Singapore B773 enroute on Jun 27th 2016, engine fuel leak into engine oil system
Last Update: March 14, 2017 / 16:35:13 GMT/Zulu time
The airline reported the aircraft returned due to an engine oil warning light, the right hand engine caught fire after the aircraft touched down, the fire was put out by airport emergency services. The passengers disembarked via stairs and were bussed to the terminal.
A replacement Boeing 777-300 registration 9V-SWF reached Milan with a delay of 8 hours.
On Aug 1st 2016 Singapore's AAIB released an interim statement stating, that fuel was found in the right hand engine's oil system. The investigation determined that the fuel entered the oil system through a crack in a tube of the engine's fuel-oil-heat-exchanger. The engine manufacturer had released a service bulletin (SB) in December 2014 requiring removal of certain fuel-oil-heat-exchangers from the engines, inspect for cracks in the fuel tubes and do needed repairs and improvements at the next engine shop visit of the engine. 9V-SWB's engine had been in the engine shop just prior to the SB being released. As an immediate safety action the AAIB recommends to accelerate the implementation of the service bulletin.
On Mar 14th 2017 Singapore's AAIB (SAAIB) released their final report concluding the probable causes of the accident were:
- The fuel leak in the occurrence flight was a result of a cracked tube within the MFOHE of the right engine. Fuel leaked into various areas of the engine through the core of the engine and the fan duct.
- When the thrust reverser was deployed during landing, the conditions at the area aft of the turkey feather seal of the core exhaust nozzle resulted in hot surface ignition of the fuel that had leaked from the MFOHE into the various areas of the right engine.
- As the fire developed, it propagated towards the forward section of the engine and entered the core of the engine through the fan booster inlet.
- The methods that were used to detect fuel leakage into the engine system by the operator and engine manufacturer were not able to detect the fuel leak that resulted from the cracked tube within the MFOHE when it occurred in that event flight.
- The engine manufacturer issued SB 79-0034 to address the issue of possible fuel leak in the MFOHE. The deadline for incorporating the SB was determined using the CAAM. The actions called for by the SB was not performed on the occurrence engine as the SB was issued after the engine’s last maintenance.
- In the course of the investigation, the engine and MFOHE manufacturers have identified that diffusion bonding can potentially cause any tube in the MFOHE to crack.
- The flight crew did not execute the steps in the FUEL DISAGREE checklist correctly.
- The flight crew depended on the fire commander as their sole information source when deciding whether an evacuation was needed. Several other resources which could have aided them in making their decision were not utilised.
The SAAIB reported the aircraft was deviating around weather while climbing to cruise level FL300. About 30 minutes after departure, after having levelled at FL300, the crew noticed that the engine oil quantities were reading 17 units for the left but only 1 unit for the right hand engine, the right hand engine oil pressure was fluctuating between 65 and 70 psi and the right hand engine oil temperature was about 10 degrees above the left hand oil temperature, however, all parameters were within the operating range. The crew consulted with the manuals but were unable to find an appropriate procedure/checklist dealing with the low engine oil quantity. The crew therefore consulted with maintenance whether it was safe to continue the flight, maintenance suspected a faulty oil quantity indication, however, after discussing the issue with service personnel recommended the aircraft should return to Singapore.
In the meantime the first officer made a fuel check while passing over waypoint VPG and found that the fuel consumption had been 600kg better than computed.
Following the recommendation to return to Singapore another conference call between captain, engineering control center and technical service personnel, that lasted 20 minutes, concluded that it was possible to continue to Milan provided the right hand engine was closely monitored.
Shortly after the conference call ended the crew felt an unusual vibration in the control column and aircraft floor. The crew in trying to identify the source of the vibration found, that when power on the right hand engine was reduced the vibrations ceased, at the same time the flight crew caught a brief wisp of burning odour in the cockpit. The captain contacted engineering control center again, it was concluded there was no need to shut the right hand engine down, the power was reduced to idle and the aircraft returned to Singapore.
A burning odour developed in the cabin, especially in the business class, where cabin crew handed out wet towels for passengers to breath through those towels. The purser informed the flight deck of the odour, the right hand bleed air system was shut down in response.
The crew descended the aircraft to 17,000 feet adopting the single engine procedures.
