Keene, NH, USA
N910SM
Robinson R-22B
The student pilot was practicing hovering autorotations with a flight instructor onboard. They were performing the practice in an open field next to a runway. At the rear of the field behind the helicopter was a snowbank with an irregular line. There was a wind of about 10 knots on the nose of the helicopter. During each hovering autorotation, the helicopter ended up further aft than the preceding one. The flight instructor looked out of his left door and based upon the part of the snowbank that he could see, he thought he had about 50-75 feet to the snow bank. However, he was unaware that the left rear skid was already under the crust of the snowbank directly behind the helicopter. He took control of the helicopter with the intent of repositioning it to the middle of the field. When the flight instructor applied the collective to lift off, the helicopter pivoted on the left rear skid, and rolled over on its left side. The flight instructor reported the accident could have been prevented had he maintained better situational awareness of the snowbank in the field where they were practicing, and if he had used a two stage pickup, where he first made the helicopter light on the skids, and then applied the collective to lift off more slowly.
On February 19, 2001, about 1430 Eastern Standard Time, a Robinson R-22B helicopter, N910SM, operated by Green River Aviation, was substantially damaged during takeoff from Dillant-Hopkins Airport, Keene, New Hampshire. The certificated flight instructor received minor injuries, and the student pilot was not injured. Visual meteorological conditions prevailed for the dual instruction flight. No flight plan had been filed for the flight that was conducted under 14 CFR Part 91. According to the flight instructor's written statement on the NTSB Form 6120.1/2: "...proceeded to south end of Runway 02 area cleared by runway snowblower down to grass. Performed at least 6 hovering autorotations, winds were from 223 degrees at 14 knots, gusts [to] 17 [knots]. Preparing student for private check ride. Started in center of area, 100 x 100 ft. Winds had pushed us back toward snowbanks. Looking out my left window, looks as though I still had 50 - 75 ft, but decided to lift off after last hover auto to return to center, not realizing left rear skid has slid under the crust of the snow. Between the winds and the lift off, helicopter went over on left side...." In a telephone interview, the flight instructor reported that he should have maintained better situational awareness of the location of the snowbank. In addition, he also said he should have used a two step pickup where the helicopter was first made light on the skids, and then lifted into the air. The 1435 weather observation at Keene recorded winds from 230 degrees at 10 knots, with gusts to 16 knots. According to Advisory Circular AC 90-87 Helicopter Dynamic Rollover: "...During normal or slope takeoffs and landings with the same degree of bank angle or side drift with one skid/wheel on the ground, the bank angle or side drift can place the helicopter in a situation where it is pivoting (rolling) about a skid/wheel which is still in contact with the ground. When this happens, lateral cyclic control response becomes more sluggish and less effective than for a free hovering helicopter. Consequently, if a roll rate is permitted to develop, a critical bank angle (the angle between the helicopter and the horizon) may be reached where roll cannot be corrected, even with full lateral cyclic, and the helicopter will roll over onto its side. As the roll rate increases, the angle at which recovery is still possible is significantly reduced. The critical rollover angle is also reduced..."
inadequate visual lookout by the flight instructor on the helicopter's proximity to the snowbank, and subsequent lift off, which resulted in a dynamic rollover.
Source: NTSB Aviation Accident Database
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