On 19 February 1999, while on the tarmac at Townsville with the auxiliary power unit (APU) operating, the crew of A320 Airbus VH-HYT observed an advisory message for "oil quantity below 1/4" on the electronic centralised aircraft monitoring (ECAM) cockpit display. The limitations section of the operator's A320 Operating Manual stated that the minimum before start APU oil quantity was 1/4. However, it also noted that with the ECAM low oil level message displayed, the APU should be considered unserviceable until an engineering inspection was conducted. This included a check of the APU oil level and inspection of the APU compartment and air intake for oil contamination.
The pilot in command notified the operator's Townsville maintenance engineer of the ECAM APU oil quantity message. The operator's procedure in response to a low APU oil quantity ECAM advisory message required that the APU be inspected for gross oil leaks and that the aircraft maintenance log APU oil servicing records be reviewed to determine oil consumption. However, because the APU bay was a controlled fire zone, the operator's maintenance procedures specified that the APU access door not be opened while the unit was operating.
The engineer opened the APU bay access door and inspected the operating APU for oil leaks. He then returned to the cockpit, where he consulted the aircraft maintenance log to review the APU oil servicing records. Noting that oil had not recently been added to the APU, the engineer advised the pilot in command that he would replenish the APU oil.
Passengers were already on board for the flight to Brisbane. Because of the prevailing hot and humid conditions, the engineer decided that, for reasons of passenger comfort, he would leave the APU running while he replenished the oil. This decision to add oil to the APU while it remained operating was in violation of the operator's standard policy and procedures, which stated that replenishment of APU oil may only be carried out while the unit is not operating.
The engineer connected the oil supply line from a mobile oil dolly to the operating APU. The oil dolly was equipped with a pressurised supply tank, with delivery of oil from the tank being controlled by a hand-operated spool valve in the oil delivery line. The engineer had determined that 1 L of oil should be added to the APU, and he calculated that it would take 15 seconds to deliver that quantity from the oil dolly into the APU. After the oil had been added, the engineer returned to the cockpit to observe whether the ECAM advisory message had extinguished, leaving the APU access hatch open and the oil delivery line still connected to the operating APU. The ECAM advisory remained illuminated, so the engineer returned to the APU bay. As he was climbing onto the workstand, a fire broke out in the APU tailpipe and the APU shut down.
The surface movement controller in the control tower observed fire and smoke coming from the tail of HYT. He sounded the crash alarm and radioed HYT on the surface movement control frequency, but there was no response. Three fire trucks responded immediately and parked in a fanned position around the aircraft's tail, with one truck being parked under the APU exhaust. Two firemen climbed onto the top of that truck to better observe the source of the fire and to determine the appropriate fire-fighting measures to be employed.
The crew had been conducting pre-flight checks when they noticed the APU shutdown. At the same time, they heard the sirens from the fire trucks, and a customer service officer entered the cockpit and notified the crew that the APU was on fire. However, the engineer also entered the cockpit and stated that the fire was under control, but he did not inform the pilot in command that the fire was located in the APU tailpipe and not in the APU bay.
HYT was equipped with a fire and overheat detection system located in the APU compartment. The system was designed to provide for automatic APU shutdown and agent discharge in the event of fire or overheat in the APU compartment while the aircraft was on the ground. A fire warning light was fitted to the overhead panel in the cockpit to alert the crew in the event of an APU fire. However, because the fire was located in the APU tailpipe, it did not activate the APU fire detection system and consequently there was no fire warning. After being advised that an APU fire had occurred, the pilot in command elected to leave the aircraft to obtain further information about the nature of the problem and its effect on the safety of the aircraft. Before leaving the cockpit, he made a public address to the passengers to advise that the aircraft had experienced a problem with its airconditioning system, and that this would delay the aircraft's departure.
The pilot in command then left HYT through the left forward cabin door (L1) but did not brief the cabin manager, who was stationed at L1, about the nature of the problem. The cabin manager was therefore unable to plan for the possible evacuation of passengers from the aircraft. The pilot in command proceeded to the rear of HYT, where one of the firecrew informed him that the fire was still burning. The firecrew were unaware that passengers were already on board and when the pilot in command asked if they wanted the passengers off the aircraft, the fire controller instructed the pilot in command to disembark the passengers immediately. However, no instruction was given regarding doors that were not to be used for the disembarkation.
