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Monday, February 1, 2010

Perspectives: Finding ‘No Fault’

Ken Anderson

As the United States fleet of military aircraft age, the maintenance of aircraft electronic Line Replaceable Units (LRU) and Weapons Replaceable Assemblies (WRA) poses an ever increasing challenge. Much of the challenge is related to intermittent faults that occur because of aging wiring and connections. These intermittent problems manifest themselves as “Bad Actor” LRUs/WRAs or as repair shop No Fault Found (NFF) and Cannot Duplicate (CND) test results.


Intermittent faults are a growing problem in electronic equipment. As integrated circuits (IC) and other electronic discrete parts become increasingly capable and reliable, an ever growing portion of the electronic equipment maintenance problems encountered are not component failure, but rather interconnectivity problems between the electronic piece parts.

The intermittent fault can be caused by a loose or corroded wire wrap, a cracked solder joint, a corroded connector contact, a loose crimp connection, a hairline crack in a printed circuit trace, or via a number of other phenomena, all very common in electronic equipment. Many times the intermittent open circuit will manifest itself during an initial troubleshooting test, such as in the electronic back shop of a military flying wing. But by the time the equipment item has been transported to a military depot for repair, the intermittent circuit is no longer open. This results in NFF. Some LRUs/WRAs now test NFF over 50 percent of the time when inducted into the repair shop.

Many of the “failed” LRUs/WRAs that end up testing NFF have no parts replaced and no other repair work performed at the depot. These boxes are turned right around and returned to the field as fully operable boxes, because the depot is unable to detect a problem. According to an inside Pentagon source, NFF and CND represent a multi-billion-dollar a year problem for our military.

Not only do these expended funds accomplish nothing useful, but the LRUs/WRAs experience the needless “wear and tear” of being shipped from the field to the depot and back again. A substantial share of these inconsistent test results and this NFF activity is attributed to intermittent circuits.

A common rule-of-thumb for older aircraft is that 40 percent to 60 percent of all pilot-reported aircraft system malfunctions that occur in the air go undetected during follow-on ground testing. These equipment items, with documented in-flight failures, operate properly, or register NFF, during post-flight testing, and thus the problems remain as undetected, unrepaired latent defects. The unit is placed back into service, hazarding the aircraft and crew, repeatedly wasting maintenance and operational resources and adversely affecting operational sortie capability. NFF resulting from age-induced intermittency is now one of the biggest aircraft maintenance cost drivers for the United States government.

The United States Air Force, working with Universal Synaptics, just completed the F-16 APG-68 Radar System Modular Low Power Radio Frequency (MLPRF) Small Business Innovative Research Grant (SBIR) joint testing project. The F-16 MLPRF chassis is the LRU with the highest NFF cost of all F-16 avionic boxes. More than $13 million in maintenance costs are incurred annually by the Air Force for just that portion of the F-16 electronic boxes that test NFF at the depot.

The $2.2 million SBIR investment has already returned over $20 million back to the Air Force. The service is taking the lead in reducing NFF by investing in a unique maintenance tool appropriately named the Intermittent Fault Detection and Isolation System (IFDIS). The system employs a thermal chamber with an integral shake table to lightly induce simultaneous thermal and vibration stress to the chassis while it is being interrogated by the Intermittent Fault Detector (IFD), which monitors all circuit paths concurrently and continuously, detecting any intermittent fault as short as 50 nanoseconds.

The temperature and vibration parameters are adjusted to levels appropriate for detecting intermittent faults in the particular item under test while strict adherence to the tolerances set by the original equipment manufacturer are followed.

With the military escalation in Afghanistan, and intelligent budget expenditures essential isn’t it about time we really examine the cost of NFF and make changes to solve it?

Ken Anderson is director of sales and marketing with Universal Synaptics Corp., of Ogden, Utah.

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