History of Flight
On July 20, 2011, about 1126 eastern daylight time, a Robinson R22 BETA helicopter, registration N83509, was substantially damaged when it collided with terrain following an uncontrolled descent while maneuvering near Dayton, New Jersey. The certificated commercial pilot and a passenger were seriously injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local photography flight conducted under Title 14 Code of Federal Regulations Part 91. The helicopter departed Princeton Airport (93N) around 1045.
Witnesses reported the helicopter orbiting at low speed and low altitude near an elementary school and commercial properties. One witness on the school roof observed the helicopter complete one and a half circuits about 150 feet above ground, with an occupant taking photographs. The engine sounded "fine" throughout, but the helicopter then descended quickly in a slow spin. Another witness noted the helicopter appeared "slightly out of control," descending in a spin, briefly regaining altitude, then descending from view.
In an interview with a Federal Aviation Administration (FAA) inspector, the pilot stated he positioned the helicopter in an out-of-ground-effect hover about 400-500 feet above ground level (agl). The photographer requested a different vantage point, but as the pilot explained that wind and high power demands would prevent accommodation, the helicopter began to "settle with power." The pilot attempted to arrest the descent with a reduction of power and forward cyclic input. The helicopter yawed right and began to rotate around the main rotor mast. He reduced throttle to idle to arrest the spin; the yaw rate slowed, but the spin continued as the helicopter descended through about 250 feet agl. The pilot remembered assuring the passenger of a safe landing but could not recall further events. In a later statement, he estimated the onset began at 150-200 feet agl, and the helicopter was "spinning" when it hit the ground.
Personnel Information
According to FAA records, the pilot held a commercial pilot certificate with a rating for rotorcraft helicopter. His most recent second-class FAA medical certificate was issued on January 31, 2011. He reported 430 total hours of flight experience, of which 90 hours were in the accident helicopter make and model.
Aircraft Information
FAA records indicate the helicopter was manufactured in 2001. It was a two-seat, two-bladed, single main rotor helicopter equipped with a Lycoming O-360 reciprocating engine. The most recent annual inspection was completed on March 16, 2011, at which time the airframe had accumulated 4,000 total hours. The maximum allowable gross weight was 1,370 pounds; the operator estimated the gross weight at the time of the accident was 1,282 pounds.
Meteorological Information
At 1153, the weather observation at Trenton Mercer Airport, about 10 miles northeast, recorded variable wind at 5 knots, clear skies, 10 miles visibility, temperature 31°C, dew point 22°C, and an altimeter setting of 29.89 inches of mercury. The calculated density altitude was 2,555 feet.
Flight Recorder and Video Evidence
A video camera, two still cameras, and several data storage devices were recovered and examined. The still cameras and storage devices yielded 46 photographs taken before the accident but no other useful information. The video camera was operating at the time of the accident, but during the actual sequence, it rested inside a camera case capturing only audio and a partial view.
The recording began at 1120:16, showing the helicopter in forward cruise flight over urban residential areas. Over about five minutes, audio and video indicated the helicopter alternately slowing and accelerating. At 1125:51, sounds of slowing were heard. Two seconds later, an 800 Hz continuous warning tone, similar to the low rotor rpm audio alarm, began and continued beyond the sound of impact at 1126:00. The alarm was silenced by emergency responders at 1132:14.
Wreckage and Impact
FAA inspectors photographed the wreckage, which rested upright on its belly with landing gear skids spread uniformly. The fuselage had significant vertical crushing into the cockpit and cabin areas. The tailboom was wrinkled at its mount and torn about three feet forward of the tailrotor and tailrotor gearbox. Both tailrotor blades were fractured near their blade grips. The main rotor blades, hub, and mast appeared undamaged.
Control continuity was established from the cockpit to the main and tail rotor through several breaks due to impact and overload. The engine sustained minimal impact damage. The airbox and air filter were deformed; the carburetor was broken into pieces, and its float bowl was void of fuel. The float, made of blue foam, was shattered, and small blue foam pieces were found in the spark plugs. The exhaust system was deformed by impact. Oil level was within limits, and the oil appeared clean with no debris. The crankshaft rotated freely, and compression was confirmed at all cylinders. Both magnetos were secure. The cooling scroll sustained impact damage; the fanwheel and upper sheave showed rotational score marks.
Additional Information
FAA Advisory Circular (AC) 90-95, "Unanticipated Right Yaw in Helicopters," describes loss of tail rotor effectiveness (LTE) as a critical low-speed aerodynamic characteristic that can result in an uncommanded rapid yaw rate. It notes that LTE may occur in all single main rotor helicopters at airspeeds less than 30 knots. Paragraph 6 states that maneuvers in a high-power, low-airspeed environment with left crosswind or tailwind may create conditions for unanticipated right yaw. Paragraph 8 notes that at airspeeds below translational lift, the tail rotor must provide nearly all directional control. Paragraph 9 advises avoiding tailwinds and out-of-ground-effect hover in winds of 8-12 knots.
The FAA Rotorcraft Flying Handbook defines vortex ring state as an aerodynamic condition where a helicopter may descend vertically with maximum power applied and little cyclic authority, also known as "settling with power." The U.S. Army Field Manual 1-203 lists three conditions: low airspeed, 20-100% engine power applied, and a descent rate of 300 feet per minute or more with insufficient remaining power.