What You Need To Know About Rogue Aircraft Parts

Pulling a component from your aircraft early costs you thousands of dollars and hours of time.

It’s frustrating. It’s infuriating. It’s stressful.

Your first instinct it’s to “blame” the maintenance professionals who repair these components.

rogue aircraft part

That would make it a lot easier if it were always correct.

But, it’s not reality.

As an operation, you must first acknowledge that you’ll own a part that becomes a rogue or chronic unit.

It’s not in your best interest to fixate on warranty claims, TATs, or poor quality that demotivates the repair professional from addressing the core issue.

Let’s first identify what a rogue unit is…

What is a “rogue aircraft part?”

According to the Aviation Pros article “Rogue Units: Focus on cost containment,” a rogue unit is defined as…

”It repeatedly experiences short service periods, manifesting the same system fault every time, and whose replacement resolves the system malfunction. The problem is that when it is sent in for repair, the standard bench or overhaul testing can't identify its unusual failure mode. Bench testing does not address 100 percent of what a component does, where it lives, or how it operates. It's not intentional; it's just a fact of life that the shop is not the same as the aircraft. Also, bench tests are designed to identify anticipated failures -- checking things that are expected to fail. So a unit that fails in an unaddressed or unanticipated way will never be resolved -- a rogue is born.

Roy Resto has a similar and more detailed definition that’s worth noting.

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He says…

The typical pattern is that initially, the shop will return the part to service with a ‘Could not duplicate’, or ‘No trouble found’, or that corrective actions did not eliminate the pesky, recurring writeup. This may occur a few times before the part is declared ‘chronic’ or ‘rogue.’ A simplistic definition for these parts is that they pass all the required Maintenance Manual tests and inspections, but do not last on the aircraft. Most shops or airlines establish a baseline, rule of thumb to make the determination that a part is chronic or rogue; for example, if a certain serial number part exhibits 3 removals in a six month period. These baselines vary widely depending on the family of parts, ATA chapter, and/or the airline or shop policy.

Now you have a distinct classification of what a rogue aircraft part is for your operation.

Articulating this into your maintenance procedures and flagging removal occurrences will help you prepare for your next rogue component.

The causes of a rogue aircraft part.


Listing all the causes of a rogue unit would be counterintuitive as there are so many variations to consider.

Roy Resto gives some great advice on the typical causes that fall into several broad categories.

Intermittent Failures

These faults occur randomly and cannot be replicated at the repair station. Mr. Resto has compiled a list:

“Variation in the manufacturing or maintenance process as previously discussed; undetected wear; corrosion; abuse such as being dropped or mishandled; ESD damage; severe events such as lightning strikes, hard landings, landing gear collapse, or collisions with ground vehicles; coffee, water, lavatory leaks or other spills; failure of other parts in the system such as an over-voltage condition, etc.”

False Indicators - a problem with the aircraft

Sometimes the problem is beyond the component and is with the aircraft itself.

Troubleshooting this issue can take months while components are being removed multiple times.


Many components are controlled by software and the integration with other systems. Software is often upgraded which can potentially cause the system to act in unexpected ways.

Variation with the OEM and repair station

“Variation exists in any process, and that variation may exhibit itself in a particular serial number that will eventually end up on someone’s chronic list.”

The OEM may have manufactured a flawed serial number or the repair technician…

“…may have accomplished a poor solder joint repair or improperly crimped a wire for example. These could lead to intermittent failures.”

Now you have the categories of what causes rogue units, let’s discuss the impact on your operation.

The impact to you and your operation.


Any time aircraft components are replaced, it costs money. Often, the time associated trying to resolve the problem requires more money than the component itself.

“Any time maintenance is performed on the aircraft, there is the direct cost of the technician, along with a host of support equipment required to facilitate the repair and system checkout.”

It goes far beyond the typical aircraft maintenance procedures.

“There is a whole infrastructure built within the maintenance organization to facilitate the movement and tracking of unserviceable units to the shop and refilling the holes on the spare shelf with serviceable ones.”

“When considering the typical life of a rogue unit: usually six more installations after becoming a rogue (before some sort of "accident" befalls it); the associated "no fault found" costs; the additional system parts replaced needlessly; their associated "no fault found" costs; and the hours of troubleshooting the system wiring and plumbing unnecessarily; the average cost for each rogue unit comes out to be around $50,000 to the maintenance division.”

The scary part is, this doesn’t even include flight delays, cancelations, and excess inventory burden.

The costs are real, and rogue unit identification and resolution program will help minimize the financial impact.

Create a rogue unit identification and resolution program.

The best rogue unit program takes a set-by-step process of identifying and scrapping the faulty part.

Step 1: Track the component.

The most important aspect of a rogue unit program is the tracking of each component by serial number.

“This would include date on/off, aircraft number, position, reason for removal, time since installed/overhauled, etc.”

Step 2: Identify what a rogue is.

“Identify the potential rogues as they develop, such as three consecutive installations that are less than 1,000 hours each.”

Step 3: Review available data.

Review all the data you have access to and see if there are any correlations.

“Are the parts coming from a single aircraft tail number; was there an event such as a new Service Bulletin or software revision that coincided with the rise in removals; is there a particular phase of flight that the problem manifests (cruise, landing or takeoff); was there any maintenance event that occurred in the history of the unit that seemed to usher in the rise of removals (such as a repair or replacement of anything); etc.”

Step 4: Test the unit in different ways.

“Rogue units require new and unique testing to identify and resolve the oddball failure, so the shop needs to understand how and where the component operates in service to mimic those conditions.”

Step 5: Swap the parts.

“This involves swapping a subassembly from one unit into another and seeing if the problem follows. If the chronic unit comes back again, at least you’ve eliminated that subassembly from the list of possibilities, and the list narrows.”

Step 6: Scrap the part.

Scrap it.

Selling it to someone else will cause new issues for that operation.

Write it off and be happy that you caught it as quickly as you could to avoid any future financial burdens.

"The 5 KEY QUESTIONS Every Repair Station Must Answer Before You Hire Them!"

Lower TATs, Reduce Costs, And Eliminate The Time Sucking Status Update Chase!

Click Here To Download [REPORT]