Air Finland B752 near Paris on Jan 15th 2011, fuel indication problem
Last Update: April 3, 2013 / 14:59:42 GMT/Zulu time
Incident Facts
Date of incident
Jan 15, 2011
Classification
Incident
Aircraft Type
Boeing 757-200
ICAO Type Designator
B752
The immediate cause of the incident was a fuel leak from a fuel tube coupling in the right engine high pressure fuel pump. The investigation group found three possible causes for the fuel leak.
The most probable cause leading to the fuel leak, as concluded by the investigation group, is that the fitting of the seal was originally too tight, for which reason the seal may have been pressed incorrectly against the edge of the groove when it was installed dur-ing maintenance before the incident flight. A contributing factor to this fuel leak mecha-nism would be that the airline had not carried out the non-mandatory modification suggested by the engine manufacturerÂ’s service bulleting SB RB.211-73-G230, which would have improved the fitting of the seal in the coupling.
Another possible cause for the fuel leak is that the bolts between the coupling flanges had not been tightened up to the instructed final value during maintenance before the incident flight. In this case, there would be several contributing factors. Firstly, it is challenging to find the correct value for the tightening torque in the tables of the manual. Secondly, there is little space in the engine compartment for turning the torque wrench, which may have made it more difficult to tighten the bolts. Thirdly, the coupling had not been secured by lockwire or marked with inspection lacquer after tightening, which hampered the inspection of the final tightness. Inadequate supervision of the mainte-nance would probably also have contributed to this mechanism.
The third possible cause for the fuel leak is that a wrong seal was inadvertently installed in the coupling during maintenance before the incident flight. The investigation group does not consider this likely, but it cannot be fully excluded. Inadequate supervision of the maintenance would probably have contributed to the installation of the wrong seal.
The in-flight fuel leak resulted in an incident. The pilots did not shut down the leaking engine during the flight, although the Quick Reference Handbook (QRH) instructs to do so in engine fuel leak situations. The pilotsÂ’ deviation from QRH instructions caused the maximum allowable fuel imbalance to be exceeded as the fuel leak continued, which led to operations outside the approved performance envelope and the incident developed into a serious incident according to ICAO classification.
The first officer (41, CPL, 3,324 hours total, 1,420 hours on type) was pilot flying, the captain (35, ATPL, 5,719 hours total, 2,805 hours on type) was pilot monitoring when the aircraft was enroute. About 70 minutes after departure the crew had compared fuel quantity indications with flight plan estimations and found the values in agreement. 117 minutes after departure the flight data recorder revealed that the right hand engine's (RB211) oil temperature dropped and the engine oil pressure increased, both values remained within permitted operating range however. 150 minutes after departure, during the next schedule manual comparism of actual fuel quantities with estimated flight plan figures, the crew detected they had 1700 kg less fuel indicated than estimated, the crew also noticed the right hand engine's fuel consumption was higher than the left hand engine's and beginning to suspect a fuel leak they shortened the monitoring interval. Another 17 minutes later the mismatch had increased by another 600 kg. The captain instructed the chief flight attendant to check the right hand engine for visible fuel leaks, after checking she reported she could not see any fuel leaking. The center fuel tank was about to run empty at that time, the captain decided to wait for the low fuel pressure light to illuminate as indication that the center tank was empty indeed in order to cross check, whether they had an indication problem or a real fuel leak. The low fuel pressure light illuminated indicating there was a real fuel leak. The captain went to check the right hand engine visually but could not find any trace of leaking fuel, too. About 30 minutes after the onset the fuel mismatch had become so large that the captain decided to discontinue the flight and divert to Paris. The crew switched roles with the captain now becoming pilot flying. While descending towards Paris, about 10 minutes prior to estimated landing, the crew received a fuel configuration alert indicating the fuel imbalance between left and right hand wing tanks had exceeded 885 kg, the captain however decided to keep the right hand engine running considering it was safer to continue on both engines running and accept the fuel imbalance, especially as the left hand engine had exhibited some vibrations earlier into the flight, than to shut the engine down and accept delays as the related checklists would not be completed in 10 minutes. The captain activated the APU before landing with the intention to shut the right hand engine down after landing. The aircraft landed safely at Paris Charles de Gaulle Airport 220 minutes after departure, the fuel imbalance had increased to 1700kg by then, the right hand engine was shut down after touchdown.
