Final Report: Caravan engine suffered fatigue failure

 - July 6, 2007, 5:49 AM

Cessna 208B Caravan, Globe, Ariz., July 22, 2005–The NTSB determined that the probable cause of the crash of Caravan N717BT was the fatigue failure of the compressor turbine stator vane, the liberation of vane material into the compressor turbine (damaging the turbine blades downstream) and the total loss of engine power. The Safety Board also listed as a factor the operator’s failure to inspect the vane during fuel nozzle checks.

The ATP pilot, who sustained minor injuries, said he heard a loud “thunk” during takeoff from San Carlos Apache Airport and noted a loss of engine power. He moved the power lever from the full-forward position to full aft (“stop-to-stop”) and found he had no power.

Examination of the Pratt & Whitney Canada PT6A-114A engine revealed that the compressor turbine vane’s outer rim liberated a section of metal that damaged the turbine blades downstream. The fracture surface of the outer rim showed evidence of fatigue with signs of oxidation, indicating the crack had existed for some time. The liberated material impact damaged the compressor turbine blades and resulted in a loss of power.

The engine was approved for an extension–to 5,100 hours–beyond the normally recommended 3,600-hour overhaul period. The engine had accumulated 4,461.3 hours at the time of the accident. In addition, the turbine section (hot section) had a recommended overhaul period of 1,800 hours; however, the operator instead used an engine trend monitoring program in accordance with a manufacturer-issued service bulletin. Investigators noted many errors with the operator’s manually recorded data used for the trend monitoring. However, the NTSB said it is not likely that the data, even correctly recorded and monitored, would have depicted the fatigue cracking.

The manufacturer issued a service information letter for the PT6A-116 on Jan. 27, 2003 (following a similar investigation), reminding operators to conduct borescope inspections of the compressor turbine vane during routine fuel nozzle maintenance, as the manufacturer’s maintenance manual recommended. There was no evidence that a borescope inspection of the accident airplane had been conducted.

CorpJet operated the airplane, which was substantially damaged, as a contract cargo flight for United Parcel Service.