Description

On 31 January 2011, an Airbus A380-800 being operated by Singapore Airlines on a scheduled passenger service from Hong Kong to Singapore and nearing the end of the cruise at night was suddenly subject to a loud noise and signs of possible fire in one of the toilet compartments accompanied by a corresponding Electronic Centralized Aircraft Monitor (ECAM) Smoke alert for the same compartment. A fire extinguisher was discharged into the apparent source area and the location carefully monitored but no further signs of combustion were evident and the flight to destination was completed. The flight was being used as a Line Check with the Check Captain occupying one of the Supernumerary crew seats.

Investigation

The Incident occurred in international waters and the Investigation was conducted by the Singapore Air Accident Investigation Bureau. It was determined that there was no DFDR information that was relevant to the Investigation and noted that the Cockpit Voice Recorder (CVR) record of the period in which the incident occurred had been over-written.

It was established that about 45 minutes before arriving in Singapore, an ECAM message was annunciated of smoke in toilet compartment LM35 situated on the left hand side of the forward main deck. At the same time, the Senior Cabin Crew Member (SCCM) had been in that compartment and he had heard a loud bang. The lights in the compartment had gone out and an “electrical burning smell” was apparent. The Flight Crew were informed and when smoke was seen coming from the base of the wall panel under the compartment basin, the SCCM positioned a fire extinguisher under the basin pointing in the direction of the smoke and discharged it. It was stated that within about 10 minutes, the smoke and smell of burning had cleared. The compartment door was left open for the rest of the flight for monitoring and no further signs of fire or smoke were seen.

Inspection of the aircraft after completion of the flight found signs of burning on the feeder terminal block for the No.1 Variable Frequency Generator (VFG) located in the forward cargo compartment below the affected toilet compartment, on the feeder cables connected to the feeder terminal block and on the insulation blankets around this feeder block. The inner surface of the cover of this feeder terminal block was partly covered in soot but there was no sign of any heat damage. The Lightning Protection Units (LPUs) connected to the feeder terminal block showed signs of melting.

After replacing the burnt components and performing a No 1 engine idle power ground run, the aircraft was released to service but when take off thrust was applied, the ECAM alert message “ELEC GEN 1 FAULT” appeared and so the take off was abandoned and the aircraft returned to the gate. Further investigation found that the No 1 VFG Main Excitation Cable, which contains two wires in which voltages are differentially varied by the associated No 1 GGPCU in order to create a means of voltage control, was degraded with the sheath and shielding of the cable and the insulation of one of the wires within it both damaged. No heat or stress damage was found in the vicinity of damaged cable and it was found that no maintenance work had been performed on it since the aircraft had been delivered.

The non-volatile memory (NVM) from the No 1 Generator and Ground Power Control Unit (GGPCU) which controls the No 1 Engine VFG was downloaded and showed that at the time of the in-flight event being investigated, the GGPCU Fast Over Voltage (FOV) protection logic had failed to trigger when the No 1 VFG output voltage reached the established maximum figure of 181 Vac because the negative wire voltage was above 19Vdc. Examination of the main excitation cable showed that the negative main excitation wire had been damaged by electrical arcing with the cable shielding which surrounded the both wires in the cable. It was not possible to be certain of the origin of the damage although it was certainly considered possible that damage or imperfection could have arisen during manufacture or installation at build.

The same NVM data also showed that the Over Current protection for the No 1 VFG output had triggered correctly so that the consequences of the fault had been contained.

It was concluded that the LPU damage was a consequence of excessive current flowing through them because they were exposed to a high voltage over a longer period than their lightning protection function requires. The No 1 VFG and the No 1 and No 6 GGPCUs were sent for examination by the respective OEMs and found serviceable.

With this information assembled, the Investigation was able to explain the recurrence of the ECAM ELEC GEN 1 FAULT messages which had appeared twice during No 1 engine starts on the day of the investigated event and, as noted above, when high thrust was first set following the initial replacement of damaged components. They had been caused by the damaged No 1 VFG main excitation wire but because the troubleshooting and test procedures in place at the time had not required a full wiring continuity and insulation check, the root cause of the ECAM indication had not been found.

On the subject of fire detection and suppression, it was concluded that:

• there was no fire detection or suppression system installed on the aircraft to detect fire in the vicinity of the feeder terminal block
• there was no way for the cabin crew to access the damaged feeder terminal block in order to fight any fire there
• the flow of the fire extinguishing agent which had been discharged from the toilet compartment could not reach a fire at the feeder terminal block
• 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.

A number of Safety Actions consequent upon the findings of the Investigation were noted:

• The Operator completed an inspection of its A380 aircraft fleet for degradation of the Main Excitation Cable and found no similar wire degradation

• Airbus issued an All-Operator Notice on 4 April 2011 “to disallow flight crew to reset the generator in the case of ELEC GEN 1(2)(3)(4) FAULT or ELEC APU GEN A(B) FAULT”

• The OEM responsible for the Fast Over Voltage (FOV) protection logic in the Generator and Ground Power Control Units (GGPCU) installed on the aircraft has redesigned it so that it is triggered when the negative Main Excitation Wire voltage is higher than 95% of the positive Main Excitation Wire voltage. This new logic has been validated for a range of short circuit impedances and will be implemented as a modification to all GGPCUs.

• Airbus issued (on 1 October 2011) a revision to the relevant troubleshooting procedure which requires an insulation check of the excitation line circuit to be performed. This circuit includes the Main Excitation Cable.

Four Safety Recommendations were made as a result of the Investigation as follows:

• that Airbus review the design of the lightning protection system to prevent short circuiting of the feeder cables when excessive voltage is output by the Variable Frequency Generator [Recommendation R-2012-002]

• that Airbus review the need for fire detection and suppression in the vicinity of the feeder terminal block. [Recommendation R-2012-003]

• that the European Aviation Safety Agency require Airbus to review the design of the lightning protection system to prevent short circuiting of the feeder cables when excessive voltage is output by the Variable Frequency Generator. [Recommendation R-2012-004]

• that the European Aviation Safety Agency require Airbus to review the need for fire detection and suppression in the vicinity of the feeder terminal block. [Recommendation R-2012-005]

The Final Report of the Investigation was published on 2 October 2012.

Further Reading

• Fire Smoke and Fumes
• Fire in the Air
• In-Flight Fire: Guidance for Flight Crews
• Passenger Cabin Fire
• Aircraft Fire Detection Systems