What happened
On a day in mid-1998, a Bell 214B-1 helicopter, registered C-GTWH, was conducting heli-logging operations near Telkwa Pass, approximately 10 nautical miles south of Smithers, British Columbia. While flying at roughly 500 feet above the ground, the aircraft was transporting a sling load of logs when the engine suddenly lost power. As the low-rotor warning horn sounded, the pilot released the external load and initiated an autorotation to land in a cleared area. Upon touchdown, the main rotor blades struck the terrain, causing the aircraft to roll onto its side. The impact resulted in substantial damage to the helicopter, though both pilots on board escaped without injury.
The investigation
Investigators examined the Honeywell T5508D turboshaft engine and its maintenance history. The engine had recently undergone an overhaul and had been installed in C-GTWH just six days prior to the accident. During its previous installation in another aircraft, the engine had been subject to several significant maintenance interventions, including the replacement of the starter gearbox, scavenge oil pump, and various wiring and oil components.
Technical analysis of the engine following the crash revealed severe internal damage within the combustion section. Specifically, the second-stage nozzle and the third-stage turbine had made contact while the engine was rotating at high RPM. Furthermore, the investigation found that the engine's post-overhaul run-up was conducted in a test cell that was not approved by the manufacturer. This unapproved facility lacked the necessary instrumentation to accurately measure critical parameters, leading to performance calculations based on inaccurate data. Additionally, it was discovered that the second-stage nozzle had previously failed a non-destructive testing (NDT) examination six months prior to the accident but was nonetheless installed in the engine.
Findings
- The engine failure was caused by the failure of the second-stage nozzle, which subsequently struck the third-stage turbine.
- The nozzle failure likely originated from extensive cracking or the separation of a braze joint.
- The engine was not airworthy because the nozzle, which had previously failed NDT, was installed without being repaired or certified.
- The engine's performance verification was compromised by the use of an unapproved test cell and inaccurate performance calculations.