Sunwing B738 at Belfast on Jul 21st 2017, overran runway on takeoff
Last Update: November 21, 2018 / 16:05:38 GMT/Zulu time
Date of incident
Jul 21, 2017
Belfast Aldergrove, United Kingdom
ICAO Type Designator
Airport ICAO Code
The Canadian TSB reported that a runway inspection found an approach light for runway 25 bent over. The aircraft was inspected at Kerkyra with no damage found. The UK AAIB have opened an investigation into the occurrence.
The occurrence aircraft remained on the ground in Kerkyra (also known as Corfu) for about 90 minutes, then departed for the return flight BY-1527.
On Sep 20th 2017 the AAIB released a special bulletin stating:
"On 21 July 2017 at 1539 hrs, C-FWGH took off from Belfast International Airport with a thrust setting which was significantly below that required for the conditions of the day. Preliminary evidence indicated that, after the aircraft lifted off from the runway, one of the aircraft tyres struck a runway approach light, which was 35 cm high and 29 m beyond the end of the runway."
The AAIB reported that neither UK's Civil Aviation Authority not the AAIB had been informed about the occurrence by captain, operator or tour operator although the Canadian TSB had been informed about the occurrence. ATC filed a Mandatory Occurrence Report which was not read by the AAIB until Jul 24th, as result the data off the flight fata and cockpit voice recorders were not available to the investigation as the aircraft had flown 16 sectors in the meantime and the data of the occurrence flight on both flight data and cockpit voice recorders were overwritten.
The AAIB reported: "The crew were cleared for takeoff on Runway 07 from Taxiway D (Figure 1), which gave a Takeoff Run Available (TORA) of 2,654 m. During the takeoff, at around 120 to 130 kt, the crew realised that the aircraft was not accelerating normally. They estimated, during post-flight interviews, that they reached V1 1 with around 900 m of the runway remaining and rotated shortly afterwards. The aircraft was seen, by multiple witnesses, during rotation and took a significant time to lift off before climbing at a very shallow angle. ... After takeoff, the crew checked the aircraft’s FMC which showed that an N1 of 81.5% had been used for the takeoff. This figure was significantly below the required N1 setting of 93.3% calculated by the operator and shown on the pre-flight paperwork."
With respect to N1 settings the AAIB reported:
Passing the upwind end of the runway the aircraft’s ACARS sent a takeoff report, which confirmed that the engines were at an N1 of approximately 81.5%. Other ACARS messages confirmed that the correct weights for the aircraft had been entered into the FMC.
The aircraft’s auto-throttle BITE8 history showed two messages generated during the climbout. Both messages were consistent with the crew having manually advanced each throttle to a power setting above an N1 of 81.5% when the aircraft was approximately 800 ft aal.
The Electronic Flight Bags (EFB) used by the crew to calculate the performance figures for entry into the FMC were provided to the AAIB. Initial examination of these devices indicated that the correct figures were calculated by the EFB performance software prior to the aircraft’s departure.
The AAIB conducted simulator tests to find out whether the aircraft would be able to climb out or stop in case of an engine failure and how such a performance of N1=81.5% could occur in the FMS. The AAIB wrote:
The AAIB and operator carried out independent assessments of how the incorrect thrust setting might have been programmed into the FMC. Both assessments concluded that the only credible way to achieve a grossly low N1 setting was to enter an extremely low value into the outside air temperature (OAT) field on the n1 limit page. It was found that the takeoff N1 setting used on the flight (81.5%) would be calculated by the FMC if:
a. The expected top-of-climb outside air temperature (OAT) was entered into the OAT field on the n1 limit page instead of the OAT at the airport (a figure of - 52°C as opposed to +16°C);
and b. The correct assumed temperature9 of 48°C was entered into the FMC. No other combination of data entries was found which would achieve the same result.
During the simulation carried out by the AAIB, the aircraft’s performance was assessed following an engine failure immediately prior to V1, with the pilot making a decision by V1 to either abandon or continue the takeoff. In the simulator, the aircraft was able to stop in the runway remaining following a decision to abandon the takeoff, but was unable to climb away safely following a decision to continue the takeoff.
The AAIB analysed:
The aircraft took off from Runway 07 with a thrust setting significantly below that required to achieve the correct takeoff performance, and struck a Runway 25 approach light shortly after lifting off.
The N1 required to achieve the required takeoff performance was 93.3% but 81.5% was used instead. Independent assessments by the AAIB and operator showed that the only credible way for this to have happened was for an error to have been made whilst entering the OAT into the FMC. If the top-of-climb OAT was mistakenly inserted into the OAT field on the n1 limit page (a figure of -52°C as opposed to +16°C), and the correct assumed temperature of 48°C was entered, the FMC would have calculated a target takeoff N1 of 81.5%. The investigation will consider how such a data entry error could have been made, and whether actual aircraft performance matched that which would be expected given the N1 power setting used.
The AAIB released two safety recommendations as result of the investigation so far:
It is recommended that the Federal Aviation Administration, mandate the use of Flight Management Computer software revision U12.0, or later revision incorporating the outside air temperature crosscheck, for operators of Boeing 737 Next Generation aircraft.
It is recommended 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.
