WPR14TA370

The failure of the engine to provide sufficient power when commanded by the pilot during the power recovery phase of a practice autorotation for reasons that could not be determined because postaccident examination revealed no mechanical malfunctions or failures that would have precluded normal operation.

The purpose of the public helicopter flight was to provide recurrent emergency procedures and night vision goggle training for the commercial pilot. Both the pilot and the flight instructor were active pilots for the law enforcement agency and current in the accident helicopter type. Preflight checks and initial training maneuvers were uneventful. During the power recovery phase of a practice autorotation, the flight instructor applied engine power by moving the throttle twist grip from the idle to the flight position; however, the engine did not respond as expected, the rotor rpm decayed, and the helicopter landed hard. The helicopter sustained substantial damage to the tailboom; neither occupant was injured. Immediately following the hard landing, the flight instructor observed that the rotor rpm was still low and that the amber-colored governor and twist grip warning lights, which should have extinguished during the power recovery, were still illuminated. He manipulated the throttle twist grip multiple times between the idle and flight detents in an attempt to extinguish the lights and increase the rotor speed without success. With the twist grip in the "flight" position, he then reached up and recycled the start selector switch on the roof panel, and the lights extinguished. After the flight instructor exited the helicopter and examined the damage, he got back in the helicopter, and the pilot then shut down the engine. This was the first training flight (requiring an autorotation with power recovery) since the helicopter manufacturer had issued a service bulletin (SB), which recommended modifying the engine control logic. The SB was issued following multiple reports of engines remaining at idle power during practice autorotation power recoveries despite the twist grip being moved to the "flight" position. Although this was similar to the accident scenario, maintenance records revealed that the SB was accomplished about 46 flight hours before the accident, and postaccident examination revealed that the SB had been complied with correctly. During the postaccident airframe examination, a small amount of play was observed in the twist grip on the flight instructor's side, which sometimes caused the governor and twist grip amber caution lights to not extinguish when the grip was in the flight detent. Wiggling the twist grip while in the flight detent resolved the issue, which the operator's chief pilot reported was not uncommon throughout the agency's fleet, and the pilots were accustomed to it. The flight instructor did not recall the status of the amber lights during the recovery phase before the hard landing. The flight procedures for autorotation training called for a confirmation that these lights were extinguished during the power recovery phase; however, the chief pilot stated that, given the minimal altitude (70 ft above ground level [agl]) that was recommended by the helicopter's manufacturer to initiate the power recovery, pilots were taught to focus on flying the helicopter and not on the lights. Ten months after the accident, the helicopter manufacturer issued a safety information notice regarding simulated engine-off landing training, which referenced the high exposure to accidents and incidents during engine-off landings, and issued a series of procedural updates, including advising that power recoveries be initiated as the helicopter passed through 200 ft agl rather than 70 ft agl. The status of the lights during the maneuver could not be determined; however, if they were illuminated, the engine would have been operating in "mixed" mode and would have exhibited a very slight delay in power recovery. A postaccident engine run revealed that the engine responded within specifications in mixed mode and in a series of other modes under multiple load conditions. Following the run, the engine's hydromechanical unit was removed and tested. It was slightly outside of specification for the "P3" module check, which affected operation in mixed mode. However, because the engine test run was successful, this adjustment variance was not considered causal to the engine providing insufficient power during the accident sequence. Additionally, the engine manufacturer stated that it was not unusual for this module to go out of adjustment in the field. A failure-mode analysis was performed in an effort to determine why the engine would not resume flight power after the hard landing and whether this was related to the engine's failure to provide sufficient power when commanded by the pilot during the power recovery phase of the practice autorotation. The results revealed that the behavior could be duplicated if the throttle input lever had become declutched from the load limiter inside the hydromechanical unit. However, for this condition to have occurred, the pilots would had to have performed a highly unusual series of nonstandard procedures before the accident. It is also possible that, if declutching occurred, it could have been caused by the hard landing, in which case, it would not explain what happened in the accident. Therefore, the reason that the engine did not provide sufficient power when commanded by the pilot during the power recovery phase of the practice autorotation could not be determined.

