Is NFF a Safety issue?

Opinion divided on whether No Fault Found affects Safety

Safety is the foremost concern in the minds of maintenance professionals everywhere, from the Aerospace, to the Oil & Gas, to the Food & Drinks sectors. So imagine our surprise when we discussed the topic with senior technical managers responsible for airworthiness and support - in the context of "first time fix" and "No Fault Found" - and discovered that they didn't think they were a safety issue.

This prompted us to ask the question to a wider pool of aerospace maintenance professionals.

This blog article gives a sample of their views.

"No Fault Found is a Safety issue"

In the real world , out there on Aircraft, on a practical point of view it is definitely a safety issue, no doubt about it, as far as I am concerned.
NFF is a safety and an airworthiness issue. Definition of Airworthiness is a condition that causes the air vehicle to be unfit for flight. Despite the digital redundancy in LRUs, unless cables and connectors have been considered in the redundant path analysis during design [no guarantees here] then a rogue bit/byte can ruin your whole day! Most avionic systems have been non-flight critical in the past but in the last 20 years, Airbus and Military weapon systems are fly-by-wire and if the wire is intermittent, how could this be other than safety ergo airworthiness?
I would say that it could be a potential safety issue, If the flight crew loses its confidence of the caution and warning system they might just as well not taking a "real" fault seriously. The ground crew on the same side could just make note that the fault have appeared again but this time a thorough fault finding could find that the intermittent fault have evolved to a reproducible fault that might lead to an airframe loss.
NFF may (most of the time does) indicate an intermittent problem that cannot be reproduced on ground. Often these are related to wiring interfering with airframe/wing parts, and we all know that these are not the same in flight as on the ground. The problems are usually not active when the aircraft is dispatched, and problems may reoccur in any phase of the flight. They become a safety problem when crews are not properly trained to recovering aircraft from abnormal circumstances combined with poor communication of problem history (logging, correct description, indicating of all relevant symptoms...). Another liability is indeed the assessment by the maintenance personnel that depends on communication, MEL, Manual information.... If they don't recognize the degree of criticality of the problem: again a safety problem. Did you notice that all these elements have 1 thing in common? Yes, the Human Factor, emphasizing the importance of support, tooling, training and experience of everyone involved.

"No Fault Found is not a Safety issue"

IMHO NFF happens after the event so how can it be a safety issue ? A NFF issue is not a root cause but a symptom of bad design and /or test. The root cause of the NFF problem may well be a safety concern, but the fact you can't find it is not. Remember, NFF is not related to the system going wrong, but the inability to detect & confirm it after it has happened.
NFF is more of a commercial than a safety or airworthiness issue. Indeed the root cause is inter-dependency of systems or components within a system. More often, this is the result of lack of aircraft ground time available to go through the full troubleshooting and re-testing drill. The cost of NFF is also different from airlines to airlines. Those who have their own shops or at least service and test facility would not be as worried as those who have a component exchange program.
Definitely not a Safety or Airworthiness issue, IMHO, since failed units have been taken out of service following established and approved maintenance procedures and certified equipment and aircraft design (with whatever level of redundancy and common cause failure protection in place to prevent a single failing unit to jeopardize Safety/Airworthiness). No Fault Found (NFF) is an economic issue caused by putting a unit through the maintenance chain (removal/replacement, packaging, shipping, testing and documenting) for no apparent reason.

"It depends..."

It all depends on what system it is on. Most if not all safety of flight systems are redundant. I think NFF is more of a cost issue. It affects maintenance Man Hours, scrubbed missions, reliability.
I believe an isolated NFF issue is not a safety or airworthiness issue. The potential causes are so numerous. It could be EMI, 'seat-to-control interface' problems, etc. It does become a safety issue when it is repetitive.
As a pilot who has spent a career flying aging helicopters and is now doing flight safety for an organisation that does not have a robust SMS the answer is both yes and no. No: If the system which is suffering the NFF problem is a minor one and the crew is able to see, pre-flight, there is a repeated snag from the aircraft paperwork, then the issue is one of distraction. Plus there is a financial penalty for doing repeat rectification (aircraft on ground is not earning revenue) if the organisation is a commercial one. Being a military operator I am worried about is the availability of the aircraft for operations. Yes: When the distraction happens is the important factor. Anything that takes the crews attention away from flying or 'operating' the aircraft might be a flight safety hazard in some circumstances.

Who is right?

The problem here seems to be one of terminology, leading to apparently different views. In some people's minds the root cause of an NFF occurrence perhaps becomes synonymous with the NFF end result. The root cause and its in-flight effects can be a safety problem: whether by causing a crew distraction (such as a transient fault affecting flight deck display units), or by affecting a safety-critical system (such as an intermittent undercarriage retraction handle). The subsequent maintenance activities that lead to an NFF decision have implications for cost, support contracts and asset availability.

Moreover, if a fault root cause remains undiagnosed then it can have implications for safety if it manifests again at a later date. Closing the loop entirely, the reliability root cause of the fault root cause will be a factor of the component's design, operating environment and/or use case.

To sum up, most would seem to agree that the NFF-related maintenance activity itself is not the safety issue; however a fault root cause could well have safety implications if left undiagnosed and unrectified.