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 3 of 6)
Whatever Raytheon's motives may have been, the company's eventual response was draconian. According to an AD issued in August 2001, each T-34 front spar and rear spar attachment would have to be subjected to a magnetic eddy-current inspection for cracks every 80 flight hours. The inspections, which are relatively difficult to perform (and also prone to yield occasional false positive findings), would cost thousands of dollars, even after initial modifications had been performed to make the suspect areas more accessible. For a heavily used airplane, the 80-hour interval could mean two or three inspections a year. Nobody would want an airplane saddled with such demanding inspection requirements; in five years, says Lima Lima's Bill Cherwin, the requirement would "turn the whole fleet into beer cans."
Spar Solutions (Harry Whitver)
Even before the FAA published the Raytheon inspection procedure, however, T-34 operators and their support organizations and businesses had begun to think about what the FAA calls alternative means of compliance. The FAA allows AMOCs as an avenue for independent solutions to engineering problems. Raytheon had not come up with a solution that T-34 owners could afford. If there was to be an affordable solution, owners would have to come up with it themselves.
In principle, two paths lay open. One would completely eliminate the suspect spar components, and therefore inspections for fatigue cracking. The other would perform one eddy-current inspection of the existing spar to ensure that it was free of cracks, and then strengthen it so as to preclude future fatigue failures.
The simplest repair, the Saunders Strap, had been around for decades-the T-34 was not the first Beech airplane to have spar problems. The first was the pre-World War II Beech 18. When Model 18 spars got into trouble, Dave Saunders, a freelance engineer, stretched a stainless steel strap under the belly of the airplane from one outer wing panel to the other. The strap took over a share of the load being carried by the lower elements in the spar, which were the only ones subject to significant fatigue.
Other Beech aircraft-Queen Airs, King Airs, and Beech 99 Airliners-experienced fatigue cracking, and Saunders adapted his straps to all of them. When it became apparent that T-34s were going to need spar work, Saunders lost no time designing a strap for them; it is also suitable for Barons and Bonanzas.
Saunders Strap (Richard Fleck)
The Saunders Strap costs only $12,000 installed, and the down time, assuming that the required eddy-current inspection doesn't turn up cracks in the spar, is only two to three weeks. While no one questions the structural effectiveness of the strap-no airplane equipped with one has ever suffered a wing failure-some T-34 owners object to the ridge it produces along the underside of the wing; purists don't like the idea of tacking a conspicuous structural Band-Aid to the outside. And, though Saunders denies it, some say that the strap saps performance.
Earle Parks' Amarillo, Texas shop is equipped to rebuild T-34s from any condition. In addition to a huge inventory of spare parts, Parks has enough tooling to build an entire airframe from scratch-if it were legal to do so.
Parks Jig (Peter Garrison)
Parks had his own ideas about the T-34 spar. He had seen enough disassembled T-34 wings to know that there was some random variation in the size and shape of the many small parts, some of them shims and spacers to bring larger members into alignment with one another. Beech had eliminated the buildup of small parts when it designed a new spar that it has installed in new Bonanzas and Barons since 1973, and Parks decided that he could do the same. He replaced the inboard section of the shear web-the thin vertical element of the I-shaped main spar-with a sheet of heavier stock, eliminating the joggled lap joint that coincided with the location of the fatigue failure in the Sky Warriors airplane. He replaced the complicated buildup of small parts in the lower spar cap-only the lower cap is subject to significant fatigue-with a single long part machined from a solid piece of aluminum.