Engine Control Operating Principles The engine is controlled by the pilot through a set of guarded starting and mode selector switches on the overhead instrument panel, and a twist grip on the collective pitch lever. The start selector has an "IDLE" position for engine autostart and ground idle, and a "FLT" position which is selected for flight. The mode selector has both an "AUTO", and "MAN" position. In AUTO mode, the digital engine control unit (DECU) controls the hydro mechanical unit's (HMU) fuel metering system by utilizing a series of external input parameters such as collective pitch angle (anticipator), engine speeds, and atmospheric conditions. In AUTO mode engine power is set to flight by rotating the twist grip to the VOL detent, and idle by selecting the MIN position. In AUTO mode, the twist grip remains in the VOL position for normal operation. The MIN position is used for training purposes only, and switching to ground idle under normal operations is accomplished by setting the overhead start selector switch to IDLE. In MAN mode, the red GOV light illuminates, and modulation of engine power is performed by the pilot, by rotating the center (pilot side) twist grip past the stop, after unlocking its gate. If the automatic governing system fails, the red GOV light is illuminated, and the fuel metering needle in the HMU is frozen. The pilot can then control the fuel flow manually by rotating the twist grip. During the power recovery phase of autorotation training, the pilot is directed to move the twist grip from MIN to VOL, thereby relinquishing full engine control to the DECU, which commands flight power. The electrical control logic in the twist grip system mechanism is designed such that flight power is commanded by the DECU as soon as the twist grip moves out of the IDLE detent (releasing the "forced-idle" microswitch), prior to it engaging the VOL detent. In this "mixed" mode, both the amber GOV and TWT GRIP lights are illuminated, and the position of the fuel metering needle is governed by the DECU. Movements of the metering needle by manual control are compensated by an internal stepper motor (until it reaches its mechanical stops). Full engine power is still available in mixed mode, however the engines reaction time to external load changes is reduced. Examinations Following the accident, the helicopter was examined by the NTSB investigator-in-charge (IIC), along with technical representatives from Airbus Helicopters and Turbomeca, and the CHP Chief Helicopter Pilot. Throttle Functional Check A functional check of the center (pilot) throttle twist grip revealed that when in the MIN position, the TWT GRP and GOV amber caution lights were on, as expected. Rotation to the VOL position resulted in the lights extinguishing, as expected. A similar functional check was performed on the left twist grip, which was used by the CFI. It was found that when moving from the MIN to VOL position, the grip could be moved past the VOL detent, causing the TWT GRP and GOV amber caution lights to briefly extinguish, and then illuminate again once the detent stop was positively reached. Easing the grip back caused the lights to extinguish. According to the CHP Chief Helicopter Pilot, this was not uncommon throughout their AS350 fleet, and all pilots knew that sometimes the grip needed to be "wiggled" in its detent at the VOL position, for the lights to be extinguished. A functional check was performed on a similarly equipped CHP sister-ship, with the same results, except that more force was needed to induce the same "over center" result on the left twist grip. Rotor Control System Alert Service Bulletin Maintenance records revealed that this was the first training flight (and thereby practice autorotation) since mandatory Alert Service Bulletin (ASB) AS350 - 67.00.43 had been accomplished in July 2014 (about 46 flight hours prior). The bulletin was issued by Airbus Helicopters on November 28, 2013, following multiple reports of engine's remaining at idle power during practice autorotation power recoveries, despite the twist grip being moved to the VOL position. The ASB called for a modification to the engine control logic, which gives priority to the engine HMU in the event that the "forced-idle" microswitch in the twist grip fails to operate correctly. The ASB documented a series of revisions to the start selector and twist grips electrical logic. This was accomplished through modifications to the wiring harness along with the addition, dependent on model, of a series of relays. Correct compliance of the ASB was confirmed by the group through examination, and completion of Airbus Helicopters testing procedure outlined in the maintenance document AMM 76-12-02, 4-3, and Safety Information Notice SIN 2569-S-00. DECU/VEMD Exam The Vehicle and Engine Multifunction Display (VEMD) was checked during the exam, and no over limits or failures were recorded for the accident flight. The Digital Engine Control Unit (DECU) and Engine were removed, and examined by the group at the facilities of Turbomeca Engines, in Grand Prairie, Texas. The DECU appeared undamaged, and was installed and tested on a DECU loading and test bench system, configured for the 2B engine. Upon initialization, the test bench indicated that the DECU was posting a "PAN 3" error, equivalent to the error which would have been reported to the helicopter as a red GOV warning light. The DECU was then powered down, and upon second initialization the error did not post, and the test continued. The unit passed the power-up test, and the fault history file was downloaded. A total of 5 errors were recovered, the most recent occurring 500 flights prior. Due to the age of the error, the group determined that it was not related to the accident, and further examination revealed that it was most likely triggered as a result of the master switch being turned on while the helicopter was undergoing a maintenance procedure. In an effort to replicate the PAN 3 error, the unit was allowed to cool down for 24 hours, and the test was repeated five more times. The unit passed on all subsequent tests. Engine Exam The engine was installed in a Turbomeca test cell, and a series of oil pressure, vibration, and coast-down time tests were performed. The engine met nominal specifications during the tests. A series of performance engine runs were then accomplished in an effort to duplicate the loss of power scenario described by the pilots. The tests included operating the engine at varying power levels in both AUTO and mixed modes. In both modes a free turbine speed (Nf) of 100 percent was accomplished, and it took 4.9 seconds for the engine to go from ground to flight idle in mixed mode. To test the engines response to throttle input in mixed mode, the gas generator speed (Ng) was set to 88 percent, and the throttle was moved in both directions in an effort to assess the HMU's ability to respond. In both directions, the change in Ng speed was 37 rpm; the maximum speed change allowed per the test standards was 250 rpm. When the emergency throttle was moved beyond the mixed mode (mechanical stops of the stepper motor within the HMU), the engine responded as expected by accelerating or decelerating. The engine responded appropriately throughout the tests, and the circumstances reported by the pilots could not be duplicated. The hydro mechanical unit (HMU) was then removed and an "HP/LP Pump and Metering Valve" acceptance test was performed utilizing a Turbomeca multi-purpose test bench configured for the 2B engine. The unit passed all tests except for the "dynamic limiter acceleration test (ALTITUDE)". The observed error was consistent with a discrepancy of the P3 module, and according to Turbomeca representatives would have resulted in a 10% reduction in fuel flow at takeoff power, but only when the engine was operating in MAN or mixed mode. The HMU wa