The purser informed the flight deck that the burning odour was still present in the cabin, in response the crew also shut down the right hand air conditioning system as well as the recirculation fans. Shortly after the odour subsided.
While on the return to Singapore, about 2 hours after the onset of trouble, the crew received a "FUEL DISAGREE" message, the quantities indicated a total of 79 tons of fuel on board while it was computed 83 tons should be on board. The FUEL DISAGREE checklist had listed that scenario indicating the FUEL LEAK checklist should be executed, however, the crew did not believe the fuel quantities indicated because changes had been made to the flight management system after the right hand engine was reduced to idle, the aircraft was no longer on the planned flight route and they had 600kg above computed on the last fuel check, the crew concluded the FUEL DISAGREE message was spurious and the FUEL LEAK checklist was not needed.
Prior to landing the crew dumped 41.5 tons of fuel. Emergency services took their stand by positions for the arrival of the aircraft in Singapore.
The aircraft touched down on runway 20C, about 20 seconds after touchdown two bangs were heard accompanied by two flashes originating from the right hand engine, the flight crew heard a soft thud. Emergency services saw fire at the right hand engine and informed tower, tower informed the crew about the fire and recommended to stop at the intersection between runway and high speed turn off E7. The crew did not receive any fire indication in the cockpit.
The first fire tender reached the aircraft after about 57 seconds and started to foam the right hand engine.
After the crew switched to the emergency frequency thus establishing direct radio contact with the emergency services, following exchange ensued between the fire commander and the captain:
06:51:50 PIC how is it looking…Is the fire contained
06:51:53 FC …we are still trying to contain the fire…the fire is pretty big…will like to advise… disembarkation on your port side
06:52:05 PIC Okay evacuate from the port side confirm
06:52:09 FC …still trying to contain the fire…still some random fire on your right hand engine but we are keeping it under control
06:52:24 PIC …do you need us to evacuate from the port side
06:52:29 FC …Singapore 368 standby standby
06:52:33 PIC Okay standby for your instructions Singapore 368 standby for your instructions
06:54:08 FC …we have kept the fire under control. We will like to advise disembarkation on your port side
06:54:20 PIC okay you want us to disembark through the slides or are you going to provide mobile stairs
06:54:38 FC …we will like to advise disembarkation on your on your port side
06:54:48 PIC okay you want us to disembark on the port side through the emergency slides can you confirm that
06:55:14 PIC …can you just confirm that we need to evacuate through the left through the emergency slides
06:55:33 FC negative negative negative we will like to advise disembarkation disembarkation no evacuation no evacuation
06:55:42 PIC okay disembarkation through mobile steps understand understand…
The fire commander queried the crew whether they had discharged the fire bottles into the right hand engine, the captain discharged both bottles after that query.
The fire was brought under control and the fire commander considered the situation safe, the passengers disembarked normally via a mobile stair 21 minutes after the aircraft touched down.
The right hand wing and right hand engine area received substantial damage, there was no damage to fuselage/fuselage windows or other parts of the aircraft.
The right hand engine showed extensive heat damage. In addition the oil tank was found full with jet fuel, jet fuel was also found in various drain positions of the engine where only oil was to be expected. Fuel streaks and stains were seen in various sections of the engine, which showed significant heat damage including the variable bleed valves and the fan section.
The SAAIB summarized the situation in the cockpit while the engine fire was still active:
According to the PIC, on being informed that there was an engine fire, he was mentally prepared to initiate an evacuation via the escape slides, even though there was no indication of fire in the cockpit. To prepare for the evacuation, he instructed the cabin crew members to position themselves at the doors as soon as the aircraft came to a stop.
During the initial stages of the fire, several cabin crew members tried to contact the flight crew through the cabin interphone. However, only one call was answered by a flight crew member and he informed the cabin crew that they were aware of the situation and were handling it.
According to one of the cabin crew members interviewed, during the initial stage of the fire, heat was felt in the cabin wall at the section of the fuselage closest to the fire. She informed the Chief Steward who then informed the IFS. The IFS could not recall whether this piece of information was conveyed to the flight crew.
According to the PIC, the flight crew were aware that cabin crew members were a source of information throughout the occurrence. However, the flight crew were not able to attend to every call from the cabin crew as they had to prioritise their tasks. In terms of obtaining information on the fire, they gave priority to the task of communicating with the FC as he was the subject matter expert and would have a better assessment of the fire from his location outside the aircraft.
