TUI B738 at Bristol on Mar 4th 2024, takeoff with insufficient thrust

Last Update: December 5, 2024 / 14:47:14 GMT/Zulu time

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Photo of TUI Airways G-FDZS, Boeing 737-800 — TUI Airways G-FDZS, Boeing 737-800 (Photo credit: BriYYZ / Flickr / License: CC by-sa)

Incident Facts

Date of incident
Mar 4, 2024

Classification
Report

Airline
TUI Airways

Flight number
BY-6114

Departure
Bristol, United Kingdom

Destination
Las Palmas, Spain

Aircraft Registration
G-FDZS

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A TUI Airways Boeing 737-800, registration G-FDZS performing flight BY-6114 from Bristol,EN (UK) to Las Palmas,CI (Spain) with 163 passengers and 6 crew, was departing Bristol's runway 09, when autothrottle disengaged upon selection of TOGA at the start of the takeoff roll. The crew manually set the thrust to 84.5% N1 (rather than 92.8% N1 as needed) and continued takeoff. The aircraft rotated about 260 meters prior to the end of the runway and crossed the end of the runway at 10 feet AGL. The crew continued the flight to Las Palmas although whenever they tried to engage autothrust, it disconnected again. The aircraft landed safely in Las Palmas.

The AAIB released their special bulletin without conclusions yet analysing:

The aircraft took off from Runway 09 with a thrust setting significantly below that required to achieve the correct takeoff performance. Rotation for the takeoff occurred only 260 m before the end of the runway and the aircraft passed over the end at a height of approximately 10 ft. The N1 required to achieve the required takeoff performance was 92.8% but, following an A/T disconnect when the crew selected TOGA, 84.5% was manually set instead. Despite an SOP requirement to check the thrust setting on takeoff, the crew did not realise that the thrust was not set correctly until after the takeoff although they had noted how close to the end of the runway they were.

The A/T had disconnected when the TOGA switch was pressed due to a fault with the ASM associated with the thrust lever for engine 1. This disconnect was a known issue with the older type ASMs fitted to the aircraft type. The manufacturer has issued a Fleet Team Digest for operators detailing the issue and the SB for replacing the ASMs with a newer model.

On Dec 5th 2024 the AAIB released their final bulletin concluding the probable causes of the serious incident were:

Having selected the TOGA switch to begin the takeoff role at Bristol, the A/T disconnected. The crew reselected the A/T, but it did not re-engage in an active mode and did not control the throttles as the crew expected. Neither crew realised that the thrust was not set as required for takeoff but was significantly less than had been calculated during the pre-flight preparation. As a result, the aircraft performance was significantly compromised. The uncommanded disconnection was likely caused by low voltage being supplied to the ASM (autothrottle servo motor) which caused it to disconnect from the throttles. The issue is a well known one for which a new model of ASM, and updated FCC Software incorporating changes to reduce the incidence of uncommanded disconnects.

The operator took action to raise awareness of the issue and what actions are expected as well as strengthening their FDM programme to better monitor the occurrences. The manufacturer encourages any operators who experience nuisance disconnects to replace the ASM with the newer model and ensure a later standard of FCC Software loaded.