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

Air Safety Week, Feb 24, 2003

Commentary By Brent Sorensen, President, Universal Synaptics Corp.

"Digital averaging" is a critical yet innocuous sounding problem. It has had a deleterious effect on aircraft safety and reliability for decades under the guise of no fault found (NFF) where fully half of all pilot-reported system failures are never duplicated on the ground or repaired. It is probable that several mysterious or suspected wiring and flight control related incidents - and possibly crashes - have at least in part been due to this major testing void that hides or masks problems rather than reporting them.

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Digital averaging, achieved through the inherent technique of digital processing, more directly by engineering design to remove unwanted system noise, consequently removes information concerning age-related failure mechanisms such as glitches or intermittency, which also looks just like noise to an electronic system. Therefore, instead of delivering prognostic information or critical data reflecting the high probability of an impending failure, digital averaging (in comparison to a raw analog signal).delivers a message that "all is well," when the opposite may be true.

The details

Some 30 to 40 years ago, practically all avionics and avionics measurement equipment used to test and diagnose aircraft systems relied on analog technology to read the various input sensors and control signals and to compute the various output functions. Analog systems delivered pretty fair results but had a high susceptibility to stray electronic noise and glitches. Since then digital technology - which works fairly well in "noisy" environments via on-off operation, periodic sampling and data averaging - has taken over progressively and has established itself in nearly every avionics and measurement function.

Part of the success of this technology is due to miniaturization and its inherent noise rejection.

The change of technology has been dramatic and impressive, except where test measurements must be taken of unexpected, randomly occurring, rapidly changing, real-world phenomena that just happen to look like noise. Such noise as would be expected is created by aging or dry solder joints, oxidized, corroded and loose connector pins and wiring crimps, poor grounding or insulation breakdown allowing arc-tracking conductors to touch other conductors or aircraft structural components.

Digital measurements may be highly accurate if the source measured is stable and/or repetitive. "Aliasing" - the problem of missing data because it is linked in frequency to the sample rate has been addressed in most newer digital test equipment. However, if the source is intermittent, noisy or unstable, it is an entirely different matter. To smooth these noisy and randomly changing signals, digital averaging is employed. The end result is that one might see a portion of the unexpected intermittent defect, or one might not see any of it depending on the frequency and duty cycle of these glitches. A little reflection on how digital measurements are taken, as well as missing data on digital flight data recorders (DFDRs) that use "averaging," confirms this (see ASW, Aug. 5, 2002). The problem here is that "unwanted" signals that get averaged out by normal digital processing may be the critical indicators of an existing system flaw - or imminent failure. Digital processing does not have the capability to discriminate electrical (or electronic) nuance.

Some in the industry have suggested that faster sampling rates can catch these minor glitches. This may be a false hope. In long runs of aircraft wire harnesses, the inherent capacitance between the suspect wires, surrounding circuits and metal aircraft surfaces will slow the meter's stimulus charge time and will limit sample rates to about 200 per second, and oftentimes much slower. Faster sampling is not a good solution when microsecond or even nanosecond disconnects are the targets of interest. They are occurring randomly and one quickly arrives at a point where the digital measurements will exceed processing rates and soon one is right back to where one started, with data being discarded or averaged out for lack of enough storage capacity. It is either that or the testing process must temporarily shut down while processing of the data catches up. Of course, during that time failures could occur and go undetected.

While the NTSB is understandably concerned about missing data as a result of "averaged" inputs to the DFDR, it also should be concerned about the source of these random glitches that likely would have been missed during any preflight, ramp, functional or depot testing as well. A simple analogy may help to illustrate the problem. Before a volcano explodes, tremors can be measured which help scientists predict when the main eruption likely will occur. The same principle applies to avionics suffering from age related inter-connectivity defects. They first become aberrant at low levels of activity and generally progress over time to higher levels of activity. However, at present digital instruments average these latent "tremors" right out of existence. The result is that important prognostic and diagnostic data is lost in the process and faultfinding then becomes the more costly exercise in post-failure diagnostics. At testing time, when one would like to know if a connector, crimp, splice, wire, solder joint, circuit breaker or other connectivity component is working properly, the unwanted spurious noise that under the right conditions could precipitate a full-blown system failure is simply filtered out by the technology and sometimes even the test programs. Especially in the early developing stages, digital measurement equipment simply cannot see all of these age-related failure modes. Technicians and test engineers, relying on the higher accuracy of digital instruments, walk away from the problem with a false belief in the safety and reliability of the systems they are testing.

Government Industry

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

Air Safety Week, Feb 24, 2003

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