The SAAIB analysed the fuel leak:
The cracking of a tube in the MFOHE allowed fuel in the fuel flow path of the MFOHE to flow into the oil flow path in the MFOHE. The investigation has not revealed other sources of fuel leak.
During all phases of the engine fuel pump operation, fuel is delivered at pressures between 400 and 1600 psi. In comparison, the pressure within the engine oil system is about 100 psi. As such, when the fuel carrying tube in the MFOHE cracked, the higher pressure fuel entered the engine oil distribution system.
During the normal operation of the engine oil system, a small amount of oil will collect in the A Sump19. However, when fuel leaked into the oil system, it filled the A Sump until its maximum storage capability. The additional quantity of leaked fuel overflowed into the booster spool cavity and started to collect there.
Once the booster spool cavity was filled up to the aft lip, the excess fuel leaked through a gap between the spool and aft stage booster vane into these areas:
- HPC through the core airflow
- Fan duct when the VBV doors are open at engine idle power
The oil tank and various engine drain points are areas where one would usually expect to find only oil. Instead, fuel was found in those locations.
Similarly, residual fuel was found in the various engine sumps. In addition, the gearboxes and the engine bearings, which are usually coated in oil, were found to be dry. These observations suggest that engine oil was displaced from the engine and fuel, in place of oil, was distributed throughout the engine oil system.
Engine oil lubricates and cools the engine bearings and gearboxes, and helps in lowering vibration at the engine bearings. Fuel is not as efficient as oil for engine lubrication. Therefore, when oil had been displaced by fuel in the occurrence engine, oil temperature increased. The temperature increase was a result of fuel in the oil system which was not able to cool the engine bearings and gearboxes as efficiently as oil.
The vibration detected by the flight crew when operating the right engine at a higher power setting was likely due to the fuel that collected in the booster spool cavity. This cavity is a dome shape space and rotational forces would have caused the fuel to be spun against the inner wall of the booster spool cavity as the engine was operating. The rotating fuel created imbalance that resulted in vibration. At higher engine power settings, the vibration would have been more pronounced as compared with the engine at idle operation. This was consistent with the flight crew’s observation that the vibration seemed to disappear when engine was at reduced power setting.
For the remainder of the return journey back to Singapore, fuel leaked through the core of the engine and the fan duct. As engine was operating at idle power, the VBVs were open, allowing the leaked fuel into the VBV ducts and the fan duct, where it could accumulate in the honeycomb core material behind the perforated walls of the thrust reverser duct.
The SAAIB analysed the fire ignited as follows:
There was no fire during airborne segment of the aircraft’s return journey to Singapore. This was due to the high velocity of the airflow over the exterior of the engine which prevented both the ignition and sustained combustion of the leaked fuel.
As the aircraft arrived to land, fuel was still leaking from the engine through various leakage areas (Figure 16). When the thrust reversers were deployed, the airflow over the core exhaust nozzle was significantly reduced. The area aft of the turkey feather seal, which is a protrusion on the core exhaust nozzle, would have experienced the most significant disruption of airflow. In addition, the accumulated fuel in the fan duct was also distributed over a wide area of the lower surface of the wing.
The investigation team believes that, with the disrupted airflow, the mixture of accumulated fuel on the core exhaust nozzle and fuel in the airflow would have been sufficiently heated to the point of ignition.
The entire fire event lasted for about five minutes before the fire was extinguished by the ARFF. Throughout this period of fire, there was no EICAS indication in the cockpit that a fire was detected.
With respect to the fuel leak detection the SAAIB analysed:
The operator’s engine oil consumption programme is based on the engine manufacturer’s recommended regime. In addition, the operator also engages the engine manufacturer’s services to monitor the health of the engines in their fleet. Both the operator and engine manufacturer did not detect the possibility of fuel leak prior to the occurrence flight.
During the occurrence flight, the sudden increase and decrease of the EICAS oil quantity indication shortly into the flight was an indication that the crack in the tube had developed sufficiently to cause the tube to be separated. However, the engine oil quantity is a parameter that is displayed on the secondary EICAS display screen and that is only required to be monitored periodically.
In addition, the flight crew shared that during the climb phase of the flight, they encountered weather and was performing weather avoidance manoeuvres. Hence, the abnormal behaviour of the oil quantity parameter which occurred over a short span of time was not detected by the flight crew.