On Nov 21st 2018 the AAIB released their final report concluding the probable causes of the serious incident were:
- An incorrect OAT was entered into the FMC, which caused the FMC to calculate an N1 setting for takeoff which was significantly below that required for the aircraft weight and environmental conditions.
- The incorrect OAT was not identified subsequently by the operating crew.
- The abnormal acceleration during the takeoff run was not identified until the aircraft was rapidly approaching the end of the runway, and no action was taken to either reject the takeoff or increase engine thrust.
- The aircraft’s FMC did not have the capability to alert the flight crew to the fact that they had entered the incorrect OAT into the FMC, although this capability existed in a later FMC software standard available at the time.
- The Electronic Flight Bags did not display N1 on their performance application (some applications do), which meant that the crew could not verify the FMC-calculated N1 against an independently-calculated value.
- The crew were unlikely to detect the abnormally low acceleration because of normal limitations in human performance.
The AAIB analysed human factors:
The human factors analysis in Appendix B suggested that the crew in this serious incident were unlikely to have sensed the abnormal takeoff acceleration. The physical limitations of the human sensory system mean that it is very hard for pilots to distinguish the difference between the acceleration of a normal takeoff and that experienced by the crew in this serious incident. Neither the vestibular system nor the visual system were likely to have been able to detect the difference in acceleration. This, combined with the pilots’ unfamiliarity with BFS Runway 07, meant that there was little or no context for the pilots to compare this takeoff with others previously experienced. It was only in the last 900 m of the runway, with the change in centreline light colours and the rapidly approaching runway end, that the crew was alerted that something was wrong.
Cockpit instrumentation did not show anything to the crew during the takeoff roll which was likely to have alerted them to the abnormal acceleration. For the crew, the indications displayed were similar to every other normal takeoff and it would have been extremely difficult to perceive any abnormality from the information displayed.
The response of the crew to the rapidly approaching runway end was the same as could be expected from many crews. They found themselves in a time critical situation but one for which they had no obvious explanation. Pilots are trained to make decisions based on evidence and review rather than reaction, with rapid action in ambiguous circumstances discouraged. Their training would have discouraged them from moving the thrust levers after V1 and they had no experience of moving the thrust levers to a higher power setting during a derated takeoff. Natural human reaction not to accelerate towards perceived danger may also have made increasing the thrust counter-intuitive as the end of the runway approached.
The acceleration clues in this case were unlikely to have alerted the pilots that there was a problem until the visual clues of the approaching runway end became apparent. Once they realised that there was an issue, their reactions in not increasing thrust were the same as could be expected from many crews.
With respect to the Electronic Flight Bag the AAIB analysed:
The EFB used to calculate the takeoff data complied with the Canadian AMC document and was approved for use as a performance tool. However, there was no requirement to display the calculated N1, the parameter which defines each engine’s thrust and, therefore, determines the aircraft’s ability to meet takeoff performance requirements. Had N1 been displayed on the EFB, it would have allowed the pilots to crosscheck the value of N1 calculated by the FMC. Had they done so, they would have noticed the significant difference between the two calculated figures and investigated the discrepancy, and this would have probably prevented this serious incident. However, whilst the aircraft manufacturer required the crews to verify the N1, there was no specified procedure to do so.
An N1 crosscheck would also highlight other errors that have caused serious incidents and accidents, including selecting the wrong fixed derate and entering an incorrect assumed temperature. Such errors would not be picked up by the automated OAT crosscheck which would only identify erroneous OAT entries. However, the errors would lead to a discrepancy between the EFB‑and FMC-calculated N1 and, if the N1 figures were crosschecked by the crew, there would be an opportunity for these additional types of errors to be picked up and corrected before they led to an incident or accident. For aircraft not equipped with EFBs, a crosscheck of FMC-calculated N1 against an alternative, independently-calculated value would increase the likelihood of identifying the error.
EGAA 211650Z 12014KT 9999 SCT026 16/11 Q1000=
EGAA 211620Z 12015KT 9999 FEW027 SCT033 15/10 Q1000=
EGAA 211550Z 12015KT 9999 SCT024 15/10 Q1000=
EGAA 211520Z 13013KT 9999 FEW024 SCT030 16/11 Q0999=
EGAA 211450Z 13014KT 9999 SCT025 15/10 Q0999=
EGAA 211420Z 13014KT 9999 SCT025 15/10 Q0999=
EGAA 211350Z 13016KT 9999 FEW023 SCT028 15/10 Q0999=
EGAA 211320Z 13016KT 9999 FEW021 SCT027 15/11 Q0999=
EGAA 211250Z 13015KT 9999 SCT026 15/10 Q0998=
EGAA 211220Z 14014KT 100V160 9999 SCT024 16/10 Q0998=
EGAA 211150Z 13016G28KT 9999 SCT022 17/12 Q0998=
EGAA 211120Z 12016KT 100V160 9999 FEW021 SCT027 17/12 Q0997=
Aircraft Registration Data
Aircraft registration data reproduced and distributed with the permission of the Government of Canada.
Date of incident
Jul 21, 2017
Belfast Aldergrove, United Kingdom
ICAO Type Designator
Airport ICAO Code
This article is published under license from Avherald.com. © of text by Avherald.com.
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