On November 9, 2001, approximately 2015 Pacific standard time, a Piper PA-28R-200 airplane, N9466K, registered to Innkeepers Investments Inc. of Puyallup, Washington, and being operated by Spanaflight (a Puyallup, Washington, fixed-base operator), experienced a loss of control and subsequently landed off-runway while landing on runway 16 at Pierce County Airport/Thun Field, Puyallup, Washington. The pilots (an airline transport pilot/certificated flight instructor, who was acting as pilot-in-command, and a private pilot receiving flight instruction in preparation for his FAA commercial pilot practical test) continued the landing rollout in the grass. During the landing roll, the airplane's nose gear collapsed, substantially damaging the airplane. Neither pilot was injured in the accident. Dark night visual meteorological conditions, with calm winds, were reported by the flight instructor, and the 14 CFR 91 local instructional flight was on a visual flight rules (VFR) flight plan. The flight instructor reported that he and the trainee flew a standard traffic pattern, with another single-engine aircraft and a helicopter entering the pattern on a long final. The flight instructor stated that he believed the helicopter to be a Robinson R22; the type and identity of this helicopter were not further established during the investigation. The instructor reported that they extended downwind to accommodate the helicopter. He stated that their approach was stabilized from about 1-mile final. The instructor reported that the helicopter cleared the active runway "about one-half to one-quarter mile before our touchdown." The instructor stated that "After crossing the runway threshold and prior to the flare, the aircraft rapidly banked approximately 45 degrees right wing down, yawed right and descended rapidly." The instructor reported that "Corrective action was immediately applied to level the wings, apply power and pitch nose-up", but that "Touchdown [occurred] almost simultaneously slightly to the right of the runway partially or totally on the grass edge." The instructor stated that "The decision was made to continue the rollout on the grass area." During the rollout, the aircraft's nose gear failed and the aircraft came to a stop in the grass. The instructor indicated in his NTSB accident report that no mechanical malfunction or failure was involved in the accident. The private pilot trainee reported that while the accident aircraft was on downwind, a light helicopter entered final approach. The trainee stated that "when turning from base to final I had him in sight near the runway threshold, slightly right of the runway", and that the helicopter then moved right to the parallel taxiway. The trainee stated, "I continued the approach not considering rotor wash to be a factor as he was now at the taxiway and standard glide path would have me above the area he had been near the runway[.]" The trainee reported that approximately 200 to 300 feet past the runway threshold, the aircraft suddenly rolled right, yawed right, and sank. He reported that opposite control input failed to arrest the roll or sink, although it did seem to slow the yaw. The trainee stated that the aircraft impacted the ground right wing low, yawed slightly to the right and nose high. He reported that the aircraft then became airborne again, and that he "was able to maintain control and land in the grass parallel to the runway." The FAA's Aeronautical Information Manual (AIM), paragraph 7-3-7, "[Wake Turbulence from] Helicopters", states that pilots of small aircraft should avoid operating within three rotor diameters of any helicopter in a slow hover taxi or stationary hover, and that pilots of small aircraft should use caution when operating behind or crossing behind landing and departing helicopters. The Robinson R22 helicopter's main rotor diameter is 25 feet 2 inches. According to the Pierce County-Thun Field airport manager, the centerline of the parallel taxiway is 180 feet from the runway centerline in the northernmost 700 feet of the 3,650-foot by 60-foot runway, and 240 feet from the runway centerline for the remainder of the runway. The flight instructor reported that the accident aircraft's maximum gross weight is 2,650 pounds. The R22's maximum gross weight is 1,300 to 1,370 pounds, depending on variant.