Safford, AZ, USA
N515D
FALLBECK DYKE DELTA JD2
The owner/builder (pilot) of a home built airplane was on a test flight to evaluate the slow speed flight characteristics of the airplane after installation of fixed tabs on the wing tip trailing edges. During the flight, the airplane crashed on the downhill side of a 50-foot knoll. The wreckage exhibited evidence of vertical impact loads and a low impact angle. The designer of the original prototype described a fully developed stall in this airplane as a mushing characteristic, resulting in a high sink rate and temporary loss of control. The designer also stated that the pilot's incorporation of a T-tail was a contravention of the original design specifications. The pilot had previously recorded stall speeds of 100 mph. The designer's intended stall speed was 75 mph for an unmodified airplane.
On October 21, 1995, around 1036 hours mountain standard time, a Fallbeck Dyke Delta JD2, N515D, collided with terrain while performing an undetermined maneuver during a flight test about 10 miles southeast of the Safford Municipal Airport, Safford, Arizona. The experimental airplane was destroyed and the certificated commercial pilot received fatal injuries. The airplane was owned, operated, and built by the pilot. Visual meteorological conditions prevailed at the time, and no flight plan had been filed. The flight originated from Safford at 1010. This was reportedly the first flight since the pilot had towed the airplane on its main landing gear wheels from St. Louis, Missouri, to Safford. The owner of Safford Aviation reported that he had spoken with the pilot prior to his departure. The pilot had recently completed a modification to the airplane's wings which consisted of the installation of fixed tabs on the wing tip's trailing edges. This was to be the first test flight since the tabs were installed and that the tabs were designed to improve the airplane's slow speed flight characteristics. The pilot indicated that he planned to fly to the designated practice area to perform a test flight. He planned to evaluate the tabs effectiveness and record any changes to the airplane's controllability at approach speed. He also mentioned that his air speed indicator might be inaccurate. After the pilot taxied for takeoff, the owner heard the pilot initiate an engine run-up, during which time he heard a magneto check and the prop being cycled. He said none of the run-up sounds were unusual. The wreckage was located the next day in open desert terrain in the airport's designated practice area which is about 3 minutes flight time from the airport. The aircraft was found on a northwest bearing along a route directly toward the airport. After arriving at the accident site, the owner of Safford Aviation also reported that the initial ground impact occurred on the downwind side of a 50-foot knoll. In his opinion, in order for the airplane to have cleared the top of the knoll and still impact on the down range slope, the airplane must have been descending at a high rate of vertical speed. He also noted that during the wreckage recovery the pilot's seat was found separated from the cockpit. The five-point restraint system had also separated from its attachment points. The aircraft's accelerometer was found registering about 11 positive and 4 negative g's. The clock was stopped at 1030. The fuel selector was on the left tank. Prior to removal the wreckage was examined by the pilot's long time friend, who is also a certificated commercial pilot as well as a airframe and powerplant mechanic. He reported that he had assisted the pilot/builder with the construction of the aircraft. According to his friend, it was his opinion that the airplane impacted the terrain during a high rate of descent while in a slightly left wing low, 5-degree nose-down attitude. The distance between the initial point of ground impact and the main wreckage was about 150 feet. All of the airplane's major structural components, including the flight control surfaces and newly installed trailing edge tabs, were found with the main wreckage. The engine was examined by the engine manufacturer's representative under the supervision of an FAA airworthiness inspector. (An extract from the representative's report and the FAA inspector's report are appended to this report). According to FAA records, the pilot also held an airframe and powerplant mechanic certificate. Using the pilot's weight and balance forms and flight planning notes from the day of the accident, his friend estimated that the aircraft's gross weight at takeoff was about 1,617 pounds with a CG of 63.3 inches. At the time of the accident, the estimated weight was 1,587 pounds with a CG of 63.0 inches. Based on his computed values, the friend concluded that according to the aircraft loading chart, the aircraft was within both maximum gross weight and CG limitations at all times during the flight. Statements from the pilot's wife to his friend report the aircraft took off with about 25 gallons of fuel. The estimated duration of the flight was 20 minutes, in addition to a 6 minute taxi and run-up. The engine manufacturer's operator's manual computes fuel consumption rates of 12.3 gph at 75 percent power (2,450 rpm) and 9.5 gph at 65 percent power (2,350 rpm). Fuel onboard at the time of the accident was estimated by Safety Board investigators to have been about 20 gallons. The friend reported that the pilot told him on his initial flight he found the aircraft's stall speed was about 160 mph. He also found the aircraft to be so nose heavy that he had about 2 inches of aft control stick movement available at slower speeds. To correct that problem, he had added approximately 35 pounds of lead to the aft fuselage and installed some lighter engine accessories. According to the friend, the pilot noted that changes to the center of gravity had resulted in an approach speed of 130 mph and a stall speed of 100 mph. The friend reported that he watched some of the test flights and noticed the aircraft had a tendency to roll, or for a wing to drop just prior to touchdown. (A witness statement from his friend is appended to this report.) The designer of the aircraft was interviewed by telephone and stated that he had met the pilot in Oshkosh, Wisconsin. He said that he had not told the pilot to add trailing edge tabs to the wings and that his design does not provide for such an installation. He also noted that his own aircraft, which was built from the same plans, has an approach speed of 100 mph and stall speed of 75 mph; whereas, the pilot reported that after adding lead and replacing accessory components had managed to reduce his aircraft's approach speed to 130 mph with a stall speed of 100 mph. The designer described the flight characteristics of a stall as an initial forward "mushing" as the stall approaches. A fully developed stall results in a high sink rate and temporary loss of aircraft control. Finally, he stated that since his design does not accommodate a '"t-tail", the pilot's incorporation of such change altered the final product and resulted in an aircraft that did not conform to the designer's plans and was in contravention to his original design specifications. An autopsy was performed by the Graham County Coroner with specimens retained for toxicological analysis by the FAA Civil Aeromedical Institute (CAMI). The results of the toxicological analysis were negative for alcohol, carbon monoxide, cyanide, and screened drugs.
Improper planning/decision by the owner/builder (pilot), which resulted in his failure or inability to recover from a stall/mush condition.
Source: NTSB Aviation Accident Database
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