Component No Fault Found

Component "No Fault Found" So then what? By Thomas Carroll July 2001 One of the biggest ballyhoos at airline and industry meetings has been the high incidence of "No Fault Found" (NFF) when components are removed from the aircraft...


Beware of rogue units
A number of NFFs are part of the natural order of things, but, there’s a more insidious perpetrator of NFFs prowling around out there — the rogue unit.
A rogue unit is an individual component that repeatedly experiences short service periods, manifests the same fault each time, and whose replacement resolves the system problem. It just doesn’t work on the aircraft. The problem is that it always checks good in the shop. Suffice it to say that a part can be defective and still score NFF. If phantom glitches in the aircraft are considered "gremlins," then rogue units are the devil incarnate!
Since the shop can be generating NFFs also, using the shop bench test as the absolute measure of NFF and the competency of line maintenance is a fallacy. The only true judge and measure of the shop or line maintenance is the big hunk, the aircraft itself.
Should the NFF be ignored? No. But further investigations need to be performed to see if there are line or shop issues, or a combination of both. To accomplish this, some fundamental data sources are required — aircraft maintenance history and component tracking by serial number. If one or the other is missing, then it would be better to ignore the NFF count than to continue focusing solely on line maintenance.
Sifting through the data
Okay, we get the data we need – then what? Post mortem analysis of aircraft maintenance history will enable the sorting out of system design, troubleshooting or BITE issues. Combining that data with component tracking by serial number will identify rogue units, as well as component design, bench test or repair issues.
The world will probably always be focused on component No Fault Found — so then what? First, stop jumping to conclusions. Take a deep breath and approach the issue from a global perspective, combining fundamental aircraft and component data and real-world experience. Only then will the NFF count be meaningful, have a chance of being understood, and quite possibly reduced to an acceptable level.
Or, we’ll talk about it again in 10 years.

Thomas Carroll is the Manager of Reliability Engineering at US Airways at Pittsburgh International Airport, Pittsburgh, PA. During his 28 years in aircraft maintenance, he has worked the line, shop, maintenance control and component reliability engineering disciplines.

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