ANC00LA132

Two commercial helicopter pilots, both certificated helicopter instructors, were in a turbine-powered helicopter practicing autorotations with a power recovery prior to touchdown. The flying pilot inadvertently activated the flight stop augmented fuel flow switch during a power recovery, and oversped the engine and main rotor. The other pilot joined him on the controls, and increased collective to reduce rotor rpm. The helicopter climbed abruptly to about 60 feet above the ground, where the tail rotor drive shaft separated. The engine subsequently lost power, and an autorotation was accomplished. Investigation disclosed that the engine and main rotor system had been exposed to significant overspeed conditions, resulting in a catastrophic failure of the turbine engine, and the tail rotor drive shaft coupling. The flight stop switch on the collective has no protective guard, and can be readily engaged, allowing the engine to enter the augmented fuel flow regime and, under certain conditions, causing the engine to overspeed. The switch has a history of inadvertent activation, and resultant engine overspeed events.

On September 23, 2000, at 1345 Alaska daylight time, a Eurocopter AS-350B-3 helicopter, N405AE, sustained substantial damage during a hard landing three miles south of the Ketchikan International Airport, Ketchikan, Alaska. The commercial pilot-in-command, and the commercial pilot-flight instructor conducting air carrier flight instruction, were not injured. The flight was being conducted under 14 CFR Part 91 to conduct differences training for the pilot. Visual meteorological conditions prevailed at the time of the accident, and a company VFR flight plan was filed.

The instructor, who was seated in the right seat, told the NTSB investigator-in-charge during a telephone interview on September 25, that while practicing a power recovery autorotation to a beach, the pilot inadvertently activated the engine manual overspeed (flight stop) switch on the collective lever. When he began to increase throttle to recover from the maneuver, the engine began to overspeed, and rotor rpm rapidly increased. The instructor stated that he joined the pilot on the flight controls, and increased collective pitch to attempt to maintain rotor rpm. He said the helicopter rapidly climbed to about 60 feet agl, and then the overspeeding engine flamed out. The instructor indicated that he then rapidly reduced collective pitch to maintain rotor rpm for an emergency autorotation landing. He told the NTSB investigator that during the overspeed condition, the tail rotor drive shaft output coupling from the engine M5 module separated, and exited the helicopter. As he increased collective pitch to cushion the final touchdown, the helicopter's nose began to yaw uncontrollably. The helicopter sustained substantial damage to the tail boom.

On November 15, 2000, the helicopter's engine, a turboshaft Turbomeca Arriel 2B, was inspected and partially disassembled. The inspection was accomplished at the Turbomeca-US facility at Grand Prarie, Texas, under the direction of an NTSB investigator from the south central regional office, Arlington, Texas. According to the NTSB investigator's attached report, the engine displayed indications of internal heat distress, and the power turbine wheel's turbine blades had all separated from the wheel at their base.

The helicopter's Vehicular Engine Multi-functional Display (VEMD) was queried to recover archived rotor data from the accident flight. The VEMD only records main rotor rpm to 511. An overspeed is defined by Turbomeca as 47,304 rpm of the power turbine, or 121%, of the standard maximum operating rpm of 39,095. At 511 main rotor rpm, the power turbine was operating at approximately 50,803 rpm. It is possible for the main rotor system to exceed the 511-recorded rpm, and for the power turbine to experience higher revolutions, but it will not be recorded in the VEMD. The accident helicopter's VEMD read-out indicated the maximum, 511-rpm.

Turbomeca personnel indicated that overspeeding of the engine results in excessive internal temperatures, and weakens the turbine power blades.

During flight, the twist grip throttle on the accident helicopter is normally positioned in the flight detent position, where the Electronic Engine Control Unit (EECU), and Full Authority Digital Engine Control (FADEC), control engine rpm. During a practice autorotation, the power is reduced to simulate a loss of engine power by twisting the throttle out of the detent to the idle position. Returning the throttle to the flight detent position restores power. The EECU and FADEC systems may be overridden, and the engine rpm increased, by moving the flight stop switch on the collective, and continuing to twist the throttle beyond the flight detent. Such action results in augmented fuel flow, and, under certain conditions, can result in engine overspeed.

The NTSB south central regional office investigator noted a history of four previous engine overspeed events with the AS-350-B3 in the two years it had been produced. She examined the twist grip throttle on the collective control, and spoke with pilots who operate the helicopter. She discovered that the manual flight stop switch is unguarded, and can readily be moved from the normal to the emergency augmented fuel flow position, increasing the potential of an inadvertent engine overspeed.