If you have read our piece on reading a six figure quote, this is the other half of that job, and one thing has changed in the meantime. Your leverage is mostly gone.
Before induction you could walk away. Now the aircraft is in their hangar, the work is done, the panels are back on, and in most states a repair facility can assert a lien against the aircraft and simply retain possession until the bill is paid. That is not a threat and it is not unusual.
It is the ordinary physics of the situation, and it is the single best reason to have the invoice conversation before the invoice arrives.
One thing to be clear about up front: none of the five items below is a scam. Every one of them exists for a real reason and appears routinely on invoices from shops with excellent reputations and technicians you would trust with your family. The difficulty is that each one has a wide zone in which legitimate and excessive look identical from the outside, and almost nobody questions them, because questioning them requires knowing what to ask.
So here is what to ask.
1. Consumables billed as a percentage of labor
What it is
Safety wire, sealant, solvent, rivets, lockwire, gloves, rags, and the hundred other things a technician goes through in a week. Rather than itemizing them, most shops bill a flat percentage of the total labor figure.
Why it is legitimate
Because the alternative is worse for everyone. Nobody is going to track a fifty cent cotter pin across a three week event, and you would not enjoy paying for the bookkeeper who did. Brian Wells of JSSI, who reviews these invoices professionally, notes that many MROs bill roughly 3 to 4 percent of the total labor amount for consumables and describes it as standard practice across the industry.
How it drifts
Three ways, and all three are quiet. The percentage sits above the customary range and was never stated anywhere you signed. It is calculated on labor plus parts rather than labor alone, which produces a strange result on a job with one expensive rotable and very few wrench hours. Or there is no cap, so a long event generates a supplies number out of all proportion to anything a human being actually consumed.
If your shop itemizes consumables instead of using a percentage, the check is different: confirm the items match the narrative of the work. Wells offers the illustration of finding ten screw extractors billed under a gear lubrication line item. Consumables that do not fit the story of the task are worth a question.
The question: What percentage is this, is it calculated on labor or on labor plus parts, and is it capped?
2. Troubleshooting labor on an open ended clock
What it is
Hours billed against diagnosis rather than against a defined repair. This is the largest of the five by a wide margin and it is where most invoice disputes actually originate. Wells is direct about it: the majority of the disputes he sees come out of time and materials defect work.
Why it is legitimate
Because intermittent faults are genuinely, structurally hard, and the numbers on this are worse than most owners realize.
Published research puts no fault found rates in avionics systems somewhere in the range of 20 to 50 percent, with figures around 40 percent or higher commonly reported, and avionics accounting for roughly three quarters of all no fault found occurrences on aircraft. Broader industry estimates suggest something like a quarter to a third of all component removals come back with no fault found at all.
Sit with that for a second. It means that a meaningful share of the diagnostic work performed on your aircraft, at any shop, by good people, will not find anything. That is not incompetence. Intermittent faults caused by connector degradation and wiring can last nanoseconds and appear only at altitude on a cold morning, and somebody has to be paid for the hours spent chasing them.
How it drifts
The real question is not whether troubleshooting costs money. It is who bears the cost of a diagnostic path that did not work, and that question is almost never answered in advance.
Wells describes the archetype. A generator fault. A frequency converter gets replaced and the fault persists. A variable speed generator gets replaced and the fault clears. Both parts appear on the invoice, and when the teardown report on the converter comes back, it reads no fault found. Every individual step in that sequence was reasonable engineering judgment. Whether you agreed to fund all of it is a separate matter entirely, and one nobody raised with you at the time.
While you are in this section, check the labor discipline on each line. Rates vary by trade, and Wells gives the example of fifteen hours of avionics time booked against a tire change, at a rate ten dollars an hour above the mechanical rate that should have applied. That is usually a coding error rather than anything deliberate. It is also money.
The question: Show me the man hours booked against each discrepancy, the corrective action narrative for each one, and the teardown reports on anything that came off the aircraft.
The genuine fix lives upstream, not on the invoice
Agree before induction that every discrepancy gets an estimated man hour figure and an individual approval or denial, and that nobody has open ended troubleshooting authority. Wells makes the same recommendation, and it is the single highest value sentence in any work authorization.
3. Parts lines with no part number and no source
What it is
A parts charge with a price and a loose description, but no part number, no condition, and no indication of where the item came from.
Why it is legitimate
Markup on parts is universal and it is earned. A shop carries inventory, finances it, absorbs the risk on returns, performs the receiving inspection, and owns the paperwork chain. Nobody is entitled to see a shop’s acquisition cost, and asking for it is not a reasonable request.
How it drifts
The problem is not the markup. It is the opacity. A line reading “parts and hardware, $18,400” cannot be verified, cannot be benchmarked, and cannot be discussed. Wells’s observation is that comparable pricing for new, repaired, and overhauled parts is available on the open market, which makes a large disparity fair game for a question. But you can only run that check if you have a part number and a condition to run it on.
Two related items live in this bucket. If you supplied any parts yourself, verify the handling fee against the percentage that was agreed at debrief. And confirm that somebody actually checked for existing warranty coverage before ordering. Wells flags warranty research as one of the most commonly skipped steps in the entire process, and it is skipped by accident far more often than on purpose. Nobody catches it if you do not ask.
The question: Part number, condition, and quantity on every parts line above my threshold, plus confirmation that warranty coverage was checked before purchase.
4. Outside services with an undisclosed handling fee
What it is
Non destructive testing, plating, machining, hydrostatic testing, wheel and brake work, upholstery, avionics bench repair. Work sent out to a specialist and passed back through to you.
Why it is legitimate
No shop does everything, and the ones that pretend otherwise are worse, not better. Specialists are usually the right answer. Coordinating a vendor, managing the logistics, standing behind the result, and carrying the cost until you pay is real work that deserves compensation.
How it drifts
The fee gets embedded rather than stated. You see a single number and have no way to tell whether it is the vendor’s invoice, the vendor’s invoice plus a disclosed percentage, or a comfortable round figure with the arithmetic left as an exercise for the reader. Duncan Aviation’s own guidance to customers tells them to ask directly whether support services like NDT, machining, and hydrostatic testing are included in the quoted number or billed on top. When a shop advises you to ask the question, ask it.
The adjacent charges worth a second look are tool rental, hangar, towing, and incoming inspection or walkaround video. Wells lists all of these as items that should prompt scrutiny when they land as customer charges. An authorized service center for your type should already own the tooling for your type. Hangar space and towing are broadly what a facility provides in exchange for having your aircraft in its building. Incoming photos and video mostly protect the shop. Sometimes there is a perfectly good explanation, such as a specialty tool rented from the manufacturer for a single task on a single day. Ask for it and you will usually get it.
The question: Who performed this work, what did they invoice, and what is the handling fee shown as a separate line?
5. Scope that was added without written approval
What it is
Work performed during the event that was never in the quote and never approved on paper. The classic while we are in there addition.
Why it is legitimate
This is the item that most deserves defending, and it is the one owners most often get wrong by treating it as presumptively bad.
A good technician with the panel already off who spots a chafed harness and corrects it has probably saved you an entire future maintenance event and possibly prevented a real problem. On most aircraft the expensive part of a repair is access, not the repair. Doing the work while the access already exists is frequently the cheapest timing that will ever be available, and a shop that ignored the chafe to stay strictly inside the quote would be doing you no favors.
How it drifts
The approval never happens. Or it happens verbally to a crew member with no authority to commit money. Or it happens by text message to a phone nobody can find three months later. Or it happens to your director of maintenance, who genuinely approved it, but against no threshold, so nobody upstream knew a decision was being made at all.
Then a line appears on the invoice that nobody remembers agreeing to, and a reasonable repair becomes an argument, which serves nobody including the shop.
The fix is a threshold and a channel, both agreed before induction. Below a stated dollar figure, use your judgment and tell me in the next progress report. Above it, written approval from a named person before work starts. This is not an exotic ask. AOPA’s guidance to maintenance providers says it plainly from the shop side: if you agree to obtain authorization before continuing past a stated cost or timeframe, honor that agreement.
The question: Where is the written approval for this item, and who gave it?
The five questions on one page
Worth keeping somewhere you can find it the week an invoice lands.
| The line item | What to ask | What a good answer sounds like |
|---|---|---|
| Consumables and shop supplies | What percentage, calculated on what base, and is there a cap? | Three and a half percent of labor only, capped, and here is where it was stated in your work authorization. |
| Troubleshooting labor | Man hours against each discrepancy, the corrective action narrative, and teardown reports on anything removed | Here is the discrepancy log with hours booked against each item, and here are the reports you asked for. |
| Parts lines | Part number, condition, and whether warranty coverage was checked before purchase | Itemized with tags attached. Two of these were still under coverage, so they have been credited. |
| Outside services | Who performed it, what did they invoice, and what is the handling fee | The vendor was named, their invoice was this, and our handling fee is the percentage we quoted you. |
| Added scope | Where is the written approval, and who gave it | Approved by email by your director of maintenance on the fourteenth. Here is the thread. |
The habit that does more than all five questions combined
Put the quote and the invoice side by side, sorted into the same categories, and look only at the differences.
Most owners read the invoice on its own, and an invoice read on its own always looks like a list of reasonable things, because it is a list of reasonable things. Every line has a plausible explanation. The document is designed to be read that way.
An invoice read against the quote stops being a list and becomes a variance report. Variance is where the information is.
You are not looking for wrongdoing. You are looking for the categories that moved and by how much, because that tells you what to ask about and, more usefully, what to write into the work authorization next time. A department that does this after every event develops something almost no flight operation has: an actual history of how a given shop estimates versus how it bills.
What to do if you actually disagree
Most of the time you will not. When you do, a few things are worth knowing before you pick up the phone, because this is an area where instinct leads people badly astray.
Raise it before the aircraft leaves. Possession is leverage and it currently belongs to the shop, but the incentive to resolve things quickly is highest while the aircraft is still on their ramp and the relationship is still live. After the airplane is home, your issue becomes an accounts receivable problem and moves at that speed.
Understand the shop’s position honestly. A maintenance provider may assert a lien against the aircraft, and depending on state law may simply retain the aircraft until the debt is paid. A recorded lien is a cloud on title, and clearing it generally requires a release or a court order. That consequence outlives the argument by a long way and can surface at the worst possible moment, which is during a sale.
Do not simply refuse to pay. The conventional approach among aviation counsel is to pay the portion that is not in dispute and handle the disputed portion separately, whether by negotiation, by bonding or escrowing the amount at issue, or with a lawyer. Paying what is not contested removes the shop’s justification for holding the aircraft and narrows the disagreement to the thing you actually disagree about.
Put it in writing, itemized, and keep the temperature down. You are far more likely to be dealing with a coding error, a rate applied to the wrong discipline, or a genuine miscommunication about approval than with anything intentional.
And it is worth remembering where the responsibility ultimately sits. Under 14 CFR 91.403, the owner or operator is primarily responsible for maintaining the aircraft in an airworthy condition. That surprises a lot of first time owners. It is also the clearest possible statement of why this is your file, your decision, and your right to understand every line in it.
If you would rather not become an invoice auditor
Everything above is doable by a determined person with a free afternoon, a printout, and a highlighter. Plenty of chiefs of staff do precisely that, and do it well.
If you would rather not spend the afternoon, invoice review is part of what we do. Send us the quote and the invoice and we will reconcile them line by line, tell you which categories moved, and tell you what we would ask. A fair number of times the answer is that everything holds up, which is genuinely worth knowing with the same confidence as the alternative.
VHMX is not a repair station and does not perform maintenance. We do not second guess the engineering judgment of the people who worked on your aircraft. We read the commercial side of the document so that you do not have to.
Sources and further reading: Brian Wells, Jet Support Services Inc., "Aircraft MRO: How Best to Review Your Invoice" and "Maintenance Debrief: Your First Opportunity to Control Costs," AvBuyer. Duncan Aviation, "Comparing Aircraft Maintenance Quotes: Avoiding Sticker Shock." AOPA Pilot Protection Services, "Aircraft Maintenance: Tips for Aircraft Owners and Maintenance Providers." Published research on No Fault Found rates in avionics maintenance, including Copernicus Technology and peer reviewed maintenance engineering literature. 14 CFR 91.403 and 14 CFR 43.11.
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