Save the Mentor!
T-34 owners are the latest to prove the value of good old-fashioned American ingenuity.
- By Peter Garrison
- Air & Space magazine, January 2005
(Page 4 of 6)
Replacing a spar sounds like a huge job. In most wings, skins and ribs are riveted directly to the spar, so removing the spar entails drilling out nearly every rivet in the wing. The Beech wing, however, is an unusual design. It consists of three separate assemblies: the D-shape leading edge, the main spar, and the main torque box, a sheet metal structure between the main and rear spars. The three assemblies are neither riveted nor bolted together; instead, they are joined by stainless steel wires, about the thickness of a wire hanger, that run the full length of the wing through interlocking piano-style hinges. To separate the spar from the rest of the wing, all you do is pull out the wires. In building the first post-World War II high-performance personal airplane, Beech seems to have anticipated that periodic spar inspections might be needed someday.
The FAA required that Parks perform a stress test to the 9 G ultimate load. Parks built a heavy steel fixture, put a wing into it, and pushed on the wing with a hydraulic ram. The steel fixture deformed, but the wing did not. Parks got his Supplemental Type Certificate.
The most technically economical response to the spar situation emerged from GAMI, George Braly and Tim Roehl's company. GAMI first did a computer survey of the T-34 spar, using the now-universal method called finite element analysis, and found a hot spot of concentrated stress at the exact point where the Sky Warriors wing had failed. Then, using electronic strain gauges affixed to wing and spar surfaces, the engineers recorded the structure's reactions to G-loads applied in flight. The results confirmed the computer's diagnosis.
GAMI gusset (Luke Kerr)
FAA Designated Engineering Representative Victor Juarez then designed a small, artfully tapered gusset that bridges the critical area, eliminating the stress concentration. The GAMI team hardened the perimeters of rivet and bolt holes in the affected area, increasing their lives several-fold. The FAA approved the modification without testing a wing, solely on the basis of extensive analytical documentation the company supplied.
GAMI intended to turn over the rights to the AMOC to the T-34 Association, but to Braly and Roehl's surprise, the association's board chose not to involve itself in the airframe repair business or to endorse any particular AMOC. So Braly and Roehl formed the T-34 Spar Corporation, which provides the required inspections and modifications at a number of sites for $14,000.
The costliest repair is offered by Nogle & Black Aviation of Tuscola, Illinois. Charlie Nogle and his son Jud are, like Earle Parks, longtime eminences in the T-34 community. The Nogles scrap the existing spars and replace them with one more massive and better made than the original. The new spar also provides a shear web that runs all the way out to the wingtip (the original T-34 web stops a little outboard of the landing gear). The full-length web looks stronger, but its real function is to provide support for supplemental rubber fuel bladders in the leading edges outboard of the standard tanks, whose 50-gallon capacity may not be enough for the bigger-engine airplanes.
New spars-two are required-cost around $12,000 each, and the additional cost of dismantling the airplane and wing and refurbishing the wings as needed (it would be foolish to take the wings apart and not bring them up to near-new standard) runs another $12,000 to $20,000. Nevertheless, says Jud Nogle, the mod sells itself. "Pilots take one look at the new spar next to an old one, and they want to have it."
T-34 owners grumbled over the cost of modifying their airplanes. Many felt that the FAA had overreacted to the cracks. After all, only one airplane had had the problem, and that airplane, they felt, had been systematically abused. The same might be true of all airplanes used in air combat schools, which also provides "upset training" to pilots wanting to learn to recover from unusual attitudes, such as those produced by an encounter with the wake of a larger airplane. Surely it was unfair to lump them together with the T-34 operators who used their airplanes primarily for weekend outings and formation flights, and who rarely, if ever, indulged in acrobatics. It was rumored that in addition to the fatal Sky Warriors accident, a couple of other T-34s had experienced partial wing failures and had flown home to be repaired, and that these too had been air combat school airplanes.