Question of the Month: Does TBO Apply to Flying Clubs?

Get any two pilots together and drop the phrase “TBO”.  Now stand back and watch the fireworks!

Without doubt, if you operate an aircraft for long enough, it will, at some time in the future, require engine and propeller work.  Whether it is a repair, overall, or replacement depends on several factors, but it will happen, and clubs must prepare for that certainty.   Although the notion of TBO is most familiar when applied to engine(s) and propeller(s), there are some other actual or experience based TBOs that warrant attention…we’ll look at some of these later in this article.

TBO, “time between overhauls”, is a simple concept that is (very) often misunderstood, which can get expensive, quickly.  If you think about it, literal TBO doesn’t make much sense.  Why would you overhaul anything if it doesn’t need it?  In fact, knowing what we now know—that unnecessarily messing with anything will very likely result in a mess—then time, on its own, doesn’t appear to be a good enough reason to fiddle with things. In a nutshell…if it ain’t broke, don’t fix it!

Now, there is a lot more science behind this than a well-worn idiom.  In fact, in aviation terms, it goes back to the Second World War, when the RAF (Royal Air Force, not the other one) realized that initial reliability after performing time-based maintenance was actually worse than (in general) not doing maintenance at all.  I use “in general” here since common sense (and indeed more science) shows that maintenance is a good thing.  The point here is that blindly tearing down a complicated machine just because “it is time” can increase the risk of a problem.  As well as the chance that the work was not performed properly, there is a finite probability that, say, a tool is left in the wrong place, or a wire is torn loose, or…  If you have experienced the chaos of everything falling apart after your IT group completes some scheduled computer maintenance, then you’ll get the point.

On the other hand, not doing any maintenance at all and just letting things wear out isn’t going to fly (literally) in the case of aviation, and not just for aeroplanes themselves—we clearly want to ensure that the ATC system is maintained to a reliable level, that NOTAMs are reliably available (oh, wait…), and so on. Without getting into the weeds, this is why there is so much attention on “aging aviation infrastructure” at the moment…the uninformed see one failure as proof that the system is worn-out, which actually does a big injustice to the dedicated people who keep it running—and keep us safe.

Moving on.  I think it is fair to say that the notion of TBO is not just a manufacturer’s ploy to get people to part with money, and that it does, indeed, have a place in the continuum of preventative maintenance.  I’ll also say that in many situations it is a good thing, especially when history and statistics show that there are more benefits than risks.  By the way, note that we are talking here about actual, invasive maintenance and not inspections, mandated or otherwise.  Clearly, annual, progressive, 100-hour inspections—or whatever—are good things, especially when some time-bomb is found, but we are talking about the risks of performing complicated maintenance tasks—and I’d argue that pulling apart an engine is a complicated task! 

BTW—If you are now pondering whether 100-hour inspections apply to flying club aircraft, please read this article.

This is not a one-or-the-other argument. Some time-based maintenance is absolutely necessary, and we all should be doing it for some things and circumstances.  Checking the tyre pressures every week, lubing-up every month, changing the oil and filter after n-hours of use…all these make sense.  Nevertheless, checking tyre pressure is not likely to induce an engine failure, so we must always look at two sides of the coin and weigh the benefits of performing maintenance that might result in unintended consequences (and indeed the severity of the consequences) with the pretty certain result when not performing any maintenance at all.

There is so much more than can be said about this, and fortunately for us, much of it has already be said by people more qualified than me…see the reference at the end of this article!

Okay – the scene is now set for the relevant part of this article…does TBO apply to club aircraft?

Well, to start with, TBO is not mandatory for (flying club) aircraft operating under Part 91, unless prescribed by an AD, but it definitely should be considered as an arrow in the quiver of club safety and equipment reliability.  It just isn’t sensible to state that “we do not have to comply with TBOs” just because it isn’t mandated under the regulations. Indeed, a counter argument is that, if does apply to other operations under the regs, why wouldn’t we comply with it?  Afterall, the aircraft doesn’t care what FAR Part it is operating under! This is the same debate for the 100-hour inspection of flying club aircraft—even if club operations do not trigger FAR 91.409(b)—and they absolutely should not—if data shows that other operations are safer because of 100-hour inspections, why wouldn’t a club do them?  Some members will argue that it will increase the cost flying, as indeed it will, but what price safety? 

