Thomson Airways Flight 1526

Map of Belfast International Airport

The outside air temperature was 16 °C (61 °F), but the operating crew entered incorrect figures (variously −47 °C (−53 °F) and −52 °C (−62 °F)) 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.[3] (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.[3] Delays around reporting the incident meant that the Cockpit Voice Recorder information was not available for the investigation.[3][4])

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

As the aircraft departed Runway 07, the crew noticed unusually slow acceleration as well as a low climb rate, a fact noted by witnesses on the ground. Shortly after lifting off the runway one of the aircraft's landing gear collided with a supplementary runway approach light. The light was knocked loose from its mounting and crushed while the 737 suffered no damage. Crewmembers were not made aware of the incident until ATC personnel at Belfast airport filed an incident report with the AAIB.[5]

The incident aircraft was a Boeing 737-86J(WL) serial number 38124, registration C-FWGH. The aircraft was delivered to Air Berlin in 2011 as registration D-ABMC. The aircraft entered Sunwing Airlines fleet in 2017 following the bankruptcy of Air Berlin. During the time of the incident Thomson Airways had leased the aircraft for the summer season from 29 April until 31 October 2017.

Usage of inaccurate takeoff data can be fatal, as it can lead to runway overruns and possibly collisions with obstructions, as happened in a 2004 accident at Halifax Airport in which seven people died.[6][7]

Neither the installed flight management computer software nor the Electronic flight bags (EFBs) in use helped in detecting the data input error.[8] 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').[3]

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

• Extended duration of the CVR (Cockpit Voice Recorder)[4]
• Takeoff Performance Monitoring System (TOPMS) or Takeoff Acceleration Monitoring System (TAMS).[3][9] A 2019 research paper explores the cause of this serious incident, caused by erroneous data entry. The paper "summarises a basic takeoff acceleration monitoring system and the effect this would have had on the July 2017 event".[10]