TTAF, SMOH, TBO: What Engine Time Actually Means When Buying

Every aircraft listing quotes three numbers: TTAF, SMOH, and TBO. Buyers treat these like price stickers — lower is better, higher means discount. That is not wrong, but it is incomplete. The relationship between these numbers matters more than any single one of them, and the story behind SMOH matters more than the number itself.

TTAF: Total Time Airframe

TTAF is the total flight hours logged on the airframe since manufacture. A 1979 Cherokee 180 with 4,200 hours TTAF has been flying for 47 years and accumulated 4,200 hours of use. The airframe itself — aluminum structure, spars, control surfaces, fuselage — does not have a mandatory retirement life for most GA single-engine aircraft. There is no FAR that says a Piper Cherokee must be retired at 10,000 hours the way transport category airplanes have life limits.

What TTAF tells you: how much wear the systems have seen, how much corrosion could have accumulated over years of operation, and how much maintenance history you are inheriting. A high-time airframe on a well-maintained aircraft is not a problem. A low-time airframe on an aircraft that sat outside in coastal Florida for 20 years may have corrosion that is worse than 4,000 hours of hard flying.

TTAF also matters for financing. Most lenders put age and time limits on aircraft they will finance. An aircraft over 30 years old with over 5,000 TTAF will get pushback from some lenders even if the aircraft is perfect.

SMOH: Since Major Overhaul

SMOH is the hours elapsed since the engine received a major overhaul. This is the number most buyers focus on — and the one most frequently misunderstood.

First: what counts as a major overhaul? Under FAR Part 43, a major overhaul means disassembling the engine, inspecting all components against serviceable limits, replacing parts that are out of limits, reassembling, and running the engine on a test stand. An engine overhauled to new limits has all internal dimensions within the manufacturer's new part tolerances. An engine overhauled to serviceable limits has internal dimensions within the larger wear tolerances — still legal, but tighter clearances than new.

This distinction matters. A Lycoming O-360 overhauled to serviceable limits at 300 hours SMOH may not perform like a factory new engine. A Lycoming O-360 factory rebuilt by Lycoming is as close to new as it gets.

SMOH also does not tell you who did the overhaul or what condition the engine was in going in. An engine overhauled because of a sudden stoppage or prop strike may have internal damage that survived the overhaul process. An engine overhauled proactively at TBO with good compressions going in is a different situation entirely.

TBO: Time Between Overhauls

TBO is the manufacturer's recommended interval between overhauls. Lycoming's TBO for the O-360 is 2,000 hours. Continental's TBO for the O-300 is 1,800 hours. These are recommendations, not legal requirements for non-commercially operated aircraft.

Under FAR 91.409 and 91.421, a privately operated aircraft is not required to have its engine overhauled at TBO. The engine must be airworthy, but an owner can legally fly past TBO on a private aircraft if an IA signs the engine airworthy at each annual. Commercially operated aircraft have different rules — TBO is mandatory for most 135 and aerial work operations.

This means a listing showing "2,200 hours SMOH, 2,000 hour TBO" is not illegal. It means the engine is 200 hours past the manufacturer's recommendation and the IA has been signing it airworthy. Whether that is fine or a problem depends entirely on compressions, oil consumption, oil analysis history, and what the borescope shows.

Reading TTAF, SMOH, and TBO Together

Here is where buyers go wrong: they look at SMOH in isolation. The right way is to read SMOH relative to TBO, then look at the evidence for engine condition regardless of either number.

Low SMOH (under 500 hours). The engine is freshly overhauled. That sounds good — but ask why. Was it overhauled proactively before TBO? Or after a sudden stoppage? An engine at 1,400 hours SMOH that has been flying well is often more trustworthy than one at 200 hours SMOH with a murky backstory. Get the overhaul records.

Mid-time SMOH (500-1,200 hours). Prime territory if the engine has documented history. Compressions, oil analysis, and consistent oil consumption matter here. This range is past break-in and still has significant life remaining.

High-time SMOH (over 1,500 hours on a 2,000-hour TBO engine). The engine has seen a lot of use since overhaul. An O-360 at 1,800 hours SMOH with 78/80 compressions on all four cylinders and clean oil analysis is a better buy than one at 400 hours SMOH with three cylinders at 68/80 and rising oil consumption. But you are buying less remaining life, and you should price that in.

Run-out or over TBO. Factor an overhaul reserve into your offer. Lycoming O-360 factory overhaul runs about $18,000-$22,000. Divide by TBO hours to get your hourly engine reserve — typically $9-$12/hr on common engines. If the engine has 400 hours left to TBO, that is $3,600-$4,800 in reserve you have not had to build yet.

Factory Rebuilt vs. Field Overhauled

This distinction can swing value by $5,000-$8,000 on the same engine.

A factory rebuilt engine (Lycoming New Rebuilt, Continental Reman) is disassembled at the factory, rebuilt to new tolerances with new or zero-time parts, and comes with a new engine log — SMOH resets to zero and the engine is treated as new for most purposes.

A field overhaul is performed by an FAA-certificated repair station or IA. Quality varies. A reputable shop with a track record on your engine type delivers results comparable to factory. A field overhaul from an unknown shop on a run-out engine is a different proposition.

Always ask: who did the overhaul? Where are the overhaul records? What were the cylinder measurements at reassembly? A legitimate overhaul has paperwork — work order, parts list, measurements, 8130 tags on new parts. If the seller does not have these records, price the engine accordingly.

The Tests That Actually Matter

SMOH is a number. These tests tell you what the engine actually is right now.

Differential compression test. Checks each cylinder against an 80 psi reference. 76/80 or better is healthy. Anything consistently under 70/80 is a conversation. Values in the 60s usually mean cylinder work. Ask for the last three annual compression readings, not just the current one.

Oil analysis. Spectrographic oil analysis costs $25-$40 per sample. Send samples every oil change and you build a trend baseline. An engine with 18 consecutive clean oil analysis reports is telling you something important. Elevated iron, chromium, or lead means look harder. Trend matters more than a single reading.

Borescope. A fiberoptic camera through the spark plug holes shows cylinder wall condition, top of piston, and valve stems. It is a 30-minute procedure that reveals scoring, corrosion, and lead deposits the compression check alone will not catch. Worth paying for on any aircraft you are serious about.

Oil consumption. A healthy Lycoming or Continental burns less than a quart every 6-8 hours. An engine burning a quart every 2-3 hours is telling you something — ring wear, valve guides, cylinder glazing. Ask the owner how often they add oil between oil changes.

What to Ask the Seller

Before you spend money on a pre-buy, get answers to these:

• Who did the overhaul, and when? Can I see the overhaul records?
• What were the compression readings at the last annual?
• Is there an oil analysis history? Can I see the reports?
• What is the typical oil consumption in quarts per hour?
• Any sudden stoppages, prop strikes, or overspeed events? Are they documented in the logs?
• Was the overhaul to new limits or serviceable limits? A seller who can answer these with documentation knows their aircraft. A seller who cannot — or whose answers do not match the logbooks — is telling you something too.

Bottom Line

TTAF tells you how long the airframe has been flying. SMOH tells you when the engine was last rebuilt. TBO tells you the manufacturer's recommended interval. The number that matters most is the one you cannot get from a listing: the actual condition of the engine right now.

Buy the paperwork, not the SMOH number.

Browse aircraft listings on List Buy Fly — every listing shows TTAF, SMOH, and TBO so you can compare before you call.

---

Written by the List Buy Fly editorial team — pilots writing for pilots.