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

Air Safety Week, Feb 24, 2003

The trusty old analog meter has been replaced by a highly sophisticated 8-digit measurement device of phenomenal accuracy. Like most things in life, good things - including accuracy - come with a penalty. The penalty for aging aircraft systems is that as accuracy has been increased, mostly through digital averaging, these measurement devices have lost their ability to see age related failure modes such as random intermittency or glitches. Inconsistencies due to aging (e.g., "poor contact") have simply been "averaged" out.

In nearly all cases, the more accurately a meter can split a fraction of an ohm or other quantity, the slower it is likely to operate and, therefore, the less likely it will be able to respond to intermittency or glitches in a meaningful way.

The two desired measurement goals, high speed and high accuracy, occupy opposite ends of the measurement bandwidth spectrum. Hitting either end has its own opposing penalty.

Digital instruments using averaging deliver high accuracy readings, but only when the signal being measured is itself steady. If the signal of interest is intermittent, generally due to the ravages of aging, you have no idea what the measurement result will be. The problem with digital measurements is that they are based on a fixed sampling rate, while the intermittency is occurring randomly. The "glitches" generated when the electromechanical connectivity elements break down momentarily simply cannot be guaranteed to occur in synchronization with the sampling pattern/measurement window. The end result is that you might catch the glitch if you are really lucky, you might catch part of it, or more likely you might catch none of it. Accuracy, then, as well as repeatability, in the presence of age-related intermittency, is a myth and the information delivered by the instrument is to some degree a lie.

The following example illustrates how random intermittency or glitches seen - or not seen - by one test instrument can make a huge and important liveor-die difference in the results. In the item being tested, 4.1-volt glitches were introduced randomly in time. The meter's inherent digital averaging has averaged the glitches right out of existence, as far as testing is concerned. The meter has completely missed the series of approximately 70 intermittent faults:

All that needs to be done to make this digital averaging problem go away is to continue to use digital-based equipment for the hard failures and add analog-based equipment for the NFF or intermittent failures.

Source: Sorensen

What Exactly Are We Testing?

There are two types of electronic failures: hard failures and intermittent failures:

Hard failures are detectable every time the unit is used or tested (e.g., blown internal fuse or failed capacitor); everything else may be considered intermittent.

Intermittent: an intermittent is any temporary deviation from nominal operating condition of a circuit or device. This definition encompasses the media-popular "short circuits" as well as the more numerous yet less understood "open circuits." There are three basic types of intermittents: