Every aircraft carries a short list of things that are not quite right. A reading light that flickers. A cabin display that needs a reset. A system that works but is due for attention. Some of those items can wait. Some cannot. The skill in maintaining an aircraft is knowing which is which, and deciding before the aircraft decides for you.
Deferral is a legitimate tool. It is also the place where costs quietly accumulate. This piece looks at how deferral is supposed to work, what it costs when it goes wrong, and why we plan maintenance around how an aircraft actually flies.
How deferral is meant to work
For operators flying under Parts 91K, 121, 125 and 135, the FAA framework for flying with inoperative equipment is the minimum equipment list, or MEL. The FAA describes an MEL as an operator-specific, FAA-approved document that lists the items that may be inoperative during a flight, and it defines deferral as the authorization to delay a repair and keep operating with a required item inoperative under that approved list 1.
Deferral is not open-ended. Items carry repair categories with fixed clocks. Under FAA guidance, a Category B item must be repaired within three consecutive calendar days, a Category C item within 10, and a Category D item within 120, in each case excluding the day of discovery. Category A items carry whatever interval the MEL itself specifies 1. The clock runs whether or not the aircraft is near a maintenance facility, whether or not parts are on the shelf, and whether or not the calendar is full of trips.
Scheduled inspections work the same way. An aircraft that carries passengers for hire must, under FAA rules, have had an annual or 100-hour inspection within the preceding 100 hours of time in service 2. Charter aircraft with ten or more passenger seats must be maintained under a formal maintenance program, while those with nine seats or fewer are maintained under Parts 91 and 43 and may use an approved aircraft inspection program 3. These limits are not suggestions. When one comes due, the aircraft stops.
The cheapest repair is almost always the one made on schedule, at home, with the part already on the shelf.
Where the cost actually lands
The invoice for a deferred repair is rarely the expensive part. The expense is in what happens when a small item becomes a grounding item away from home. The industry term is AOG, aircraft on ground.
Published estimates of AOG costs come mostly from commercial aviation, where fleets are larger and schedules tighter, so they are not a direct measure for a private aircraft. They are still useful for showing where the money goes. One 2025 industry analysis put aircraft repositioning costs at roughly US$15,000 to upwards of US$75,000, shipping and logistics for AOG parts at US$10,000 to US$100,000 or more, and labor for AOG teams at US$5,000 to US$25,000 per incident 4. The same analysis estimated that a single day on the ground can cost between US$50,000 and US$150,000 once lost revenue is included 4. A business jet will sit at a different point on those ranges, but the categories are the same.
Parts are a particular pressure point. In AIN’s 2025 product support survey of business aircraft operators, parts availability was described as a low point in most of the business jet manufacturer ratings, and cost of parts was among the weakest-scoring categories 5. A part ordered on a planned schedule moves at a planned price. The same part needed tonight, at an airport without a stocking distributor, moves by courier or a dedicated flight.
People are the other constraint. Boeing’s 2025 outlook projects demand for 710,000 new maintenance technicians across the global commercial fleet through 2044 6. That forecast does not cover business aviation directly, but the two sectors draw from the same labor pool. When a technician has to be flown to a stranded aircraft, the cost includes travel, time and whatever work that technician is not doing elsewhere.
The costs that do not appear on an invoice
Crew time is finite. Under Part 135, a one-pilot crew on unscheduled operations is generally limited to eight hours of flight time in a 24-hour period and a two-pilot crew to ten, with at least ten consecutive hours of rest required before the planned completion of an assignment 7. A two-hour delay to troubleshoot a fault can push a crew past its limits. The trip then waits for rest, or a second crew is positioned, with hotels and travel on top.
Then there is the trip itself. A charter flight that cannot depart is revenue the operator does not earn, and often a replacement aircraft it must source at short notice. For an owner, an aircraft that is down is an aircraft that cannot be flown for its owner or offered for charter. For a passenger, it is a missed meeting or a shortened weekend. None of those costs appears on the maintenance invoice, and all of them are larger than the original repair.
Deferral also compounds. An item left open on one system can complicate troubleshooting on another. A few open items reduce the margin for anything else to go wrong on a given day. The aircraft is still legal to fly. It is simply less forgiving.
Building maintenance around utilization
The better approach is to plan maintenance around how the aircraft actually flies. Hours, cycles and calendar time drive different inspections, and an aircraft flying many short legs ages differently from one flying a few long ones. A maintenance plan that tracks real utilization can bring inspections forward into natural gaps in the schedule, rather than letting them land on a busy week.
That is how we approach aircraft management at Aukeo Air. We look at the coming months of flying alongside the inspection and MEL picture, and we schedule work into the quiet periods. We prefer to close deferred items at home base, with the right technicians and parts on hand, rather than on the road. We order long-lead parts against forecast hours, not against a failure. And we treat an open MEL item as a reason to plan, not as permission to wait.
None of this removes the possibility of an unscheduled event. Aircraft are complex, and things break. What changes is the proportion of maintenance that happens on the operator’s terms.
What this means for you
If you own an aircraft, ask your manager three questions. How many deferred items are open right now, and when does each one expire? Which inspections fall due in the next 90 days, and where will they be done? Which long-lead parts are already on order? Clear answers usually signal a program that is managing cost rather than postponing it.
If you charter, it is reasonable to ask an operator how it plans maintenance around its flying and how it handles a mechanical issue away from base. The rules referenced here are summarized for general understanding, and owners and operators should confirm specifics with their own maintenance advisors or the FAA.
The cheapest repair is almost always the one made on schedule, at home, with the part already on the shelf.