Finding a Cure for No Fault Found

Finding a Cure for No Fault Found

Departments - Ensuring Excellence

Intermittent Fault Detectors address the all too familiar No Fault Found and Cannot Duplicate situations within the avionics repair and maintenance business.

November 11, 2009

Intermittent Fault Detectors address the all too familiar No Fault Found and Cannot Duplicate situations within the avionics repair and maintenance business.

Aging aircraft electronic boxes often pose huge maintenance challenges, and maintenance cost management challenges, across all aviation operations. Subsequent to a malfunction during flight, an avionics assembly often tests "good" or "No Fault Found" when bench tested. The avionics repair and maintenance shop dispositions the box No Fault Found (NFF) or Cannot Duplicate (CND) during the ground test, and sends it back into service, only to have the intermittent malfunction reoccur. This creates the situation where the box is again pulled and tested, hoping that this time the cause of the malfunction will be found. However, quite often it again is chalked up to NFF or CND - adding to growing maintenance costs.

Unfortunately, this scenario is repeated all too often; but the U.S. Air Force is now leading the way in attacking this problem. Because more than $2 million in maintenance costs are incurred annually for just that portion of the Modular Lower Power Radio Frequency (MLPRF) units (one of the many F-16 electronic boxes) that continually test NFF at the depots (the main maintenance and repair organizations in the U.S. Air Force), the MLPRF was their first target.

Working with the U.S. Air Force to address the problem, are two companies, Total Quality Systems and Universal Synaptics, with their Intermittent Fault Detection and Isolation System (IFDIS). The IFDIS is now successfully enabling avionics repair and maintenance personnel to find and isolate intermittent conditions in the MLPRF, proving the effectiveness of this new technique.

But, you ask, what exactly is intermittence?

Testing Issues

Intermittence occurs randomly and often the intermittence is of short duration. You can only detect intermittence when it occurs and sometimes those occurrences are so short - happening in the nanosecond range - that you only have a very small window of opportunity to catch and trace the intermittence. Simply put, it is similar to the check engine light coming on in your car. Once that check engine light comes on, you take your car to a mechanic to get it checked. They hook it up to a computer, run diagnostic tests, check the codes, but often find no trouble with the car. So, they reset the check engine light and send you on your way. That is intermittence.

Something, somewhere in your car - if only for a few nanoseconds - ceased to work properly somewhere in the system, but the mechanic could not duplicate the problem so they call it NFF.

Founded by Brent Sorensen, Universal Synaptics offers the devices necessary to quickly and safely address these intermittent issues. Sorensen spent 29 years working with the US Air Force and was heavily involved with the day to day issues of dealing with NFF and CND problems. Upon his retirement, he founded Universal Synaptics, and today Sorensen and his company are helping address the $1 billion per year NFF issue our military faces.

Total Quality Systems and Universal Synaptics teamed to develop the IFDIS under a Total Quality Systems Small Business Innovation Research (SBIR) project with a goal of developing a system to test for and find the intermittent conditions in U.S. Air Force avionics systems. Universal Synaptics provided the Intermittent Fault Detection devices and Total Quality Systems integrated the Intermittent Fault Detectors (IFD) into a temperature and vibration stimulus test system and developed the required Interface Test Adapters for MLPRF testing.

How it Works

Conventional test equipment is very effective in trouble-shooting hard failures, but is severely limited when applied to testing for intermittent problems. The intermittent failing event rarely synchronizes with the measurement window during test time.

This testing blind spot is believed to be a leading contributor to the extensive NFF and CND problem. The IFD, or in the case of the U.S. Air Force's issue - the IFDIS - was specifically designed to overcome these limitations. The neural-analog IFD tests all lines all the time in a simultaneous and continuous manner. "The result of this type of testing is that intermittent events can readily be detected by the IFD," explains Ken Anderson, director of sales and marketing at Universal Synaptics. "Our sensitive analog technology detects low amplitude, high speed (nanosecond) impedance changes." The neural architecture of the IFD monitors all of the potential failure points at the same time and in parallel. The number of circuits or channels that can be tested simultaneously is virtually unlimited and are installed in 256-channel, module increments.

