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Friday, 18 March 2011

Airbus Updates No.128

Report: Air Canada A333 near Montreal on Apr 30th 2008, inflight engine fuel starvation

By Simon Hradecky, created Tuesday, Mar 15th 2011 12:05Z, last updated Tuesday, Mar 15th 2011 12:06Z
An Air Canada A330-300, registration C-GFAH performing flight AC-418 from Toronto,ON to Montreal,QC (Canada) with 228 passengers and 10 crew, was enroute when several fuel pump low pressure warning appeared. The crew switched those fuel pumps off according to checklists. During the descent into Montreal low pressure warnings also appeared on the remaining fuel pumps and were switched off too, the engines (Trent 772) continued to run with fuel fed by gravity. When the aircraft levelled off at 11000 feet the left hand engine rolled back below idle causing an engine stall followed by an engine fail message on the ECAM. The crew declared emergency, switched all fuel pumps back on, the left engine recovered shortly thereafter. The crew continued for a safe landing with 4500 kg (9910 lbs) of fuel remaining.

The Canadian Transportation Safety Board released their
final report releasing the following findings:
Findings as to Causes and Contributing Factors

1. The main fuel pump low-pressure warnings were erroneously generated by the possible freezing of fuel pressure switches. The result was that normally operating fuel pumps were switched off as per the displayed electronic centralized aircraft monitor (ECAM) procedure.

2. Standby pump low pressure occurred due to air ingestion into the pump inlet port at fuel quantities slightly above 2500 kg. This may occur with a fuel quantity up to 2750 kg in an inner tank when the main fuel pumps are inoperative.

3. The A330 fuel system design results in air ingestion through the standby fuel pump inlet port during gravity fuel feeding operations with inner tank fuel quantities below 2500 kg. As a consequence, the left engine rollback occurred.

Findings as to Risk

1. The position of the fuel low-level sensors below the standby pump inlet port will result in standby pump starvation before ECAM fuel low-level activation when the main fuel pumps are inoperative. Therefore, a crew may be unaware of an imminent engine failure due to air ingestion into the fuel line with fuel quantities below 2750 kg.

2. The current A330 documentation does not alert crews of the fuel system vulnerabilities at low fuel quantities. Therefore, under certain failure conditions, crew actions that initiate gravity fuel feeding operations may result in an engine failure.

3. The company anticipated low fuel ECAM (ALFE) procedure contains items that may be in conflict with Airbus recommended procedures. Therefore, crews may be confused and omit critical items from the Airbus recommended procedures, thereby increasing the risk of fuel starvation.

4. Dispatch under minimum equipment list (MEL) with one main fuel pump inoperative will expose the flight to a risk of fuel starvation when the inner tank fuel quantity falls below 2500 kg and the remaining main fuel pump also becomes inoperative.

5. The fuel tanks had not been water drained as per the company procedure. The aircraft operated for an extended period of time without the draining of the fuel tanks, thereby increasing the risk of water contamination.

6. The cockpit voice recorder (CVR) was not deactivated and preserved following the event, resulting in the cockpit conversations being overwritten. Consequently, CVR information relevant to the occurrence was not available to TSB investigators.

Other Finding

1. Following the event, the aircraft was inspected, repaired, refuelled, test flown and returned to service without prior coordination with the TSB. Therefore, much of the evidence related to this incident was handled by several parties before the TSB investigation began.

The aircraft had been dispatched under Minimum Equipment List requirements with the right hand integrated drive generator inoperative, requiring the APU to run throughout the flight.

Air Canada's flight planning system therefore computed the airplane should carry 9800 kg (21600 lbs) of fuel for departure for the 51 minutes long flight. 3854kg of fuel were uploaded in Toronto bringing the total quantity of fuel to 9900kg with 450kg distributed into the outer left and right hand tank each and 4500 kg of fuel distributed into the inner left and right hand tanks with no fuel in the trim tanks.

The captain (ATPL, 15882 hours total, 660 hours on A330 and A340 with 350 hours as captain) was pilot monitoring, the first officer (ATPL, 14370 hours total, 807 hours on A330 and A340) was pilot flying.

The initial climb was without event. About 7 minutes into the flight a right hand fuel standby pump low pressure message appeared briefly on the ECAM. When the aircraft climbed through FL250 a right main fuel pump #2 low pressure warning occurred, the stand by pump automatically activated. The crew worked the according checklists and turned right pump #2 off. A minute later a right main fuel pump #1 low pressure warning activated, the pump was also switched off according to checklists. The crew performed a fuel leak check and determined that a fuel leak was not the cause of the low pressure warnings. At that time the left fuel pump fault light began to flicker.

The aircraft levelled off at FL350 and the crew turned the right hand fuel pumps back on to see whether they would perform better with a lower nose attitude, the low pressure warnings however continued prompting the crew to turn the pumps back off.

