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B738, Belfast International UK, 2017
From SKYbrary Wiki
|On 21 July 2017, a Boeing 737-800 took off from Belfast with a significantly lower thrust setting than that intended. In the absence of any response from the pilots who had detected that the achieved acceleration along the runway was below that expected, the aircraft became airborne just before the end of the runway but only climbed at a very shallow angle and continued to do so for almost a mile. The Investigation is continuing but has found that the low thrust setting resulted from crew FMS input of the expected top-of-climb temperature in place of the surface temperature.|
|Actual or Potential
|Controlled Flight Into Terrain (CFIT), Human Factors|
|Type of Flight|
|Origin||Belfast International Airport|
|Intended Destination||Corfu International Airport|
|Take off Commenced||Yes|
|Flight Phase||Take Off|
|Location - Airport|
|Airport||Belfast International Airport|
|Tag(s)||Event reporting non compliant,|
Use of Erroneous Performance Data,
Delayed Accident/Incident Reporting
|Tag(s)||Pre Flight Data Input Error,|
Procedural non compliance,
|Damage or injury||Yes|
|Causal Factor Group(s)|
On 21 July 2017, a Boeing 737-800 (C-FWGH) being operated by the Canadian operator Sunwing on a non-scheduled international passenger flight from Belfast International to Corfu for a UK airline began its takeoff from intersection ‘D’ on runway 07 in day VMC and after using almost all the available 2654 metre TORA then climbed away at a very shallow angle over generally flat terrain.
An Investigation is being carried out by the UK AAIB which has noted with considerable concern that it was unaware of the event until three days later and as a consequence was unable to obtain relevant data from either the FDR or the CVR but limited data was available from other sources including ACARS messaging and the EFBs used by the crew to calculate the takeoff performance figures for entry into the FMC. The aircraft was also fitted with a QAR but the Investigation learned that “the operator was troubleshooting this installation and the memory cards contained no data”. Fortunately, recorded ATC and ADS-B transmissions were available and these data proved critical to determining the takeoff roll and the vertical profile flown once airborne.
Although the Canadian aircraft operator did eventually report the event to the Canadian TSB, the aircraft commander failed to report it to either the UK or Canadian Investigation agencies and Belfast ATC, who had been sufficiently aware of the event to know that it was likely to be considered to have been a Serious Incident, merely raised an out of office hours MOR. The inaction of the aircraft commander and the inappropriate response of Belfast ATC were noted to have both been in direct violation of the applicable regulatory requirements.
The aircraft involved was operating for a UK airline on a seasonal wet lease. It was noted that the 38 year-old Captain had 8,234 total flying hours of which 2,817 hours were on type. It was established that V1 for the takeoff had been 144 KIAS and Vr had been 146 KIAS. The crew stated that around 120 to 130 KIAS, they had realised that the aircraft was “not accelerating normally” but rotation was commenced with about 600 metres of runway left (see the first illustration below) and a very shallow angle climb was achieved (see the second illustration below). Almost immediately, part of the landing gear struck a runway 25 approach light which was 35 cm high and 29 metres beyond the end of the runway. Approximately 500 metres beyond the end of the runway, the aircraft was still only approximately 40 feet aal, and still only approximately 220 feet aal when 1,500 metres beyond the end of the runway. At an undetermined point after getting airborne, the crew stated that they had checked the FMC and found that it showed that an N1 of 81.5% had been used for takeoff, significantly less than that required and was shown on pre-flight paperwork. This 81.5 % figure was also contained in the ACARS takeoff report and other ACARS messages confirmed that the correct weights had been entered into the FMC. The BITE history for the A/T showed that not until the aircraft had reached approximately 800 feet aal did the crew manually advance the thrust levers to an N1 greater than 81.5%. Examination of the EFBs used by the crew to calculate takeoff performance showed that they had been used to calculate the correct performance figures.
Simulator assessments were used to help establish how the incorrect thrust setting might have been programmed into the FMC and it was found that the only plausible way to obtain a grossly low N1 was by entering an extremely low OAT figure in place of the correct one. On this basis, an N1 of 81 5 % could only be obtained by entering the expected top of climb temperature (which was -52°C) instead of the actual OAT (which was +16°C) whilst correctly entering the assumed temperature (which was 48°C).
A simulator assessment based on the N1 of 81.5% was then used to examine the consequences of an engine failure immediately prior to V1 and it was found that the aircraft could be stopped safely in the remaining runway distance available but was unable to climb safely if the takeoff was continued. It was recognised that this trial was indicative of what the actual situation might be rather than definitive.
It was noted that after previous events in which incorrect OAT figures had been entered into the FMC prior to takeoff, Boeing had issued an FCOM Bulletin on the subject which included the reminder that ‘An incorrect reduced thrust target may result in slower acceleration to V1, which may invalidate the takeoff performance calculations and/or result in decreased obstacle clearance margins after liftoff’ ’and the need for flight crews to verify the OAT entry on the N1 LIMIT page. However, it was considered unfortunate that because FCOM Bulletins, which are by nature temporary, are removed once the Manual is next routinely updated (and in this case the Operator’s FCOM had been updated to guide crews on how to verify their FMC OAT entry), the removal of the whole Bulletin had meant that the background to why this check was necessary was no longer available and wider appreciation of the circumstances in this latest event was desirable.
It was also found that the current version of the FMC software (available from February 2016 but which also required 737NG aircraft to have an updated version of the ‘Common Display System’ not made available until January 2017) was not fitted to the aircraft under investigation. Had it been, there would have been an automatic crosscheck between the OAT entered by the crew and the OAT detected by the external sensor which would have precluded the error that was made in the event under investigation.
Two Safety Recommendations have been made so far as follows:
- that the Federal Aviation Administration (FAA) mandate the use of Flight Management Computer software revision U12.0 or a later revision incorporating the outside air temperature crosscheck, for operators of Boeing 737 Next Generation aircraft. [2017-016]
- that the Boeing Company promulgates to all 737 operators the information contained within this Special Bulletin and reminds them of previous similar occurrences reported in the Boeing 737 Flight Crew Operations Manual Bulletin dated December 2014. [2017-017]
A Special Bulletin was published on 20 September 2017. The Investigation is ongoing.