History of Flight
On August 28, 2023, about 0844 eastern daylight time, a Eurocopter EC135T1, registration N109BC, was destroyed when it crashed into an apartment building near Pompano Beach, Florida. The helicopter was operating as a Title 14 Code of Federal Regulations Part 135 air medical flight. The commercial pilot reported that during initial climb at about 300 to 400 feet above ground level, approximately 90 seconds after liftoff, a “bang” was heard from the rear of the helicopter. The turbine outlet temperature (TOT) on the No. 1 engine began rising but remained within limits. The pilot reduced the No. 1 engine throttle to idle, declared an emergency, and reversed direction toward the airport. The No. 1 engine fire button illuminated; the pilot activated the fire suppression system, but the TOT continued to rise near 1,000° C (maximum limit 895°C). A second “bang” was heard, and control was lost; the helicopter spun and descended into an apartment building. Witness video showed an inflight fire near the No. 1 engine exhaust and air conditioner condensing fans. The fire spread to the tail boom attach point, causing a partial tailboom separation and an uncontrolled right spin.
Pilot Information
The pilot held a commercial pilot certificate with rotorcraft helicopter and instrument helicopter ratings, and a second-class medical certificate. Total helicopter experience was 3,895 hours, with 272 hours in the same make and model as the accident helicopter. The pilot had been flying for the operator for over four years.
Aircraft Information
The helicopter, manufactured in 1999, was powered by two Turbomeca Arrius 2B1 engines (670 shaft horsepower each). It was maintained under a manufacturer’s approved inspection program; the most recent 100-hour inspection was completed on May 23, 2023, at 5,557.1 total airframe hours. The engines had accrued 5,327.2 hours since new (2,251.3 hours since overhaul in 2016). The helicopter operated about 24 hours after the last inspection. An air conditioner was installed under supplemental type certificate on August 1, 2023, about 16 flight hours prior to the accident.
Wreckage and Impact
The wreckage came to rest on its left side, facing south, through the roof of a one-story apartment building. A postcrash fire consumed most of the airframe. The tailboom was located about 30 feet south of the main wreckage; the fenestron (tailrotor) remained intact. All four main rotor blades separated near the blade root, consistent with impact damage. Both engines and their electronic engine control units (EECUs) were retained for examination. The No. 1 engine fuel shutoff valve was also retained.
Engine Examination and Data Analysis
Examination of the No. 1 engine at the manufacturer’s facility revealed five turbine blades fractured below the blade platform. Metallurgical analysis showed their inner walls exhibited dissolution of material precipitates (microstructural transformation) consistent with overheating beyond 1,295° C and fatigue cracking due to excessive temperatures. No evidence of fire was found inside the No. 1 engine, but exhaust gases in excess of 1,000° C were consistent with an ignition source. A fiberglass air conditioner housing and composite tailboom fuselage near the No. 1 engine exhaust were not certified to withstand temperatures above 1,000° C.
Data downloaded from the No. 1 engine EECU showed a simultaneous double N1 and double N2 failure recorded about 67 seconds after liftoff, and about 25 seconds before the first “bang.” This failure would result in a “FADEC FAIL” cockpit caution and would freeze the fuel control unit (FCU) at the fuel flow at the time of failure, which was 123 l/h. Postaccident testing showed the FCU resolver remained at that position. The reason for the failure could not be determined. Setting the engine throttle to idle had no effect on fuel flow; manual control via the engine twist grip was required.
Computed tomography scanning of the No. 1 emergency fuel shutoff valve showed it was in the open position. Examination of the cockpit revealed that the No. 1 and No. 2 engine fire buttons’ breakable safety wire was unbroken and the buttons showed no inward deformation. The No. 1 engine air inlet had no blockages, but that area was subjected to postcrash fire.