Pilot Debrief

BELL 407 near Big Lake, TX — 2012-12-29

Final reportCEN13LA119
Sign in to save
Date
2012-12-29
Location
Big Lake, TX, USA
Airport
E41
Aircraft
BELL 407
Registration
N534MT
Category
Helicopter
Highest injury
None
Fatalities
0
Phase of flight
Cruise

Probable cause

The loss of engine power due to the fatigue failure of the outer combustion case at the fuel nozzle port. Contributing to the accident was the operator’s failure to conduct the recommended routine inspections of the outer combustion case.

NTSB narrative

The pilot reported that, while in cruise flight, the helicopter suddenly yawed 10 degrees right. He was able to control the yaw and subsequently heard an alarm and observed an rpm warning light. After silencing the alarm, he noticed a 2- to 3-percent decrease in rpm for the engine and rotor. As the pilot began to scan the flight controls and instruments, he saw the "check instrument segment" illuminate, and the measured gas temperature gauge begin to flash and display a reading of "E 920." The pilot stated that all of the other engine indications were in the normal operating ranges. During the precautionary landing, when the helicopter was about 10 to 15 ft above ground level and 20 knots, the engine "quit." The pilot responded by moving the throttle to idle and increasing the collective pitch. The helicopter subsequently hit the ground hard, which resulted in substantial damage to the tailboom. During the postaccident examination of the engine, three fractures were found in the outer combustion case at the fuel nozzle port. Two of the three fractures originated at the intersection of the circumferential weld that joined the reinforcing ring to the main skin and exhibited heat tinting consistent with the cracks having been open during engine operation. The third fracture ran adjacent to the reinforcing ring to the main skin interface. All three fractures exhibited signatures consistent with high-cycle fatigue. The investigation determined that the operator was not conducting the manufacturer-recommended postflight and scheduled maintenance inspections of the outer combustion case; these cracks would likely have been detected during such inspections.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Failure to conduct post-flight and scheduled maintenance inspections as recommended.

NTSB coding

Evidence available

  • 5 docket documents
View NTSB final reportView NTSB docket

Docket documents5

Related mishaps