Why are airplane engine TBO hours so low?

WDD said:
Double the displacement; larger diameter pistons and/or longer stroke and/or another two cylinders. But that would be a lot more piston mass running at high RPM's; all sorts of problems with that, including new crankshaft, bearings to handle the load. Probably need a new valve design to handle the air mass / velocity. Unlike the lower RPM Continentals, the higher RPM Rotax might need 4 valves per cylinder valves to breath. Cooling would have to be re-done. New design on oil circulation / lubrication / oil cooling. Reconfigure the case to handle the higher propeller load.

As much as I'd like to see it, I wouldn't think it would be that easy.
Click to expand...
They should just slap another one on the other side of the reduction gear... just like that 300hp and v8 sound.
 
Adam Weiss said:
Coming at this from a completely different angle.
I put 100 to 150 hrs/yr on my plane.
That means the engine will last 13-20 yrs.

I’m good with my engine lasting ~15 years.

If I ran my car as hard as I run my plane, I’d be OK replacing the engine after 15 years too.
Click to expand...
But replacing the engine in your car is generally less than $5K for many of them, $10K at most with labor, not $40K+.
 
Jim K said:
They should just slap another one on the other side of the reduction gear... just like that 300hp and v8 sound.
Click to expand...
A twin flat four, eight cylinder dual crank high revving monster!!
 
But for the performance we're getting out of the 360/520/540, it's not comparable to a high performance Porsche. It's comparable to a 70s 454. The complexity isn't Ferrari, it's a basic push rod V8. Hell, even the Rotax is cheaper and it's far more complex than the Lycoming/Conti variants.
 
Tantalum said:
they haven't had innovation. My friend's partner nearly went for a swim in Long Island sound because one of the plastic magnetos failed in a peculiar fashion that was causing bad misfirings, limped home and the engine died on the ground. I've got the email from a mechanic somewhere. That's inexcusable that we still rely on tech like that. Our spark plug gaps are so itsy bitsy that the slightest bit of fowling ruins it. Even a 50 year old car engine runs better, check out the guys at Corvair..

..and they are cheaply built, sand casting a mold with crummy metallurgy is cheap, requiring careful break ins.

..and they are low volume

I don't know why some engineers take this so personally. There's no defendable objective argument that can be made to justify the "low tech" nature of these engines.
Click to expand...

What’s inexcusable is that your friend did not properly troubleshoot the engine and shut off the bad mag. The other things you point out are because they are air cooled engines that must operate over big deltas of temperature and pressure. They have to maintain cooling at widely different air speeds and continue to operate from -3 to 9 G’s in all attitudes. They have to operate at a RPM that keeps prop tips subsonic.
Many aircraft engines have been used in other applications especially generators. J79 jet engines that ran for 600 hours in a F4 run for a decade continuously in a power plant. Lycoming Piston engines have been purpose built to run ground generators or aircraft engines modified to do the same run almost forever.
Ask Honda to build you a lightweight engine with the degree of reliability required and the ability to operate through extremes of temperature, pressure, 2700 max rpm, unusual attitudes ect..
You will find they don’t even want to try.
 
Last edited:
Tantalum said:
they haven't had innovation. My friend's partner nearly went for a swim in Long Island sound because one of the plastic magnetos failed in a peculiar fashion that was causing bad misfirings, limped home and the engine died on the ground. I've got the email from a mechanic somewhere. That's inexcusable that we still rely on tech like that. Our spark plug gaps are so itsy bitsy that the slightest bit of fowling ruins it. Even a 50 year old car engine runs better, check out the guys at Corvair..

..and they are cheaply built, sand casting a mold with crummy metallurgy is cheap, requiring careful break ins.

..and they are low volume

I don't know why some engineers take this so personally. There's no defendable objective argument that can be made to justify the "low tech" nature of these engines.
Click to expand...

There were 1,111 single engine piston aircraft sold in 2019. 234 twin engine sold. So roughly 1500 total new engine sales for new aircraft. Chevy alone sells about 1,000 Corvettes every month. Low volume and strict regulation has forced the lack of innovation. Low volume is partially on us. But whats new is old and what's old is new. Why buy new when the same thing made in the 70's is what you'd be buying today and save 300k.

With that, I was leading a race one time when the engine started backfiring and missing. It got progressively worse. Actually improved lap times because it slowed my entry and could go faster in corners. Exited track after winning, and it died on the ramp. Ignition was toast. In our planes we, can switch a mag. Or switch to a mag if running surefly/electroair and have that fail.

Look at Rotax engines.


