Take-off problems: Boeing passenger plane almost crashed due to software error
During take-off of a Boeing 737-800 in Bristol, the autothrottle system failed and the thrust was almost insufficient, reports the British regulatory authority.
A Boeing 737-800 in the service of TUI, symbolic photo.
(Image: TUI, Archiv)
New trouble for Boeing due to technical problems: an almost fully occupied 737-800 passenger plane recently narrowly escaped disaster due to a software error and human error of judgment. The aircraft, operated by TUI, took off from Bristol Airport to Las Palmas on Gran Canaria on March 9, 2024 with 163 passengers on board, but ran into major difficulties on take-off. It left runway 9 just before the end of the runway at an altitude of just three meters. The medium-haul aircraft then flew over the nearby A38 highway at an altitude of just 30 meters at a speed of around 150 knots (around 278 kilometers per hour).
According to an initial report by the British Air Accidents Investigation Branch (AAIB), the pre-set thrust was clearly too low to achieve the prescribed performance. According to the report, the autothrottle system, which can control the engine power from take-off to landing, switched off after a warning. The autothrottle (A/T) was set to take-off mode at this point. The crew then manually set the thrust to 84.5 percent of the engine fan speed (N1), which was lower than the required 92.8 percent. None of the pilots noticed the error. It was also not detected by the standard operating procedures of air traffic control.
The AAIB considers the incident to be serious and requires further analysis. Its special bulletin contains preliminary information for pilots and operators of the Boeing 737 Next Generation (737NG) about the occurrence of the A/T failure and the actions the manufacturer expects the crew to take in such a case.
Problems with the autothrottle are not new
The autothrottle is designed to significantly reduce pilot workload, save fuel and protect the engine. The N1 limits are normally provided by the flight management computer. Each thrust lever is moved by an independent autothrottle servo motor (ASM). The throttles can be controlled manually or, when the A/T is active, the servo motors will position the handles to meet the calculated thrust requirements. The autothrottle is reactivated during flight at the latest when the pilot selects N1 mode at thrust reduction altitude to set the climb thrust. It is automatically deactivated if a system error is detected.
According to the AAIB, problems with the A/T on the current 737 aircraft have long been known. The manufacturer itself speaks of a system "that has a long history of annoying interruptions during take-off mode". The error history of the throttle valve contains many messages for the ASM for both throttle levers. Normally, no errors would be found during subsequent functional tests of the system. However, investigations by the aircraft manufacturer together with the ASM manufacturer had shown that an earlier model of the engine was more susceptible to switch-on or power transfer events. These could have caused the ASM to shut down internally and cause an interruption of the A/T when the automatic thrust control button was pressed.
Boeing safety standards in the crossfire of criticism
According to the regulator, a newer ASM model is more robust against the power quality problems associated with power-on and power transfer events. The manufacturer is therefore recommending that all 737NG operators affected by the fault retrofit their aircraft with the newer ASM variant and the associated flight control software. In October 2021, he described a guide including a service bulletin for the replacement. However, at the time of the incident at Bristol, the aircraft was equipped with the earlier ASM model.
The flight was part of a training segment for a new captain, who was assisted by the aircraft commander. When the A/T switch was reactivated after the initial deactivation of the autothrottle, the system did not control the thrust lever servos as expected by the pilots, writes the AAIB. Instead, it switched to an emergency mode. "As a result, the thrust levers did not move to the required thrust setting" and none of the pilots corrected this. As a result, the performance of the aircraft was reduced. A TUI spokesperson assured the Independent that his company was working closely with the authorities. The AAIB recommendations are helpful for the entire aviation sector.
Boeing's safety standards have been the focus of criticism for some time. In January, for example, a door fell off a brand new 737 Max shortly after take-off in Portland. In April, a Boeing cargo plane landed in Istanbul without its front landing gear extended, but managed to stay on the runway. Five years ago, the aviation giant paid 2.5 billion US dollars after two new 737 Max jets crashed, killing a total of 346 people.
(mack)
