Aircraft maintenance forecasting turns each item’s limit into a date on the calendar. Take what is left before the item comes due (days, flight hours or cycles), divide the hours and cycles by how much the aircraft has been flying per day lately, and add the result to today. An inspection due in 48 hours on an aircraft averaging 1.2 hours a day comes due in about 40 days. Items with more than one limit come due at whichever limit arrives first, so you forecast each limit and keep the earliest date.
What does aircraft maintenance forecasting mean?
Every maintenance item on an aircraft has a limit: a date, a counter reading, or both. Tracking tells you how much is left today. Forecasting tells the scheduler and the director of maintenance the day the aircraft will run out.
For a calendar item, the answer is already a date. For an hour or cycle item, it depends on how the aircraft gets used between now and then, so you need a rate. The forecast is only as good as that rate, which is why most of the work goes into choosing it.
The output is a planning date. The item’s limit stays what the maintenance program says it is, and the aircraft’s records have to show it. Under 14 CFR 91.417(a)(2), the owner or operator keeps records of “the total time in service of the airframe, each engine, each propeller, and each rotor,” “the current inspection status of the aircraft,” and, for an airworthiness directive with recurring action, “the time and date when the next action is required.” A forecast sits on top of those records. It never replaces them.
How do maintenance items come due?
Inspection and maintenance programs express intervals in a small number of units. The rule for an owner’s inspection program approved under 91.409(g) lists them. The program needs a schedule “expressed in terms of the time in service, calendar time, number of system operations, or any combination of these” (91.409(g)(2)).
| Limit type | Counted in | Example from the rules | How you forecast it |
|---|---|---|---|
| Calendar | Days or calendar months | Annual inspection: “within the preceding 12 calendar months” (91.409(a)) | Already a date; no rate needed |
| Hours | Time in service, usually airframe or engine hours | 100-hour inspection: “within the preceding 100 hours of time in service” (91.409(b)) | Hours remaining ÷ hours per day |
| Cycles | Landings, engine starts, pressurization cycles | Items your program counts in “system operations” (91.409(g)(2)) | Cycles remaining ÷ cycles per day |
| Combination | Two or more of the above | Any item your program sets as “X months or Y hours, whichever comes first” | Forecast each limit, keep the earliest date |
Hours here means time in service, which 14 CFR 1.1 defines as “the time from the moment an aircraft leaves the surface of the earth until it touches it at the next point of landing.” Make sure your rate is built from the same counter the limit uses. A rate from block time applied to a limit in time in service will pull every forecast earlier than it should be, and a rate from airframe hours applied to an engine that was swapped last month will be wrong in a different way.
Which program sets your intervals depends on the aircraft and the operation. Large airplanes, turbojet and turboprop multiengine airplanes and turbine rotorcraft (unless the rotorcraft operator elects the annual, 100-hour or progressive options) are inspected under a program selected under 91.409(f), such as “a current inspection program recommended by the manufacturer.” Aircraft on an approved inspection program under Part 125 or 135 and named in the certificate holder’s operations specifications are exempt from the annual and 100-hour rules (91.409(c)(2)) and follow their program’s intervals instead. The forecasting method is the same either way; only the list of items and limits changes. If you’re working out which rules apply to your flying, our comparison of Part 91, Part 135 and Part 91K covers the split.
How do you calculate average daily utilization?
Average daily utilization is the usage between two counter readings divided by the days between them:
Rate per day = (latest reading − earlier reading) ÷ days between the readings
The arithmetic is easy. The choices around it decide whether the forecast is any good.
- Pick a window of recent history. Too short and one long trip or one week in the hangar swings the rate. Too long and you’re forecasting next month from last winter’s flying. Somewhere around 60 to 90 days is a reasonable starting point for most operations; compare it now and then with a longer view to catch a season the window misses.
- Use readings from the same counter as the limit. Airframe hours for airframe items, each engine’s hours for that engine, landings for landing items.
- Make sure the window has enough readings. Two readings a week apart don’t make a rate. Readings logged leg by leg give the best series; readings typed in once a month work but lag.
