Digital Averaging - The Smoking Gun Behind 'No Fault Found'

Air Safety Week, Feb 24, 2003

Nonetheless, avionics testing experts agree that the only way to find these random intermittent connectivity failures is to use massively parallel and analog-based equipment, which is sensitive over a smooth and uninterrupted continuum and capable of sensing all critical points all the time, and at a high level of sensitivity. The increased probability of being able to detect these random defects in a multiwire system is some three million times greater than is possible with single-point-at-a-time digital based testing.

If any of these intermittent problems were lurking in the Flight 587 accident aircraft, they likely were not being properly tested for anywhere in the aircraft or in the chain of avionics maintenance. All the relevant flight control system (FCS) and BITE functions (built-in test equipment) are digital.

Related concern

Federal Aviation Administration (FAA) directives attempt to deal with the intermittency problem by requiring "repetitive testing." It would be a great leap forward if the FAA also required a testing process capable of actually seeing these expected failures at a threshold below that which would be required to trigger an actual system failure. Generally, there are many more micro-breaks that will occur in an intermittent situation than major breaks sufficient to cause a system to fail. By vaguely calling for "repetitive testing, the FAA pretty much guarantees that repair is no more of a guarantee of reliability or safety than the original test that obviously let the latent condition slip past to begin with.

Key question

The term expected has appeared numerous times in the foregoing discussion as it relates to aging and intermittency. The NTSB expected certain data to be on the Flight 587 DFDR and it was not. Intermittency is expected when the FAA calls for repetitive testing. Test engineers are sufficiently concerned about expected false failures to program software routines that, on an initial failure, loop back and test the wiring or LRU again and again to eliminate any so-called "false failures." These "false failures" of course could as easily be real intermittent failures occurring in the products they are supposed to be testing.

Why, then, with this insidious and potentially dangerous type of defect so universally expected, does routine in situ testing continue to be done exclusively with digital equipment that cannot possibly be expected to detect these kinds of problems. After all, many government agencies, task forces, advisory committees and whatnot have spent considerable time looking into the aging wiring problem, and a great deal of money over a long period of time has been spent looking for answers to this testing problem.

There is a saying that "you can't dig a new hole by digging the same hole deeper." The phrase is directly applicable, as the "aging wiring" problem has been around for a long time, and not one testing equipment upgrade over the last 35-year period seems to have even made a dent in it. In fact, most "upgrades" striving to take advantage of the advances in accuracy that digital technology affords may actually have made the problem worse by giving false-positive indications of functionality and reliability - masking the effects of aging rather than reporting it.