To TBO or not to TBO?

[jetdriven]

I replaced my functioning Slick mags with two rebuilt Bendix units, then one failed 7 hours later. IN that case,

I was probably safer before doing the swap.

Still, when it comes tk mags, and especially that Bendix dual magneto, I strictly follow the 500hr IRAN schedule. But everything else I run until it falls off. Just make sure there are no consequences of it falling off.

[pinerunner]

While the data used at the start of this thread can be criticized for its lack of a denominator, Busch admits this and points out that its the best he could do and why. I think it does a great job of backing up the adage "Be on guard right after work has been done."

[DrBill]

The flight school at my airport routinely goes 3000-4000 hrs.  Of course thosePiper Cherokee 160HP planes are flown daily and maintained every 100 hrs but in the 9 years that I've been there not ONE engine related mishap on any of the 4 aircraft.  The only mishaps were two pilots landing fast and long  and ran off the end of the runway rather than do a go around.. The common thread ?  "I never had to do a go-around before !"

BILL

[WardHolbrook]

The flight school at my airport routinely goes 3000-4000 hrs.  Of course thosePiper Cherokee 160HP planes are flown daily and maintained every 100 hrs but in the 9 years that I've been there not ONE engine related mishap on any of the 4 aircraft.  The only mishaps were two pilots landing fast and long  and ran off the end of the runway rather than do a go around.. The common thread ?  "I never had to do a go-around before !"

BILL

 

It probably wouldn't be the best thing to extrapolate a flight school's experience to what the "average" private owner might expect. For operations under Part 91, the published TBO is rather meaningless, but it does provide us with a convenient reference point for budgeting. Although Part 135 operators must honor the published TBOs, we are allowed to perform our maintenance "On Condition", This puts the onus on us to see that appropriate maintenance is performed whenever necessary. In the case of a busy flight school or other high usage operation, they typically perform oil analysis and have some sort of trend monitoring program in place and the engines get a good looking at on a very regular basis (both hours and calendar). For the average privately owned and operated aircraft that's normally not the case. When it comes to engine longevity, there's a big difference between 50 to 100 hours per month and 50 to 100 hours per year.

[DrBill]

It probably wouldn't be the best thing to extrapolate a flight school's experience to what the "average" private owner might expect. For operations under Part 91, the published TBO is rather meaningless, but it does provide us with a convenient reference point for budgeting. Although Part 135 operators must honor the published TBOs, we are allowed to perform our maintenance "On Condition", This puts the onus on us to see that appropriate maintenance is performed whenever necessary. In the case of a busy flight school or other high usage operation, they typically perform oil analysis and have some sort of trend monitoring program in place and the engines get a good looking at on a very regular basis (both hours and calendar). For the average privately owned and operated aircraft that's normally not the case. When it comes to engine longevity, there's a big difference between 50 to 100 hours per month and 50 to 100 hours per year.

EXactly why I mentioned it.

BILL

[WardHolbrook]

You can't assume that your engine will be one of those which can be safely and successfully operated beyond TBO. If you're going to play that game you'll need to monitor the health of your engine using all the available tools such as oil-filter inspection, oil analysis, monitoring oil-consumption and oil-pressure trends, compression tests, borescope inspections, spark-plug inspections, and digital engine-monitor data. This is really something that ideally should have been going on since the last overhaul. Even if you do all of that, there are simply no guarantees that your engine will let you know, in advance, that it's had enough.

 

It may be false economy to try to get a few more hours out of a run-out personally owned aircraft that only gets flown a hundred hours a year or so if those extra hours end up increasing your costs when overhaul time finally arrives. It would surprise most guys what actually needs to be replaced on an engine in order for it to be labeled as overhauled. A lot of things can legally be pulled out inspected and reused and overhaul quotes are made with that assumption. If you end up ruining something like a crankshaft you'll find that they don't come cheap and can easily end up costing you more than what you saved by running past TBO.

My experience tells me that as you approach TBO you'd better have the money set aside for the overhaul.

[ryoder]

Ward I am sure you know more about airplanes than I do but this quote from Mike Bush is interesting and refutes the notion that flying over tbo is false economy.

"Continuing to fly an engine beyond TBO is false economy, because doing so just makes the inevitable major overhaul more expensive."

This old wives tale probably originated back in the days when new cylinders were very expensive and most engines were field overhauled using reconditioned (chromed or oversized) jugs. In those days, if you pushed an engine to the point that its cylinders could not be reconditioned, you'd have to spend more at overhaul to buy new ones. Nowadays, however, the cost of new cylinders has come down to the point where most major overhauls include all new jugs as standard procedure. Consequently, there's no longer any real advantage to overhauling sooner rather than later. The only things that will impact the overhaul cost are an unserviceable crankshaft or a cracked crankcase, and neither of those items are any more probable for an engine operated beyond TBO.

By the way, it's not just owners who hold these misconceptions. Plenty of A&P mechanics believe these things, too.