Piper PA-22 accident at Near Dover, Delaware, 23 Sept 1958 (N2945P)

Accident narrative

On September 23, 1958, at about 1400 e.s.t., a Piper PA-22 (N2945P) crashed following an in-flight structural failure near Dover, Delaware. The pilot, who was the sole occupant, was killed. The Civil Aeronautics Board investigated the accident and concluded the aircraft was subjected to aerodynamic overloads caused by the wake vortex of a large aircraft.

**The flight** The pilot, 31-year-old William Wilson Miller, held a single-engine land rating with 339 total piloting hours, including 332 hours in the Piper PA-22. He departed Zahn's Airport in Amityville, Long Island, New York, bound for Charlottesville, Virginia, where he was a law student. The aircraft carried ample fuel for the 350-mile nonstop flight, and its gross weight and center of gravity were within limits. The pilot was transporting fragile and expensive phonograph equipment in the rear seat.

No flight plan was filed, but witnesses to his flight planning stated the route was to proceed south from New York, east of McGuire Air Force Base, and then via Victor 16 airway to Charlottesville. Weather over the entire route was ideal for visual flight, with light and variable winds. The Board noted that winds aloft offered the most help at 2,000 feet, and air traffic rules for flights above 3,000 feet would have required flying at 4,500 or 6,500 feet. Because it is customary for pilots of such aircraft to fly under 3,000 feet in good weather over flat country, the Board believed it is likely the pilot was flying at approximately 2,000 feet.

**The accident** Based on the departure time and the crash location about 160 miles along the route, the aircraft's groundspeed was calculated to be about 120 m.p.h. The aircraft's omni receiver was found tuned to the Kenton VOR, located about one mile north of the crash site.

Five lay witnesses in the area observed the aircraft falling and shedding parts, with some reporting a loud noise beforehand. None saw the aircraft prior to the failure. All described the fall as "straight down," and one noted that only one wing appeared attached and folded back. Three witnesses saw no other aircraft, but two reported seeing a "large" airplane in the vicinity. One of these witnesses stated the large airplane was so close they initially thought it was towing a target that was falling. No smoke or fire was seen.

**What the investigation found** The main wreckage was found embedded in Muddy Branch Swamp, about six miles northeast of Dover Air Force Base and just south of the Kenton VOR. The nearly vertical impact buried the engine and propeller. The right wing and right lift struts remained with the fuselage. The left wing structure, left aileron, right aileron, right fuel tank, and pieces of fabric were found in a markedly localized area about a quarter-mile north of the main wreckage.

Examination of the wreckage revealed no evidence of a mid-air collision, bird strike, fatigue, or faulty construction. The powerplant and control system appeared to have been operating normally prior to the crash. The Board found that the primary structure failed from downward-acting airloads, noting that damage to both wing panels and their lift struts was markedly bilateral.

The Board ruled out intentional acrobatics, noting the aircraft was placarded against them, the pilot had a reputation for conservative flying, and he was carrying fragile cargo. Official weather reports indicated no appreciable natural turbulence in the area.

Investigators examined operations at nearby Dover Air Force Base, which operated four-engine transports weighing up to 185,000 pounds. These aircraft frequently used the Kenton VOR for practice instrument approaches. Loaded C-12s departing Dover reached the Kenton VOR at altitudes between 1,300 and 2,000 feet, crossing the Victor 16 airway at approximately a 90-degree angle.

Engineering studies by the NACA, Douglas, and Beech Aircraft Corporation indicated that wingtip vortices from large aircraft can persist for several miles and minutes in still air. The Board noted that penetrating a pair of vortices imposes sudden up, down, down, and up loads. The Piper PA-22 was designed to an ultimate maneuvering load factor of 5.7g's and a negative ultimate maneuvering load factor of 2.28g's. The Board surmised that if a pilot moved the elevator control forward upon hitting the initial updraft, the subsequent downward forces would be greatly intensified, potentially reaching or exceeding the negative ultimate maneuvering load factors of normal category aircraft.

The Board concluded that the pilot was cruising at 2,000 feet or less near the Kenton VOR when the aircraft penetrated a wingtip vortex of a large aircraft, probably a C-12h leaving or approaching Dover Air Force Base. The violent artificial turbulence caused simultaneous downward failures of both wings.

**Probable cause** The Board determines that the probable cause of this accident was "structural failure of a Piper PA-22 aircraft resulting from excessive airloads created by wing tip vortices behind a large aircraft."