Thomson Airways flight BY-1526

Thomson Airways flight BY-1526

Incident
Date	21 July 2017
Summary	Due to bad cockpit data entry, aircraft took off with insufficient power, and struck a runway approach light.
Site	Belfast International Airport
Aircraft
Aircraft type	Boeing 737-800
Operator	Sunwing Airlines
Registration	C-FWGH
Flight origin	Belfast International Airport
Destination	Corfu, Greece
Passengers	185
Fatalities	None

A Sunwing Boeing 737 NG underperformed at takeoff on July 21, 2017, from Belfast International Airport, due to an incorrectly entered Outside Air Temperature (OAT). The engines operated at 81.5% speed, rather than 92.7% as required, giving 60% thrust. There were 185 people aboard. The area beyond the runway and the terrain beyond were largely unobstructed, and the aircraft eventually climbed away undamaged. The remainder of the flight, to Corfu, was uneventful. The "serious incident" at take-off was recognised and reported when airport staff subsequently observed minor ground damage. The incident was due to a data-entry error by aircrew, into the flight management computer, and the investigation Report concluded that the crew could not reasonably have been expected to recognise the anomalously low speed sooner or intervene more effectively. The incident was reported in industry and enthusiast sources, (identified in one as "significant") and, following publication of the Report, in mainstream media around the world. The Report listed several examples of aircraft underperformance at takeoff, reviewed the history of relevant industry efforts, and recommended that a Takeoff Acceleration Monitoring System and associated certification standards should be developed without further delay.

The outside air temperature was 16°C, but the operating crew entered incorrect figures (variously -47°C and -52°C) for the airport outside air temperature, into the flight management computer, resulting in miscalculation of the required N₁, the engine fan speed (low-pressure compressor speed) required on the take-off run. (Both incorrect figures appeared as OATs on the flight plan: -47°C at the first waypoint after top-of-climb, -52°C at top-of-climb. Delays around reporting the incident meant that the Cockpit Voice Recorder information was not available for the investigation.)

When the co-pilot returned from the exterior inspection, he listened to the ATIS to find out the runway in use and the weather conditions. Using the passenger and baggage figures from the ground handling company, and the weather information from the ATIS, each pilot completed weight and balance, and performance calculations independently on his EFB. These calculations were then crosschecked before the information was entered into the FMC. The flight crew then completed a taxi and takeoff briefing covering items such as the expected taxi and departure routing and including a discussion on the handling of emergencies during the takeoff and departure. At some point during the cockpit preparation, [the first incorrect] figure of -47 °C was entered into the FMC as the outside air temperature (OAT).

— AAIB, Report on the serious incident to Boeing 737-86J, C-FWGH | Belfast International Airport on 21 July 2017

Damage

"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".

Analysis

Bad takeoff data in the Flight Management Computer can be fatal. This incident is a case where checklists and other safeguards have failed the pilots. Once airborne the crew checked the take-off performance data: the N₁ level was 81.5%, far below the required level of 92.7%. Thrust was only increased when the aircraft reached 800 feet, about 4 km after becoming airborne.

Neither the installed flight management computer software nor the Electronic flight bags (EFBs) in use helped in detecting the data input error. A recent software release had not yet been installed, and the software omitted the cross-check of the pilot input data against the outside air temperature actually measured. The pilot became aware of the aircraft’s underperformance late in the take-off run but did not intervene effectively. The Report explored various human-factors aspects of the incident, concluding that the pilots could not reasonably have been expected to respond more quickly to the developing situation, either before or after becoming airborne. It reviews and lists recent incidents of aircraft underperformance at take-off, reviews industry efforts to provide automatic warning in such situations, and calls for closer regulatory attention to pilots' portable computers ('electronic flight bags').

Relevance to aircraft systems

This incident was included in discussions of proposed new aircraft equipment:

• Extended duration Cockpit Voice Recorder (CVR)
• Takeoff Performance Monitoring System (TOPMS) or Takeoff Acceleration Monitoring System (TAMS)

References

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