ERA15FA085

A main rotor stall due to low rotor rpm, which resulted in an uncontrolled descent into terrain. Contributing to the accident was the flight instructor’s delayed remedial action.

The purpose of the instructional helicopter flight was to review advanced maneuvers in preparation for the student pilot's upcoming stage check. The student stated that he and the flight instructor had conducted 3 approaches and landings before initiating a practice autorotation. He stated that as the helicopter descended through 100 ft in the autorotation, the instructor applied throttle in an attempt to recover, but the engine did not respond. A witness stated that, about halfway through the helicopter's autorotative descent, it appeared to momentarily level off before abruptly entering a nose-down attitude and descending to ground contact. A surveillance video showed the helicopter descending rapidly at a steep angle in the last 2 seconds before impact. The helicopter impacted terrain about 700 ft north of the runway threshold, fatally injuring the instructor and seriously injuring the student. Postaccident examination of the helicopter and a test run of the engine revealed no mechanical anomalies that would have precluded normal operation. The flight school's published procedure for practice autorotations instructed the pilot to initiate the maneuver first by lowering the collective, then reducing the throttle to idle. The practice of reducing the throttle to idle was contrary to manufacturer guidance for this maneuver, which stated that the throttle should be adjusted only enough to allow for a small tachometer needle separation in order to reduce the chance of inadvertent engine stoppage during the maneuver. The practice of reducing throttle to idle introduced greater susceptibility to a loss of engine power, though it could not be determined whether a loss of power occurred before the accident. Based on conflicting statements from the student, it could not be determined who was controlling the helicopter during the entry into and throughout the autorotation before about 100 ft. The helicopter's trajectory described by a witness and as captured on surveillance video suggested a rapid, uncontrolled descent during the final portion of the autorotation, consistent with a main rotor stall; likely as a result of a premature application of collective pitch. This allowed the rotor rpm to decay below the normal operating range at an altitude that was insufficient for power recovery. In the event that the helicopter did experience a loss of power during the maneuver, the helicopter should have been able to attain a safe landing following a steady-state autorotation. The helicopter manufacturer published notices to pilots warning that main rotor stall due to low rotor rpm could occur rapidly, at any airspeed, and that if allowed to develop, recovery could become "virtually impossible."

