On 25 August 2010, a Boeing 757-200 being operated by UK airline Astraeus on a passenger flight from Freetown Sierra Leone to London Heathrow was in the cruise at night in Instrument Meteorological Conditions (IMC) at FL370 when vibration levels on both engines increased. When the prescribed ice shedding drill was followed, one engine malfunctioned and vibration on the other remained abnormally high and so a MAYDAY was declared and a diversion to Nouakchott, Mauritania was made without further event. None of the 103 occupants were injured and there was no engine damage.
An Investigation was carried our by the UK AAIB. It was noted that reporting of the incident by the Operator was delayed and that the failure to secure Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) data was not in accordance with applicable regulations. It was also observed that, although QAR data had been recovered for use in the Investigation, provision of such data did not constitute an AMC compatible with regulatory requirements. It was found that at the time of the incident, Astraeus did not have a formal procedure for the preservation of the FDR or CVR but noted that this has since been rectified. During the replay of the QAR, it was found that there was an error on the Boeing FDR data frame conversion in respect of a parameter relevant to the Investigation.
An analysis of an infrared satellite image in conjunction with the QAR data identified the aircraft track in relation to weather characterised as Mesoscale Convective Systems and it was estimated that the aircraft had made prolonged transits through two of these and been within the second when the sudden increase in engine vibration occurred despite having altered course earlier to avoid significant weather radar returns. SAT at the onset of the vibration had been MS 47, which is below the AFM temperature (MS40) at which engine anti icing (EAI) is required to be selected on. However, observing light icing on the windshield by torchlight, the flight crew had first selected the EAI on and then carried out the Quick Reference Handbook (QRH) procedure for ice shedding on the left engine As engine thrust was reduced, the vibration level increased rapidly to the maximum EICAS reading. An attempt to restore normal function was not successful and engine malfunction followed.
The right engine was not subjected to the same drill and continued to operate normally although at the continued abnormal vibration level. Prior to the diversion landing, normal response to left engine thrust lever movement returned and a 2 engine landing was made. Subsequent examination found no sign of damage to either engine.
The engines fitted to the aircraft were Rolls Royce RB211-535E4. Rolls Royce advised the Investigation of their awareness of several low pressure shaft vibration events on the same engine type which had been attributed to ice and noted that all had occurred on aircraft which had either not had a specific SB in respect of the fan spinner embodied ahead of a latest-compliance date of March 2015 or where prior seal damage in this same area had been found. In this case, the incident aircraft engines had no pre-event seal damage but were in a ‘pre-mod’ condition.
The Investigation concluded that the left engine had “entered a surge or stall condition following the action of retarding the thrust lever and then increasing thrust. There was no damage evident within the engine and the vibration condition was attributed by the engine manufacturer to an asymmetric ice build up under the spinner fairing.”
The reason why the engine responded abnormally to the ice shedding procedure was considered to have been a consequence of thrust lever movements as part of the drill being made more abruptly than the engine manufacturer had anticipated. A review of the existing Engine Operating Instructions (EOI) by Rolls Royce concluded that they should be improved and an addendum made to the effect that the ice shedding procedure should not be used when at climb or cruise thrust in high altitude ice crystal clouds above FL250 where the most appropriate action would be to descend to warmer conditions. The Investigation was advised that the proposed EOI changes have been agreed to by the airframe manufacturer and will be embodied in the QRH and FCOM in their 2011 revision. Because of this, no Safety Recommendation to correct the inappropriate QRH drill was considered necessary.
However, three other Safety Recommendations were made by UK AAIB as a result if the Investigation recommending that:
- The Civil Aviation Authority ensures that United Kingdom operators have procedures for preventing the loss of Cockpit Voice Recorder and Flight Data Recorder recordings, following an occurrence subject to mandatory reporting, in accordance with the legislative requirements of EU-OPS 1.160 and EU OPS 1.085. (2011-022)
- Boeing advises all operators utilising the Flight Data Recording 757-2 Data Frame of the need to correct the conversion of the left and right engine vibration parameters. (2011-021)
- Boeing provides updated documentation that corrects the Flight Data Recording 757-2 Data Frame conversion information for the left and right engine vibration parameters. (2011-022)
The Final Report: AAIB Bulletin: 6/2011 EW/C2010/08/15 of the Investigation was published on 9 June 2011. It contained the following ‘Comment’ in respect of the icing environment which led to the event:
“The aircraft experienced a prolonged exposure to an area where ice crystal concentrations may have been present. Although ice crystals may not have been the cause of this event it is an atmospheric condition that is not yet well understood. This has been recognised and a data collection programme is presently underway in the USA to increase the understanding of high water content ice crystal conditions. These are conditions where strong convective weather activity lifts high concentrations of ice crystals to high altitude. The crystals can partly melt and stick to internal engine surfaces causing power loss and/or surge/stall to occur. Data indicates that there have been at least 100 events of jet engine power loss due to core-icing during the last 30 years. The data gathered should enable certification authorities to define new parameters for developing and certifying engines. It may also lead to better forecasting of the presence of such areas, which are not detectable by existing aircraft weather radars. The knowledge of these conditions, and their effect upon various aircraft systems, is at present limited. Proposed guidance to flight crew is restricted to avoiding such weather or flying clear if encountered”'.
The publication in March 2011 of the EASA NPAs 2011-03 and 2011-04, entitled ‘Large Aeroplane Certification Specifications in Supercooled Large Drop, Mixed phase, and Ice Crystal Icing Conditions’ and ‘Turbine Engine Certification Specifications in Icing Conditions’ respectively was also noted. These recognise that the icing environment used for certification of large aircraft and their turbine engines needs to be expanded in order to include severe high altitude icing conditions which can occur at very low temperatures and which are not included in the current certification icing environment.