The crew decided to not use reverse thrust in order to not blow leaked fuel inside the engine into the hot section.
Maintenance soon located an actual fuel leak at a coupling of the fuel tube leading to the right hand engine's high pressure fuel pump. An O-ring was found partly bulged out of its groove permitting 4100 kg of fuel to leak out. The investigation determined about 63kg/min were released.
The AIBF reported that prior to the flight the aircraft had undergone daily checks and additional maintenance tasks. The intended maintenance facility FTS (Finnair Technical Services) at Helsinki Vantaa Airport however did not have sufficient staff available to carry out the additional maintenance items, that included the replacement of the right hand engine's accessory gear box and the left hand engine's thrust reverser and rearrangement of the left hand engine's fan blades. The airline's second contracted maintenance partner ITS (Icelandair Technical Services) provided maintenance staff to carry out the additional maintenance work at the FTS facility at Vantaa Airport under the lead and quality assurance by FTS. FTS viewed the ITS staff as "other working team" but did not assess the qualification/competence of the ITS staff, in addition the supervisory actions were not documented. The work cards required as a last item to conduct an engine run up, the AIBF stated: "Due to inadequate documentation, the extent of engine run-up could not be determined." and finished: "Airworthiness documents for the spare parts used were not included in the work package. There were no markings by the continuing airworthiness management organisation of acceptance of the maintenance work."
The AIBF analysed that the changes in engine oil temperature and pressure marked the start of the fuel leak. A higher fuel flow through the fuel/oil heat exchanger means the engine oil would receive more cooling, the cooler oil would show higher viscosity resulting in higher oil pressure readings - this connection is widely known in aviation. As both values remained within permitted operational range, the crew however did not regard the change in indication signficant.
When the crew noticed the discrepancy between estimated and actual fuel figures they decided to shorten the monitoring interval, however, this is not part of any checklist procedure. After establishing the discrepancy the crew should have actioned the relevant engine fuel leak checklist.
The AIBF further analysed, that after the center fuel tank had run empty and the low fuel pressure light illuminated in accordance with the fuel quantity indications confirming there was an actual fuel leak, the crew should have shut down the right hand engine according to the engine fuel leak checklist.
The AIBF continued analysis: "Because the pilots deviated from the QRH instructions, a Fuel Config warning was illuminated in the cockpit. QRH also gives separate instructions for those situations. The maximum fuel imbalance allowed by the Aircraft Flight Manual is limited to 885 kg. Although the fuel imbalance increased all the time, the pilots took no action. As the flight was continued while the fuel imbalance increased after the Fuel Config warning the flight was then outside the approved performance envelope and the situation developed into a serious incident according to the ICAO classification."
The AIBF continued: "The threshold to shut down an operating engine is high. In this case the vibration in the left engine and the need to consume extra fuel for landing raised pilotsÂ’ already high threshold to shut down the leaking right engine. If the pilots had shut it down and flown with a single engine, it would have been more difficult to control the aircraft. However, the pilotsÂ’ decision not to shut down an engine that was leaking fuel and deviate from instructions was not justified. In the investigatorsÂ’ opinion, B757 aircraft is not considered particularly challenging to fly in a single-engine situation. In pilotsÂ’ opinion the actions chosen by them were safer than the procedure described in the QRH."
The AIBF reasoned that the crew did not know where the fuel was leaking and it was therefore impossible for the crew to assess the risk of a fire associated with the leaking fuel concluding that "an inflight fuel leak should always be considered to involve a risk of fire." and thus reaching the conclusion: "If the situation of the incident flight was simulated on a proficiency check, deviating from the check list procedures would cause the check flight to be failed. Consequently, the actions of the pilots cannot be considered fully acceptable. Deviating from QRH procedures is only allowed if it is absolutely necessary for flight safety reasons."
The AIBF thundered with regards to the maintenance activity prior to the flight in their analysis: "The deficiencies in maintenance documentation related to work order WO #85 and the work package associated with the daily check were not detected before or after maintenance. The certificate release to service issued did not meet the EASA requirements. As a result, the aircraft was not airworthy as regards the maintenance actions carried out."
Incident Facts
Date of incident
Jan 15, 2011
Classification
Incident
Aircraft Type
Boeing 757-200
ICAO Type Designator
B752
This article is published under license from Avherald.com. © of text by Avherald.com.
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