The Avcon Industries Learjet 25 aft fuselage delta fin development program came to an abrupt halt last June 12 when, during a key pre-modification flight test, the aircraft was destroyed in a crash landing, seriously injuring the two pilots–company president Larry Franke and contract test pilot Bob Fisher.
Franke and Fisher were supposed to take the Learjet 25D out to the “knee,” a point where Md and Vd, max dive speeds, are reached concurrently. That’s also where maximum dynamic pressure on the airframe occurs. The recovery would put a 1.5-g load on the 1978 airplane.
Descending from a higher altitude, the jet would approach the knee at about 20,500 ft. But at just below 24,000 ft the crew experienced high-speed elevator flutter. The pilots reduced power and slowly tried to pull the nose of the airplane up. Then they heard a bang in the back of the airplane, followed by loss of elevator control. Franke surmised that a clevis bolt in the control system had broken, rendering the elevator useless.
The crew trimmed the airplane, declared an emergency and prepared for a landing at Salina, Kan. Using horizontal stabilizer pitch for vertical control, “everything seemed fine.” But it was a typical gusty summer Kansas day.
At a quarter mile from the runway and about 150 ft agl, a gust of wind hit, the nose dropped and the airplane slammed into a muddy wheat field that immediately sheared off the nose gear. They slid for about 1,000 ft on the mains and the nose until smashing into a berm, launching the Learjet airborne again. That’s when the engines and wings separated from the fuselage, which did a roll in the air before crashing back down to the ground. Franke suffered five broken ribs, but remained conscious. Fisher had numerous cuts and was knocked unconscious. They were extracted from the airplane by airport rescue personnel. Fisher regained consciousness on the way to the hospital.
To prevent a replay of what happened on June 12, an FAA-approved flight-test profile will not take Avcon’s recently acquired Learjet 25D to the knee. The company is still going to have to do Md and Vd data gathering, but it doesn’t have to hit those numbers concurrently.
“It’s going to cost us more to do,” Franke said. “But I don’t want to crash again. I always wondered what it would be like right before you hit the ground. Now I know, and I don’t like it.”