I suggest a better approach is to consider the non-mandatory aspect of TBO—and 100-hour inspections—as offering a level of consciously and carefully applied flexibility.  If a club plane flies 120- hours between (the non-negotiable) annuals, then perhaps it is reasonable for a club to exercise its right of not performing 100-hour inspections.  But what about the club plane that flies 200, 300, 400-hours between annuals?  If data (and the regulations so derived) state that aircraft operations for compensation or hire, or that are provided for purposes of flight training require 100-hour inspections to safeguard the public, why wouldn’t club members want to be equally safeguarded?

At the end of the day for flying clubs, the 100-hour argument probably comes down to the cost of flying—but again, what price safety?  If a club plane flies less than 100-hours between annuals, then no worries.  If a club plane flies, say, 120-hours between annuals, club members should be given the option (by voting) to wait for the annual, but if a plane regularly flies well in excess of 100-hours (and the actual threshold should be openly debated and agreed), then a payment plan should be put in place.  In reality, it doesn’t add much to the cost of flying as, by definition, a 100-hour inspection is amortized over 100 flight hours.  Let’s say a 100-hour inspection costs $1,500.  Add $15 per hour (equivalent to around 2-galls of fuel) to the usage rate, such that money is in the bank when the plane reaches 100-hours.  If for some reason it doesn’t reach 100-hours by the annual, use the extra money for an improvement fund, or provide members with a refund.  If you still decide not to do time-based inspections between annuals, at least be extra diligent at oil changes, such that you can track trends and evolving situations.

I propose that the situation of engine and propeller TBO is very similar to the 100-hour case.  Plan and budget based on the manufacturers TBO, but exercise flexibility by carefully monitoring actual conditions.  Let’s say that the recommended engine TBO is 2,000 hours and let’s assume that there is 1,000-hours “left” before that time is reached.  For a $30,000 overhaul (or replacement or whatever), you need to be saving $30 per hour.  If 1,500 hours are left on the engine, this drops to $20 per hour and with the new engine in place, the per-hour engine charge will drop to $15, so, start saving as soon as you can.  The same applies to propeller TBO, so tack on a few more dollars for that.

A quick clarification, here.  It is true that standard-category (airworthiness) club aircraft operating under Part 91 do not “need” to comply with TBO recommendations, unless mandated by an FAA-issued AD.  If you operate an LSA, you’ll need to be a bit more careful, as under the airworthiness consensus standards, manufacturers’ Service Bulletins carry the same weight and requirement for compliance as do ADs.


The big remaining question regarding TBO revolves around liability.  I get several calls a month from club officers enquiring about the risk of exceeding recommended TBO—could the club be found liable if something bad happens and the cause is traced back to a recommended TBO not being followed?    

The trite answer is “yes, there is always the potential of liability, since we fly airplanes”, but there is more to it than that.  A better, but equally unsatisfactory answer is “maybe”, but that begs the real question “What can we do to limit our liability if we decide to go beyond manufacturers’ recommended TBO?

I’m not going offer any legal advice here—for that, talk to our AOPA lawyers (you are a member of the AOPA Legal Services Plan, aren’t you? ), but a lot of this comes down to common-sense.  Exercising the privilege (under Part 91 as previously explained) does not relieve the obligation to maintain your aircraft in an airworthy condition.  If your A&P/IA determines that your beyond-TBO (or even before-TBO, for that matter) engine is not airworthy, then you’ll have choices to make.  On the other hand, if the engine is considered airworthy, I suggest there are still some sensible safeguards you can take to limit potential liability exposure—in fact, perhaps to stifle any suggestion that the club is negligent in any way by not adhering to the TBO—and that is to go above and beyond with preventative maintenance and record keeping.

Here are some suggested actions:

