NFF (No Fault Found) has the major contribution to reduce the operational availability, resources and increase the cost of Maintenance of any aircraft in aviation. The likely causes are human factors, maintenance training, faults reporting, fault analysis, corrective Maintenance and procedures. However, mitigating these issues are completely a tedious process wherein management skill can't achieve the desired results. So, what are the other parameters/ technical factors that need to be considered?
It costs Airlines lot of money to deal with NFF spares and worse is when they are fitted into the product (aircraft) and reintroduces the fault. Mitigation can be :
1. Ensure the complete print outs of the trouble shooting data is attached to the component which identifies the exact cause of failure.
2. Component hold procedure can also help in reducing the return of the bad spare into the aircraft or stops it from being declared as inoperative. 3 to 4 flight days are good length of time to observe.
When a fault is "flagged" the maintainer is often provided with the 3 most likely fixes, replaces all 3 "faulty" components to fix the problem and sends all 3 back for maintenance/repair. That scenario leaves the repair facility with 2 of the 3 components replaced with NFF indications. The repair facility then performs the required maintenance, usually quite costly, on all 3 units to return them to service. When a unit is removed it has to be returned to a repair facility, it cant be put back on the shelf to be reused.
Blindly "shotgunning" a problem by swapping out modules may return a unit to service, but inflicts a heavy burden on the support infrastructure. It is likely more cost-effective to hire talent who are true "troubleshooters" -- who can drill down to the smallest Field Replaceable Unit, rather than introducing more issues into the logistics chain than they fix. A false economy at work.
One of the most important things to remember as a maintenance technician is following the tech-data and knowing the system. Sometimes even skilled maintenance mechanics don't get it right the first time because troubleshooting is a trial and error task, but aircraft that have integrated fault reporting are making it easier. Avionics test stations cost several million or more, but they speed up the process if they are available within the immediate organization. The component is removed from the aircraft and sent to the back-shop where the black box is hooked up to the station and the stimulus and measurement software program is run to find erroneous values from the box. There are avionics components that flag themselves. There are also bus controllers that will say which avionics component on their bus has failed. Sometimes you don't have a highly integrated system or the luxury of a test station or its not a black box issue and you have to get into the diagrams but management can pressure you to "fix it now!" (definitely not my current employer).
If a repair facility is used, a part must be rented or cannibalized in the interim. We expect the new part will fix the issue, but often it doesn't. Sometimes there are factors which contribute to not being able to perform the troubleshooting as written in the fault isolation manual. Other maintenance being performed can inhibit the process. You can't open the engine compartment because someone is working up on the pylon. A single wire can travel the entire length of the aircraft. Poor manufacturing can cause electrical wear issues that must be mitigated on the flight line. Extreme safety protocols like writing a ton of overlapping warning tags and forms discrepancies and having quality assurance stand over you and make sure every "i" is dotted and every "t" is crossed (a previous employer) is definitely an inhibitor to technicians to want to do the full fault tree. Instead, they read through each step and try to figure out from the fault tree what the problem is by the process of elimination "we know its not that, so it must be this."
Not having the complete diagrams can be an inhibitor. I worked in a back-shop where they sent me all of the flight control avionics components from the aircraft. I looked in the diagram of inside the flight control panel which they did not have access to and was able to determine they were missing a weight-on-wheels signal. It turned out I was correct and didn't have to spend two entire shifts testing the components.
Aircraft are highly complex beasts - one particular aircraft I worked had around 50 avionics components, literally hundreds of relays, and a thousand circuit breakers. There was actually a published list of "nuisance faults" because the system was so integrated. Having full documentation is important - every technician needs a theory of operation or function for a system or component to have an understanding of how it works, a job guide or manual for performing the work they can follow to prevent mistakes, and a fault isolation manual where engineers have figured out the best way to find the issue.
Many technicians will troubleshoot and order parts, then wait for the parts to come in - they have plenty of other work on other aircraft. When those parts don't fix the issue, then the process repeats itself. Technician knowledge and skill, available functional and calibrated tools, available proper maintenance manual libraries (iPads!), a good work atmosphere, proper technician manning, a well designed integrated fault reporting system, a well-designed aircraft, an aircraft designed with the maintenance technician in mind, and available test equipment all contribute to the pace of operations when it comes to aircraft maintenance, and ultimately the bottom line for the organization.
Thank you so much Mr. Anthony Dean. Your all points are actually valid and recommended.
One of the biggest reasons for NFF from a practical standpoint is connectors and cabling.
Thank you Mr. Russel King.
Could you please elaborate your response regarding connectors and cabling responsible for NFF?
The maintainer will remove and replace a "defective" unit to repair a fault. The unit replaced does not go through any troubleshooting procedures, it is sent for maintenance. The indication of NFF is often a result of loose connections, not the unit itself.
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