- Throw out the bad steps, not the whole window. A typo that adds 1,000 hours, a correction that takes it back, or an engine change that resets an engine’s counter will wreck an average. Drop those steps and keep the rest.
- Count days with no flying. A month on the ground is part of how the aircraft gets used. If you leave those days out, the rate overstates use and every forecast comes in early.
- Recompute as readings come in. A rate is only current as of its last reading. Update it each time you log flight time.
Check the rate against what you know is coming. If the schedule shows a heavy month ahead, or the aircraft spends the next three weeks in paint, the past 90 days won’t describe it. Treat the history as a default and override it when the calendar says otherwise.
Worked example: forecasting a 100-hour inspection
Every number in this example is illustrative. The aircraft, readings and dates are invented to make the arithmetic easy to follow.
An aircraft carries passengers for hire and is on 100-hour inspections under 91.409(b). Its last 100-hour inspection was done at 2,350.0 airframe hours, so the next one is due at 2,450.0.
Step 1: find what is left
Today is October 28. The airframe reads 2,401.6 hours.
2,450.0 − 2,401.6 = 48.4 hours remaining
Step 2: work out the rate
Ninety days ago, on July 30, the airframe read 2,293.6 hours.
| Reading date | Airframe hours |
|---|---|
| July 30 | 2,293.6 |
| October 28 | 2,401.6 |
| Change over 90 days | 108.0 |
108.0 hours ÷ 90 days = 1.2 hours per day
Step 3: turn hours into days
48.4 ÷ 1.2 = 40.3 days. Round up, because a forecast that lands a day late is worse than one that lands a day early: 41 days.
Step 4: add the days to today
October 28 + 41 days = December 8.
That is the planning date. If the aircraft keeps flying at its recent rate, the 100-hour inspection comes due around December 8, and any trip on the schedule after that date needs the inspection done first.
Step 5: check the other limits on the same item
Suppose the same aircraft also has an item on its program due every 12 months or 300 hours, whichever comes first. It was last done in April at 2,180.0 hours, so it is due at 2,480.0 hours or next April, whichever comes first.
- By hours: 2,480.0 − 2,401.6 = 78.4 hours, and 78.4 ÷ 1.2 = 65.3, so 66 days. October 28 + 66 days = January 2.
- By calendar: next April.
January comes first, so January 2 is the forecast date for that item, and the hour limit is the one to watch.
Step 6: see how sensitive it is
Run the same 100-hour inspection at a slower and a faster rate:
| Rate assumed | Days to 48.4 hours | Forecast date |
|---|---|---|
| 0.8 hours per day | 61 | December 28 |
| 1.2 hours per day (recent average) | 41 | December 8 |
| 1.6 hours per day | 31 | November 28 |
A full month separates the slow and fast cases. If there is a big trip planned in late November, don’t plan around the average alone: either book the inspection before the trip or watch the hours closely as it approaches.
Can you fly past a 100-hour inspection?
Only in one narrow case. Section 91.409(b) says: “The 100-hour limitation may be exceeded by not more than 10 hours while en route to reach a place where the inspection can be done. The excess time used to reach a place where the inspection can be done must be included in computing the next 100 hours of time in service.”
Two things follow for forecasting.
- It’s not planning margin. The allowance covers getting the aircraft to a place where the inspection can be done. Scheduling a trip that you know will run the aircraft past 100 hours and then calling the extra time an en route allowance doesn’t fit the wording.
- The overrun comes out of the next interval. In the example above, if the inspection is done at 2,456.0 hours after a 6-hour ferry to the shop, the next 100-hour is due at 2,550.0, not 2,556.0. Your forecast for the next cycle has to start from the original due point.
Progressive inspection programs have a similar provision: the inspection procedures manual must include “instructions for exceeding an inspection interval by not more than 10 hours while en route” (91.409(d)(2)(ii)).