About 2 minutes after reaching FL350 the trim tank fuel pump low pressure warning activated confirming the trim tank was empty. Another minute later both left hand fuel main pump low pressure warnings activated. Both left hand main fuel pumps were turned off, fuel was now fed from both tanks via the respective stand by pump remaining.

The crew advised dispatch and considered a diversion but rejected a diversion due to the proximity of the destination airport.

After reaching the top of descent and initiating the descent towards Montreal the crew attempted again to turn on all fuel pumps to see whether the lower nose attitude would allow the pumps to operate normally, the low pressure warnings however persisted and all main pumps were turned off again.

When the airplane descended through FL200 about 8 minutes later the crew performed the gravity fuel feed procedure was carried out as a precaution in case one of the stand by pumps would fail. The TSB could not determine whether the cross feed valve was closed at that time as required by the procedure.

3 minutes later, the aircraft descended through 16,000 feet the low pressure warnings for both stand by pumps activated. The crew carried the according checklists out, opened the cross feed valve and switched the fuel pumps off resulting in gravity fuel feeding to the engines.

Another 3 minutes later the airplane levelled off at 11,000 feet on autopilot and autothrust commanded the engines to accelerate to maintain speed. When both engines accelerated through about 65% N1, the left engine's fuel flow suddenly decreased and the N1 reduced to below flight idle resulting in an engine stall warning on the ECAM shortly followed by an engine fail warning.

The crew declared emergency, the autopilot was disconnected, both thrust levers brought to the Maximum Continuous Thrust detent (MCT), and all fuel pumps were turned on. The left engine remained below idle for about 30 seconds before the engine accelerated again and reached the same speed as the right hand engine at approx. 85% N1. Although all fuel pumps were switched on and both engines were running the fuel pump indications remained amber on the ECAM continuing to indicate a fault condition. At that point there were about 5000kg of fuel on board.

During the approach the crew manually increased thrust to verify the engine response and saw a synchronous acceleration of both engines. The approach was continued for a safe landing with 4500 kg of fuel remaining, which matched predictions of the flight plan.

The cockpit voice recorder was not secured, the operator however downloaded the flight data recorder, the TSB later retrieved these data from a secure website of the operator. The aircraft was examined, refuelled, test-flown and returned to service without coordination with the TSB.

The TSB reported that the operator performed a water draining procedure at some stage following the event retrieving 4.5 liters of water from each inner tank. Airbus considers this amount normal. Air Canada's procedures require the draining procedure to be conducted at each service check (about every 7 days). The last water draining procedure had been performed 26 days prior to the incident however.

Fuel samples were taken from all 6 fuel pressure switch sensing lines and revealed the presence of suspended particles. Free water droplets were also observed in the fuel samples.

All fuel pumps were found operating normally, however were replaced as a precaution. Following the replacement and disassembly cavitation marks were found inside the incident fuel pumps.

Both left and right hand fuel metering units, fuel control and monitoring computers and engine electronic controls were also replaced as a precaution. All units were sent to the manufacturers for examination and were found free of fault.

The fuel pressure switches were found fault free and remained on the aircraft.

Following the troubleshooting the aircraft performed a test flight on May 4th and returned to service on May 5th. In June Airbus recommended to replace the non-return valves which was promptly carried out. The aircraft had flown 611 hours in 85 cycles since the incident by then with no further abnormal fuel indications.

The TSB analysed that although both left and right hand standby fuel pump low pressure warnings occurred simultaneously, only the left hand engine rolled back. The TSB determined therefore that the fuel quantity in the left hand inner tank was lower than in the right hand inner tank permitting the right hand engine to continue to run.

The TSB further analysed that the MEL did not take into account possible air ingestion at fuel quantities below 2750 kg in the inner tanks and identified other system failures like a single bus failure to produce a similiar vulnerability to the fuel system operating under MEL conditions. The documentation did not identify any of these vulnerabilities leading to a possible engine failure.

The TSB further analysed that the stand by pump design did not meet expectations as the stand by pumps could become starved with 2750 kg of fuel remaining in the inner tanks while the main fuel pumps were capable of extracting fuel down to 8.3kg remaining in the inner tanks. The stand by fuel pump feed port allowed air ingestion at fuel levels below 2500kg.

The investigation in the end determined that the main fuel pump low pressure warnings were most likely erroneous but the stand by fuel pump low pressure warnings were authentic. The investigation also concluded that the roll back of the right hand engine was imminent had the crew not switched all fuel pumps back on.

As a result of the investigation Airbus revised documentation in November 2008 advising, that 2000kg of fuel in the affected fuel tank can not be used in gravity fuel feeding. Airbus further changed the Minimum Equipment List requirements in October 2009.

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