What happened
On 7 January 2015, a Pacific Aerospace 750XL, registered ZK-SDT, was conducting tandem skydiving operations at Taupō aerodrome. During its fourth flight of the day, while climbing at approximately 2,100 feet, the aircraft's Pratt & Whitney Canada PT6A-34 engine suffered a sudden failure accompanied by a loud bang and visible sparks.
Recognizing the severity of the engine failure, the pilot declared a 'Mayday' and transitioned the aircraft into a glide. In the ensuing emergency, the tandem masters and their passengers began exiting the aircraft. During the evacuation, the pilot exited through the cockpit door; however, the pilot struck the aircraft wing before falling away. The remaining passengers successfully exited the plane and landed on nearby dry land, escaping serious injury. The aircraft subsequently struck Lake Taupō at high speed and was destroyed.
The investigation
Investigators focused on the mechanical failure of the engine and the circumstances of the emergency evacuation. Laboratory examinations conducted by Pratt & Whitney Canada revealed that the failure originated from a fatigue crack in a compressor turbine blade.
Technical analysis showed that several blades in the set were undersized in certain dimensions compared to new components. This discrepancy, resulting from a specific repair process, likely contributed to the fatigue failure. Furthermore, the investigation looked into maintenance practices, noting that while the engine was under a specialized maintenance program, the inspection of the turbine blades may not have been as thorough as recommended, as it was unclear if both sides of the blades had been inspected during the previous borescope check.
Findings
- The primary cause of the engine failure was a fatigue crack in a compressor turbine blade caused by dimensional discrepancies in overhauled parts.
- Maintenance staff may have failed to follow the most current instructions for airworthiness, potentially due to familiarity with older procedures.
- The pilot's emergency exit procedure was rushed and did not follow the operator's recommended protocols, which created a risk of injury to the occupants.
- The operator's training for pilots regarding emergency procedures was found to be inadequate for managing a mid-air evacuation.
- There was a lack of necessary flotation equipment for the pilot, as the operation was previously assumed to stay within gliding distance of land.
Safety action
Following the inquiry, several improvements were implemented:
- Pratt & Whitney Canada updated its measurement specifications for repaired blades to ensure they meet the same criteria as new blades.
- The engine manufacturer introduced additional scheduled inspections for PT6A-34 engines used in single-engine skydiving configurations, as these engines were being operated at higher power settings than originally assumed.
- The operator updated its aircraft by painting rear bulkheads white to improve cockpit visibility of passengers during emergencies and improved pilot training, including voluntary tandem jumps.
- The operator now provides lifejackets for tandem masters.