What happened
On 10 February 2010, a Cessna C208 operating a scheduled commercial service departed Nelson Aerodrome bound for Wellington. Shortly after takeoff, the flight crew detected a noticeable drop in engine performance accompanied by a heavy smell of fuel inside the cabin. The aircraft was carrying two pilots and four passengers at the time of the incident.
Following the discovery, the pilot communicated with the aerodrome controller to coordinate a return to Nelson. Notably, the crew did not declare an urgency or distress situation during these communications. The aircraft successfully landed back at Nelson Aerodrome with the engine still experiencing reduced power. The event resulted in no injuries and no damage to the ZK-TZR.
The investigation
Investigators examined the mechanical state of the engine and the fuel system to identify the source of the leak. The inquiry established that fuel was escaping past compromised o-rings intended to seal the delivery of fuel to the engine. The investigation traced the damage to the o-rings back to the movement of fuel-transfer tubes.
It was discovered that these tubes had been subjected to a chemical milling process during a previous maintenance period. This process had inadvertently reduced the size of the tubes by removing the protective anodic coating, which subsequently led to the mechanical failure of the seals. Additionally, the investigation looked into the regulatory response and the crew's communication protocols during the emergency.
Findings
- The primary cause of the engine performance loss was fuel leaking past damaged o-rings.
- The o-rings failed because the fuel-transfer tubes had been undersized due to an improper chemical milling process during maintenance.
- The flight crew failed to declare an urgency or distress situation, which prevented emergency services from being placed on standby.
- The Civil Aviation Authority of New Zealand (CAA) incorrectly classified the initial notification of the defect, delaying the assignment of an investigation for two months.