"hours" Definitions

[Pom pom]

Hi everyone.

Could you please give me information about the terms used when describing "hours" for a particular aircraft.

I've seen things like SMOH and the like, but would really appreciate a list of all such abbreviations, what they stand for, what they actually mean (in plain English), and what restrictions/implications each has in respect of types of operation, insurance, maintenance (in general terms) etc.

I'm at the very earliest stage of contemplating purchasing my own aircraft (old, SEP, perhaps 172 or PA28) and need help understanding the most basic before I can make any realistic assessment of whether or not it's a possibility.

Cheers, PP :)

[Pilot DAR]

Pom,

Aircraft ownership is fantastic, as long as you have allocated adequate funds, and will use the plane enought to justify it.

Your question is fair and reasonable, and you're going to get good information here. I'll give you some basics:

TTAF = Total Time Airframe = the number of hours it has been in the air since new from the factory. No maintenance will change this number ever. It applies to the whole airplane, until engine(s) and/or propeller overhaul, then those times will never correspond to TTAF again in the aircraft's life.

SMOH = Since Major Overhaul = and can be applicable to the engine(s) and propellor, but not the airframe. If the manufacturer's specified overhaul procedure, and mandatory parts replacement is performed on an engine or prop by a qualified person/shop, the engine and or prop can get a zero SMOH, which is a definate positive, though very certainly drives up the aircraft purchase price that much.

STOH = Since Top Overhaul (or other references to "top overhaul" means the engine cylinders have received major maintenance, but not the "lower end" which is the crankshaft, crankcase, camshaft etc. This work is often accomplished to return to original, the engine's performance, but will not address any proplems associated with "making metal" (wear).

There are other "hours", but for a light aircraft, those are the most important to consider when purchasing.

That said, don't always look for the lowest time airframe in an "older" aircraft. I'd rather buy an aircraft which has an annual average use of 50 to 200 hours, than one which has sat doing nothing for long periods.

I'm much more concerned about the actual inspected condition of the aircraft, than it's airframe time. The two can be surprizingly un related.

That should get to started, but there certainly is more to this than meets the eye. Your questions about purchasing are the tip of the iceburg!

[Pom pom]

Thanks Pilot DAR, that's a great answer - just what I needed! One more for you (or anyone!) :

What does ARC mean? I've seen it in some ads, and I think it pertains to engines with quite some hours on them, but I don't really know. It may be a CAA/JAR thing so, looking at your location, not sure if you can help, but thanks again for your original answer.

Always trying to learn, PP. :ok:

[A and C]

ARC = Airworthiness Review Certificate.

It's an EASA thing and requires to be renewed annualy, most people tie it to the annual check but this is not a requirment.

[Pom pom]

Thanks A and C. I've googed the full term and found lots of useful starting points.

One more question if I may : what is the relationship between 2000 engine hours /2500 hours / training (flying school) / public transport / aerial work? It was suggested to me that leasing my (hypothetical) aircraft to a school would be a way of off-setting some of the running costs, but some complications exist re engine hours - hence my further queries.

:confused:

[Maoraigh1]

If looking at an aircraft to buy, SMOH etc can be meaningless. Check in the log book what happened at the major overhaul. Understand Factory New Tolerances and Permitted Tolerances. Get YOUR engineer to look at the aircraft. The cheap way is to look at aircraft and logbooks yourself first, then pay your engineer to go and examine what you think is O.K.
If you can find problems, take your engineer's advice as to whether it's worth getting him to look at it.
And why did a LyCon with 300 hours need a top head overhaul??

[englishal]

If the engine has had a "major overhaul" and hence zero timed (meeting or exceeding all manufacturers specs and tolerances and mandatory parts replaced), then it should be issued with a new logbook starting at zero hours. This is the only overhaul I'd be concerned with, if it had an "overhaul" but not zero timed then the engine should be considered to have run the same number of hours as before the overhaul for the purpose of financial negotiations if you get what I mean !?!

There is also TBO which is the Time Between (major) Overhaul recommended by the manufacturer. An engine near to TBO should be considered not worth much above its core value, even if the aeroplane is running fine. You can go beyond TBO for private ops and run the engine "on condition" if you want, though personally I'd get a major overhaul done by TBO. Engine overhauls cost anywhere from £15000 to £30000 for typical 150-250HP types. Our TO360 cost £28,000 to overhaul (zero time), though we had it balanced at the same time. So when factoring cost of the aeroplane you should take this rebuild figure into account and an aeroplane with low hours on the engine could be worth say £25000 more than one at TBO.

[Duchess_Driver]

In general terms if you want to 'make money' from your aeroplane you need to have it maintained to what is often referred to a 'public transport standards'. (We're talking EASA here...????).

To lease your aircraft back to a flying school, for them to make most use of it it would need to be maintained in accordance with those 'Public' standards rather than the lower standards required for private ownership. Some private owners would maintain to public standards anyway.

Part of those 'public transport' requirements is that the engine is lifed either on hours or years. Each engine has its own TBO, typically 1600-2000hours - plus an allowable extension to those hours (as englishal says) "on condition" that the compressions are okay. Usually, no matter how few hours the aircraft has done if it goes beyond (IIRC) 12 years old it needs to be overhauled.

The only way to make a small fortune in aviation? Start with a large one!

[Pom pom]

Thank you guys/gals.

Fantastic info from all of you. First rung on the ladder (very, very tall ladder I'm beginning to realise) of aircraft ownership now climbed! But, as the song goes : "if you don't have a dream....."

And I have a deeper respect for flying clubs now!

But why are aircraft engines so flippin' expensive!? I guess it's the economy of scale (inverse, if you see what I mean).

Cheers, PP :eek:

[Tinstaafl]

Not just economies of scale. There is also the paper trail that approved aviation parts must have. Quality control tracking mandates that aviation products have to be able to be tracked from manufacture to service. Sometimes even down to the raw material batch.

Limited competition doesn't help either.