels National Airport. This gave the pilots relatively little time to assess the situation and a number of non-normal checklists actions were not carried out; consequently, the aircraft was incorrectly configured for the approach and landing.
At a late stage of the approach, the pilots lost visual references and executed a go-around. The aircraft then orbited while the thunderstorms cleared the airfield and the pilots used the time to further analyse the failure. The second approach and landing were uneventful. Data recording on the Flight Data Recorder and Cockpit Voice Recorder stopped when power was lost as a result of the failure of the electrical failure.
The AAIB examined the 737 G-JMCR at Brussels National Airport with support from the operator and a local maintenance organisation. The aircraft was connected to an electrical ground power supply and the inoperative systems were confirmed. Circuit breaker C819, which is a 35-amp circuit breaker located between Generator Bus 1 and Transfer Relay 1, was found open as a result of an internal short circuit in Transfer Relay 1. C819 was found to be serviceable. Transfer Relay 1 was replaced, and the aircraft electrical system was tested and found serviceable; the relay had been manufactured in 1985 and been in operation for 22 years before being fitted to G-JMCR in October 2018. No anomalies were found in the service history of the relay.
The electrical failure was caused by a fault in the transfer relay which resulted in the loss of power to a number of electrical buses. Following the electrical failure, the commander’s assessment was that the aircraft was in a stable condition so continued the approach to land at Brussels National Airport. This gave the pilots relatively little time to assess the situation and a number of non-normal checklists actions were not carried out; consequently, the aircraft was incorrectly configured for the approach and landing. 
Safety action has been taken by the operator to provide clarity in the aircraft documentation.














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