What happened
On 2 August 2003, an Aero Vodochody L-39ZO Albatros, registration G-OTAF, was performing a private flight near Duxford Airfield. While rejoining the circuit to land on Runway 24, the pilot was executing a 'run and break' maneuver. During the process of reducing speed and extending speed brakes, the pilot noted an unusual sound from the engine. Upon attempting to advance the throttle to maintain speed, the engine failed to accelerate, leading the pilot to conclude that a flame-out or engine failure had occurred.
As the aircraft descended through 850 feet, the pilot considered ejecting but ultimately decided against it due to the low altitude and rapid descent. The pilot instead committed to a forced landing in a recently harvested wheat field. During the landing roll, the aircraft struck a hedge and the nose gear collapsed, though the airframe remained largely intact. The pilot was able to vacate the aircraft unassisted without injury.
The investigation
The investigation examined the aircraft's engine, the cockpit configuration, and maintenance history. Post-accident engine testing revealed that while the engine could produce full power on the ground, the Inlet Directing Body (IDB) of the high-pressure compressor was stuck at a fixed angle. This mechanism is designed to vary blade angles to stabilize airflow during RPM changes. Additionally, investigators found that several circuit breakers, including those for the fuel pump and ignition, were in the OFF position, though it was noted that the seat harness could potentially reach and displace these switches.
Maintenance records showed that the engine had not been overhauled since its installation in 1991, despite the manufacturer's recommendation for a six-year calendar limit. Furthermore, a critical service bulletin regarding the torque check of the IDB mechanism was only available in Russian and Spanish, meaning the maintenance organization was unaware of the requirement.
Findings
- The engine failure likely involved a sub-idle compressor stall or surge caused by the seized IDB blades being unable to compensate for airflow changes during throttle reduction.
- The IDB blades were found to be seized, potentially due to corrosion or lack of use, as the engine had been significantly under-utilized.
- The maintenance organization was unaware of a necessary torque check for the IDB mechanism because the relevant service bulletin was not available in English.
- There was a lack of evidence regarding a recent engine overhaul, as the engine's age exceeded the manufacturer's recommended six-year calendar limit, though no specific calendar limit was documented in the aircraft's UK airworthiness approval.
Safety action
- The UK CAA is advised to consider mandating a calendar time limitation for overhauls of Ivchenko AI-25TL engines.
- The CAA should ensure L-39 operators are aware of the requirement to check the IDB mechanism.
- The CAA should remind 'Permit to Fly' operators of their responsibility to monitor and, if necessary, translate all published manufacturer service information.