N2755UCessna 172D · 1963

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 for flight , which originated from Ferguson Airport (82J), Pensacola, Florida and was destined for CQF. The personal flight was conducted under the provisions of Title 14 Code of Federal Regulations Part 91.

The pilot was returning to CQF after completing youth introductory flights at 82J. The enroute portion of the flight was uneventful, and upon arriving in the vicinity of CQF, the pilot entered the downwind leg of the traffic pattern for runway 19. While on the downwind leg of the traffic pattern the pilot observed a large helicopter that was approaching the airport and was "almost on the ground." As the pilot was on the short final leg of the approach to the runway, at an altitude of about 30 feet agl, he noted that the helicopter was flying over the runway's parallel taxiway at about the mid-field point. Once the airplane reached the approach end of the runway the pilot observed the helicopter just past the mid-field point of the runway. About that time, one of the airplane's wings "went straight up," which is the last thing the pilot remembered about the flight. The pilot reported that prior to the upset, there were no mechanical malfunctions or other anomalies of the airplane that would have precluded normal operation

According to witnesses, the accident airplane was following the large helicopter in the airport traffic pattern. While on final approach, the accident airplane appeared to be "way too close" to the helicopter and was observed performing S turn maneuvers. The pilot of the helicopter then made a radio call to announce that he was going to 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 of the runway and, at an approximate altitude of 30 feet above ground level (agl), the airplane suddenly rolled into a right wing low attitude followed by a left wing low attitude. The airplane then leveled out, impacted the ground in a level attitude forward of the runway threshold and a post impact fire ensued near the engine cowling. A witness reported that the large helicopter was "50 ft above the ground" when the accident airplane was upset.

According to the pilot of the helicopter, following an initial low approach to the runway that terminated in a hover, the co-pilot maneuvered the helicopter around the traffic pattern for a second landing. The crew observed the accident airplane while it was on the downwind leg of its approach, and commented that the airplane seemed to be flying closer than customary to the runway and the helicopter. The pilot subsequently advised the copilot to stop his descent at an altitude of between 300 and 500 feet agl, and commence his final landing approach further down the runway. As the helicopter continued its final descent, this time about 3/4 down the runway the crew heard via radio that the accident airplane had crashed.

The pilot held a private pilot certificate with a rating for airplane single engine land. He had accumulated about 633 total hours of flight time, of which 173 hours were in the accident airplane make and model.

The 2000 recorded weather observation at the MBLA1 buoy, located 7 nautical miles southeast of the accident site, included wind from 120 degrees at 5 knots, temperature 15 degrees C, and an altimeter setting of 30.15 inches of mercury.

Two witnesses reported winds at the time of the accident at CQF were from the south straight down the runway at approximately 5-7 knots.

The airframe and engine were examined at the accident site by representatives of the airframe and engine manufacturers under the supervision of a Federal Aviation Administration inspector. The initial impact point was slightly forward of the runway 19 numbers and the main wreckage was located 800 feet past the initial impact point and 50 feet west of the runway edge. Abrasions of the runway surface consisting of brake rotor scars and propeller strikes were traced from the initial impact point to the location of the main wreckage.

The cabin, tailcone, and wing roots exhibited signs of thermal damage. All flight control surfaces remained attached to their respective airframe components, and flight control continuity was traced from each flight control surface to the cockpit. The flap handle was observed in the retracted position and due to post-crash fire the flap setting could not be determined at the accident site. The elevator trim actuator was in the neutral position, but the elevator trim indicator could not be examined due to fire damage. The fuel system was also damaged from the post-crash fire and could not be examined. Both propeller blades remained attached to the hub, which was found attached to the crankshaft. One blade displayed thermal damage and twisting. The other blade was separated at the tip and exhibited chordwise scoring and curling.

There was significant fire and impact damage to the engine. The carburetor separated from the engine and was melted by postcrash fire. Both magnetos remained attached to the crankcase. All spark plugs contained varying amount of thermal and impact damage, but also remained installed on the engine.

The top spark plugs were removed and examined, with all exhibiting normal wear signatures. The bottom spark plugs were inspected with a borescope and no anomalies were noted. The magnetos exhibited significant thermal damage; one magneto could not be rotated by hand and the other magneto was rotated, but did not produce a spark. Both magnetos were disassembled and no abnormalities were noted.

The cylinders were examined using a borescope. The pistons and cylinder bores exhibited signs of normal combustion and all of the valve faces and seats were in place. The crankshaft was rotated at the propeller flange, and thumb compression was confirmed on each of the six cylinders. All rocker arms and valves operated normally during crankshaft rotation. Valvetrain continuity was confirmed on all of the cylinders and to the accessory drive gears.

The oil screen was found on the runway about halfway between the initial impact point and the main wreckage. The screen did not contain any contaminates.

Rotorcraft Trailing Vortices

The flight profile of the large helicopter, a CH-53E, provided by the helicopter pilot was sent to Sikorsky Aircraft Corporation and a proprietary analysis was conducted to examine the trailing vortex dissipation of the helicopter. The data, which included a heading of 190 degrees magnetic, 80 knots airspeed, altitude of 400 ft. agl, and a gross weight of 48,100 lbs. were programmed in a simulation. Data points for the blade radius and tip vortex core radius were then estimated and two videos were produced. One video was used to simulate 5 knot winds out of the southeast and another video was used to simulate a no wind scenario.

The wake evolution simulation was modeled as an unsteady two-dimensional vortex interaction problem. A differential equation was used to update the tip vortex position given its initial position and strength, which then produced the vortex trajectory. The helicopter heading and wind direction were used to map the two-dimensional wake to three dimensions including the runway position.

The results of both simulations showed a main rotor blade wind velocity of 60 knots. The simulations 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 above the ground for a vortex interaction to occur. A similar outcome would be expected for winds aligned with the runway heading at a velocity of 0 knots.

Given the weather conditions at the time of the accident, along with the gross weight of the helicopter, 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, Helicopter Trailing Vortices

According to the FAA's Aeronautical Information Manual, section 7-3-7:

"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."