  • Increase oil change frequency, and of course change the filter every time
    • Yes – this is to keep the oil nice and clean, but it is also as an opportunity to…
  • Cut open the old oil filter on every change
    • Keep dated and annotated photos of the opened-up filter element—both sides
    • Identify and catalog any particles found in the element’s folds
    • You’ll likely find some carbon, but if you find “metal”, follow the engine manufacturers recommendations, to the letter
  • When draining the oil, take a sample and send it off to your favorite analysis lab (read the instructions before starting to drain the oil!)
    • Don’t hop between different labs.The point is to keep a growing record of trends, rather than just spot checks
    • Carefully read the analyses reports and keep them with your maintenance records
    • If you’re energetic, enter the data into a spreadsheet and plot some graphs, as it is easier to see emerging trends in a picture
  • Remove and check the oil screen for particles.This is a coarse trap to stop nasties from circulating though the oil system on wet sump engines—read your engine operator’s manual for more details. The screen is usually the first line of defense, whilst a full-flow filter is the second.Oils screens vary in location with different engines—even from the same manufacturer.The engine illustrated part manual is a good source to find out where it is—could be on the side of the crankcase or perhaps on the bottom.On my Bat’s O-235-L2C, the screen is on the bottom and is held in place with a 1” nut and crush washer.Be sure to have the right tools and a stock of crush washers before jumping in.You probably won’t fine a torque setting for the nut/new washer combo, but conventional wisdom says hand tighten until the washer mates with the crankcase, then add an extra three-quarters to one turn to squash the washer enough to form a good seal.Then safety wire the nut in place.If you don’t know what I’m talking about, pay your A&P to show you so you can do it next time!
  • Depending on the oil change interval, either at every change or every other, remove the (top) spark plugs and take compression readings, but perhaps more useful and important, use a borescope and take photos (or videos) of all valves and upper cylinder (head) areas as well as the condition of the cylinder walls
  • Keep impeccable records.Keep a binder with:
    • Tach and Hobbs times
    • Dated photos of the oil filter element and borescope results
    • It is not a bad idea to take photos of the electrode ends of all spark plugs, as they come out of the heads.This can be useful to determine emerging issues with piston rings, valve guides, etc.It is also a good way to visualize how members are leaning the engine—or not.For more about leaning, see here and here.
    • Oil analyses results…read and understand.Again, plotting results will help visualize emerging trends
    • Oil added between changes and keep a running tab of oil consumption (per hour)
    • Any other notes and observations
    • By the way, remember that most club management tools (such as Flight Circle) allow you to set reminders/notifications for imminent tasks, such as oils changes.

Other TBOs for Flying Clubs:

TBO is a concept that doesn’t only apply to engines and propellers.  Pretty much every part in every machine will have a useful life and that is why we do time-based inspections.  Indeed, some modern composite airframes have time limitations, beyond which “heavy” maintenance is required.  

Now, although I’m a strong follower of condition-based maintenance, there are two parts that I regularly change/overhaul on a 500-hour schedule, and they are the vacuum pump (yeah… I know), and the magnetos.  These parts are just too important to leave to chance, and there is not much that you can see, externally, to help determine condition.  I just add the cost of these, divided by 500, into the per-hour cost of my plane, so that the money is there when I need it.

Take a long hard look at your airplanes operator’s manual for maintenance schedules—what the manufacturer recommends doing every 50-hours, 100-hours, etc.  Every airplane is unique—so listen to it and get to know its quirks beyond “just” the recommendations.  For example, I’ve found that the shimmy dampener on my Aerobat needs a service every 55-hours to prevent the sudden onset of shaking…all the seals and so on are in good shape—its just what this unit does!

Beyond the generally accepted understanding of TBO, why not stretch the notion a bit further to include all other time-based club tasks?  These could include:

  • Annual planning and budgeting
  • Updating members’ information – things like flight hours, sure, but also a skills and talents spreadsheet to make it easier to find members to help with various tasks and projects
  • Periodic review of bylaws and operating rules—they get outdated and stale surprisingly quickly
  • Membership of various committees to ensure good representation of members


For clubs operating under Part 91, there is no obligation or requirement to conform to manufacturers’ recommended time between overhauls unless accompanied by an FAA-issued AD, or manufacturer-issued SB if the plane is certificated as an LSA.  Nevertheless, unless you really believe that TBO is just a ploy to get you to spend more money, you’d be wise to consider it as being based on data derived over many thousands of operational hours, so establishing a statistical window.  Rather than just ripping out the engine at exactly TBO, think of it as an alarm call to get your attention. As your engine or whatever approaches the stated TBO, start taking extra precautions and ramp-up inspections and preventative maintenance as, statistically at least, things will start to degrade.  Getting ahead of the curve will allow you to establish “normal” behavior, so making it easier to detect the inevitable decline. 

As you go through the process of planning this out, think about other aspects of flying clubs that have some notion of time sensitivity, and include them in your planning and notification system—you’ll be glad that you did!

See this month’s Club Spotlight and Resource Spotlight for other ideas to sensibly take your engines beyond TBO.

As always, fly lots and fly safely!


Here are some good resources to review:

Stephen Bateman
Contributor, You Can Fly Program
Steve retired from AOPA in April 2024, but continues to contribute to You Can Fly programs. Contact Steve at [email protected]

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