Published on October 18th, 2017 | by Mark Dwyer0
AAIB Publish Special Bulletin into Serious Incident at Belfast
The UK Air Accident Investigation Branch (AAIB) have issued a Special Bulletin following a serious incident involving a Sunwing Boeing 737-86J at Belfast during the summer. The aircraft (C-FWGH) departed with a thrust setting which was significantly below that required for the conditions of the day resulting in a longer than normal takeoff run. 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 aircraft was registered in Canada but was operating on behalf of TUI, a UK tour operator. The Canadian operator supplied the aircraft and flight crew to support the tour operator for the summer season. The crew were cleared for takeoff on Runway 07 from Taxiway D, 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 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 the takeoff, airport operations staff conducted a runway inspection and a runway approach light for Runway 25 was found to be broken. Preliminary evidence indicated that the aircraft struck the light, which was 35 cm high, 29 m beyond the end of the runway in the stopway. 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.
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:
- 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
- The correct assumed temperature 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 FMC software fitted to C-FWGH, U10.8A, predated revision U12.0, which features the crosscheck between the OAT entered by the crew and that sensed by the external temperature sensor. In this event, had C-FWGH been updated with U12.0 and CDS BP15, the entry of a top-of-climb OAT instead of the ambient OAT at ground level would have been prevented, the crew would have received feedback on their erroneous entry, and the serious incident would have been prevented. The updates to the CDS and FMC software are offered by Boeing as upgrade service bulletins at nominal cost. Fleet embodiment of software revision U12.0 (or later revision incorporating the outside air temperature crosscheck) would reduce the likelihood that this type of data entry error was repeated.
The Special Bulletin makes two safety recommendations:
Safety Recommendation 2017-016: 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.
Safety Recommendation 2017-017: 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.
The full bulletin can be read here.