Cell Phone Information The CFI's cell phone was retained for examination in the NTSB vehicle recorders laboratory; however, the device was passcode-protected and could not be unlocked. Usage records obtained from the cellular service provider could not conclusively determine whether the phone was in use at the time of the accident. Practice Autorotations According to the pilot training syllabus provided by the flight school, the objective of Stage 3, Lesson 2 was for the student to practice advanced maneuvers and procedures, including normal and steep approach, normal and maximum performance takeoff, hovering, hover taxi, air taxi, ground reference maneuvers, emergency operations, autorotation to a power recovery, loss of tail rotor, stuck pedal, hovering out of ground effect, and confined area operation. Review of the student's training record indicated that he completed Stage 2 of the syllabus on December 23, 2014. Instructor notes for the previous flight, dated December 22, 2014, indicated, "gap in training is evident in proficiency, basic straight [and] level unsatisfactory, [aeronautical decision making] needs work, approaches need work." The flight school specified that all 180-degree and straight-in autorotations be terminated with a power recovery throughout the private pilot training course. School policy stated that all landings and practice autorotations were to be performed to a hard-surfaced runway or taxiway. The flight school's written procedures for a straight-in autorotation with power recovery indicated that the maneuver should be initiated at an altitude of 700 feet agl after clearing the area for potential traffic conflicts and applying carburetor heat. Upon selecting a landing site, the autorotation was entered by lowering the collective to its full-down position, then rolling the throttle to the full idle position, where the procedure specified it should be held "firmly against the stop for the remainder of the autorotation." About 40 feet agl, the recovery was initiated by applying aft cyclic to bring the helicopter to a skids-level attitude, where it was held for 3 seconds prior to entering a flare. At that time, the throttle was "crack[ed] open" to allow the rpm governor to operate, forward cyclic applied, and the collective raised to bring the helicopter to a hover about 5 feet agl. The maneuver guide also stated, "Make an IMMEDIATE power recovery if the following conditions do not exist through 100' AGL: Aircraft aligned with touchdown point; Rotor RPM in the green; Airspeed within +/- 5 [knots] of 65 [knots]; Rate of descent <1,500 fpm". The manufacturer's Pilots Operating Handbook outlined the following procedure for a practice autorotation with a power recovery: "1. Adjust carb heat as required. 2. Lower collective to down stop and adjust throttle as required for small tachometer needle separation. CAUTION: To avoid inadvertent engine stoppage, do not chop throttle to simulate a power failure. Always roll throttle off smoothly for a small visible needle split. 3. Adjust collective to keep rotor RPM in green arc and adjust throttle for small needle separation. 4. Keep airspeed 60 to 70 KIAS. 5. At about 40 feet AGL, begin cyclic flare to reduce rate of descent and forward speed. 6. At about 8 feet AGL, apply forward cyclic to level aircraft and raise collective to control descent. Add throttle if required to keep RPM in green arc." Robinson Helicopter Company Safety Notice SN-38, "Practice Autorotations Cause Many Training Accidents," stated, "There have been instances when the engine has quit during practice autorotation. To avoid inadvertent engine stoppage, do not roll throttle to full idle. Reduce throttle smoothly for a small visible needle split, then hold throttle firmly to override governor. Recover immediately if engine is rough or engine RPM continues to drop." Safety Notice SN-24, "Low RPM Rotor Stall Can Be Fatal," stated, "Rotor stall is very similar to the stall of an airplane wings at low airspeeds. As the airspeed of an airplane gets lower…the angle of attack of the wing must be higher for the wing to produce the lift required to support the weight of the airplane…The same thing happens during rotor stall with a helicopter except it occurs due to low rotor RPM instead of low airspeed. As the RPM of the rotor gets lower, the angle of attack of the rotor blades must be higher to generate the lift required to support the weight of the helicopter…Even if the collective is not raised by the pilot to provide the higher blade angle, the helicopter will start to descend until the upward movement of air to the rotor provides the necessary increase in blade angle of attack…The increased drag on the blades acts like a huge rotor brake causing the rotor RPM to rapidly decrease, further increasing the rotor stall. As the helicopter begins to fall, the upward rushing air continues to increase the angle of attack on the slowly-rotating blades, making recovery virtually impossible, even with full down collective." FAA publication P-8740-71, "Planning Autorotations,"was intended to raise flight instructor awareness to the hazards of training students in autorotations and provide guidelines and parameters for conducting practice autorotations. The pamphlet concluded, "The number one error in practice autorotations is the failure of the flight instructor to take control of the aircraft and terminate the maneuver before it progresses to a point where the flight instructor is not capable of recovering the aircraft in time to prevent damage to the aircraft or injury to personnel. REMEMBER: As a flight instructor, you are the most knowledgeable and experienced person in that helicopter. Do not let your student fly the helicopter into some corner of its performance envelope where it is not recoverable." An autopsy was performed on the CFI by the Office of the District Medical Examiner, District 15, Palm Beach County, Florida. The cause of death was identified as blunt force injuries. Toxicological testing was performed by the FAA Bioaeronautical Sciences Research Laboratory in Oklahoma City, Oklahoma. Testing was negative for carbon monoxide, ethanol, and all tested-for drugs and their metabolites. The helicopter came to rest on its left side in a grassy area about 700 feet northwest of the runway 15 threshold at LNA. The wreckage path was oriented approximately 140 degrees magnetic, and extended about 75 feet from the initial impact point to where the fuselage came to rest. The initial impact point was identified as a large metal stake, about 3 feet in height. The second point of impact was a small crater measuring about 4 feet long and 1 foot deep, located about 15 feet past the initial impact point. A portion of the forward skid crosstube was located in the crater. The vertical stabilizer, horizontal stabilizer, and portion of the tail rotor came to rest next to the crater. The tail boom was separated from the fuselage, and fractured into several pieces, some of which displayed signatures consistent with main rotor blade contact. The skids separated from the fuselage and were fractured into several sections, which were located along the wreckage path. The fuselage exhibited downward crushing, and the cabin was fractured aft of the seat structures. Both the auxiliary and main fuel tanks were intact and contained fuel, and both fuel caps were secure. The fuel vent tubes were separated from the auxiliary tank by impact, which allowed fuel to drain from the tanks following the accident. The main rotor remained attached to the fuselage. One blade was bent up and displayed several chordwise creases along its span. The second blade was bent up about 45 degrees near its root. Neither blade displayed significant leading edge damage. The tail rotor drive shaft separated from the helicopter during the accident sequence, and a 5-foot portion was located about 500 feet west of the main w