For other items, any tolerance comes from the program that sets the interval. If your manufacturer’s program or approved program allows an item to be done early or late within a stated window, the program’s own wording governs. Forecast to the limit itself unless you have that wording in front of you.
How to plan maintenance downtime around trips
A forecast date is useful when it reaches the people who build the schedule. A workable routine:
- List every item coming due in the next 90 days, with its forecast date and the limit that drives it.
- Group items that fall close together. If the 100-hour lands December 8 and another item lands December 20, doing both in one visit may cost less downtime than two shop visits. Check how your program treats early compliance. For the annual and 100-hour inspections, the next interval runs from when the inspection was done, so an early inspection gives up the unused part of the old one.
- Put the downtime on the schedule as soon as it’s booked. Block the aircraft for the days the shop expects, plus travel to and from the shop.
- Check the trips on either side. Look for trips booked inside the window, and trips just before it that could add enough hours to pull the due date forward.
- Recompute after every flight. A trip that adds 12 hours the average didn’t expect moves an hour item forward by about 10 days at 1.2 hours a day.
- Keep a backup plan. If an item comes due away from base, know where the nearest place is that can do it.
If your trips live in a spreadsheet and the maintenance list lives in another one, step 4 is where things slip, because nobody sees both at once. Our guide to moving off a flight scheduling spreadsheet covers that problem in more detail.
Once an inspection turns up a defect that can’t be fixed right away, the question shifts from forecasting to whether the aircraft can fly with it. That decision is covered in our guide to inoperative equipment under 91.213.
Common forecasting mistakes
- Forecasting from a rate nobody updated. A rate from last spring says little about this month.
- Mixing counters. Block time against a time-in-service limit, or airframe hours against an engine item after an engine change.
- Ignoring the calendar limit on a combination item. An aircraft that sits a lot will reach the date long before the hours.
- Rounding down. Always round the days up, so the forecast never shows a day you don’t have.
- Treating the forecast as the limit. The program sets the limit. If the aircraft flies more than expected, the item comes due earlier than the forecast said.
- Booking the overfly in advance. The 10-hour allowance in 91.409(b) is for reaching a place where the inspection can be done, and it comes out of the next interval.
For the record-keeping side of the same work, our guide to aircraft maintenance tracking covers what to keep for each item.
Maintenance forecasting in Avion Desk
Avion Desk tracks hours, cycles and other counters for each aircraft, and a forecast from recent use shows when items come due. Each maintenance item is due by a date or by a counter, gets signed off when it’s done, and its status is calculated for you. You can plan downtime for each aircraft in the same place. See the aircraft and maintenance features and pricing. To try it, start a free trial and add an aircraft with its maintenance items and counter readings to see when each item comes due, or request a demo.
Frequently asked questions
How do you forecast when an aircraft inspection is due?
Divide the hours or cycles left before the limit by the aircraft’s average daily use over a recent window, round up, and add that many days to today. For an item with both a calendar and an hour or cycle limit, forecast each one and use the earliest date.
What is average daily utilization?
The hours or cycles an aircraft accumulated between two readings, divided by the days between them, including days it didn’t fly. An aircraft that flew 108 hours over 90 days averaged 1.2 hours a day.
Can a 100-hour inspection be overflown?
By not more than 10 hours, and only while en route to reach a place where the inspection can be done. The extra time counts toward the next 100 hours (91.409(b)).
What does “whichever comes first” mean for maintenance items?
The item is due at the first of its limits to be reached. An item due at 12 months or 300 hours is due at 300 hours on an aircraft that flies a lot, and at 12 months on one that mostly sits.
How often should a maintenance forecast be updated?
Each time new hours or cycles are logged, since every reading changes both what is left and the rate. Also revisit it whenever a large trip or a long stretch on the ground is added to the schedule.
Does a forecast change when an item is legally due?
No. The inspection or maintenance program sets the limit, and the records under 91.417 show the aircraft’s status against it. A forecast is a planning estimate of when the aircraft will reach that limit.
This article explains the rules in plain language and isn’t legal advice; the regulation text governs.
