History of the flight
On 25 August 2010, a Boeing 757-28A, registration G-STRY, was operating a scheduled passenger service from Freetown, Sierra Leone, to London Heathrow. The aircraft departed at 2343 hrs UTC and climbed to FL370. En route, the crew encountered convective weather and deviated east of the planned track to avoid strong activity.
Approximately one hour and forty minutes into the flight, the flight crew observed a sustained increase in both engine vibration levels to about 2.3 units. The commander noticed ice on the windscreen wiper; the static air temperature was -47°C. The crew selected Engine Anti-Icing (EAI) on, but vibrations continued to gradually increase over ten minutes to between 3 and 3.5 units. They consulted the Quick Reference Handbook (QRH) non-normal checklist for engine vibration and, believing icing conditions were present, took no further action.
As vibrations became noticeable through the airframe, the commander decided to perform the manufacturer's Fan Ice Removal procedure described in the Operations Manual (OM). The autothrottle was disconnected and the left engine thrust lever retarded. As thrust reduced, left engine vibration immediately increased to a recalled 5 to 6 units. Thrust was reapplied, but the engine did not respond normally; exhaust gas temperature (EGT) peaked at 803°C. The crew reduced thrust to 1.2 EPR to prevent an EGT exceedance, stabilizing vibration at about 2.5 units. The right engine, while indicating 3 units of vibration, operated as expected.
A MAYDAY was declared, and the aircraft descended to FL250, diverting towards Nouakchott, Mauritania. The right engine was kept at medium power during descent. Several attempts to increase left engine power again resulted in high vibration and EGT. The crew chose not to shut down the left engine in case a similar issue occurred with the right. During the descent, the left engine eventually began responding normally to thrust lever inputs. A normal two-engine landing was made at Nouakchott at 0229 hrs. Subsequent ground examination found no external or internal damage to either engine.
Meteorological conditions
Pre-flight planning included an Africa High Level Significant Weather Forecast chart showing isolated cumulonimbus clouds with tops up to FL500. Upper winds were from the east. En route, the crew used weather radar to deviate east of the flight path. A colour-enhanced infrared satellite image at 0100 hrs, with the aircraft track superimposed, showed two Mesoscale Convective Systems (MCS) along the route. The aircraft flew within one MCS for over 500 nm over two hours.
Flight recorder data
The aircraft was equipped with a Flight Data Recorder (FDR), Quick Access Recorder (QAR), and a 30-minute Cockpit Voice Recorder (CVR). At Nouakchott, the operator secured QAR data after confirming it recorded the same parameters as the FDR, so the FDR was not removed. The CVR was not removed as the operator believed the relevant period would have been overwritten. The operator has since implemented a formal procedure for recorder preservation.
QAR data included EAI selection, thrust lever position, engine pressure ratio (EPR), EGT, and vibration. Only the shaft (N1, N2, or N3) with the highest vibration was recorded. Thrust lever and EPR were logged once per second; EGT and vibration once every 64 seconds.
Data showed that after takeoff, left engine vibration (predominantly N1) was about 0.9 units, right (predominantly N3) about 1.2. At 0020 hrs, as the aircraft resumed a northerly track, N1 vibration on both engines began increasing. At 0035 hrs, during light turbulence, the autothrottle reduced EPR from 1.6 to 1.4; left engine peak vibration shifted from N1 to N3, dropping from 2 to 1.2 units. When EPR increased to 1.6, right engine peak vibration returned to N1 at 1.7 units. At 0041 hrs, entering the second MCS, vibration increased further. As EPR increased to 1.7, both engines' N1 vibration trends rose; at left 2 units and right 2.5 units, the crew selected EAI on.
Investigation findings
The investigation determined that the left engine entered a surge or stall condition following the retarding of its thrust lever and subsequent increase in thrust. No damage was found, and the vibration was attributed by the engine manufacturer to asymmetric ice buildup under the spinner fairing. The manufacturer's Fan Ice Removal procedure, as described in the OM, was found inappropriate for the prevailing conditions. The AAIB issued three Safety Recommendations.