by Paul Bertorelli
As a second choice after what do you do if the engine quits, the dumbest question we hear from non-pilots making small talk at parties is: Dont you worry that those old airplanes you fly will come apart in the air? Like every other owner or pilot, we sigh heavily and spout the party line. If properly maintained, old airplanes should last forever. And like every other pilot, we say that because weve been told that. Yet have barely a shred of compelling evidence-other than a fleet of ancient airplanes that havent fallen apart yet-that its true.
But we now have reason to believe that in addition to proper maintenance, theres another requirement for airframe longevity: a kinder, gentler flight envelope or at least one that doesnt take the structure to its ragged limits on every other flight. Owners of aerobatic aircraft that are flown hard expect stuff to crack on their airplanes-spar structures, control surfaces, longerons, attach fittings and so forth. These often fail in characteristic ways so the owner who wishes to survive will know where to look and what to look for in preemptive maintenance work.
But there are occasionally surprises and last December, owners of T-34 Mentors-and by extension, perhaps Bonanza owners-got a nasty one. Mentors had returned to flight status after a brief grounding and a protracted, expensive effort to devise a fix for known cracks in T-34 wing spars following two fatal crashes. Yet on December 7, a third T-34 crashed after a wing departed in flight. Ominously, the wing broke in an area beyond the bounds of the original spar fix. The fleet has been grounded since.
The potential diamond in this ugly rough is that the T-34 crashes have inspired a research project that has yielded a potentially seminal model for an owner association to work wisely in its own interest and enlist the aid of state-of-the-art technology as a means of avoiding expensive, overkill ADs that could make entire classes of aircraft uneconomic to maintain.
Recall that the T-34s recent troubles first surfaced in 1999 when a Mentor flown by Sky Warriors, a mock air combat business, lost a wing in flight, killing both occupants. It didnt take long to find out why the wing departed: Cracks were found in the main wing spar a foot outboard of whats known on Beech airplanes as a bathtub fitting. Its a receptacle on the underside of the wing that provides access to the wing attach bolts. The FAA responded by issuing an AD limiting T-34s to 2.5 Gs and 175 MPH pending further investigation .
The issue languished for two years while Raytheon reviewed the T-34s structure with the goal of recommending a fix. Raytheons solution was harsh and economically untenable: a 2001 AD required extensive eddy current inspection of the front and rear spar at 80-hour intervals.
Even ahead of Raytheons drastic recommendation, the T-34 Association and several associated companies had been exploring ways to beef up the Mentors spar and these morphed into AMOCs, or alternate means of compliance, to sidestep the onerous inspection requirement.
Four AMOCs were eventually developed and although each fix came at the problem differently, all accomplished the same thing by redistributing loads around the known trouble spot in the spar. Owners grumbled and wrung their hands, but those who wished to keep legally flying their beloved T-34s paid their money and took their choices of AMOCs. Case closed.
Except it wasnt. In November 2003, another air combat T-34, this one flown by Texas Air Aces, lost a wing in an accident that was nearly a carbon copy of the first. Again, both occupants were killed. That airplane hadnt been modified with any of the AMOCs and the NTSBs preliminary report suggests that the airplane had been flown well beyond the 2.5G/175 MPH AD-specified limits. In its post-accident probe, the FAA cited cracks in points of the wing outside the focus of the original AD and it infuriated T-34 owners by canceling the AMOCs and grounding the fleet. But the AMOC holders moved swiftly to satisfy the FAAs concerns and the grounding was short-lived. But worse was ahead.
On December 7, 2004, T-34 owners heard stunning news: a Mentor on a mock air combat flight, again, with Texas Air Aces, shed its left wing and crashed, killing both occupants. Mentor owners held their collective breath.Had the AMOC not worked? Had something been missed? Would the airplanes again be grounded?
The answer was quick and unpleasant: The wing had broken at a different point, in the center spar section well inboard of the bathtub fitting and an area outside the primary focus of the previous investigation. The fleet was again hit with a grounding AD while the T-34 community-spearheaded by a savvy, politically connected organization called T-34 Association, Inc.-sought another solution to the T-34s woes. Significantly, all three T-34s which shed wings had been used in mock air combat operations.
Lead by the associations vice-president, Dan Thomas, and member Jud Noggle, an expert in Mentors, T-34 Association, Inc. undertook an aggressive, well-managed approach to returning the airplane to flight status. George Braly and Tim Roehl, principles in General Aviation Modifications, Inc. a company that has become a leading center for light aircraft engine R&D, agreed to provide engineering resources. (Braly and Roehl own a T-34 and developed an AMOC of their own, an elegant, minimal gusset to strengthen the affected area of the spar.)
After the first T-34 crash, the NTSB argued-and the T-34 Association agreed-that there were really two fleets of T-34s, the ones exposed to the brutal rigors of mock air combat and the rest of the fleet, which can best be described as Saturday-morning aerobats. A third segment-the six-member Lima Lima demonstration team and Julie Clarks aerobatic business-could be said to reside between the two, in terms of G-loading profiles.
Braly and Roehl had the ammunition to prove it, too. During the development of their own AMOC, they inspected 5400 bolt holes in 100 T-34 wings at points known to be sensitive to fatigue cracking. Only one crack was found-in an airplane that had been used in mock air combat. If the FAA had rejected the dual-fleet notion during the first grounding, it was warming to the idea as a way out of the second.
But more convincing was needed. In an example of the sort of Cinderella projects owner associations seldom undertake, the T-34 Association, aided by GAMI, set off on a project that may ultimately predict the safe lifespan of the T-34 fleet. But the broader issue is what light this work will shed on a far larger fleet: the Bonanza and Baron series, which have essentially the same wing structure as the T-34. Moreover, if the FAA buys off on this method of predicting lifespan as a means of avoiding overkill ADs, it could be applied more widely as other aging airframe structural issues come to light.
