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 2 of 6)
T-34 Wing (Harry Whitver)
Eventually, the civil fleet grew to nearly 500. Owners banded together in a T-34 Association, which organizes fly-ins and formation flights and publishes a quarterly magazine. A six-plane T-34 acrobatic team, Lima Lima, maintains a year-round schedule of performances, as does T-34 airshow performer Julie Clark.
Inflight structural failures are rare events. Usually they occur when a pilot loses control in clouds, emerges in a spin, and, in a desperate effort to recover, overstresses the airplane. Occasionally, an airplane is torn apart by turbulence in a thunderstorm. But the Sky Warriors accident was obviously in a different category.
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.