Eurocopter EC135 N109BC

The helicopter was dispatched to pick up a victim from an automobile accident. Electronic devices onboard the helicopter recorded that about 67 seconds after liftoff, the No. 1 electronic engine control unit reported a simultaneous double N1 and double N2 failure. While this failure would have been expected to result in a “FADEC FAIL” cockpit caution, the pilot did not recall seeing or hearing any cockpit caution or warning indications. This condition would also have frozen the fuel control unit to the fuel flow at the time of the failure until the end of the flight, which was 123 l/h, consistent with a climb power setting. The reason for the failure could not be determined. About 90 seconds after liftoff, at 300 to 400 ft above ground level, the pilot heard a “bang” from the rear of the helicopter and noticed that the turbine outlet temperature (TOT) was rising on the No. 1 engine, but still within limits. He set the No.1 engine throttle to idle, declared an emergency to air traffic control, and reversed direction to return to the airport. Unbeknownst to the pilot, due to the FADEC FAIL condition, setting the engine throttle to idle would have had no effect on fuel flow, but rather the engine twist grip would need to be manipulated to manually control fuel flow to that engine. Despite this condition, fuel flow maintained at that level would not be expected to result in an overtemperature condition in the engine. The pilot next scanned the cockpit instrument panel and noticed that the No.1 engine fire button had illuminated. He stated that he pressed the button to activate the fire suppression system; however, the TOT continued to rise near 1,000° C (maximum limit 895°C) on the No. 1 engine. The pilot subsequently heard a second “bang” (about 90 seconds after the first “bang”) and was unable to control the helicopter. It spun and descended into an apartment building. Review of witness video revealed an in-flight fire near the area of the No. 1 engine exhaust and the air conditioner condenser fans. The tailboom partially separated in flight and the helicopter descended in a right spin. Examination of the No. 1 engine revealed that five turbine blades had fractured consistent with overheat fatigue from temperatures in excess of 1,295° C. There was no evidence of fire within the No. 1 engine compartment prior to ground impact; however, exhaust gases in excess of 1,000° C could have been a factor in the initiation of the inflight airframe fire outside of the No. 1 engine compartment. Specifically, a fiberglass air conditioner housing and composite tailboom fuselage were located near the No.1 engine exhaust. While the composite fuselage offered more fire resistance than the fiberglass housing, neither were certified to withstand temperatures in excess of 1,000° C. While the No. 1 engine fire warning light could provide indications of fires within the engine compartment, this fire was outside the engine compartment. As such, the pilot had no caution and warning indicators of an inflight fire that may have forced a land immediately action, therefore, his decision to return to the airport was reasonable, rather than risk an off-airport emergency landing to a confined area with one engine inoperative. The No. 1 emergency fuel shutoff valve was in the open position. In the cockpit, the No. 1 engine fire button’s breakable safety wire was found unbroken and the button did not exhibit inward deformation. Although the pilot stated that he pressed the fire button, he likely did not. Additionally, the fire suppression system was for inside the engine compartment and would not have extinguished a fire outside of the engine compartment, but pressing the button would have closed the fuel shutoff valve for the No. 1 engine. In summary, the accident was the result of an inflight fire in the vicinity aft of the helicopter’s No. 1 engine exhaust, near the air conditioner condenser fans, and the origin of the fire was likely the result of the engine overheating. The only plausible explanations the investigation could determine for only the No. 1 engine to overheat were foreign object debris, blockage of the No. 1 engine air inlet, or hot gas or combustible fluid ingestion. Because the wreckage was subjected to a postimpact fire, the source of the overtemperature could not be determined.

Quoted verbatim from the NTSB record.