21st of December 2009
On this day in aviation history in 2009, a DHC-8-402 (Dash 8) suffered a serious electrical failure after take-off from London Gatwick (UK), en route to Düsseldorf Germany.
The incident aircraft on an earlier flight.
With the aircraft climbing through 6000 feet the crew was presented with several cautions appearing nearly simultaneously;
L AC BUS R AC BUS L TRU R TRU #1 AC GEN #2 AC GEN
Along with that a series of system failures as a result of the serious electrical failures listed above. Among the failures also some of the icing protection systems were affected, limiting the ice protection of the aircraft. For that reason, the crew asked for a lower flight level than the cleared flight level of FL120. ATC cleared them to descend to FL110. When levelling off at FL110 they were still in icing conditions, with no possibility to get a lower altitude assigned to clear the icing conditions the crew were only left with the option to declare “PAN PAN” and asked for further descend and a Dash 8 AC Electrical System return to London Gatwick. During the subsequent descent, they cleared icing conditions around FL100.
The commander of the flight actioned the Emergency Checklist for the different cautions presented to them, after which he briefed the Senior Cabin Crew Member on their problems and intentions to return to Gatwick. Although they appeared to have a clean airframe (no ice built-up), the crew decided to fly the approach with flap 35, with an increased Vref. This in accordance with company procedures for flight in icing conditions. The approach was flown without further problems, followed by an uneventful landing.
After the passengers had disembarked and the crew was debriefed the maintenance department for the airline began troubleshooting the electrical system, which included engine runs and replacing both left and right AC Generators Control Units. A fault was found logged in the EPCU (Electrical Power Control Unit) for the left hand AC Generator. On replacing the left-hand AC generator the driveshaft was found to have sheared (a fail-safe feature to protect a damaged generator causing an undue drag on the engine). For troubleshooting purposes the right-hand AC generator was installed on the left engine, subsequent testing caused the driveshaft to fail on this generator as well.
Upon a check of the AC wiring in the aircraft extensive damage was found to a wiring loom in the left-hand centre wing trailing edge. Wires in this bundle (containing 22 wires) provided power to the systems that had failed in flight including the Left propeller anti-icing and several contactors and relays.
The damaged wiring loom (Left) and the blind rivet (Right)
Further investigation revealed that the wires had been chafing against the head of a blind rivet of a loom support bracket. As a result of this finding, a modification program was rolled out by the manufacturer to mitigate this issue and avoid reoccurrence on other Dash 8 aircraft.
The full Air Accidents Investigation Branch report can be found by clicking here.