Avcon Industries, manufacturer of aft fuselage delta fins and other mods to improve directional stability and increase payload/range capability for the Learjet 35/36, will resume work to obtain STCs for similar mods on the Learjet 25. The Newton, Kan. company’s Learjet 25 development program was abruptly shattered on June 12 when, during a key flight test, the aircraft was destroyed in a crash landing, seriously injuring the two pilots.
Larry Franke, Avcon president and one of the two pilots on the accident flight, said, “We’ve lost at least six months momentum.” The company has recently bought another Learjet 25D (S/N 330) and is preparing to duplicate all the prior pre-fin baseline numbers, but “we aren’t going to start test flying until after the NBAA Convention in New Orleans this month.” (Avcon intends to exhibit at the event.)
“Realistically, test flying won’t begin until early next year,” Franke said. He now anticipates receiving approval for the fins in mid- to late-April, with the cost at or below the $82,500 installed price of the Avcon fins on the Lear 35/36 (a $79,500 special price will be available for the balance of the year at any domestic Avcon sales and installation center).
Franke recounted the accident flight, one of the last pre-fin data-gathering tests. “We were supposed to take the aircraft out to the knee, the point where you reach Md and Vd, max dive speeds, concurrently. That’s where you reach maximum dynamic pressures on the aircraft.” The recovery from such a maneuver would put a 1.5-g load on the airplane. Franke said the company accomplished the same test successfully on a Learjet 35 before obtaining the Avcon fin STC.
“We were approaching the knee, planning to reach it at roughly 20,500 ft. At just below 24,000 ft we experienced high-speed elevator flutter.” Franke and company contract test pilot Bob Fisher began to reduce power and slowly tried to pull the nose of the airplane up “as gently as we could as we didn’t want to overstress anything.” Then they heard a bang in the back of the airplane. Franke surmised that a clevis bolt in the control system broke, rendering the elevator useless. The bolt connects the elevator-control-cable turnbuckles to the push rods. The NTSB is still several weeks from reaching a final conclusion about what happened and how it happened.
After elevator control was lost, Franke continued, “We trimmed the airplane out, declared an emergency and decided to go into Salina. It has a long runway and we were headed there anyway from the test.” Fisher and Franke did slow flight, dirtied up the airplane, flew it around using horizontal stabilizer pitch and “everything was just fine.” Established on a five-mile final, they were set up with one notch of flaps, “which gave us high-speed trim.” 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 and the nose dropped. The pilots were unable to raise the nose using trim and power and the airplane slammed nose down into a muddy wheatfield, shearing off the nose gear. They slid for about 1,000 ft on the mains and the nose and “everything was going pretty well” until the airplane went through an airport perimeter fence and slammed into a berm. Franke remembered that striking the berm “launched us airborne again, and that’s when the engines and wings departed from the fuselage, which did a roll in the air before crashing back down to the ground.” Franke suffered five broken ribs but retained consciousness. Fisher had numerous cuts and bruises and was knocked unconscious.
They were extricated from the airplane by airport rescue personnel about 40 min after the crash. Fisher regained consciousness on the way to the hospital.
Fins Approved Since 1996
Some 40 Learjet 35s and 36s have had the Avcon fins installed since they received STC approval in mid-1996 (see AIN July 1, 1996, page 101 for a full flight evaluation report). Simply described, the fins improve directional stability by channeling and smoothing the airflow around the tailcone. Their installation eliminates the FAA stipulation that inoperable yaw dampers are a no-go item. The mod incorporates additional nose-up trim capability (much like Learjet did on its fin-equipped Learjet 31) as a result of early flight tests. At forward c.g. and low airspeeds, the fins were generating enough lift that there was insufficient nose-up trim to counter the force.
About half of the modified Learjets are also fitted with Avcon’s $56,000 R/X mod, a 38-in. plug in each tip tank that accommodates 375 lb more fuel each side (for a total addition of 750 lb more fuel). Aircraft with the fins, the R/X tip tanks or both are also eligible to purchase the following three weight increase options: max ramp/max takeoff of 19,850 lb and 19,600 lb, respectively; max landing weight upped to 16,000 lb; and zero fuel weight and tip-tank fuel weight of 14,400 lb. These options are priced at $10,000 each. Before any Avcon mod can be installed, the aircraft must first have several Learjet Service Bulletins incorporated. Franke said Avcon intends to offer fuel loading and weight increases for the Learjet 25 series, as well as the fin mod.
To prevent a replay of what happened on June 12, Franke said the FAA has concurred with a flight-test profile that will not take its recently acquired Learjet 25D out to the “knee,” which would theoretically subject the airplane to the same maximum dynamic pressure most likely experienced in that fateful June 12 flight. “We are talking about a 25-year-old airplane. It’s a lot different from taking a brand-new airplane that’s fresh off the line,” Franke explained. Avcon is still going to have to do Md and Vd data gathering, but it doesn’t have to hit those numbers concurrently. “We can do the Md at say 35,000 ft and then drop down into the upper teens and do the Vd.”
In addition, the test aircraft will be more heavily instrumented and “we will be better able to tell if we start getting an onset of any trouble,” said Franke. “It’s going to cost us more to do, 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.”