The association and GAMI developed a series of flight load profiles based on how the airplanes were flown, with estimated G-loads per hour of flight estimated through interviews with pilots, from military specs current at the time the T-34 was designed and, critically, from videotapes of mock air combat sorties. A virtual model of the center section spar carry through where the cracks had been found was devised and given to RaeTech for finite element analysis, a computerized method to calculate how structures carry loads. This data was then fed into a crack propagation simulation program called AFGROW, in effect, exposing the spar to each flight profile to the point of detectable failures.
Finite element analysis has been used for at least three decades as a means of predicting the fatigue lifespan of everything from bridges to buildings while computational-based failure analysis has filtered down to the desktop computer level. (Its still not cheap, however.)
Braly told us certain assumptions were made with regard to frequency and intensity of the T-34s spar loading and although these are best-available-data estimates that may not be accurate in the absolute, its the ratio of one loading scenario compared to another that tells the story.
And the fatigue propagation and failure ratios on the T-34 spar were startling beyond any expectation. Any A&P who has fixed an aerobatic airplane knows that G forces break things, but its less intuitive that an airplane flown repeatedly to near its structural limits may simply wear out beyond economic repair in a fraction of the time an airplane flown less aggressively might endure. GAMIs research suggests that with detailed flight profile data and the right computational scheme, its possible and practical to predict when fatigue-sensitive structures will fail.
GAMIs modeling explored five flight profiles, the most realistic of which was the Lima Lima flight teams typical show or practice. At the opposite end of the spectrum was an aggressive mock air combat mission, which assumed 18 minutes per hour of load cycles between at least 6 positive Gs and 2 negative Gs. According to review of air combat tapes, the real numbers were almost certainly higher than that and the modeling assumed only straight pulls, not the rolling G loading that the airplanes were exposed to. Rolling pulls load spars asymmetrically, concentrating loads rather than allowing the entire spar structure to carry the stress. The AFGROW program predicted crack propagation and how many hours would elapse before half the spars would fail.
Not surprisingly, the low-G profile suggested that barring corrosion or other insults, the spar would essentially last forever, with some 880,000 hours required to reach a 50 percent failure rate. The shocker was that when G loading was frequently pushed to nearer the airplanes structural limit, spar life declined to a tiny fraction of that for an airplane flown more gently, specifically a mere 6000 hours, a total eerily close to the total time on the crashed T-34s. Although 880,000 sounds like (and is) a lot of hours, its the ratio between the highs and the lows thats most telling.Put another way, in terms of wear and tear on the spar, one hour of high-G aerobatics-between 5G and 6Gs-is equivalent to seven hours of less vigorous flying and perhaps more.
Moreover, the model revealed a substantially lower incidence of failure in spars exposed to 4Gs to 5Gs than those which endured the positive 6Gs most of us assume to be the aerobatic limit. The study suggests that avoiding excursions from 5Gs positive to 1 or 2Gs negative reduces wear and tear on structures by a factor of two to one or more.
Failures are non-linear with G loading, Braly told us. Lop off the high-G end of the spectrum and you double the spar life. Limit it to under 5Gs and you might quadruple it.
The lesson buried in the T-34 Associations numbers shouldnt be lost on anyone: If you fly an aerobatic airplane rated for 6Gs positive and 3Gs negative, avoiding those max G limits will substantially increase airframe life. Indeed, at least one POH for an aerobatic airplane-a Citabria-recommends limits of plus 5 and negative 2 Gs, even though the airplane is certified to the aerobatic standard of plus 6 and negative 3Gs.
Casting the harsh light of economics on the T-34s fate, the associations research represents a lavish amount of time and money focused on a tiny problem. Only a small number of GA owners would be impacted if the airplane never flew again, an unlikely eventuality anyway.
As we go to press, the FAA had rescinded the grounding, granting T-34s one-time permission to fly for 60 hours, pending further developments. Braly was preparing to fly his own T-34 to collect load data to validate the computer modeling. When that data is refined further, it will be used to carry out real-world fatigue testing on actual metal parts simulating T-34 spar material. The intent is that the accumulated data will convince the FAA to approve a less intrusive long-term eddy current inspection solution for the spar, but the implications of the T-34 project are more far reaching.
The larger lesson is how what GAMI has learned may eventually apply to Bonanzas and Barons, of which there are thousands flying. While its true that not many Bonanzas are routinely wrung out to 6Gs, its also true that many have experienced unintended overloads in turbulence or been beaten up in many hours of routine operation. Braly points out that 5.5Gs in a T-34 is the loading equivalent of 4.4Gs in a 36 Bonanza, which has an identical spar.Could cracks be hiding in those spars as a result?
No one has looked, Braly told us. And no one is raising suspicions, either. But if and when cracks are found, the T-34 study could well form the basis of an economical fix that doesnt involve half the airframes value in metal work. The Mentor project may further establish two other firsts that could spare owners of older airplanes onerous costs: It could convince the FAA that its possible to sort fleets into at-risk and no-risk categories, thus avoiding expensive, shotgun fixes for airplanes that dont need them.
Last, the T-34 project will almost certainly pioneer a means of confidently estimating the remaining life of older, orphaned aircraft. Other owners would do well to use the T-34 Associations competent management of this crisis as a model when their own airplanes come under the gun. In that sense, the Mentor is still teaching important lessons.
Also With This Article
“How Much Abuse Can a Spar Take?”
“T-34 Flight Status”