National Transportation Safety Board and Federal Aviation Administration accident investigators found that the main beam, or spar, of the Sky Warriors airplane's wing had failed about a foot outboard of the fuselage. If the spar had failed from simple overstress, investigators would have concluded that McFann had pulled too hard, and that would have been that. Instead, they found that the fracture surface showed clear signs of metal fatigue. A crack had been developing in the spar for some time before the accident. Detailed examination of the entire wreckage uncovered fatigue cracking in the lower rear attachment fitting of the rear spar as well.
"Metal fatigue goes on continually in aluminum airplanes," says aeronautical engineer George Braly, a partner in General Aviation Modifications, Inc., an Ada, Oklahoma developer of equipment to improve the performance of general aviation engines (see "First Church of Combustion," June/July 2004). As co-owner, with business partner Tim Roehl, of a T-34, Braly has a personal interest in the airplane's fate.
Contrary to what many pilots believe, it's not only intermittent high loading that fatigues metal; it's any flexing due to changes in loading, even the small changes that turbulence causes during routine cruising flight. Given enough time in service, all aluminum wings will eventually fail from fatigue, but airplane structures are designed to support many tens of thousands of hours of flexing.
"The amount of fatigue that occurs," Braly says, "depends both on the [magnitude of] stresses the structure experiences and on the number of times they occur." In other words, thousands of hours of cross-country cruising will fatigue a structure as much as repeated high-G loadings occurring a few times a day. Unfortunately, fatigued material looks the same as new material, until it's far enough gone for cracks to appear.
Evidence of fatigue cracking in even a single airplane raises a red flag with the FAA. If one airplane has cracks, it's likely that others of that type do as well. Within a month of the Sky Warriors accident, the FAA issued an emergency Airworthiness Directive, or AD, temporarily limiting all civil T-34s to 2.5 Gs positive and prohibiting them from exceeding 175 mph. The emergency action was not so drastic as some the FAA had taken, completely grounding Learjets in one case and Cessna 441 Conquests in another, but it was still a burden for airplanes that are widely used for aerobatics.
The FAA enlisted Raytheon Corporation, the parent of Beech Aircraft, to determine how best to ensure the future safety of the T-34 fleet. Raytheon spent almost two years on the problem while T-34 owners dangled in suspense. From the first, some owners darkly suspected that it was probably in Raytheon's interest, from the standpoint of limiting its liability exposure, to wipe out the whole fleet. Others, more charitable, thought that Raytheon's Beech engineers were as eager as anyone to keep the fine old airplanes flying, and that the length of time they spent coming up with a prescription was really intended to give beleaguered T-34 owners, who were faced with the possible reduction to junk-bond status of their $200,000 investments, a little breathing room.
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.