Regulatory Report

The Big Stick Approach

The type certificate (TC) holder, which is to say the manufacturer in this case, must ensure that transport category aircraft (weighing more than 75,000 pounds) are free of widespread fatigue damage (WFD), or steps have been taken to assure this structural integrity, otherwise the aircraft must be retired.

By this sweeping 52-page Notice of Proposed Rulemaking (NPRM), the TC holder is directly responsible for locating possible confluences of multiple site damage (MSD) and other unspeakable horrors under the heading of WFD, and recommending inspection and maintenance protocols to prevent them from occurring. In effect, the TC holder must create a safety net, in the form of a program to replace (or supplant) the piecemeal, slapdash and patchwork quilt "discover then fix" approach that has heretofore prevailed in the form of service bulletins (SBs) and airworthiness directives (ADs). As the FAA's announcement says, "ADs are reactive and address only known instances of WFD." The FAA also acknowledges in the NPRM that advisory circulars (ACs) are ineffectual. Similarly, the NPRM says, "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." The FAA now proposes to enforce attention to WFD.

Details are in the FAA's proposed rule, for which comments are due on or before July 17 (see Federal Register, April 18, FR Doc 06-3621). As the announcement says, "Operation of these airplanes beyond their initial operational limit would be prohibited, unless operators have incorporated an extended operational limit into their maintenance programs." The notice pointedly remarks that "all airplanes in the fleet are susceptible to multiple site damage or multiple element damage." And, by the way, the announcement adds, "It would also require actions to prevent WFD in repairs, alterations, and modifications to these airplanes."

The 75,000 pound cutoff excludes regional jets, which are newer and therefore not as susceptible to WFD.

According to the FAA, this initiative will cost about $360 million, but it will prevent aircraft maintenance/downtime and accidents that could be worth $809 million.

TC holders must produce compliance plans by Dec. 18, 2009.

Is this effort going to be manageable? Perhaps, and only because there is a big-stick clause that basically says many valuable airframes, of a specific type and model, will eventually become valueless, good only as scrap metal, unless a program is put in place to assure freedom from WFD. Therefore, for new aircraft the manufacturer will be thinking hard about:

a. Making new designs more robust (i.e., beefier) to avoid this problem.

b. Avoiding materials, such as light-gauge aluminum, that have a propensity to ultimately crack and corrode.

c. According greater accessibility for inspection.

d. Using built-in strain gauges and such for capturing evidence of overstress, fatigue and corrosion.

e. Utilizing primers, sealants and gap-fillers to protect the underlying structure from the environment.

f. Incorporating modular design so that whole structural sections can be replaced without great hassle.

Whereas in the past the competition between manufacturers has been all about load/range/fuel efficiency and economy, these factors will now have to be weighed against longevity.

For existing airframes, the headache will be in deciding at what stage a particular type is no longer economically supportable. While there are many factors in that decision, an easy example has been the DC-10 and MD-11. They lost favor as passenger planes and now, except for a very few, are either laid up or hauling freight. Other examples include early models of the B747 and B737, because of center wing tank flammability issues or other structural problems.

The manufacturer's first task will be to decide at what point existing airframes should become unsupportable (which will have to be specified in the instructions for continued airworthiness, or ICA); the manufacturer's longer term interests will be vested in a fundamental change in design philosophy - one that addresses longevity, modularity and accessible serviceability based on inspectability.

Those commenting on this proposal may wish to touch on the need to integrate all aging concerns into one program. Note that four previous aging aircraft programs have been mandated by the FAA: 1) supplemental structural inspection program, 2) corrosion prevention program, 3) repair assessment program, and 4) the mandatory modification program. To this, perhaps a fifth should be included: the likely FAA requirement to inspect aging electrical wiring. There's no mention of wiring as an aging factor in the document as it now stands.

One wonders about the proliferation of programs, and whether there is too much here for the manufacturers and operators to keep tied together. One program, including WFD, beyond a certain point, may be more manageable from a maintenance standpoint.

Tighten the Standards

"There were changes to the maintenance schedules without design considerations," said the National Transportation Safety Board's (NTSB) Dr. Bruce Coury, of the jackscew controlling the horizontal stabilizer on the Alaska Airlines MD-83 that crashed into the waters off Los Angeles in 2002. Recall that the remains of the jackscrew were pulled out of the water, and there was no evidence that the jackscrew had been lubricated.

At a recent NTSB hearing on certification standards, and how they need to be tightened in light of numerous accidents where inadequate standards were brought to light, it was pointed out that design assumptions are not considered in maintenance decisions. In the case of the MD-83, which is based on a 1965 DC-9 type design, the jackscrew's proper functioning was utterly dependent on adequate periodic maintenance, yet the intervals between servicing were extended with no justification.

Moreover, the jackscrew was treated for certification purposes as part of the airplane's structure, not systems. Such logical disconnects and neglected maintenance stem from two of numerous factors, according to the NTSB:

1. "The lack of a requirement to prepare a list of safety-critical systems during type certification (which) compromises the ongoing assessment of these systems."

2. "Consideration of structural failure conditions and human/system interaction failure are not required in the certification-related assessments of safety-related systems, and these exclusions limit the scope of the failure conditions considered."

Therefore, the NTSB wants safety-critical systems like the jackscrew defined, and failure from inadequate maintance considered in the safety assessment process.