N217HP

HISTORY OF FLIGHTOn September 4, 2014, at 2005 Pacific daylight time, a Eurocopter AS 350 B3, N217HP, landed hard during a practice autorotation at Lincoln Regional Airport/Karl Harder Field, Lincoln, California. The helicopter was registered to and operated by the California Highway Patrol (CHP) as a public aircraft, training flight. The commercial pilot and flight instructor (CFI) were not injured. The helicopter sustained substantial damage during the accident sequence. The local flight departed Auburn Municipal Airport, Auburn, California, about 1952. Visual meteorological conditions prevailed, and no flight plan had been filed.

The purpose of the flight was to provide recurrent emergency procedure and night vision goggle (NVG) training for the pilot, who was positioned in the right seat. Prior to departure, the crew discussed the plans for the flight, and the pilot performed a preflight inspection. The decision was made to perform a full landing at Lincoln, followed by a practice autorotation with power recovery, and then once ambient light had diminished, transition into NVG training. Because they departed during daylight the pilot turned off the NVG unit's battery pack, and moved the goggles to the up position on his helmet.

Once the startup and departure checklists were complete, the CFI, who was positioned in the left seat, conducted a throttle check in accordance with the autorotation training before take-off check list. He ran the engine to full power while stationary on the helipad, and then rolled the twist grip to idle (MIN position), and received confirmation that the low RPM horn was functioning. The engine stabilized at idle power, and he then rolled the twist grip up to flight (VOL position). Both the amber governor (GOV) and twist grip (TWT GRIP) caution lights extinguished, and the engine responded appropriately by reaching full power RPM within about nine seconds.

With all systems normal, they departed, and the pilot performed an uneventful landing on Runway 15 at Lincoln. They then departed to practice the autorotation on the same runway. The CFI stated that he was the sole manipulator of the throttle twist grip throughout the flight, and that he instructed the pilot to pay attention to work on attaining the appropriate rotor and airspeeds rather than focusing on a specific landing spot. Once they were both ready, the CFI rolled the twist grip to the MIN position to initiate the maneuver. The pilot lowered the collective and the helicopter descended; once they reached an altitude of about 50 feet above ground level (agl), the pilot began to initiate the flare, with the CFI countering by rolling the twist grip back to the VOL position. They heard the engine respond along with an accompanying yaw motion, and the CFI announced "power recovery."

The pilot stated that he held the helicopter in the flare about 25 ft agl, and the rotor speed started to increase, so he pulled up lightly on the collective control to prevent a main rotor overspeed. The helicopter then "ballooned" slightly, and he lowered the collective to recover. The forward speed decayed, and he moved the helicopter forward in anticipation of the hover. As he started to raise the collective control, the low rotor speed horn sounded and the helicopter began to rapidly descend. He pulled up the collective in an effort to arrest the descent, and the helicopter hit the ground hard. His NVG goggles flipped down over his eyes, and his forward vision became effectively blocked. He then perceived forward and nose-low motion as the CFI took control of the helicopter. He felt the cyclic pull full aft, and the helicopter came to rest.

Once on the ground, the CFI pushed down on the collective and the rotor speed returned to about 360 RPM, which was below the normal operating speed range indicated on the RPM gauge. He then noticed that the amber governor (GOV) and twist grip (TWT GRIP) caution lights were still on. He looked down and confirmed the twist grip was in the VOL detent and against the stop. He then tried to move it, confirming that it was fully against the stop. He then "jiggled" the control in an attempt to extinguish the caution lights, stating that in his experience the lights do not always immediately extinguish. Again they did not turn off, so he rolled the twist grip down to IDLE, and then back to VOL, but the rotor RPM again stopped short below the green arc of the normal operating RPM range, at about 360 RPM.

He then reached over to unlock the twist grip locking device ("gate") on the pilot's side, with the intention of manually controlling fuel flow to the engine. He did so, rotated the twist grip, and the engine RPM increased slightly. He then decided to discontinue further troubleshooting steps.

With the twist grip back in the VOL detent, he reached up to the start selector switch in the roof panel, and turned it to IDLE, and then back to FLT, at which time the amber warning lights extinguished. He then then asked the pilot to take the controls, and he exited the helicopter to examine if any damage had occurred.

Examination revealed that the tailboom had bent downwards at its intersection with the aft bulkhead, just below the engine exhaust outlet. The aft bulkhead sustained wrinkling damage, and the skids had spread, bending both aft landing skid support tubes. The CFI got back into the helicopter and the pilot initiated an engine shutdown. Both pilots reported that at no time in the flight did they see the red GOV warning light illuminate. PERSONNEL INFORMATIONBoth crew members were full-time active pilots for the CHP.

The flight instructor held a commercial pilot certificate with ratings for helicopter, instrument helicopter, along with a flight instructor certificate with ratings for helicopter. He reported a total flight time of 3,943 flight hours, with 2,943 as pilot-in-command in the accident make and model, and 79 hours in the 30 days prior to the accident. His most recent flight review took place on August 27, 2014, and was performed in the accident make and model.

The pilot held a commercial pilot certificate with ratings for helicopter and instrument helicopter. He reported a total flight time of 2,988 flight hours, with 2,635 as pilot-in-command in the accident make and model, and 42 hours in the 30 days prior to the accident. His most recent flight review took place on June 14, 2014, and was performed in the accident make and model. AIRCRAFT INFORMATIONThe helicopter, serial number 3628, was manufactured in 2002 and equipped with a Turbomeca Arriel 2B engine. The helicopter was maintained under a continuous airworthiness program, and the last inspection occurred twelve flight hours prior to the accident. AIRPORT INFORMATIONThe helicopter, serial number 3628, was manufactured in 2002 and equipped with a Turbomeca Arriel 2B engine. The helicopter was maintained under a continuous airworthiness program, and the last inspection occurred twelve flight hours prior to the accident. ADDITIONAL INFORMATIONThe flight manual gave specific instructions for autorotation training procedures. Specifically, that the power recovery should be initiated about 70 ft agl, and after the twist grip has been turned to the VOL detent, the engine should accelerate to its normal governed Nf speed, and the pilot should confirm the amber GOV and TWT GRIP lights have extinguished.

The CFI stated that he did not recall the status of the amber lights during the recovery phase prior to the hard landing, and that checking their status is not normally part of his instrument scan during the recovery maneuver. The CHP's Chief Helicopter Pilot stated that with the power recovery performed at 70 ft per flight manual recommendations, minimal time is available and the decision to focus on "flying the aircraft" is given priority over a visual scan of the GOV and TWT GRIP lights, particularly in considering the fleet's history regarding the tendency of the lights to not always extinguish.

Ten months after the accident, Airbus Helicopters issued Safety Information Notice 2896-S-00, applicable to the B, BA, BB, B1, B2, B3, D models of the AS 350. The notice covered simulated engine-off landing training, and stated the following,

"Current helicopter accident/incident statistics indicate that the greatest exposure to accidents or incidents is during simulated engine-off landing (EOL). The purpose of this Safety Information Notice is to raise the level of awareness of Flight Instructors involved in simulated EOL training and to stress on key points." The notice included an update, advising that a power recovery now be initiated as the helicopter passed through 200 ft agl.

The helicopter was equipped with an AeroComputers digital mapping system, which was capable of recording GPS based flight track information. The data from the accident flight was recovered and analyzed, and revealed a flight track that closely matched the pilot's statements. TESTS AND RESEARCHEngine 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 ...