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B733, vicinity Bournemouth UK, 2007 (LOC AW HF)

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Summary
On 23 September 2007, a Boeing 737-300 operated by Thomsonfly, on routine ILS approach at night to Bournemouth Airport, experienced a stall during early stage of the approach. The auto-throttle disengaged with the thrust levers in the idle thrust position. The disengagement was neither commanded nor recognised by the crew and the thrust levers remained at idle throughout the approach. As result of the stall, the commander took control and initiated a go-around. During the go-around the aircraft pitched up excessively; flight crew attempts to reduce the aircraft’s pitch were largely ineffective. The aircraft reached a maximum pitch of 44° nose-up and the indicated airspeed reduced to 82 kt. The flight crew, however, were able to recover control of the aircraft and complete a subsequent approach and landing at without further incident.
Event Details
When September 2007
Event Type AW, HF, LOC
Day/Night Night
Flight Conditions IMC
Flight Details
Aircraft BOEING 737-300
Operator Thomsonfly
Domicile United Kingdom
Type of Flight Public Transport (Passenger)
Origin Faro
Intended Destination Bournemouth
Flight Phase Missed Approach
APR
Location - Airport
Airport vicinity Bournemouth
General
Tag(s) Approach not stabilised
Event reporting non compliant
Inadequate Aircraft Operator Procedures
HF
Tag(s) Ineffective Monitoring
Manual Handling
Procedural non compliance
Inappropriate crew response (automatics)
Inappropriate crew response - skills deficiency
LOC
Tag(s) Flight Management Error
Flight Control Error
Temporary Control Loss
Extreme Pitch
AW
System(s) Engine Fuel and Control
Contributor(s) OEM Design fault
Safety Net Mitigations
Malfunction of Relevant Safety Net No
Stall Protection Partially effective
Outcome
Damage or injury No
Aircraft damage Nonewarning.png"None" is not in the list of possible values (Minor, Major, Hull loss) for this property.
Injuries Nonewarning.png"None" is not in the list of possible values (Few occupants, Many occupants, Most or all occupants) for this property.
Fatalities Nonewarning.png"None" is not in the list of possible values (Few occupants, Many occupants, Most or all occupants) for this property.
Causal Factor Group(s)
Group(s) Aircraft Operation
Safety Recommendation(s)
Group(s) Aircraft Operation
Aircraft Airworthiness
Investigation Type
Type Independent

Contents

Description

On 23 September 2007, a Boeing 737-300 being operated by Thomsonfly, on a public transport flight from Faro to Bournemouth was making a night ILS approach at destination with the First Officer as PF when the auto-throttle disengaged early in the approach with the thrust levers at the flight idle thrust position. The disengagement was neither commanded nor recognised by the crew and the thrust levers remained at idle throughout the approach. After the indicated speed had dropped significantly below Vref, the low and reducing speed was eventually recognised and the aircraft commander took control and initiated a go-around. A number of stall warnings occurred during the go-around and at one point, the aircraft pitched up excessively. Flight crew attempts to reduce the aircraft pitch were largely ineffective and a maximum pitch of 44 degrees nose-up was reached and indicated airspeed reduced to 82 kts151.864 km/h
42.148 m/s. The flight crew, however, were finally able to recover control of the aircraft and complete a subsequent approach and landing without further event. The reporting of the an event by the crew to the Operator was prompt but not adequate and there was a significant delay before the Operator identified the seriousness of what had occurred and actioned external notification.

The Investigation

The Investigation of the Serious Incident led by AAIB UK identified the following causal factors:

  • "The aircraft decelerated during an instrument approach, to an airspeed significantly below the commanded speed, with the engines at idle thrust. Despite the application of full thrust, the aircraft stalled, after which the appropriate recovery actions were not followed.
  • The trimmed position of the stabiliser, combined with the selection of maximum thrust, overwhelmed the available elevator authority."

The investigation identified the following contributory factors:

  • "The autothrottle warning system on the Boeing 737-300, although working as designed, did not alert the crew to the disengagement of the autothrottle system.
  • The flight crew did not recognise the disengagement of the autothrottle system and allowed the airspeed to decrease 20 kts37.04 km/h
    10.28 m/s     below Vref before recovery was initiated."

The following three recommendations are produced by the Report:

  • "It is recommended that Boeing, in conjunction with the Federal Aviation Administration, conduct a study of the efficacy of the Boeing 737-300/400/500 autothrottle warning system and if necessary take steps to improve crew alerting.
  • It is recommended that the European Aviation Safety Agency review the requirements of Certification Standard 25 to ensure that the disengagement of autoflight controls, including autothrottle, is suitably alerted to flight crews.
  • It is recommended that Boeing clarify the wording of the Boeing 737 300-500 approach to stall recovery Quick Reference Handbook Non-normal Manoeuvres to ensure that pilots are aware that trimming forward may be required to enhance pitch control authority."

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