In May 1999, in Plattsmouth, Nebraska, a two-seat homebuilt GP-4 crashed shortly after takeoff, killing its 73-year-old builder and 60-year-old passenger. Both men were licensed pilots. The builder had logged 181.4 hours in the airplane since first flying it 15 months earlier.
The weather at the time of the accident was clear and mild. According to witnesses, the airplane had climbed to about 500 to 700 feet agl after takeoff when it pitched over and entered a spin. The wreckage, approximately two miles from the end of the runway, gave indications of a near-vertical attitude at the time of impact. The engine and propeller were partially buried in the ground, with the oil pan, injector manifold and starter motor separated from the case and the crankshaft broken at the propeller flange. The engine was too badly damaged to be test run, but investigators could find nothing to suggest engine trouble as a possible cause of the crash. There was fuel-or at least "a fluid, blue in color"-in the fuel tank. Control continuity could be established, but it was impossible to determine the positions of the gear and flaps.
In the absence of any indication of mechanical failure, the next hypothesis is pilot error.
The pilot held a commercial certificate and instrument rating; his total time was just shy of 2,000 hours. He participated in the FAA's Pilot Proficiency Award Program, which is designed to encourage recurrent training, and had recently completed his fourth annual segment. The toxicology report following the accident was clean, and there is no indication in the National Transportation Safety Board's accident report of a potentially incapacitating medical condition. There was therefore nothing in the pilot's history or physical condition to explain a stall-spin shortly after takeoff.
The GP-4 is a compact high performance low-wing airplane of wood construction created by George Pereira, whose Osprey amphibian has been a popular plans-built design for many years. It uses a four-cylinder Lycoming engine of 200 hp and has a gross weight of 2,000 pounds. Its tricycle landing gear is retractable. It seats two side-by-side under a bubble canopy. A recognition point is the unusual, almost triangular vertical tail. The specifications published on Pereira's website, ospreyaircraft.com, report a range of 1,100 nautical miles at 75 percent of power cruising speed of 209 knots. The website states that the "GP-4 is a high performance aircraft and it's not more difficult to fly the GP-4 than any other high performance aircraft."
Designer Pereira provided accident investigators with a written statement regarding the flying qualities of the GP-4. Regarding the power-on stall he wrote, with exemplary candor, "Power set at 20 inches, propeller at 2400 a very steep nose high angle is attained before the stall at 70 mph IAS … the right wing drops almost vertical. Recovery is very fast with rudder and stick application. You can expect to lose 300 to 500 feet in the recovery with full flaps, 32 degrees, gear down the stall is 62 to 65 mph." The garbled last sentence may be a transcription error; most likely, it should have a period after "recovery." An accelerated clean stall, Pereira continued, resulted in a "fast wing rotation or snap" opposite to the direction of the turn.
Regarding other flying qualities, Pereira noted that the GP-4 is neutrally stable in pitch. "If you … set up a climb or descent, the aircraft … will not return to its pre-trimmed condition without a control input." As would be expected in a neutrally stable airplane, "[t]he elevator is very responsive in pitch control ... Care must be taken not to bump the stick while retracting the gear with the Johnson bar. Since the GP-4 accelerates rapidly after liftoff the gear should be retracted before much over 100 mph. The faster you go the harder it is to retract the gear due to the low pressure area on the main gear doors."
"Johnson bar" is a mock-technical term for any large handle or lever. Like that of early Mooneys, the standard landing gear retraction system of the GP-4 is manual. A hydraulic alternative is available, but was not installed on the accident airplane.
The builder of the accident airplane had developed a Pilot's Operating Handbook for it, although there was no regulatory requirement that he do so. In it, he mentioned "rapid decay of control effectiveness" as the stall approaches, followed by "rapid loss of altitude with the control stick aft," but suggested that pre-stall excursions could be controlled with "precise control coordination" and use of rudder. If the airplane dropped a wing, it could go either way. "Intentional spins are prohibited," he wrote, but "if stall recovery is delayed or if the airplane is held in stall in an uncoordinated manner, it will likely go into a spin."
Regarding takeoff procedure, the builder wrote, "Retract gear when safely airborne and in control, after centering nose wheel and applying brakes to stop wheel rotation. (May be necessary to climb to safe altitude, and slow to 80 mph to retract.)"
This last, somewhat unusual requirement was confirmed by another pilot who had logged 51.1 hours in the accident airplane. The gear retraction procedure, he reported, was to climb to 1,500 agl, retard the throttle to idle, slow the aircraft to 80 mph, then swing the "Johnson bar" to retract the landing gear. Concerning the stall, he confirmed the POH description but stated that the airplane "banked sharply to the left" if stalled.
