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24th of January 1993

A Rolls Royce RB 211 powered Boeing 747-236B had taken off from London Gatwick on a scheduled passenger flight to Lagos, Nigeria. Onboard 296, 278 passengers and 18 crewmembers. With Outside Air Temperature below +10ºC and showers in the area, the engine anti-ice was selected "ON" for take-off, as per operating procedures. As the aircraft remained clear of the clouds and showers during the climb the engine anti-ice was selected "OFF".

A Rolls Royce RB 211 jet engine (source public domain)

During the initial climb, the aircraft received clearance from ATC to fly direct to the Amboise VOR-DME in France (112 miles southwest of Paris (France) . Upon crossing the French coast the aircraft entered a cloud layer that spread from FL150 to FL200 (~4500 meters to 6000 meters). The engine anti0-ice was switched to "ON" again and it remained one until the aircraft was clear of the cloud layer, during the time flying through the cloud layer, no ice build-up on the airframe had been observed, and the flight continued its climb. No further clouds were encountered and the aircraft levelled off at its initial cruise lever, FL290 (~8800 meters). Less than two minutes after reaching FL290, with the speed stable at Mach 0.845, the Flight Engineer observed an increase in engine number 4 (right-hand outboard engine) vibration level. It continued to increase to above 2.5 units, at which a vibration warning light came on notifying the crew of the high vibration level.

With an audible rumble, the crew performed the required memory items as per the Flight Manual. This confirmed the increased vibration level above the 2.5 units level, in accordance with the procedures the engine was throttled back and shit down. The remaining three engines were operating as normal.

Boeing 747 Flight Engineer panel (source; public domain.

The flight was close to the Amboise VOR and the commander decided a return to London Gatwick was the best option the diversion to Gatwick was requested as well as a descent to FL180 (~5400 meters), which were both approved by ATC With one engine out the aircraft drifted down to FL180 and upon reaching FL180 the thrust on the operative engines was increased to attain the right setting for the 3 engine cruise.

However when the thrust was increased the number 1 engine (lefthand outboard) vibration level increased to a level that set off the vibration warning while all other engine parameters were normal, Again an audible rumble was heard, The crew carried out the required procedures as they did just a short time ago, with the engine at idle the thrust lever was moved and the vibration level was found to be adversely affected by an increase in thrust, Subsequently, the engine was shut down.

With two of the four engines shut down an emergency was declared and a diversion was initiated to Paris Charles de Gaulle Airport. During the diversion, 60 tons of fuel was jettisoned to get the aircraft gross weight below the maximum landing weight. After completing the fuel dump an uneventful dual-engine approach to, and landing on Runway 28 at Charles de Gaulle Airport was made. After landing the aircraft taxied under its own power to a parking position.

After extensive checks of both engines several engine runs were carried out, some minor findings were done, but none that could explain the high vibration levels. Out of an abundance of caution, the Variable Inlet Guide Vane Controler and the Fuel Flow Regulator on engine number 1 were changed. Another extensive engine run was carried out, and all indications were satisfactory, after which the aircraft was ferried to London Heathrow. Several checks were carried out during the flight without any abnormalities being found. After arrival in London, the aircraft went into the hangar for a major check (which was already scheduled). Extensive checks were carried out and (summarised) the following results were recorded, related to the vibration issues;

  • #4 engine fanblades out of balance

  • #4 engine fanblade clappers (mid-span shrouds) were dry (little to no lubricant, {An fanblade clapper is shown on the picture adjacent circled in red} This could cause them to stick together, causing a rise in vibration

  • #1 engine fanblades showed wear, related to lack of lubrication on the clappers This could have caused them to stick together, causing a rise in vibration

An investigation by the engine manufacturer into the incident using all available data concluded that the lack of lubrication on the fan blade clappers could have been the cause of the vibration,

The extensive report by the AAIB is available by clicking here.

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