History of the Flight
The aircraft, a LET L-410 UVP-E Turbolet registered as OK-ASA, was operating a scheduled passenger flight from Isle of Man to Blackpool. The commander was the pilot flying. Weather conditions were fine and dry with a surface wind from 340° at 5 kt. At 1546 hrs, the flight crew requested taxi clearance and received instructions to taxi to holding point A1 for Runway 26. At 1551 hrs, they were cleared to line up and wait, and at 1553 hrs, takeoff clearance was issued.
The aircraft was configured with 18° flap. At V1/VR speed of 81 kt, the co-pilot called "V one rotate" and the commander began raising the nose. Both pilots then heard a loud noise. The commander checked the engine instruments but observed no unusual indications, and there was no yaw or abnormal aircraft behavior. The aircraft had become airborne. The commander retarded both power levers to idle, lowered the nose, and landed ahead on the remaining runway. The co-pilot advised ATC they were aborting the takeoff. Maximum groundspeed recorded was 101 kt. The aircraft touched down near the intersection of Runway 03/21 and slowed to taxi speed before vacating at the end of Runway 26. The noise persisted at reduced levels during taxi. The aircraft taxied to the parking area and a normal shutdown was performed.
During the incident, ATC instructed one other aircraft to go around. The runway was inspected after OK-ASA vacated; no debris was found, and the runway was re-opened.
Aircraft and Crew Information
The LET 410 UVP-E is a 19-seat passenger aircraft powered by two Walter M601E turboprop engines. The aircraft was manufactured in 1990 (Serial Number 902439). On board were two crew and ten passengers (including one infant). The actual takeoff weight was 5,943 kg; maximum takeoff weight is 6,400 kg. The aircraft had undergone a routine maintenance inspection at 0630 hrs and flown six sectors prior to the incident with no abnormalities reported.
The commander, aged 40, held an Airline Transport Pilot's Licence and had 3,221 hours total flying experience, with 2,827 hours on type. In the last 90 days he flew 157 hours, and 51 hours in the last 28 days. He had been based at Isle of Man for several months. After the incident, he commented that the event was unlike any he had experienced, noting the high noise level and absence of yaw. The co-pilot had 785 hours on type and described the sound as "a terrible noise" without recollection of vibration.
Engineering Investigation
The aircraft was inspected the day after the incident. The left engine showed no external damage or leaks, but when the propeller was turned by hand, a metallic rubbing noise was heard from the power turbine section. No other defects were found. The left engine was removed and sent to the manufacturer for detailed examination.
Examination revealed that a balance plug had broken and released from the centrifugal compressor disc. Balance plugs are screwed into the disc beneath compressor blade roots and secured with thread-locking adhesive and center-punch indentations. The detached plug traveled through the gas path, damaging the centrifugal compressor, gas generator and power turbine nozzle guide vanes and blades, as well as nine Intermediate Turbine Temperature (ITT) thermocouples. Damage was contained within the engine casing and was insufficient to cause significant loss of power.
The manufacturer concluded that the broken balance plug failed due to a fatigue crack originating from the thread root of the plug's uppermost screw thread. The fracture surface exhibited crack initiation from multiple sites, consistent with intensive cyclical loading caused by the plug becoming loose. Metallurgical examination showed the plug was made of the required grade of steel with no anomalies in heat treatment, hardness, or microstructure.
Possible causes for the plug becoming loose were identified as either insufficient assembly torque or ineffective securing after installation. The type of balance plug that failed has been withdrawn from use, and the manufacturer is evaluating design changes to remaining balance plug types, thread-locking adhesive compositions, and plug tightening procedures.
Recorded Data
The aircraft was fitted with a Cockpit Voice Recorder (CVR) and Flight Data Recorder (FDR). The FDR, of Czech origin, was not familiar to the AAIB. The operator provided an avionics engineer with the manufacturer's interface equipment. Data from 76 flights were downloaded, but not from the incident flight. The CVR recording captured the event but suffered from a fault on the cockpit area microphone channel that attenuated the recording. The operator committed to resolving the flight recorder problems, and EASA is assessing requirements for flight recorder checking.
Summary
The loud noise heard by the crew during takeoff was traced to damage in the left engine's gas generator and power turbine stages, caused by a broken balance plug released from the centrifugal compressor disc. The damage did not cause significant power loss, so the usual cues of engine failure—power loss and yaw—were absent. The crew were startled by the noise, which was outside their previous experience. The commander suspected left engine failure but was uncertain; seeing sufficient runway ahead, he closed the power levers and aborted the takeoff, landing safely.