On 30 March 2008, a privately operated Cessna Citation 500 which had just taken off from Biggin Hill UK for Pau, France in day Visual Meteorological Conditions (VMC) reported ‘engine vibration’. Whilst positioning for a return to land, the aircraft descended and the pilots reported a major power problem just before it struck the side of a house killing all five occupants and destroying the house and adjacent property in the intense fire which followed.
An Investigation was carried out by the UK AAIB. In the absence of both Flight Data Recorder (FDR) and Cockpit Voice Recorder (CVR) which were not required to be fitted, the investigation was difficult. There were two pilots on board but it was not possible to ascertain the exact role of pilot during the flight. The one in the left hand seat had recently qualified on type and was employed to fly the aircraft on behalf of the owners and there was another pilot with more experience on-type in the right hand seat who was believed to have been functioning as a mentor.
The track of the aircraft constructed from radar records is shown on the map below, taken from the AAIB Report.
Aircraft recorded flight path, tranmissions and witness locations (click to enlarge)
The Investigation noted that the aircraft had been returning to the departure airfield because of reported “ENGINE VIBRATION” and therefore an attempt was made to identify the source of this vibration. It was noted that in normal operation, bleed air from both engines is used to drive a single Air Cycle Machine (ACM) and that therefore movement of either engine throttle would result in a change in the bleed air supply and hence the rotational speed of the ACM shaft. It was additionally noted that, in the event that the shaft was vibrating, any change of thrust on either engine would result in a change in the characteristics of the vibration being produced and because of the high speed at which ACM shafts operate, any vibration would be similar to the vibration frequency produced by a malfunctioning engine. The investigation therefore concluded that vibration of the ACM shaft and inlet fan was the most probable source of vibration that the pilots described as “ENGINE VIBRATION”.
The evidence available suggested that there had been a loss of thrust output from both engines despite their lack of faults and that relights had been attempted. It was considered likely that had a starter-assisted, single engine relight been initiated shortly after the loss of thrust from both engines, the aircraft could have achieved a positive rate of climb in the time available. A full explanation of what had happened could not be established. However, the following factors contributory to the loss of control through loss of thrust were identified:
- “It is probable that a mechanical failure within the air cycle machine caused the vibration which led to the crew attempting to return to the departure airfield.”
- “A missing rivet head on the left engine fuel shut-off lever may have led to an inadvertent shutdown of that engine.”
- “Approximately 70 seconds prior to impact, neither engine was producing any thrust.”
- “A relight attempt on the second engine was probably started before the relit first engine had reached idle speed, resulting in insufficient time for enough thrust to be developed to arrest the aircraft’s rate of descent before ground impact.”
The full AAIB Final Report was published on 21 May 2010 and may be seen at SKYbrary bookshelf: Aircraft Accident Report 3/2010
The Report contained three Safety Recommendations.
Safety Recommendation 2010-014: It is recommended that the Federal Aviation Administration require that Cessna Aircraft Inc introduce a scheduled inspection of the Cessna Citation 1 throttle quadrant assembly to ensure the integrity of the riveted joints securing the fuel shut-off levers to the throttle levers.
Safety Recommendation 2010-015: It is recommended that the Federal Aviation Administration require Cessna Aircraft Inc to amend the ‘EMERGENCY RESTART -TWO ENGINE’ checklist to emphasise the significance of only restarting one engine at a time.
Safety Recommendation 2010-016: It is recommended that the International Civil Aviation Organisation adopt the proposals of its Flight Recorder Panel for the requirement to install flight recorders on turbine engine powered aeroplanes of a maximum certified takeoff mass of 5,700 kg or less.