History of the Flight
The instructor pilot was conducting circuit training with a student. After two successful touch-and-go departures from Runway 20, during the climb-out from the third touch-and-go at about 200 ft above airfield level, the engine experienced a sudden and significant loss of power. The aircraft began to sink. The instructor moved the throttle lever but received no response from the engine and decided to land in the nearest safe area, making a MAYDAY call. The aircraft touched down in a field south of the runway and the railway track, just outside the airport perimeter. It then struck a hedge and a fence and continued over a large mound of earth. The impact caused the landing gear to collapse and all three wheels to separate. The instructor followed shut-down procedures and exited with the student through the main door.
Engine Examination
Examination revealed that a clamp between the turbocharger compressor outlet and the turbo pipe assembly was missing. This clamp, referred to as a Wiggins clamp, was later found in a field near the extended runway centerline. It had fractured and broken apart. Loss of this clamp in flight would result in loss of compressed air from the turbocharger to the intake manifold and a significant power reduction. The maintenance organisation later found another cracked clamp on a similar aircraft (G-BZLH) during an inspection of their fleet.
Metallurgical Findings
Both clamps were sent for metallurgical examination. They were manufactured from aluminium alloy (2024-T4 or T351) to specification Mil-C-22263. Microscopic examination revealed that cracks initiated at multiple sites on the inner surface and propagated in fatigue through the thickness of the sidewall. The crack on the G-BZLH clamp, if undetected, would have eventually caused failure in the same manner as the G-TLET clamp. No pre-existing material defects were identified. Fretting wear was observed on the inner surface of both clamps, indicating vibration during operation. The metallurgist concluded that the G-TLET clamp separated due to fatigue fracture under engine vibrations; the duration of crack growth could not be estimated.
Maintenance Requirements
The maintenance requirements for the TAE 125-02-99 engine on the PA-28-161 are defined in a supplement to the aircraft maintenance manual authored by the engine manufacturer, Thielert Aircraft Engines GmbH. This supplement specifies that the clampshell coupling (Wiggins clamp) should be inspected every 300 hours (with removal) and replaced every 1,200 hours. Additionally, a 100-hour in-situ visual inspection is required per the engine Operation and Maintenance Manual. The maintenance organisation responsible for G-TLET had been following only the 100-hour in-situ inspection requirement and was unaware of the 300-hour removal inspection and 1,200-hour replacement requirement. The maintenance history of the clamp could only establish at least 793 hours of operation, but possibly up to 1,647 hours, exceeding the replacement interval.
Conclusion
The accident was caused by a fatigue failure of the Wiggins clamp, leading to a sudden loss of engine power during climb. The maintenance organisation subsequently found a similar crack on another aircraft. Three Safety Recommendations were issued.