TIME TO RUN (TTR) vs TIME SINCE OVERHAUL (TSOH)

This week I thought we would take an in-depth exploration of the difference between expressing engine time in TTR and TSOH. And hopefully I'll refrain from an impassioned rant about why brokers and other aircraft websites should only use TSOH!! Stick with me here - I have lost sleep over this.

The reason for my hoo-ha about the issue? I still commonly see advertising for aircraft with reference to TTR, and I scratch my head in wonder. Resultantly, "How long does the engine have to run?" is a common question asked by Buyers. "How long is any piece of string", is my (perhaps overly sarcastic) response. It's simply not all that important. Let's explore why.

What's the difference?

TSOH, as the name suggests, is the number of hours since a MAJOR overhaul was completed. And
similarly, NOT a bulk strip. Instead, a major overhaul is when the engine is completely overhauled (to a 'new' state) and the time is 'reset' in the engine logbooks by way of calendar and operating time, in line with the manufacturer's prescribed requirements. A bulk strip is not an overhaul. It is a 'tear-down' of the engine for diagnostic purposes, and may not involve the replacement of ANY parts!

Additionally, it's not referencing time since a top-end overhaul. A top-end overhaul is only related to work conducted on the upper 'half' of the engine, generally only above the engine block. In short, the major factor affecting value of the engine (and determination of likely future cost), is the running time completed SMOH!

Taking it '10% past' is the limit? Really?

For most Australian private and aerial work piston aircraft, the manufacturer's TBO is a recommendation rather than a mandatory retirement limit. In many circumstances, an engine can continue operating beyond its published TBO provided it remains airworthy, and the maintenance requirements are satisfied.

You'll often hear people talk about "10% past TBO" as though it represents some hard and fast limit. In my experience, that figure is more of an industry 'rule-of-thumb' than a magic number supported by compelling engineering evidence.

It's important to listen to your LAME's advice, but I'm going to put my neck on the block here and say that the '10%' is just a number that is 'bandied around' the industry; it's not based on evidential proof that the risk of failure increases exponentially past it.

Now, before every LAME in Australia writes me an angry email, let me be clear: there are perfectly good reasons why engineers may become increasingly cautious as an engine moves beyond TBO. Risk management, maintenance planning, insurance considerations, resale value and operational experience all play a role.

But the idea that an engine is perfectly safe at 9.9% beyond TBO and suddenly dangerous at 10.1% has never made sense to me.

An engine is like a living, breathing thing

We all have different health conditions, we 'age' at different rates, and our living environment, and how we chose to live, can often determine our overall health! An engine is similar.

As we know, the engine is one mechanical component of an aircraft and its health can be assessed by a licensed LAME. It is always, therefore, effectively 'on condition'. It can be signed-off to operate for another 100 hours at an annual, or condemned to an overhaul immediately. Or condemned at ANY point in between! TBO is only a manufacturer's recommendation, and most light aircraft in Australia are flying past their calendar TBO (at very least), and many past their TBO by hours. It's like a 'best before' date on a packet of potato chips. We don't look at it, because the chips are sealed and still good often months (I won't say years, but you get it) after the 'best before' date.

Two identical engines may leave the factory on the same day and have dramatically different lives. One might spend twenty years flying regularly, achieving full operating temperatures and receiving meticulous maintenance. The other might spend years sitting idle in a humid hangar near the coast, started and run for a few minutes occasionally (this is not great for the engine, but that's another story for another day). Which one would you rather own?

This is why TBO should be viewed as an important planning guide, rather than a crystal ball.

Engine manufacturer's reputation / legal liability

Let's be honest; if you were manufacturing engines, it's likely that you would choose a conservative recommended number within which you estimate an overhaul should happen. That's because you don't want liability or bad press and reputational damage. Don't you agree?

Where can I easily find the TSOH?

This is a little hack that I want to share. Open your engine logbook, and at every service interval, there should be a CASA Form 728  Piston Engine Condition Report! The information is on the form! You'll see the DATE that the engine was overhauled (major overhaul), and the time since that overhaul!

Now, manufacturer's TBO shouldn't be ignored. If an engine is approaching TBO (by time), it's an indication that the time to overhaul is LIKELY to happen in the next few years, so planning for it would be prudent. But, I've seen engines that were overhauled 30 years ago exceed TBO and fly on happily, for hundreds of hours. Having an honest discussion with your LAME on the facts is important here.

The wash out

So what's the main message here? Let's only reference TSOH in advertising, as the determinative number, NOT TTR!! For the buyers of aircraft intended to be used in CHARTER (or RPT), the date of last overhaul and the TTR is important. But they can easily work it out for themselves.

For the rest of us - it's as useful as runway behind us, fuel on the ground, or 20:20 hindsight after an incident.

What really matters is:
• How many hours since major overhaul?
• How has the engine been operated?
• How has it been maintained?
• What do the records tell us about its condition?

Those questions will usually tell you far more about the future cost and reliability of an engine than a simple TTR figure ever could. It serves to complicate and confuse what should be a very simple indication of how much the engine has worked (wear and tear), since it was made 'factory new' again.

And that's why I keep banging on about TSOH.