The IFD's digital processing provides fast, precise data-handling and failure location graphics to facilitate the rapid isolation of the failure source, which translates into a significant savings of time, manpower, and resources. The temperature and vibration stimulus test system simulates the conditions the electronic boxes on an aircraft would be exposed to in actual flight.

Over $10 Million Saved

In February 2009, Total Quality Systems and Universal Synaptics completed the MLPRF joint testing project with the U.S.

Air Force. Over $10 million worth of MLPRF assets have already been recovered due to the $2.2 million SBIR investment.

"The project was started because the MLPRF on the F-16 has one of the highest NFF rates of all of the electronic boxes on the aircraft. There are numerous boxes, referred to as line replaceable units (LRUs), which contain shop replaceable units (SRUs), which make up the aircraft avionics suite," Anderson says.

"Not counting regular repairs and maintenance, but merely the time spent pulling the chassis boxes and testing them only to get an NFF result, the U.S. Air Force was spending more than $2 million per year.

There are over 1,000 MLPRF boxes in the U.S. Air Force inventory."

The Diagnosis

Once the IFDIS hardware had been assembled, Total Quality Systems asked the U.S. Air Force for their worst boxes, the ones that were un-repairable and slated to be condemned, as well as the boxes that had been in and out of the depot multiple times with repeated NFF or CND results.

"All told, 90 boxes have thus far been tested, 71 of which had been slated for condemnation. The value of an MLPRF is $307,000 each," Anderson states. "All 90 MLPRF chassis have been diagnosed by the IFDIS; 48 have already been repaired and put back into service while the remaining 42 are awaiting repair parts." Of the 90 MLPRF chassis that have been IFDIS tested:

  • 71 had been considered un-repairable
  • 28 had been in the shop for over one year - completely out of service
  • 7 had been in the shop for over three years - completely out of service
  • 70 had one or more intermittent circuits
  • 30 had "opens"
  • 8 had "shorts"
  • 4 were incorrectly wired

"The 71 boxes considered un-repairable had averaged over 24 hours of shop operating time, and still were not repaired," Anderson explains. "The repeat issue of checking these chassis and not being able to find and fix these elusive problems is what adds up to millions in maintenance costs." Salvaging and repairing the previously un-repairable chassis is what has resulted in recovering over $10 million worth of MLPRF assets. The IFDIS intermittent, continuity and short tests diagnosed most of the problems in minutes. In addition to diagnosing existing NFF problems, the IFDIS also caught developing problems resulting in failure avoidance and opening the door for the IFDIS to function as a preventative maintenance tool.

Present Day and Beyond

Since Total Quality Systems delivered the IFDIS to the Ogden Depot in February, the testing and success continues. U.S.

Air Force technicians are continuing to recover otherwise un-repairable MLPRF chassis and are returning them to service.

Additionally, many more boxes with intermittent faults will be tested, repaired, and returned to service by the U.S. Air Force. "Millions of dollars in maintenance cost avoidance are being realized" Anderson states. "Just getting those unrepairable boxes tested, repaired, and back into service amounts to a huge benefit for our military." Once plugged in and run through its operational profiles, an IFDIS unit can detect the intermittent problem(s) in a matter of minutes. The speed for detection is impressive, but also the fact that this is not just product-specific to the F-16 MLPRF chassis is also appealing.

"Essentially, anything that has complex wiring could have these intermittent issues, and an IFD can identify, isolate and deliver the information to the technician to fix the problem," Anderson says. "This technology has application to other F-16 LRUs, other US Air Force weapon systems and other arms of the military, and is equally applicable to commercial aviation and aerospace operations as well as consumer electronics.

Universal Synaptics
Ogden, UT
usynaptics.com