What happened
On a morning in British Columbia, a Bell 205A1 helicopter, registered as N205HQ, was engaged in heli-slinging operations approximately 21 nm southeast of Tumbler Ridge. After successfully moving 24 loads of drilling equipment using a 150-foot longline, the pilot attempted to maneuver a 2,900-pound load. During this maneuver, at an altitude of roughly 20 feet, a loud popping noise occurred, accompanied by a flash of fire from the tailpipe and a noticeable drop in rotor speed. The engine, a Lycoming/Honeywell T53-17A, lost power. The pilot responded by lowering the collective and releasing the load, but the aircraft descended into a stand of short trees. The pilot sustained minor injuries, while the aircraft suffered substantial damage.
The investigation
Investigators examined the engine and identified a significant 46-inch circumferential crack in the combustion chamber housing (CCH). This crack, which spanned approximately 210 degrees of the chamber's circumference, allowed high-pressure air to escape from the compressor section, causing the power loss. The investigation established that the crack originated from fatigue cracks linked to corrosion pits located on the internal surface of a doubler. This internal gap allowed moisture and combustion byproducts to accumulate, accelerating the degradation process.
Furthermore, the investigation found that standard external inspections were insufficient for detecting this type of failure. Because the cracking occurred internally, there were often no visible signs of heat damage or external cracking prior to the complete failure of the housing. The investigation also noted that existing service bulletins did not provide a reliable method for detecting such internal cracks through visual means.
Findings
- The engine failure was caused by the fracture of the combustion chamber housing, which led to the loss of high-pressure combustion air.
- Internal corrosion and pitting within the housing went undetected because they were not visible during external inspections.
- Existing inspection protocols, such as Service Bulletin T53-142, lacked the necessary detail to identify all instances of corrosion cracking in these components.