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Narrative:
While en route during night visual conditions to a hospital helipad with two crewmembers and a patient, the commercial pilot noticed a twist grip caution indication on the left engine (No. 1) cockpit display system (CDS) panel. The pilot also noticed a second indication but could not recall the specific caution message. He stated that he then grabbed each engine throttle twist grip individually to gently verify if he could feel they were in or out of position (neutral detent) but did not notice any significant changes to the throttle position. The pilot decided to divert to a nearby airport, and, as he executed a turn toward the airport, he noticed the No. 2 engine indication no longer matched the No. 1 engine indication; he stated that "it was lower and oscillating." Within about 1 minute of the turn, the pilot "heard the low rotor [rpm] horn," and he lowered the collective to maintain rotor speed. The pilot located a "dark spot" on the ground, which he determined would give him the best opportunity to complete a full autorotation. As he started a turn toward his intended landing location, he felt the tail oscillate to the right and back and heard an increase and decrease in engine speed. When the helicopter was about 200 ft above ground level, he thought he may land short of the intended location and adjusted the collective and cyclic to maintain rotor rpm and airspeed. The helicopter impacted terrain, rotated 180°, and came to rest upright. Surveillance video from a rail platform near the accident site showed a fire near the right (No. 2) engine during the autorotation and a flame burst after the impact with terrain.
Examination of the throttles, throttle linkages, engines, control systems, CDS, and the electronic engine control (EEC) units revealed no evidence of preimpact mechanical malfunctions or failures that would have precluded normal operation. Analysis of data retrieved from the CDS and EEC units revealed that, about 4 minutes after takeoff, the No. 1 engine was placed in manual mode and out of EEC control, which indicates that the pilot had likely inadvertently moved the No. 1 engine throttle out of its neutral detent. The No. 1 engine was in manual mode for about 7 minutes before the pilot noted the CDS twist grip caution indication. The data showed that as the pilot continued to manually control the No. 1 engine, the No. 2 engine was also placed in manual mode and out of EEC control, which indicates that the pilot moved the No. 2 throttle out of its neutral detent. The pilot attempted to maintain rotor and engine rpms while controlling both engines manually; it is not likely that he fully understood the nature of the problem. The pilot misinterpreted an aural alert (low rotor rpm as opposed to high rotor rpm) when high rotor rpm existed and then lowered the collective, which created a rotor overspeed condition. This configuration resulted in a high-workload scenario in which it would be particularly challenging for the pilot to control the helicopter while maneuvering in low altitude and night visual conditions.
The pilot had accumulated about 300 hours of flight experience in EC135s, with about 11 hours in the accident make and model EC135 P1. The accident helicopter was the only EC135 P1 variant in the operator's fleet. Its engines, displays, and throttle controls differed from the EC135 P2 variant in which the pilot was formally trained. The investigation revealed the pilot completed a basic online (self-study) differences training presentation and some informal familiarization training with other company pilots. No formal flight training was part of the differences training curriculum. Because the throttle (twist grip) differs between the P1 and P2 variants, it is likely that the pilot moved it into manual mode without realizing it; he likely did not recognize this issue because he did not have as much experience or formal training in the P1 variant. Because the displays also differed between the variants, it could have been more difficult for the pilot to recognize and understand the indications
Probable Cause: The pilot's inadvertent disabling of the No. 1 and No. 2 engines' electronic engine control systems, which resulted in engine and rotor overspeed conditions, a subsequent autorotation, and a hard landing. Contributing to the accident were the pilot's inexperience with the helicopter variant and the operator’s lack of a more robust helicopter differences training program.
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#N312SA
9 окт 2024
