Free tool

Engine overhaul reserve calculator

The overhaul is the largest single bill in piston ownership, and it arrives all at once unless you have been putting money away by the hour. This works out the hourly number.

Your engine

Overhaul cost ($)

What a shop will charge at TBO, parts and labor.

TBO (hours)

Manufacturer recommendation for your engine.

Hours since overhaul

Hours flown per year

Reserve per hour

$19.00

Set this aside for every hour flown

Per year

$1,900

At 100 hours a year

Time to TBO

1,350 hrs

About 13.5 years at this rate

Where you should be: reserving from the first hour after overhaul, an engine at 650 hours should have $12,350 banked. If the account is short of that, the gap is what you are financing at overhaul time.

TBO is a recommendation, not a limit. Under Part 91 you can run past it on condition. Reserving does not commit you to overhauling on the hour, it just means the money is there when the decision arrives.

Reserve on tach hours rather than Hobbs — manufacturers specify TBO in tach time, and on an aircraft with a lot of taxi time the two drift apart.

Aloft360 tracks engine time against actual Hobbs and tach entries, so the reserve balance moves with the airplane rather than with a spreadsheet someone updates monthly. Start a free trial.

Engine reserve FAQ

How do you calculate an engine reserve per hour?

Divide the expected overhaul cost by hours to TBO. A $38,000 overhaul on a 2,000-hour TBO is $19 an hour. Set that aside for every hour flown and the money is there when the engine reaches the end of its run.

Should the reserve run on tach time or Hobbs?

Tach. Lycoming and Continental specify TBO in tach hours. On an aircraft with a lot of taxi and run-up time, Hobbs runs ahead of tach by roughly 5 to 8 percent over a TBO, so reserving on Hobbs collects more than you need and reserving on tach matches the wear.

What does an overhaul actually cost?

It depends on the engine and whether you go field overhaul, factory overhaul, or factory new. A four-cylinder Lycoming or Continental typically runs in the $30,000 to $45,000 range, a six-cylinder IO-550 in the $55,000 to $75,000 range. Get a current quote from the shop you would actually use rather than relying on a national average.

Do I have to overhaul at TBO?

Not under Part 91. TBO is a manufacturer recommendation and a privately operated aircraft can run past it on condition, guided by oil analysis, compressions, and borescope results. Commercial operations under Part 135 are a different matter. Reserving is about having the money ready for the decision, not about forcing it.

What if I have not been reserving?

The calculator shows what should be banked at your current engine time if you had reserved from the first hour. The gap between that and your actual balance is what you will be financing when the overhaul comes due. Starting late means a higher per-hour rate to catch up before TBO.