What happened
On 5 November 2012, a LET L-410 UVP-E aircraft, registration OK-ASA, was performing a scheduled passenger flight from Isle of Man (Ronaldsway) Airport to Blackpool. The flight was operating under clear, dry weather conditions with a light surface wind.
As the aircraft reached the rotation speed of 81 kt on Runway 26, the crew heard a sudden and extremely loud noise. Although the commander initially checked the engine instruments and found no abnormal indications, yaw, or changes in aircraft behavior, the noise was highly unusual. Suspecting a potential engine failure, the commander closed the throttles and decided to abort the takeoff. The aircraft landed safely on the remaining runway with no injuries to the two crew members or the ten passengers on board.
The investigation
Following the incident, the aircraft was inspected at Isle of Man Airport. While there was no visible external damage or leaks, a metallic rubbing sound was noted in the power turbine section of the left engine when the propeller was turned by hand.
Detailed examination of the left engine revealed significant internal damage to the gas generator and power turbine stages, including damage to the power turbine nozzle guide vanes, turbine blades, and nine intermediate turbine temperature thermocouples. The investigation established that this damage was caused by a broken balance plug that had released from the centrifugal compressor disc and traveled through the engine's gas path.
Findings
- The source of the loud noise was internal damage to the left engine's gas generator and power turbine stages.
- The damage was caused by a broken balance plug that had detached from the centrifugal compressor disc.
- The engine damage was contained within the engine casing and was not severe enough to cause a significant loss of power or the typical physical cues of engine failure, such as yaw.
- Metallurgical analysis showed the balance plug was made of the correct steel grade with no material defects, but it failed due to a fatigue crack at the thread root.
- The manufacturer suggested the plug may have become loose due to insufficient assembly torque or ineffective use of thread-locking adhesive during installation.
Safety action
Following the incident, the specific type of balance plug involved in the failure was withdrawn from use. The manufacturer is currently evaluating design changes for other M601-series engine balance plugs, as well as improvements to tightening procedures and adhesive compositions.