By John Crawley

Reuters
Tuesday, April 18, 2006; 6:38 PM

WASHINGTON (Reuters) - U.S. aviation regulators proposed for the first time on Tuesday operating limits for commercial aircraft to help avoid the most serious age-related metal fatigue cracks and other damage.

The change would exceed long-standing regulations on aging aircraft, mainly concerning maintenance, and apply to thousands of airliners already in service and those on the drawing board, a draft Federal Aviation Administration rule showed.

It could take months -- or even years -- for the agency to adopt a final rule. Manufacturers would work with the FAA to establish operating limits based on thousands of takeoffs and landings.

The FAA estimates the cost to industry at $360 million over 20 years. Plane makers like Boeing Co. and Europe's Airbus would incur about 10 percent of this, while airlines and other operators would pay the rest.

But regulators say airlines would save hundreds of millions of dollars on maintenance and other expenses.

The proposal covers planes like the workhorse McDonnell Douglas MD-80 series, first popular in the early 1980s and still flown domestically, and the newest Boeing 777, a wide-body that flies premium international service. The rule would also apply to next-generation aircraft like Boeing's 787 and the superjumbo Airbus A380.

It took several years to conclude an operating limit was necessary. The proposal comes as the average age of many planes in the U.S. fleet is on the way down.

Nevertheless, the FAA concluded that "all airplanes in the fleet are susceptible" to the most serious kind of fatigue.

Currently, manufacturers must determine an expected service life for an airliner, and for new designs, they must show that serious fatigue damage will not occur. But there is no rule that restricts or prohibits operation once a plane exceeds its estimated service life and fatigue becomes a greater concern.

Boeing says its planes are built to be commercially viable for 25 years but airlines can fly them longer if they satisfy airworthiness regulations.

Commercial planes are generally made of aluminum and include fiberglass and some carbon-based composites. Most big planes, except very new ones, have some minor fatigue cracking that is caused by expansion and contraction of the fuselage during changes in cabin pressure and repaired during maintenance.

"As long as it's monitored carefully that's perfectly safe," said Charles Eastlake, an aerospace engineering expert at Embry-Riddle Aeronautical University.

Serious problems arise when tiny cracks -- often too small to be detected during an inspection -- begin to multiply. "These cracks could grow together very rapidly so that failure could occur before another inspection is performed to detect them," the FAA said.

Cracks can occur in an aircraft's skin or on structural parts like frames and spread to different areas.

Fatigue issues received closer attention in 1988 when a section of upper fuselage on an Aloha Airlines 737 peeled away during flight. One flight attendant was sucked out of the aircraft while in flight, before the plane landed safely.

As a result the FAA stepped up fatigue inspections, corrosion prevention, and repairs on older planes to reduce the number of repetitive checks. However, none of the programs address the most serious type of fatigue damage.

from this link

What they're addressing is a deeper inspection for an aged airplane than what's currently done in a major overhaul. CI-611 you may recall didn't even have the crud cleaned from its bilges during its major - so they had Buckley's chance of ever finding multiple site damage/WFD. Superficiality was the name of their game, both in the original temporary repair (that became permanent) and the ongoing inspections. Not a lot to do with other than human error/slackness. The fault was there, they only had to go look - but didn't.

When you look at the venerable DC3/C47 Dakota/DC4 they simply don't have any significant aging issues. They were built very robustly and so they've lasted forever. Their fatigue and corrosion footprints hardly show on the radar screen. Once it became necessary to pressurize, pare weight, "optimize" designs, save fuel and be more economical than the next guy, fatigue and "aging" entered the equation. So it's more or less self-inflicted. It's now fairly up to the TC holders to create a safety net - and this is precisely what is being proposed. Such a formalized program would replace the piecemeal, slapdash and patchwork quilt "stumble-over/discover then fix" approach that's been used ever since the start of the jet era (i.e. the SB/AD Approach). As they say in the NPRM: "ADs are reactive and address only known instances of WFD".  FAA also acknowledges in the NPRM that Advisory Circulars are ineffectual.

Similarly: "Voluntary safety assessments, such as those relating to the thrust reverser and cargo door reviews, have been difficult to complete in a timely manner because they lacked enforceability."

And of course wiring would have to be part of it - but there's no mention of wiring as an aging structures factor in the whole document now (as it stands).

They are using the big stick approach i.e. aircraft would be grounded at their "design service goal" (or initial operational limit) under the a/c's ICA, unless the TC holder was to formulate and implement integrity assurance measures acceptable to the regulator. I can't see anything wrong with that approach to making such a program compulsory. You can't expect voluntary participation. But any deeper inspection would have to be very formulaic and keep Xray and photographic and dye penetrant and acoustic records. The FAA would also have to eliminate "grandfathering" of (for instance) dangerous wiring (such as PVC) for a life-extended airplane - under any such rule.

What are they planning for the unknown fatigue profiles of the new generation composite machines? It will be very hard to write an applicable regulation that will adequately cover these new designs. The Aloha failure had more to do with the type of construction (bonding) than it did metal fatigue due to cracking (i.e. WFD) or corrosion. The CI-611 747 breakup was more to do with a poorly fabricated temporary repair (as did JAL123's loss of its tail). Catastrophic failures in the absence of compounding human factors and human error failures are (in fact) quite rare. "[Since 1978, 14 CFR 25.571 has required applicants for new type certificates for transport category airplanes to establish inspections to detect fatigue cracks before they can grow to the point of catastrophic failure]" There would have to be a section for compulsory ongoing reinspection of individual aircraft's major structural repairs (after a tailstrike for instance). There is only a partial addressing of the effect of repairs in this NPRM.

From below: "Most big planes, except very new ones, have some minor fatigue cracking that is caused by expansion and contraction of the fuselage during changes in cabin pressure and it's repaired during maintenance." "As long as it's monitored carefully, that's perfectly safe," said Charles Eastlake, an aerospace engineering expert at Embry-Riddle Aeronautical University. I tend to agree, but he's not addressing the overall compliance problem.

"proposal would require type certificate (TC) holders to
establish an initial operational limit on certain airplanes. Operation
of these airplanes beyond the initial operational limit would be
prohibited, unless operators have incorporated an extended operational
limit into their maintenance programs. Type certificate holders would
be required to develop the initial

[[Page 19929]]

operational limits based on an evaluation of WFD susceptibility, both
for existing airplanes and for proposed future certifications."

Faced with the alternative (and real possibility) of whole fleets of a certain type being grounded, I can see that compliance wouldn't be a big problem for manufacturers. The Design Service Goal and Extended Service Goal seem to me like realistic concepts.

I wonder how this sits with what the military is planning to do about their aging jet problem?