Date: 4th March 2024
Location: Bristol Airport, England, UK
Aircraft: Boeing 737-800, registration G-FDZS
Operator: TUI Airways
Flight Number: BY-6114
Route: Bristol, England, to Las Palmas, Canary Islands, Spain
Occupants: 163 passengers, 6 crew
Injuries: None reported
Summary of Events
On 4th March 2024, a TUI Airways Boeing 737-800, operating flight BY-6114 from Bristol to Las Palmas, commenced takeoff from Runway 09 with an insufficient thrust setting. During the takeoff roll, the autothrottle (A/T) disengaged after selecting the Takeoff/Go Around (TOGA) switch due to a known issue with the Autothrottle Servo Motor (ASM). The crew manually set thrust but inadvertently selected a setting of 84.5% N1 instead of the required 92.8% N1 for takeoff.
The aircraft rotated just 260 metres before the end of the runway and crossed the runway threshold at an altitude of 10 feet above ground level (AGL). Despite the suboptimal performance, the flight continued to Las Palmas without further incident. However, the crew experienced repeated autothrottle disengagements during the remainder of the flight.
The UK’s Air Accidents Investigation Branch (AAIB) classified this occurrence as a serious incident.
Sequence of Events
1. Takeoff Roll:
• Upon selecting TOGA, the autothrottle disconnected unexpectedly.
• The crew reselected the A/T, but it did not re-engage.
2. Manual Thrust Setting:
• The crew manually set thrust to 84.5% N1, unaware that this was significantly lower than the required 92.8% N1.
3. Runway Usage:
• The aircraft rotated 260 metres before the runway end and passed the threshold at only 10 feet AGL.
4. Flight Continuation:
• The crew noted the proximity to the runway end but did not realise the thrust discrepancy until airborne.
• The A/T continued to disengage when reselected during the flight.
5. Landing:
• The aircraft landed safely in Las Palmas.
Aircraft Information
The Boeing 737-800 is a widely used narrow-body jet equipped with an autothrottle system designed to automate thrust control during critical phases of flight. The ASM, a key component, adjusts the thrust lever positions to maintain target N1 settings.
Initial Findings
Autothrottle System Malfunction
• The A/T disengaged upon TOGA selection due to a fault in the ASM associated with engine 1.
• This issue was linked to low voltage supplied to the ASM, causing it to disconnect.
Crew Actions
• Despite an SOP requirement to verify thrust settings during the takeoff roll, the crew did not detect the incorrect thrust until after takeoff.
• The absence of sufficient thrust significantly compromised the aircraft’s takeoff performance.
Known Issue with ASM
• The malfunctioning ASM was of an older type known for nuisance disconnects.
• Boeing had issued a Fleet Team Digest and Service Bulletin advising operators to replace older ASMs with an updated model.
Safety Risks
1. Runway Overrun:
• The insufficient thrust setting resulted in a dangerously reduced margin for takeoff.
2. Performance Margins:
• Crossing the runway threshold at only 10 feet AGL left minimal room for error.
3. Crew Workload:
• Repeated A/T disengagements during the flight increased crew workload and distraction.
Recommendations
1. Component Upgrades:
• Operators should replace older ASM models with the newer version to reduce the likelihood of autothrottle disconnects.
2. Software Updates:
• Install the latest Flight Control Computer (FCC) software to address known A/T issues.
3. Crew Awareness and Training:
• Reinforce the importance of verifying thrust settings during the takeoff roll as per SOPs.
4. Flight Data Monitoring (FDM):
• Expand FDM programmes to monitor and address autothrottle-related anomalies.
Safety Actions
• Operator Actions:
• TUI Airways issued internal safety advisories and strengthened its FDM monitoring to identify autothrottle issues more effectively.
• Manufacturer Actions:
• Boeing continues to encourage operators to upgrade ASMs and FCC software across their fleets.
Safety Lessons
This incident highlights the critical importance of adherence to SOPs, particularly during the takeoff roll. It also underscores the need for proactive maintenance and system upgrades to address known technical vulnerabilities.
Disclaimer
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Our investigation reports are based on all the evidence and facts available at the time of writing and posting. We apologise if any details are missed or are not fully accurate.
