Singapore A388 near Singapore on Jan 31st 2011, burned wires in forward cargo hold
Last Update: October 8, 2012 / 11:33:11 GMT/Zulu time
Incident Facts
Date of incident
Jan 31, 2011
Classification
Incident
Airline
Singapore Airlines
Aircraft Registration
9V-SKD
Aircraft Type
Airbus A380-800
ICAO Type Designator
A388
The AAIB reported in addition to the preliminary report, that a "ELEC GEN 1 FAULT" message had appeared during engine start for the flight from Singapore to Hong Kong already. The Variable Frequency Generator (VFG) on the electric generator was reset, the fault cleared and the aircraft departed.
During engine start in Hong Kong the fault message occurred again, but did not clear upon resetting the VFG. The engines were shut down and the aircraft towed back to the gate. Maintenance subsequently interchanged the generator control units of engine #1 and APU, tests of both were subsequently satisfactory and the aircraft was released to the flight with no fault message occurring during engine start and departure.
About 45 minutes prior to estimated landing in Singapore the crew received an ECAM message indicating smoke in lavatory LM35. At the same time the purser was in the lavatory LM35 and heard a loud bang, the lights went out in the lavatory and an electrical burning smell appeared. The purser asked two flight attendants to take care of the lavatory while he reported the occurrence to the flight crew. The flight attendants attempted to find any source of heat but did not detect any heat. When the purser returned to the lavatory he saw smoke coming from the base of the wall panel underneath the sink but could not locate the source of the smoke. He discharged a fire extinguisher towards the bottom of the wall panel. Smoke and smell cleared about 10 minutes later. There was no further incident.
None of the 381 people on board were injured.
Maintenance found the feeder terminal block for VFG #1 in the forward cargo compartment underneath lavatory LM35 damaged showing signs of burning as well as on the feeder cables to that block and the insulation around the block. The inner surface of the cover of the block did not show heat damage but some soot. The lightning protection units (LPU) connected to the feeder terminal block showed signs of melting. The burnt components were all replaced, a functional test was concluded satisfactorily and the aircraft was dispatched for a test flight. While the engines accelerated for the test flight on Feb 20th 2011 however a ECAM message "ELEC GEN 1 FAULT" appeared again, the takeoff was rejected.
During subsequent troubleshooting maintenance discovered the main excitation cable was damaged. The cable had been installed since delivery of the aircraft, no work had been performed on it since delivery.
Subsequent checks of the non-volatile memory of the generator control units revealed that a peak amperage of 1511A had been reached with the over-current protection logic inhibiting the output of the VFG with currents above 435A.
Failure analysis of the excitation cable identified the cable had been damaged by arcing between the negative (blue) wire and the shield of the cable.
The LPUs, effectively Zener diodes, would permit any surges of voltage above 270V to flow to the electrical ground in order to protect damage to the electrical system. The damage caused to the LPUs was identified as result of excessive currents through the LPA.
The fire extinguishing agent discharged by the purser had not reached the terminal block.
The AAIB analysed that there was a short circuit between negative wire and shield of the excitation cable which caused voltage output by the VFG #1 to exceed the nominal 143 Vac. The over voltage protection logic however did not trigger due to the negative wire's voltage being above 19 Vdc. The voltage continued to increase until the LPUs, designed to safe guard in case of lightning strikes, conducted for longer than they were designed for, which resulted in a short circuit between the feeder cables attached to the terminal block and electric ground causing excessive currents through the feeder cables. "The design of the lightning protection system was such that it did not prevent the feeder cables from being shorted to the electrical ground when excessive voltage is output by the VFG."
The excessive current through the feeder cables overheated and damaged the feeder block and the LPUs. Eventually the over-current protection triggered and limited the damage.
The electrical arcing at the excitation cable suggests there may have been a prior damage to the cable, this however could not be ascertained.
The AAIB further analysed: "The fire that damaged the feeder terminal block had probably extinguished by itself but it remains a concern that there is no sure way of detecting and extinguishing a fire in that area."
2 safety recommendations were released each to Airbus and EASA as result of the report recommending to review the design the lightning protection system with regards to excessive voltage released by the VFG and review the need of a fire detection and suppression system at the feeder block.
Immediate safety actions included the aircraft manufacturer issuing a notice on Apr 4th 2011 disallowing resetting the generators in case of "ELEC GEN x FAULT" messages. The generator control unit's over-voltage protection logic was redesigned.
Incident Facts
Date of incident
Jan 31, 2011
Classification
Incident
Airline
Singapore Airlines
Aircraft Registration
9V-SKD
Aircraft Type
Airbus A380-800
ICAO Type Designator
A388
This article is published under license from Avherald.com. © of text by Avherald.com.
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