Rex says
Money math without the spreadsheet headache. Plug in your numbers and I'll show you exactly where the dollars land.
Try a scenario
Click to load — tweak from there.Inputs
Result
Reserve to set aside per hour
$26.67
Annual reserve amount
$2,667
Hours remaining to TBO
1,200
Years to reach TBO at this pace
12.0
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How to use this
- 1Enter expected overhaul cost ($).
- 2Enter current hours since last overhaul.
- 3Enter tbo (hours).
- 4Enter hours flown per year.
- 5Read your reserve to set aside per hour on the right — it updates as you type.
- 6Hit Share to keep the scenario or send it to someone.
About this calculator
An engine reaching TBO (time between overhauls) without a funded reserve forces owners into an unplanned $25,000-45,000 expense at the worst possible time, right when the airplane is grounded. This calculator works backward from overhaul cost and remaining hours to TBO to compute the per-hour reserve rate needed, and shows how the fund balance grows if you actually set the money aside rather than spending it. It also flags whether an engine flying below roughly 30-40 hours per year, which risks corrosion-driven early overhaul rather than a clean run to TBO.
Worked example
Using the values the calculator loads with:
Inputs
- Expected overhaul cost: 32000 $
- Current hours since last overhaul: 800
- TBO (hours): 2000
- Hours flown per year: 100
Results
- Reserve to set aside per hour: $26.67
- Annual reserve amount: $2,667
- Hours remaining to TBO: 1,200
- Years to reach TBO at this pace: 12
What each field means
Inputs
- Expected overhaul cost ($)
- The expected overhaul cost used in the calculation, measured in $. Starts at 32000 $ so you have a working example on load.
- Current hours since last overhaul
- The current hours since last overhaul used in the calculation. Starts at 800 so you have a working example on load.
- TBO (hours)
- The tbo (hours) used in the calculation. Starts at 2000 so you have a working example on load.
- Hours flown per year
- The hours flown per year used in the calculation. Starts at 100 so you have a working example on load.
Results
- Reserve to set aside per hour
- Returned as a money amount in US dollars and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Annual reserve amount
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Hours remaining to TBO
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Years to reach TBO at this pace
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why does low annual usage matter for engine health?
Engines that sit idle for weeks between flights are more prone to internal corrosion on cam lobes, lifters, and cylinder walls than engines flown regularly, because moisture from combustion byproducts doesn't fully evaporate off internal surfaces without sustained heat. Many mechanics see engines flown under 40 hours/year require overhaul on corrosion grounds well before reaching TBO hours.
Is TBO a hard limit I must meet?
For most Part 91 private operations, TBO is a manufacturer recommendation, not a mandatory replacement time — you can legally run past TBO if the engine passes inspections (compression checks, oil analysis, borescope) and remains airworthy. Commercial operations under certain Part 135/121 contracts, and some insurance policies, may require overhaul at or before TBO regardless of condition.
Does this account for prop overhaul too?
No — this is engine-only. Propeller overhaul is a separate expense on its own TBO cycle (often shorter, like 2,000 hours or 6 years calendar limit, whichever comes first, for many fixed-pitch or simple constant-speed props), typically running $2,500-6,000, and should get its own reserve line alongside the engine reserve.
What if I buy an aircraft mid-cycle on the engine?
Set current hours to the logbook time since major overhaul (SMOH), not total airframe time, since that's what determines remaining life to TBO. If the engine's maintenance and oil analysis history looks rough for its hours, consider a shorter effective remaining-hours estimate than the raw TBO math suggests to build reserve faster.
Accuracy and limitations
- Results are estimates before tax, fees, and inflation unless an input explicitly covers them.
- Rates are treated as fixed for the whole period — variable-rate products will drift from this projection.
- This is educational maths, not financial advice. Check anything contractual with the lender or your accountant.
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Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Engine Reserve Fund Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/engine-reserve-fund
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/engine-reserve-fund" target="_blank" rel="noopener">Engine Reserve Fund Calculator — RevenueLab</a> (2026).</p>
Source: [Engine Reserve Fund Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/engine-reserve-fund) (2026).