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12th of May 21016, Fractured Fuel Line, Blog #651

With one pilot and 17 passengers (parachutists), a Cessna 208B was rolling down runway 26 at Lodi Airport (California, USA) for a skydiving flight on this day in aviation history in 2016. The weather was good with the flight being operated under visual meteorological conditions.

The aircraft on its back in the vineyard (Source; baaa-acro.com © unknown)


The take-off run and initial climb were uneventful, a turn to the right was made to continue the climb to the skydiver drop altitude. When climbing through 1000 ft AGL the engine (Pratt & Whitney Canada PT6A-42A) suddenly lost power. The pilot initiated a return to the field, however, it became immediately apparent that the runway was unreachable.

An off-field landing at an open field was initiated.


During the landing, the airplane crossed a road after exiting the field, collided with a truck, entered a vineyard and flipped over. The pilot received minor injuries, but the 17 parachutists remained uninjured.


Examination of the wreckage, by an FAA inspector revealed extensive damage to the left wing and the aircraft fuselage. The wreckage was removed from the accident location for further investigation into the accident. During this investigation, it was found that the fuel line between the Fuel Control Unit (FCU) and the fuel pressure transducer was cracked at the swage, directly below the FCU. A support clamp of the fuel line was also found fractured. In addition, the P3 airline for the vacuum system was found with a hole in it. Maintenance records revealed that the fuel line was replaced on the 11th of April, and had accumulated approximately 4 Flight Hours since installation.

The aircraft on its back in the vineyard (Source; baaa-acro.com © unknown)


A detailed analysis of the fuel line at the National Transportation Safety Board Materials Laboratory revealed that the old, and the recently installed fuel line exhibited features consistent with crack initiation due to reverse bending fatigue. The clamp was found to exhibit features caused by bending fatigue. The orientation of the reverse bending fatigue cracks and the spacing of the fatigue striations on the tube fracture surfaces were consistent with high-cycle bending fatigue due to a vibration of the tube. The cushioned support clamp is designed to prevent such vibrations from occurring. However, if the clamp tab is fractured, it cannot properly clamp the tube and will be unable to prevent the vibration. The presence of the fractured clamp combined with the fact that the two pressure tubes failed in similar modes in short succession indicated that the clamp most likely failed first, resulting in the subsequent failure of the tubes. Since the clamp was likely fractured when the first fractured fuel pressure line was replaced, the clamp was either not inspected or inadequately inspected at the time of the maintenance.


In their Aviation Accident Final Report, the National Transportation Safety Board (NTSB) concluded that the probable cause(s) of this accident was;

A total loss of engine power due to a fatigue fracture of the fuel pressure line that connected the fuel control unit and the fuel flow transducer due to vibration as the result of a fatigue fracture of an associated support clamp. Contributing to the accident was the mechanic's inadequate inspection of the fuel line support clamp during the previous replacement of the fuel line.

Close-up of the aircraft on its back in the vineyard (Source; baaa-acro.com © unknown)


The Aviation Accident Final Report as published by the NTSB, which served as the source for this blog can be accessed by clicking on the .PDF file below;


CESSNA 208B 12May2016 Fuel Line Fractured
.pdf
Download PDF • 105KB

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