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5th of November 2012, Compressor Failure, Blog #624

With a crew of two a LET L-410 UVP-E Turbolet was operating a passenger flight between the Isle of Man and Blackpool (United Kingdom) with ten passengers. The weather that day was fine with light winds (340º at 5 knots) and no precipitation.

The damaged power turbine (Source & © AAIB)

After a successful engine start taxi clearance was received at 15.46 lt (local time), subsequently the aircraft taxied to runway 26. At 15.53 lt ATC issued the take-off clearance, With the flaps set to 18º, the decision speed and rotation speed (V1/Vr) had been calculated to be 81 knots. As the aircraft accelerated the required callouts were made by the pilot monitoring (PM, the first officer). At 81 knots the PM called out "ROTATE" at which point the pilot flying (PF, the commander) raised the nose of the aircraft. At that moment in time, a loud noise was heard by both pilots, a quick scan of the engine instruments did not reveal anything abnormal. Also, no yaw of abnormal aircraft behaviour was observed. As the aircraft had just become airborne the PF retarded the power levers to idle and landed the aircraft on the remaining runway. The PM informed ATC about the aborted take-off as the aircraft touched down again and slowed down to taxi speed, exiting the runway at the end. With the throttles at idle, the noise was still audible but much less than at the take-off power setting. As the aircraft was taxied back to the parking stand a runway inspection was performed, and no debris was found. After returning to the parking stand the aircraft was shut down using the normal shut-down checklists.

Location of the missing balance plug (Source & © AAIB)

Engineers were flown in to inspect the aircraft the next day and found a metallic rubbing noise emanating from the power turbine when rotating the left engine propeller. There were signs of damage or leaks from the engine, a Walter M601E turboprop. The engine was removed and sent to the manufacturer for a detailed examination and stripdown. During this stripdown, it was found that a balance plug had been released from the centrifugal compressor disc.

{Balance plugs are used to balance the compressor disc and are screwed into the disc beneath the compressor blade roots. They are secured to the disc by thread-locking adhesive in addition to centre-punch indentations at the edge of the balance plug holes.} The balance plug had travelled through the engine gas path causing damage to the centrifugal compressor, gas generator, and power turbine.

Nine of the ITT (Intermediate Turbine Temperature) thermocouples were also found damaged. All damage was contained within the engine casing but was insufficient to cause a loss of power.

A Walter M601 engine (Source & © The balance plug was retrieved from within the engine and after a metallurgical examination it was determined that it was manufactured according to the specifications. Insufficient torque or inadequate securing of the plug were identified as possible causes for the balance plug becoming loose. The Air Accident Investigation Branch investigated this incident, their report served as the source. It can be accessed by clicking on the .pdf file below;

Download PDF • 1.34MB

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