Tuesday, June 1, 2004
On April 10, 2003 at about 5:30 p.m. in Auburn, Calif., a Hughes 269B helicopter was destroyed when it collided with terrain while on an instructional flight.
The cause is believed to have been from practicing autorotations. A witness, located about 3 mi. from the accident site, stated the helicopter was maneuvering over his farm. He stated the engine sounded normal as though it was running at full power, then became quiet. The helicopter then came to a hover, powered up again and subsequently dropped into a ravine area. As the witness was walking back to his house, he heard the helicopter again. He looked back and saw that the helicopter had regained some altitude and was headed back toward the airport.
Another witness said he saw the helicopter drop below a tree line, and he didn't see it again. The accident site was located on a sand bar approximately one mile south of the Auburn Municipal Airport (AUN), Auburn, Calif. A preliminary investigation has revealed no mechanical discrepancies. The certified flight instructor and student pilot were fatally injured.
Ironically, many more accidents happen each year from practice autorotations than from actual engine failures. In fact, this maneuver appears frequently in NTSB accident reports regarding flight instruction.
Surprise throttle cuts are especially dangerous because they can startle students and cause them to make sudden incorrect control movements. Inadvertently raising collective, pressing the wrong pedal or lowering the nose can drop the rotor rpm perilously close to a stall. A student who simultaneously performs two or more of these movements could quickly stall the rotor system.
Before introducing forced landings, it is critical that the CFI and student establish a strong understanding of what is expected and what can happen. Then, the CFI should introduce simulated power failures slowly by telling the student in advance of rolling off the throttle. At first, this should be practiced at very low power settings to allow extra reaction time. Only after the student's reactions are correct and predictable should the difficulty level be increased. Even then, the CFI should always plan to initiate the autorotation and completely guard all the flight controls.
Careful attention should be given to high-time fixed wing pilots transitioning to helicopters. Their ingrained reactions to a sudden emergency can be deadly in a helicopter. For example, reacting to the stall warning horn in an airplane, a pilot would immediately lower the nose and add power. A power failure in a helicopter would likely trigger the lower rotor rpm horn and a pilot who lowers the nose and raises collective in response will likely stall the rotor system.
As the following accident demonstrates, CFIs should completely brief pilot examiners on their student's background and habits. According to the NTSB, on June 28, 2003 a R22 helicopter impacted the ground and rolled over during a practice autorotation during a private pilot check ride. After completing a series of maneuvers, an autorotation was initiated. According to the designated pilot examiner, the entry and flare were uneventful, and as the pilot applied power to recover the engine and rotor rpm needles were in the green. The pilot then began to cushion the descent with collective and the low rotor rpm horn activated. The check pilot expected the pilot to lower the collective slightly and roll on additional throttle; instead, the pilot lowered the collective almost all the way down and rolled some throttle off. At this point the helicopter was very close to the ground. The check pilot told the pilot "I have the controls." The helicopter entered a right turn, then the check pilot felt the right pedal move against his foot, and the helicopter settled to the ground at an angle. After bouncing once, it touched down on the left skid and rolled over.
Neither the pilot or the examiner were injured and after the accident, the DPE asked the pilot if he had been rolling the throttle off as he was pulling the collective up during the cushion portion of the autorotation. The pilot reported that in the past he had been over-speeding the engine, so he would slightly reduce the throttle to compensate. The density altitude at the time of the accident was 8,393 ft., leaving little margin for errors.
This mishap is typical of autorotation accidents in that the helicopter normally rolls over on ground contact and there are minor, if any, injuries. Nevertheless, the accident in California is a stark reminder that they can become deadly. Flight instructors, check airmen and examiners should exercise extreme vigilance and effective communication, and be prepared to handle any unexpected reaction when practicing autorotations.