History of Flight
On March 8, 2014, about 1400 central standard time, a Cessna 172D airplane, N2755U, was destroyed when it impacted terrain while attempting to land at H L Sonny Callahan Airport (CQF), Fairhope, Alabama. The private pilot sustained serious injuries. Visual meteorological conditions prevailed, and no flight plan was filed. The flight originated from Ferguson Airport (82J), Pensacola, Florida, and was destined for CQF. The personal flight was conducted under Part 91.
The pilot was returning to CQF after completing youth introductory flights at 82J. The enroute portion was uneventful. Upon arriving near CQF, the pilot entered the downwind leg of the traffic pattern for runway 19. While on downwind, he observed a large helicopter approaching the airport and "almost on the ground." On short final at about 30 feet agl, he noted the helicopter flying over the parallel taxiway at midfield. When the airplane reached the approach end of the runway, the pilot saw the helicopter just past midfield. About that time, one wing "went straight up," which was the last thing the pilot remembered. He reported no mechanical malfunctions prior to the upset.
According to witnesses, the accident airplane was following the large helicopter in the traffic pattern. On final approach, it appeared "way too close" to the helicopter and was observed performing S-turns. The helicopter pilot then announced via radio that he would fly to the south end of the field in a hover. While the helicopter was still in forward flight over the runway, the airplane reached the approach end and, at about 30 feet agl, suddenly rolled right wing low, then left wing low, then leveled out and impacted the ground forward of the runway threshold. A post-impact fire ensued near the engine cowling. A witness reported the helicopter was "50 ft above the ground" when the airplane was upset.
The helicopter pilot stated that after an initial low approach ending in a hover, the copilot maneuvered around the pattern for a second landing. The crew observed the accident airplane on downwind and noted it seemed closer than customary to the runway and helicopter. The pilot advised the copilot to stop descent at 300-500 feet agl and commence final approach further down the runway. As the helicopter descended about 3/4 down the runway, the crew heard via radio that the accident airplane had crashed.
Personnel Information
The pilot held a private pilot certificate with an airplane single-engine land rating. He had about 633 total flight hours, with 173 hours in the accident airplane make and model.
Meteorological Information
The 2000 recorded weather observation at the MBLA1 buoy, 7 nautical miles southeast of the accident site, included wind from 120 degrees at 5 knots, temperature 15°C, and altimeter setting 30.15 inches of mercury. Two witnesses reported winds at CQF from the south straight down the runway at about 5-7 knots.
Wreckage and Impact Information
The airframe and engine were examined at the accident site. The initial impact point was slightly forward of the runway 19 numbers, and the main wreckage was 800 feet past the initial impact point and 50 feet west of the runway edge. Abrasions on the runway surface (brake rotor scars and propeller strikes) were traced from initial impact to main wreckage.
The cabin, tailcone, and wing roots showed thermal damage. All flight control surfaces remained attached, and control continuity was traced to the cockpit. The flap handle was retracted, but the flap setting could not be determined due to post-crash fire. The elevator trim actuator was neutral, but the indicator could not be examined due to fire. The fuel system was fire-damaged and unexaminable. Both propeller blades remained attached; one had thermal damage and twisting, the other separated at the tip with chordwise scoring and curling.
The engine sustained significant fire and impact damage. The carburetor separated and melted. Both magnetos remained attached; one could not be rotated by hand, the other rotated but did not spark. Disassembly showed no abnormalities. Cylinders examined via borescope showed normal combustion; all valve faces and seats were in place. Crankshaft rotation produced thumb compression on all six cylinders. All rocker arms and valves operated normally. Valvetrain continuity was confirmed. The oil screen, found halfway between initial impact and main wreckage, contained no contaminants.
Additional Information
Rotorcraft Trailing Vortices
The flight profile of the large helicopter (CH-53E) was provided by its pilot to Sikorsky Aircraft Corporation for a proprietary analysis of trailing vortex dissipation. Data included heading 190° magnetic, 80 knots airspeed, altitude 400 ft agl, and gross weight 48,100 lbs. A simulation estimated blade radius and tip vortex core radius. Two videos were produced: one simulating 5-knot winds out of the southeast, another with no wind.
The wake evolution model was an unsteady two-dimensional vortex interaction problem. The simulation assumed the accident airplane trailed about 30 seconds (+/- 6 seconds) behind the helicopter. The study found that based on the airplane's altitude, the helicopter would need to be less than 200 ft agl for a vortex interaction to occur. A similar outcome would be expected for winds aligned with the runway heading at 0 knots. Given the weather conditions and helicopter gross weight, the analysis concluded that trailing vortices were likely present over the approach end of the runway at the time of the accident.
FAA Aeronautical Information Manual
Section 7-3-7 of the FAA's Aeronautical Information Manual states: "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."