What happened
On July 7, 2018, at approximately 21:23 CDT, a Eurocopter Deutschland GMBH EC135 P1 helicopter, registration N312SA, impacted terrain near Chicago, Illinois, while performing an autorotation. The aircraft was operating as an air ambulance, flying from St. Mary Medical Center in Hobart, Indiana, to Advocate Christ Medical Center in Oak Lawn, Illinois.
During the flight, the pilot noticed a "Twist Grip" warning on the left engine side of the cockpit display. The pilot attempted to verify the position of the engine throttles by manually manipulating the twist grips. Shortly after, the pilot requested a diversion to Gary International Airport (GYY) in Indiana. During the turn toward the diversion airport, the pilot observed the No. 2 engine indication oscillating and noted a low rotor RPM horn.
In an attempt to maintain rotor speed, the pilot lowered the collective. The helicopter subsequently entered an autorotation. Surveillance video from a nearby rail platform captured a fire near the No. 2 engine during the descent and a flame burst following the impact. The helicopter came to rest upright in a grass area between the Interstate 94 and Interstate 57 interchange. The pilot, flight paramedic, and flight nurse sustained 3 serious injuries, while the patient was not injured.
The investigation
Investigators examined the wreckage, engines, and cockpit systems. The engines were found to have experienced an overspeed condition; specifically, the power turbine blades on both engines were fractured at the 1/3 span due to tensile overload. The No. 2 engine also showed evidence of a nacelle fire and internal fire near the exhaust area.
Analysis of the Cockpit Display System (CDS) and Electronic Engine Control (EEC) units revealed that the No. 1 engine had been placed in manual mode approximately four minutes after takeoff, likely due to the pilot inadvertently moving the throttle out of its neutral detent. The data further showed that the No. 2 engine was also subsequently placed in manual mode.
Review of the pilot's training records indicated that while the pilot had approximately 319 flight hours in the EC135 P2+ variant, he had only about 11 total hours in the N312SA EC135 P1 variant. His training for the P1 model consisted of an online self-study course and informal familiarization, but lacked formal simulator or flight training specifically for the P1's different cockpit displays and throttle controls.