Measuring how airframe fatigue affects aircraft
There's no disagreement from anyone — AOPA, the FAA, owners groups, and airplane manufacturers — that airframe fatigue must be addressed. But it isn't an easy task. There have been a number of newsworthy fatal accidents related to fatigue. These include three Beech T-34s, a North American SNJ (a two-seat trainer that's a close cousin to the better-known T-6) a twin-engine Cessna 402, and a Cessna 160 (the 160 was built for a short run in 1962 with a 125-horsepower Franklin engine) that appeared on the 6 o'clock news as it shed a wing during a firefighting run in California. These catastrophes — all the result of metal fatigue — have happened only to airplanes that have lived long, hard lives.
The FAA is working for all it is worth — and inviting those who think they have an answer for this dilemma to submit their ideas — to try to determine how to quantify the severity of the fatigue threat to the general aviation fleet.
Each airplane that suffered an in-flight breakup from fatigue had a his-tory of severe use, such as mock aerial combat, military aerobatic training, and flight sightseeing, typically involving heavy loads, numerous takeoffs and landings, and a daily schedule of low-altitude thermal turbulence.
In addition to determining the effect that type of use has on fatigue, innumerable other variables may affect structural fatigue — the alloy and heat treatment of the aluminum used, corrosion, and modifications or fabrication techniques on an airplane's structure, such as tool marks and rivet installation, that may have occurred during repair work.
Owners of similar airplanes point at the airplane wings that have failed and say, "I never haul heavy loads on a consistent basis, or conduct mock aerial combat, and my airplane has never been flown like the accident airplanes, so why is my airplane a part of this airworthiness directive? Because of this AD I'm stuck between a rock and a hard place."
Owners are frustrated, but they're not alone. Everyone involved in trying to unravel the effects of structural fatigue on aging airplanes is in the same boat. The FAA has approved repair schemes submitted by private companies — schemes approved under the alternate method of compliance (AMOC). However, an AMOC-equipped T-34 crashed because of failure of a wing support structure not impacted by the AMOC change. The FAA approved a service-kit upgrade designed by Cessna to address fatigue cracking in the inboard section of twin-engine 402-model wing-spar caps, only to discover significant cracks in other wing locations that had been deemed, after complex computer-aided studies of the wing structure, to have a low risk for cracking.
Metal fatigue in older airplanes is a complex issue not only because of the time, money, and energy that are required to study and create repairs for the affected airplanes, but also because the way this issue is handled may have long-reaching effects for future ADs.
CAR 3 and FAR 23
All general aviation airplane designs approved before 1972 under CAR 3 were not required to undergo fatigue-analysis studies. New rules came along in 1969 when the FAA implemented FAR 23. FAR 23.572 requires metal structures to be evaluated for fatigue. One way this requirement is met is by establishing life limits on airframe structures. For instance, the wing structure of the 2005 AOPA Sweepstakes Rockwell Commander 112A must be removed from service at 6,945 hours. This method is termed "safety by retirement." Most likely, the service life will be extended as we begin to understand the effects of age on these airplanes. The FAA is not looking at safety by retirement as a solution to metal-fatigue problems in CAR 3 GA airplanes.
Predicting the effects of fatigue
Since the Aloha Airlines disaster in 1988, when a section of the upper fuselage of a Boeing 737 blew off because of a fatigue-related failure of fuselage-skin lap joints, manufacturers have been studying metal fatigue. One of the most active companies has been Cessna Aircraft. It conducted a complete FAR 23.572-style fatigue evaluation on two Cessna 402s using computer-modeling techniques. The company found that almost all of the structure passed the fatigue life requirements of 23.572 — except for the wing structures of the older 402, 402A, and 402B models. These models, also known as the "tip-tank versions," did not pass the fail-safe provision in the regulations.
The fail-safe provision is one of the three methods available for dealing with the fatigue life requirement in FAR 23.572. The fail-safe method is the most desirable certification standard because it certifies that the structure will be able to support continued safe flight, even after a fatigue failure of a principal structural element.
The safety-by-retirement method has already been mentioned. The last option is the damage-tolerant method in which the manufacturer creates an inspection program to detect fatigue damage. The idea behind this method is that regularly scheduled visual and nondestructive inspection (NDI) techniques are sufficient to provide an adequate warning between the time when a crack first appears and when it becomes critical with respect to structural integrity. The tip-tank Cessnas flew under damage-tolerant inspection programs for many years. Things changed when it was discovered that the size of a critical crack was much smaller than originally thought, and that standard crack-detection techniques in use were unable to detect a crack before it grew to critical length.
