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

Air Safety Week,  Feb 24, 2003  

• E-mail
• Print
• Link

These random intermittent glitches occur mostly in older aircraft as a result of the normal process of electronics aging, brought about by years of exposure to vibration, oxidation, heat cycling, spark-erosion, etc., which cause the micro-surface of the various connectivity elements to degrade gradually over time. The broadly held opinion that wiring and other electronics either works or it does not, and when it does not that the result is a testable hard failure, is false. Connectivity components become intermittent, generally at a reduced level, long before they actually fail hard.

The decades old NFF problem, where approximately 50 percent of all pilot reported system failures are never duplicated or repaired on the ground, indicates the scope of the digital testing void. No test method can state that a unit is reliable and safe to fly without first testing for random intermittency with equipment that can actually detect it at the low levels necessary. Single channel, scanning, digital sampling equipment, the kind almost universally in use today, simply cannot.

Most automatic test equipment (ATE) used to test avionics, including wiring testers, will generally not report the defect to the operators unless failure occurs over several repetitive tests, essentially by becoming a semipermanent hard failure. If at any time during these repetitive tests any measurement should test "good," the original failure data is discarded and the testing proceeds as if nothing had happened. The conceptual blind spot is programmed into the testing software in an effort to reduce what test engineers fear might be "false failures," which can result in additional testing and repair costs.

Accident investigators as well as test technicians and engineers should never lose sight of the fact that an intermittent connection one moment, possibly seen as a small, one-time, inconsequential system glitch may, under the right stress the next moment, become a full-blown failure that could develop into an accident. This being the case, no level of intermittency should be tolerated.

With respect to the fatal Nov. 12, 2001, crash of American Airlines [AMR] Flight 587, involving an A300-600 twinjet, 12 incidents over the previous year in the airplane's rudder control system were reported, with four of those being NFF, and another NFF breaker reset just prior to takeoff. With the exclusive use of digital based testing, any intermittent defects on the accident airplane, as would be expected in an airplane with extensive use over 13 years of service, as in this case, would have gone undetected.

Back to the future

Analog technology does have the advantage of not using sampling, but it has always had its own limitations, including susceptibility to noise - requiring filtering and signal conditioning likely to cause high-end data loss as well. In addition, frequency response limitations would make the sensing and recording of nanosecond pulses quite challenging - and perhaps unreliable in affordable analog equipment.