What happened
On 25 August 2010, a Boeing 757-28A, registration G-STRY, was operating a commercial passenger flight from Freetown, Sierra Leone, to London Heathrow. While cruising at FL370 over Mauritania, the crew observed a sustained increase in vibration levels across both engines. The crew noted ice on the windscreen wiper, leading them to activate the Engine Anti-Icing system.
In an attempt to mitigate the vibration, the crew attempted a fan ice removal procedure. During this process, the thrust lever on the left engine was retarded and then rapidly advanced. This movement triggered a surge or stall condition in the left engine, causing a sharp rise in Exhaust Gas Temperature (EGT) to 803°C and a significant spike in vibration. The crew subsequently descended to FL250 and diverted to Nouakchott, where a safe two-engine landing was completed.
The investigation
The AAIB examined the aircraft's flight data and engine components. Analysis of the Quick Access Recorder (QAR) showed that the vibration source moved between the N1 and N3 shafts. The investigation also identified that the engine manufacturer's recommended ice shedding procedure was not performed correctly, as the thrust was not reduced to idle and held for the required duration.
Furthermore, the investigation revealed a technical error in the Boeing 75/2 Data Frame, where the conversion of engine vibration parameters from the Flight Data Recorder (FDR) to engineering units was incorrectly documented, resulting in displayed values being half of the actual magnitude. The investigation also noted that the operator lacked formal procedures for preserving CVR and FDR data following an incident.
Findings
- The primary cause of the vibration was likely an accumulation of ice under the engine spinner fairing.
- The engine surge/stall was caused by the rapid deceleration and subsequent sudden re-acceleration of the left engine during the attempted ice removal.
- The crew's decision to use the ice removal procedure was influenced by visible icing on the windscreen wiper, despite the procedure being inappropriate for the prevailing high-altitude conditions.
- The aircraft had been flying through areas of significant convective activity where high-altitude ice crystals were present.
Safety action
- Safety Recommendation 2011-020: The CAA should ensure UK operators have robust procedures to prevent the loss of CVR and FDR recordings after a mandatory reportable incident.
- Safety Recommendation 2011-021: Boeing should advise operators to correct the conversion of engine vibration parameters in the 757-2 Data Frame.
- Safety Recommendation 2011-022: Boeing should provide updated documentation regarding the correction of these vibration parameters.