On December 9, 2000, about 1100 Eastern Standard Time, a Mooney M-20J, N201MP, was substantially damaged while landing at the Trenton Mercer Airport (TTN), Trenton, New Jersey. The certificated private pilot and flight instructor were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local training flight conducted under 14 CFR Part 91.

According to the flight instructor, the pilot executed a normal takeoff from Runway 34 for right closed traffic. The pilot flew the downwind at 1,200 feet, completed the base leg portion of the traffic pattern, and then turned final about a 1/2 mile from the landing threshold and 700 feet msl. Because a UH-60 helicopter was executing an approach from the southwest to the threshold of Runway 34, the flight instructor advised the pilot to extend the touchdown point by 1,000 feet. The helicopter transitioned to hovering flight near the threshold, about 30 seconds ahead of the accident airplane. Due to another airplane holding short of Runway 34 on taxiway "echo," the UH-60 had to air-taxi to another section of the taxiway.

The accident airplane was approximately 20 feet above the ground with landing flaps and 90 mph, and the UH-60 was hovering 100 feet to the north, when the accident airplane suddenly rolled right (faster than a full application of aileron could counteract.) The pilot and flight instructor simultaneously applied full left aileron, and the flight instructor applied full power. The right wing contacted the ground and the nose of the airplane rotated 90 degrees to the right. The flight instructor then closed the throttle and the airplane impacted the runway landing gear first. The airplane came to rest upright on its landing gear.

This was the third flight the flight instructor conducted with the pilot in the accident airplane. The flight instructor added that the pilot did a good job of listening and executing instructions. In addition, the flight instructor had approximately 3,300 hours of total flight experience, with 500 hours of that in make and model. Include in that time was 300 hours of flight instruction.

According to a witness approximately 680 feet to the northeast of the accident site, the helicopter was hovering over taxiway "echo," when he noticed a low-wing airplane on a right base for Runway 34. The bank angle for the airplane was more than 40 degrees, and the airplane appeared to be low. The witness than lost sight of the airplane as it passed behind a building. At the time of the accident, the helicopter was transitioning to the ramp, located approximately 600 from the accident site.

According to the Aeronautical Information Manual, every aircraft generates rotating vortices that trail from their wing tips. The vortices from larger aircraft pose problems to encountering aircraft. For instance, the wake of an aircraft can impose rolling moments exceeding the roll-control authority of the encountering aircraft. Further, turbulence generated within the vortices can damage aircraft components and equipment if encountered at close range. The pilot must learn to envision the location of the vortexes generated by larger aircraft, and adjust the flight path accordingly.

A crosswind will decrease the lateral movement of the upwind vortex and increase the movement of the downwind vortex. Thus a light wind with a cross runway component of 1 to 5 knots could result in the upwind vortex remaining in the touchdown zone for a period of time and hasten the drift of the downwind vortex toward another runway.

A helicopter in a slow hover taxi or stationary hover near the surface, generates downwash producing high velocity outwash vortices to a distance approximately three times the diameter of the rotor. When rotor downwash hits the surface, the resulting outwash vortices have behavioral characteristics similar to wing tip vortices produced by fixed wing aircraft. However, the vortex circulation is outward, upward, around, and away from the main rotor(s) in all directions. Pilots of small aircraft should avoid operating within three rotor diameters of any helicopter in a slow hover taxi or stationary hover. In forward flight, departing or landing helicopters produce a pair of strong, high-speed trailing vortices similar to wing tip vortices of larger fixed wing aircraft. Pilots of small aircraft should use caution when operating behind or crossing behind landing and departing helicopters.

About 7 minutes before the accident, Trenton reported wind 320 degrees at 10 knots, visibility 10 miles, clear skies, temperature 34 degrees Fahrenheit, dew point 16 degrees Fahrenheit, and an altimeter setting of 30.40 inches of mercury.

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