The maintenance crew who performed the sniff check using his nose prior to the departure of the occurrence flight did not detect the presence of fuel odour. However, as suggested by the informal study mentioned in 126.96.36.199.4, a combustible gas detector may be more sensitive than a person’s nose for fuel odour detection.
Had the aircraft operator prescribed the use of combustible gas detector (which is an alternate means recommended by the aircraft manufacturer) in its engine servicing operation, the fuel leakage might have been detected.
The SAAIB analysed that a Service Bulletin had been released by the engine manufacturer to address cracks in the crimped area of a tube in the MFOHE fuel oil heat exchanger, that had been discovered on other aircraft. A deadline was recommended, however, the SB was not implemented on the accident engine which had its last workshop visit prior to the release of the Service Bulletin. The SAAIB analysed:
Had the SB been incorporated in the occurrence MFOHE, the fuel leak would not have occurred and the fire event would have been avoided. There may be room for the FAA to review its airworthiness control system to ensure that corrective actions can be implemented more expeditiously to prevent the recurrence of unsafe conditions.
The SAAIB analysed with respect to the FUEL DISAGREE checklist:
The FUEL DISAGREE message that the flight crew encountered was a result of the fuel leak after the tube had cracked in the MFOHE. The FUEL DISAGREE checklist suggested four scenarios in which a fuel leak should be suspected and thus the FUEL LEAK checklist should be performed. One such scenario is when the TOTALIZER fuel quantity is less than the CALCULATED fuel quantity. Given that the TOTALIZER fuel quantity was about 79 tonnes and the CALCULATED fuel quantity about 83 tonnes, the flight crew should have concluded that they had to proceed on to the FUEL LEAK checklist.
The investigation team notes that the FUEL LEAK checklist has to be performed with both engines maintained at the same power setting. In the event flight, the thrust of both engines were unequal as the flight crew had set the right engine to idle power setting in response to the vibration felt, as advised by the technical service personnel. There is no procedures for pilots to follow to perform a fuel leak check with the engines in different power settings. It may be useful for the aircraft manufacturer to evaluate the need for such a procedure.
As mentioned in paragraphs 1.1.16 and 1.1.17, the flight crew’s own assessment and fuel calculation put the remaining fuel quantity at about 79 tonnes, which tallied well with the TOTALIZER fuel quantity figure. This gave the flight crew the confidence that the TOTALIZER fuel quantity was accurate and they were not experiencing a fuel leak. They decided that the FUEL DISAGREE message was a spurious one and that, therefore, there was no need to conduct a fuel leak check.
With respect to the decision to not evacuate the SAAIB analysed:
PIC was aware that the decision to evacuate lay with him and that he could order an evacuation even if the FC advised a disembarkation. Although the PIC was the only person actively communicating with the FC, the rest of the flight crew members were listening to the communication and the decision not to evacuate was reached collectively.
The flight crew will have to balance the pros and cons of a decision to evacuate given the situation picture that they have. So, it cannot be overemphasised that the flight crew need to exhaust all possibilities and all available resources to try to build up a situation picture that is as accurate as possible.
At 0651:53 hrs, the FC informed the flight crew that the ARFF was trying to contain the fire and described the fire as “pretty big”. At 0654:08 hrs the FC confirmed to the PIC that the fire was under control and advised disembarkation on the port side. During this interval of over two minutes, while it is appreciated that the FC would have a better view of the external environment and the flight crew ought to seek his input in deciding whether an evacuation was needed, the flight crew could also have explored other ways of ascertaining the fire situation.
The SAAIB continued that the crew could have used several resources including the taxi camera, the cockpit crew escape window and the cabin crew for assessing the engine fire. The SAAIB continued:
Admittedly, the situation that the flight crew faced was a stressful one, especially in light of the fact that there was no indication of fire from the aircraft’s fire detection system. In that situation, the flight crew depended on the FC as the sole source for information collection and it may have slipped their mind to consider alternative ways of gathering information as suggested in paragraph 2.9.5. Research has shown that decision making under stress may become less systematic and more hurried, and that fewer alternative choices are considered. The flight crew’s behaviour was consistent with the research findings.
13 safety recommendations were released as result of the investigation.
This article is published under license from Avherald.com. © of text by Avherald.com.
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