Summary
- The UK Air Accidents Investigation Branch (AAIB) issued a preliminary incident report involving a TUI Airways Boeing 737-800 at Bristol Airport (BRS) in March 2024.
- Then, the aircraft’s autothrottle system failed, with the pilots also entering the wrong thrust setting, which resulted within the aircraft taking off from the runway at a height of around 10 feet (3 meters).
- After three days of maintenance after the incident, the Boeing 737-800 has been operating flights on behalf of TUI Airways as much as at the present time.
The UK Air Accidents Investigation Branch (AAIB) issued a special bulletin containing preliminary details about an incident at Bristol Airport (BRS) on March 4, 2024. Then, a TUI Airways Boeing 737-800, registered as G-FDZS, took off from the airport with insufficient thrust and crossed the tip of the runway at the peak of around 10 feet (3 meters).
Incorrect thrust settings
The investigators noted that the manually set thrust – 84.5% N1 – was below the required takeoff thrust – 92.8% N1 – and was not noticed by the pilots and never picked up through the usual operating procedures (SOP).
In line with SKYbrary, N1 is the rotational speed of the low-pressure (low-speed) engine spool. Nevertheless, the AAIB also identified that the autothrottle (A/T) disengaged when pilots chosen takeoff mode in the beginning of the takeoff roll.
Photo:Â Photofex_AUT | Shutterstock
The TUI Airways Boeing 737-800 was operating a flight between BRS and Las Palmas Gran Canaria Airport (LPA) with six crew members and 163 passengers onboard the aircraft. The flight was also a line training sector for a brand new captain, who was within the left-hand seat of the cockpit, with a line training captain sitting beside him within the right-hand seat. The latter pilot was the investigators noted.
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Disengaging autothrottle
The AAIB’s preliminary report depicted the events of the flight, noting that the aircraft left its stand to taxi onto the runway at BRS at 10:41 UTC. The pilots armed the A/T on the activate the mode control panel (MCP) through the pre-takeoff procedures, per TUI Airways’ SOP. Subsequently, the aircraft taxied onto the runway at 11:04, and the air traffic control (ATC) cleared the Boeing 737-800 to take off shortly after.
The control of the aircraft was handed over to the aircraft commander, who was the pilot flying (PF) for the flight to LPA. The PF proceeded to advance the thrust levels to 40% N1 and proceeded to attend for the engines to stabilize before pressing the Takeoff/Go-Around switch (TOGA).
Per the AAIB, the TOGA switch engages each the A/T in N1 mode and the autopilot/flight director system (AFDS) in takeoff mode. Nevertheless, the investigators stated that at this point, the A/T disengaged with an associated warning.
The PF responded by arming the A/T on the MCP once more, and at the identical time, the pilot advanced the thrust levers manually toward the required takeoff setting before releasing the thrust levers. The AAIB highlighted that per TUI Airways’ SOP, the pilot monitoring (PM) needed to take over the controls of the thrust lever, as they did so during this flight.
Significantly less thrust
Nevertheless, when the PM re-armed the A/T, the system didn’t control the thrust lever servos, something that the pilots didn’t expect to occur. As a substitute, the A/T entered into armed mode and, thus, didn’t advance to the required thrust settings. The pilots didn’t move the thrust levers, which had remained at the extent that the PF moved them to.
Photo:Â Grosey | Shutterstock
The AAIB highlighted that the rotation point was 260 m (853 ft) from the tip of the runway, with the Boeing 737-800 crossing the runway at a height of around 10 ft (3 m). While the rest of the flight to LPA was uneventful, the investigators did note that the pilots had attempted to interact the A/T several times throughout the flight to the Spanish island.
As well as, the AAIB stated that while the thrust was increased at 450 ft (137.1 m) above aerodrome level (AAL), the required thrust was not attained until the Boeing 737-800 reached an AAL of around 900 ft (274.3 m), leading to the aircraft overflying a road adjoining to BRS at lower than 100 ft (30.4 m).
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After the investigators had undergone the Multipurpose Control and Display Unit (MCDU), they stated that the MCDU showed 11 faults on the flight to LPA, with no faults on the return flight or the preceding day. Five of the 11 faults were related to the uncommanded A/T disengagements, with the primary being logged at 11:04 UTC. At that time, the TUI Airways aircraft was on the runway at BRS in the beginning of its takeoff roll.
The AAIB said the fault record indicated that the suspected explanation for the uncommanded A/T disconnect was ASM 1, the A/T servo motor for the number one engine throttle lever. 4 further disengagements happened on the flight to LPA, with two through the initial climb phase.
Photo:Â Vytautas Kielaitis | Shutterstock
Moreover, the MCDU logged other faults after the initial A/T disconnect because the Boeing 737-800 accelerated on the runway. The investigators noted that these happened after the aircraft passed 90 knots airspeed and were all related to an incorrect thrust setting.
Boeing’s quick reference handbook (QRH) lists the explanation why pilots should reject a takeoff at 80 knots, including a system failure(s). In line with the investigators, the disconnection of the A/T when the TOGA switch was pressed was one in all the explanations, yet the flight crew continued with their takeoff roll with a thrust setting below that was calculated for his or her takeoff performance.
A previous AAIB suggestion urged operators to implement flight data monitoring (FDM) algorithms to detect takeoff events that otherwise could go unnoticed and unreported. While the incident at BRS was reported by TUI Airways, the airline’s retrospective evaluation showed that the height longitudinal acceleration through the incident had deviated from the everyday peak longitudinal acceleration, which was exhibited on 99.7% of TUI Airways flights departing BRS.
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Long history of nuisance disconnects
The AAIB explained that the A/T system can control the thrust from takeoff to landing on the Boeing 737 Next Generation (NG), including the 737-800. In brief, each thrust lever is moved by an independent autothrottle servo motor (ASM), they usually could be either moved manually or if the A/T is engaged, the ASMs will move the levers in keeping with the computed thrust requirements.
Photo:Â Markus Mainka | Shutterstock
Nevertheless, the AAIB stated that Boeing has said that the A/T system on the 737 NG has had a protracted history of during takeoff engagements. When the fault history is checked, it often shows an issue with the ASM for either the number one or number 2 lever, with subsequent checks typically showing no faults.
In line with the investigators, the TUI Airways Boeing 737-800 was equipped with older ASMs. In October 2021, Boeing issued a Fleet Team Digest to airlines operating the 737 NG, detailing the issue and the service bulletin (SB) to exchange the ASMs and apply the required flight control computer (FCC) software.
Noticing at the tip of the runway
The AAIB concluded its preliminary report that humans struggle to detect acceleration rates and realize that their takeoff run doesn’t match the calculated performance, which could be invisible until very late within the takeoff roll. On this instance, the flight crew realized that the thrust was too low after they took off from BRS, even in the event that they had noticed how close the aircraft was to the tip of the runway.
Photo:Â LIAL | Shutterstock
In line with Flightradar24 data, G-FDZS has continued flying for TUI Airways because the incident, having operated three flights on May 31 on the time of writing. Nevertheless, because it returned to BRS following the incident flight to LPA, it was taken out of economic service for 3 days before it operated a flight to Lanzarote Airport (ACE) on March 7.
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