Accident Overview
On December 29, 2012, at 1148 central standard time, a Bell 407 helicopter, registered as N534MT, was substantially damaged during a hard landing at Reagan County Airport (E41), Big Lake, Texas. The helicopter was operated by Med-Trans Air Medical Transport under 14 CFR Part 135 as a non-scheduled domestic passenger flight. Visual meteorological conditions prevailed, and the flight was operating 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. The pilot, flight nurse, flight paramedic, and patient were not injured.
Pilot's Account
According to the pilot's written statement, the helicopter was in cruise flight at an altitude of 1,500 feet mean sea level when it suddenly yawed 10 degrees to the right. The pilot controlled the yaw and then heard an audio alarm. He observed a RPM warning light and a 2 to 3 percent decrease in engine and rotor RPM. As he scanned his flight controls and instruments, the "check instrument segment" illuminated, and the measured gas temperature gauge began to flash, displaying "E 920." All other engine indications were in normal ranges, but the crew decided to land. The pilot slowed the helicopter to about 80 knots, prepared for a running landing, and continued cross-checking instruments, preparing for an autorotation if necessary. As the helicopter crossed the runway threshold, he smelled "a burning grease or oil product" but saw no smoke or fire. The flight nurse reported an increase in temperature at her crew station.
At an altitude of 10 to 15 feet above ground level and a speed of 20 knots, the engine quit. The pilot moved the throttle to idle and increased collective pitch. The helicopter "fell vertically" and hit the ground hard. During impact, the skids spread horizontally, and the main rotor blades flexed down and severed the tail boom.
Engine Examination
Post-accident examination of the engine revealed three fractures in the outer combustion case around the fuel nozzle port. Two fractures originated at the intersection of the circumferential weld joining the reinforcing ring to the main skin. The third fracture ran adjacent to the reinforcing ring-to-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.
Two fractures originating from the reinforcing ring to the main skin exhibited darker heat tinting, consistent with these areas being open during engine operation. The third fracture did not show this tinting. Investigators also documented contact between the aft firewall on the engine bay and the fuel spray nozzle.
Maintenance Findings
Investigators established that the operator was not conducting post-flight and scheduled maintenance inspections of the outer combustion case as recommended by Rolls Royce in the M250-C47B Operations and Maintenance Manual.
Additional Examination
Examination of the remaining engine sections revealed no anomalies. Airframe examination showed evidence of contact between the aft firewall of the engine bay and the engine's fuel spray nozzle. No other anomalies were found in the airframe or related systems.