The pilot in command then ran back and instructed the cabin manager stationed at door L1 to disembark the passengers. Up until that point, the only information that the cabin manager had been given about the fire was from the customer service officer when the officer had entered the aircraft to advise the pilots of the problem. None of the other flight attendants were aware of the situation until the order to disembark the passengers was given. All passengers and crew were then disembarked through both the front and rear entry doors on the left side of HYT.
The airport fire crew discharged three 5 kg carbon dioxide bottles into the APU exhaust and the fire was extinguished. When the fire controller determined that HYT was safe, he released it to the crew. The engineer conducted a damage inspection of the aircraft and it was dispatched with the APU inoperative.
On arrival at Brisbane, the pilot in command lodged a general flight report stating that HYT had sustained an APU tailpipe fire at Townsville. The report noted that there was no fire warning or ECAM display associated with the APU tailpipe fire. The same day, the engineer reported to the operator's Melbourne maintenance base that HYT had sustained an APU tailpipe fire. Neither of these reports mentioned that the APU oil had been replenished while the unit was operating.
On 4 March 1999, the engineer lodged an accident/injury report of the event with the operator, noting that an APU tailpipe fire had occurred. On 5 March 1999, BASI received an air safety incident report from the company concerning the occurrence and on 8 March 1999, BASI also received an air safety occurrence report from RAAF Townsville.
On 10 March 1999, the operator interviewed the engineer. During the interview, it was established for the first time that the APU oil had been replenished while the unit was operating. The engineer advised the operator that on 21 February 1999, he and another engineer had inspected the spool valve of the oil dolly. The inspection was conducted to determine if oil continued to flow from the delivery hose with the spool valve in the closed position. The inspection revealed that the spool valve was faulty and that it had probably been faulty at the time of the occurrence.
It is likely that during the time the engineer was away from the APU, with the oil dolly still connected, the faulty spool valve in the oil delivery line permitted the oil to continue to flow into the APU, resulting in an overfilled condition with excess oil draining into the tailpipe. The APU then surged and automatically shut down. The surge was probably caused by some of the excess oil within the APU escaping through the bearings and entering into the combustion chamber, where it was ignited. This would have resulted in a rapid increase in the exhaust gas temperature. Flame from the combustion would then have "torched" through the turbine stage into the tailpipe, where it ignited the overflow oil that had drained into the tailpipe.
The pilot in command was initially provided inadequate information regarding the APU problem. He was aware that the APU had shut down, and was informed that there was an APU fire. However, this was not confirmed by an ECAM message due to the location of the fire in the tailpipe. This series of events was an unusual situation and did not fit with the pilot in command's expectations of an APU fire. Had he been properly informed of the circumstances of the fire, it is unlikely that he would have considered it necessary to inspect the APU. Consequently, he would have been able to more rapidly respond to ensure the safety of the passengers and crew.
The operator was unable to immediately follow its post-occurrence investigation procedures due to delayed and incomplete reporting of the circumstances of the occurrence.
Local safety action
As a result of this investigation the operator advised that the following actions were taken:
- All maintenance personnel were made aware of approved company maintenance procedures relating to APU's.
- Company personnel were instructed on the proper reporting procedures to ensure prevention of any further breakdown in the reporting chain.
As a result of this investigation the operator advised the following:
- The role and responsibility of personnel posted at reduced or single-engineer bases are to be clarified and reviewed.
- Arrangements for the ongoing supervision of personnel posted at reduced or single-engineer bases are to be reviewed.
- The occurrence is to be reviewed with respect to the company's standard crew resource management policy.
- The role and responsibility of ground personnel are to be reviewed and clarified with respect to non-normal and emergency situations.
|Date:||19 February 1999||Investigation status:||Completed|
|Time:||1700 hours EST|
|Release date:||10 April 2000||Occurrence class:||Operational|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Airbus Industrie|
|Type of operation||Air Transport High Capacity|
|Damage to aircraft||Minor|