Big Lake, TX, USA
N534MT
BELL 407
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.
On December 29, 2012, at 1148 central standard time, a Bell 407 helicopter, N534MT, was substantially damaged during a hard landing at Reagan County Airport (E41), Big Lake, Texas. The pilot, flight nurse, flight paramedic, and patient were not injured. The helicopter was registered to and operated by Med-Trans Air Medical Transport under the provisions of 14 Code of Federal Regulations Part 135 as a non-scheduled domestic passenger flight. Visual meteorological conditions prevailed for the flight, which operated on a company visual flight rules flight plan. The flight originated from Fort Stockton, Texas, at 1114, and was en route to the Shannon Medical Center Heliport (03TS), San Angelo, Texas.According to the written statement provided by the pilot, the helicopter was in cruise flight, at an altitude of 1,500 feet mean sea level, when helicopter suddenly yawed 10 degrees to the right. The pilot was able to control the yaw and subsequently heard an audio alarm. He observed a RPM warning light and a 2 to 3 percent decrease in RPM for the engine and rotor. As the pilot began to scan his flight controls and instruments, the "check instrument segment" illuminated, and the measured gas temperature gauge began to flash and display a reading of "E 920." The pilot stated that all other engine indications were in normal ranges; however, the entire crew agreed that they should land. The pilot slowed the helicopter to about 80 knots, prepared for a running landing, and continued to cross-check his instruments and prepare for an autorotation if necessary. As the helicopter crossed the threshold to the runway, he smelled "a burning grease or oil product" but did not observe smoke or fire. The flight nurse reported an increase in temperature at her crew station. At 10 to 15 feet 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 "fell vertically" and hit the ground hard. During the impact the skids spread out horizontally and the main rotor blades flexed down and severed the tail boom. During the post-accident examination of the engine, three fractures in the outer combustion case, around the fuel nozzle port, were observed. Two of the three fractures originated at the intersection of the circumferential weld that joined the reinforcing ring to the main skin. 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 welds and material on the outer combustion case were to engineering specifications. Further examination of the two fractures, which originated from the reinforcing ring to the main skin, exhibited darker heat tinting consistent with this area being open during engine operation. The darker tinting was not observed in the third fracture. In addition, investigators documented contact between the aft firewall on the engine bay and the fuel spray nozzle. Investigators established that the operator was not conducting the post-flight and scheduled maintenance inspections of the outer combustion case as recommended by Rolls Royce in M250-C47B Operations and Maintenance Manual. The examination of the remaining engine sections revealed no anomalies. An examination of the airframe revealed evidence of contact between the aft firewall of the engine bay and the engine's fuel spray nozzle. The examination of the remaining airframe and related systems revealed no anomalies.
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.
Source: NTSB Aviation Accident Database
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