On 7 June 2016, a GE90-115B-engined Boeing 777-300 (S2-AFP) being operated by Biman Bangladesh Airlines on a scheduled international passenger flight from Dhaka to Dammam as BG-049 made a high speed rejected takeoff in normal daylight when an ENGINE FAIL R warning was annunciated. The extent of external engine damage to the rear part of the low pressure turbine section was not appreciated until after the aircraft had returned to the terminal, and the runway was not closed for inspection and debris clearance until 12 more aircraft had used it.
An Investigation was carried out by the Bangladesh Aircraft Accident Investigation Group (AAIG). Relevant recorded flight data was available from the FDR and the CVR. It was noted that the 60 year-old Captain had a total of 23,512 hours flying experience of which 3,637 hours were on type and the 38 year-old First Officer had a total of 6,951 hours flying experience of which 1,869 hours were on type.
It was noted that light rain had been falling prior to the reject and that the runway surface was “wet to damp” at the time it had occurred. It was established that there was no indication of any engine malfunctions until almost at V1 at which point ‘ENG FAIL R’ was annunciated and the crew took immediate action to reject the takeoff 31 seconds after the takeoff roll had commenced and come to a stop on the runway as prescribed. Once stopped, there were no other abnormal flight deck indications or any cabin reports of obvious engine distress and after approximately 2 minutes, when the TWR was advised of the reject, they simply “instructed the aircraft to vacate the runway and taxi back via the taxiway”.
Having cleared the runway via an RET, the crew then asked for clearance to stop on it, which was approved with a request to be told what the problem was. When told that it was an “engine problem”, TWR transferred them to GND and the taxi in to parking was completed. No action was taken to carry out a runway inspection before permitting further use of the runway and it remained in use until it was realised that debris was likely to be on the runway. A runway inspection was then ordered and a considerable quantity of engine debris was found which, when collected, totalled 13.5 kg and included debris sizes ranging from small granules to larger pieces up to 15 cm long and 5 cm wide.
Attendance of the Emergency Services to conduct an external inspection was at no stage offered by ATC or requested by the crew. It was also noted that although the Airport Fire Fighting section had a multi-frequency radio set, only the GND frequency is monitored and that even if directed to attend and externally inspect an aircraft, their vehicles were not equipped to facilitate direct communication with aircraft.
Establishing the cause of the engine failure
An initial inspection of the failed engine found that the N2 rotor system was still rotating normally whereas N1 rotor system rotation “was restricted to about 120 degrees” and caused “metallic sounds” to occur. Borescope inspections of the Low Pressure Turbine (LPT) module subsequently found severe blade and vane damage beginning at stage 3 and continuing rearwards, but there were no abnormal findings from similar inspections of the Low Pressure Compressor (LPC), High Pressure Compressor (HPC) and high pressure turbine (HPT) modules. Extensive disassembly of the LPT module found that the full extent of the damage was:
- Thermal damage to stage 2, 3 and 4 stators locally around the 5 o’clock radial position, reducing in that stage sequence.
- Mechanical damage caused by the liberated vanes beginning at the stage 2 rotor which increased from stage 3 onwards.
The Investigation as to the cause of this damage eventually led to a focus on the 30 LPT module fuel spray nozzles. It was found that they had varying hysteresis values and an attempt to establish a reason for this, with a suspicion of contamination, led to a focus on their only movable component, their valves. Residues eventually identified as very small size particles of Super Absorbent Polymer (SAP) were found in nozzles in the vicinity of the 5 o’clock radial sector already associated with the damage. The particle sizes - mainly between 5µm to 20µm - were below the size which the engine fuel filter and the fuel nozzles strainers were designed to catch.
It was concluded that, since there was also evidence of a high probability that fuel contamination in the engine’s Hydro Mechanical Unit (HMU) valve and in the fuel tank sample taken was also SAP, the contamination had “affected the complete fuel system and therefore must have been introduced by the fuel supply”.
It was noted that SAP, which readily absorb water, are commonly used in the filter monitors installed in airport fuel handling (whether this is refuelling vehicles, hydrant dispensers or other mobile fuelling equipment) to ensure fuel uploaded is free of water in order to minimise the risk of ice formation in the engine fuel system and are typically globules in the same size range as those found during the Investigation.
The equipment and procedures of the Dhaka aviation fuel supplier were investigated and it was noted that it was “technically supervised and supported by Shell” and was in compliance with their standards and its own documented procedures. The type of fuel filter elements used in their equipment was recorded as having dispensed 16,753,420 litres of fuel to about 20 different air carriers in the 14 day period around the date of the investigated event “with no adverse report”.
The Investigation noted that operators of aircraft with GE90-94B and GE90-115B engines had previously been informed of two GE90-94B in-flight shutdown events which occurred in 2010 (one of these affected the function of both engines in flight and was extremely serious) had been directly related to a pre-flight fuel uplift which investigations had concluded were most likely caused by SAP contamination causing fuel nozzle valves to stick. It was also noted that evidence from these two investigations had shown that a SAP contamination during fuelling “does not necessarily result in an immediate event” and could then occur “an unknown number of cycles later”.
Operational aspects of the event
The following observations were made without further comment:
- The R/T phraseology used by the flight crew following the rejected takeoff was improper.
- Use of improper R/T phraseology by the flight crew following the rejected takeoff might have negated an effective response from the controller.
- ATC did not have a procedure to apply in the event of a rejected take-off.
- The controller failed to assist the flight crew following the rejected take-off.
- The controller also failed to assess the consequences of a heavy aircraft of the size of a Boeing 777-300 ER rejecting take-off from a high speed.
- The aircraft operator’s SOPs use different terms for a rejected takeoff for different aircraft of the fleet.
- Aerodrome Fire Fighting did not proceed to the aircraft after it had come to a stop on the runway in spite of the rejected takeoff and the fact that it then held on the active runway for about two minutes.
The Cause of the engine failure was identified as Super Absorbent Polymer (SAP) contamination of some of the fuel nozzle valves which prevented the normal function of their valves and led to thermal and then consequential mechanical damage to the Low Pressure Turbine (LPT). The mechanical damage was caused by liberated turbine blades passing into the engine.
One Safety Recommendation was made as a result of the Investigation as follows:
- that Aircraft Operators should investigate their refuelling operations at the airports where uploads are accomplished to ensure proper maintenance and monitoring programs are being conducted in accordance with the use of filter monitors which contain SAP.
The Final Report was completed on 17 July 2018 and subsequently published. Note that the Attachments to the Report, whether referred to in its text or not, were not published in or with it.