After more than two years of cooperative study by industry experts, manufacturers, and AOPA, the FAA issued two AD notes in August 2004 against Cessna 400-series twins. These mandated that owners comply with a set of service instructions to address fatigue cracking by inspecting the lower spar caps and then installing additional structures called "spar straps" on various Cessna 400-series twins. The compliance times for strap installations are on a sliding time schedule. Tip-tank 401A/B, 402A/B, and 411/A models with airframe times between 6,500 and 10,000 airframe hours have 800 hours to comply while the 402C and 414A airplanes with between 15,000 and 18,000 airframe hours have 1,500 hours to comply.
One of the big sticking points on these ADs — for imaginative pilots — is the fact that these compliance times are based on computer-generated finite-element-analysis predictions by the airplane manufacturer. They're predictions. Since these ADs establish a precedent by predicting when fatigue failures will occur, more than a few owners are wondering if this precedent will open the door for the issuance of more ADs based solely on computer-generated predictions — especially when there's evidence that the predictions aren't 100-percent reliable.
Up jumps the exception
On February 11, 2005, an airline transport pilot flying for a short-hop commuter-style airline asked the company maintenance department to determine why the Cessna 402C he was flying required excessive right aileron to maintain wings-level flight. The maintenance staff found a large crack at wing station 114 — and subsequent visual inspections of a nearby fleet of 402Cs uncovered two more airplanes with similar cracks. The finite-element-analysis studies by Cessna — the studies that were the basis for issuing the AD notes against these airplanes — never predicted cracking in this location.
Yet big cracks in the wings of three airplanes — two with more than 20,000 airframe hours, operated and well maintained by reputable and fiscally sound companies — seemed to bear stark testimony to the idea that there's more to predicting fatigue cracks than anyone realized. It's no wonder that dealing with fatigue issues is creating some gray hair on the heads of some of the brightest engineers in the industry.
Needed: inexpensive fatigue meter
There's no provision for discriminating between a 400-series Cessna that's a cream puff — in other words, a lightly stressed airplane — and a short-haul, heavy-load commuter. According to Tom P. Turner, technical consultant at the American Bonanza Society, "The current feeling is that the in-flight failures of the T-34s are the result of accelerated fatigue action brought on by flying aerial combat-type maneuvers."
T-34s sprang from the same Beechcraft factories as Bonanzas, Debonairs, Travel Airs, and Barons. While the T-34s were originally sold to the military as trainers, today there is a fleet of nearly 500 T-34s in private hands. In the past three years, three T-34s have augered in because of in-flight wing separations. In every instance, the airplanes were engaged in mock aerial combat.
George Braly is an aeronautical engineer, owner of General Aviation Modifications Inc. (GAMI), and owner of a T-34 with partner Tim Roehl. Braly and other owners of T-34s make a convincing argument that no more than six T-34s in the fleet have ever engaged in mock aerial combat.
And three of those have suffered wing structure failures. Experts from the Cessna Pilots Association have also made the argument that the four Cessna 402s that have been found with cracks all have a history of severe service.
Both of these arguments seem to support the idea that owner-flown airplanes are not subject to the stresses of the airplanes that have prompted the ADs. Unfortunately there's no easy way to determine how fatigue has affected a particular airplane. And that's the sticking point for the FAA.
During an August 2004 meeting in Kansas City, Missouri, however, Mike Ciholas of Ciholas Enterprises suggested a potential solution: a "fatigue meter." He recommended that it consist of a vertical accelerometer to measure the actual loads experienced by the airplane in flight, and strain gauges mounted on the lower wing-spar caps to measure the actual stress on the wings. Data from the fatigue meter would be collected and compared to the conditions Cessna specified in its safe-flight calculations to arrive at a "fatigue hour." This fatigue hour would permit the FAA to adjust AD compliance hours based on actual conditions rather than the present method.
Mike Busch, twin Cessna expert at the Cessna Pilots Association, reports that his conversations with the FAA staff at the Small Airplane Directorate uncovered interest in the fatigue-meter concept.
For the time being, however, the task of developing solutions for metal fatigue remains daunting. AOPA members can add their input and support to this important effort by joining and participating in the type club or association ( www.aopa.org/members/clubs/) for their airplane, and by updating the registration address for their airplane to ensure they are added to the mailing list for special airworthiness information bulletins (SAIBs). These bulletins are informal bulletins the FAA mails to owners about maintenance issues regarding their airplanes.
As it stands now, fatigue affects every aluminum airplane in the general aviation fleet — but there is scant evidence that the average lightplane is anywhere near its fatigue life limit.
E-mail the author at [email protected].
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