Still a 2000 hour tbo despite some innovation. Why? People really don't like the possibility of falling out of the sky.
 
SoonerAviator said:
But for the performance we're getting out of the 360/520/540, it's not comparable to a high performance Porsche. It's comparable to a 70s 454. The complexity isn't Ferrari, it's a basic push rod V8. Hell, even the Rotax is cheaper and it's far more complex than the Lycoming/Conti variants.
Click to expand...

The performance is better than the Porsche at 2700 RPM. Those pesky airflow, noise and drag rules with supersonic prop tips do force design changes.
 
Last edited:
Idle thought: Why complain about a 2000 hour TBO for an engine in the airplane that flies less than 30 hours per month? How many of the small GA engines reach calendar TBO before operating hour TBO?
 
SoonerAviator said:
But for the performance we're getting out of the 360/520/540, it's not comparable to a high performance Porsche. It's comparable to a 70s 454. The complexity isn't Ferrari, it's a basic push rod V8. Hell, even the Rotax is cheaper and it's far more complex than the Lycoming/Conti variants.
Click to expand...

Dunno. With my IO-320, I can zip around at 180 mph all day long AND get 23 mpg doing it!
 
WDD said:
Again - rotax is a modern engine. Has a 1500 hour TBO. HP is 100.
Click to expand...
Lycoming's O-235, built in models from 100 to 118 HP, have a TBO of 2400 hours. They also have three O-235's in 125-HP versions that have a TBO of 2000 hours.
 
Jeff767 said:
The performance is better than the Porsche at 2700 RPM. Those pesky airflow, noise and drag rules with supersonic prop tips due force design changes.
Click to expand...
Lol, yeah well factor in displacement differences and it doesn't look quite as amazing. Getting 200HP out of a 3.0L versus a 5.9L (IO-360) puts it into perspective a bit more, neither engine being stressed to do so. My point isn't that the tried-n-true air cooled engines are terrible, just that there are obviously a number of giant advances in engine design that could allow for better ease of operation (FADEC, Hot/Cold starts) as well as longer service lives and small fuel efficiency increases due to liquid cooling and tighter tolerances. The finances don't justify anyone investing the money to bring a modern product to market, but that doesn't mean there wouldn't be a significant improvement in piston aviation engines if they did so. I still think a modern all aluminum turbo diesel would be a perfect fit. Miserly fuel burn on cheap Jet A, great power/tq at low rpm, can be designed to run at low rpm all day long. The DA-62 is a great example of it, even though it's not a clean sheet Aviation diesel.
 
NordicDave said:
The only new modern engine to take to the market the Rotax.Yeah, I know small displacement primarily in LSA's and the Sling 4.

Terrific engine with FEDEC with models including fuel injection and turbo-normalized. Wish other manufacturers would take this lead and innovate.
Click to expand...
Why bother if Rotax already has done it?

If Rotax built 200- and 300-HP engines with FADEC and stuff like their little engines, maybe they'd take over the GA market. They'd have to do STC's to replace all those legacy engines out there, just like Lycoming or Continental would if they built such engines, and there's the problem. A much more expensive engine plus a bunch of expensive R&D and paperwork. Would anyone really want to buy a new type of engine (for a gazillion dollars) to install on their airplane when no one has done any testing of the engine and prop combination on that airframe?
 
Last edited:
SoonerAviator said:
I'm sure if the GA gods could go back in time to the 30's/40's when the transition from radials occurred (know what we know now), they would have started designing diesel aircraft engines and we'd be fine. Diesel engine tech, while it has improved, hasn't changed a ton over the decades, and we'd all be on Jet-A with no lead necessary. We'd have FADEC liquid-cooled all-aluminum turbo diesels that are perfectly suited for the job. However, there's not much money into converting things at this point, so we get what we get.
Click to expand...
You can have an SMA diesel conversion for $80K. It's an opposed four-cylinder 230-hp turbo diesel with a big intercooler and a huge oil cooler, since the heads are air- and oil-cooled, cylinders air-cooled. Comes with a three-composite-bladed constant-speed prop, FADEC and all the rest. STC'd for the 182Q. It's available now. Has been for 22 years now. Few buy it, of course, just as few buy Lycoming's ie2 engines. SMA, when I was working on fixing one of their engines ten years ago, had spent $1 Billion dollars on the program and had 50 of them flying worldwide at the time. A lesson in aviation economics.

Owners are full of ideas about how other people should spend great buckets of money to develop and sell engines for the same price as a car engine.
 
Last edited:
Chip Sylverne said:
Folks were all hot and bothered about the Dyna-Cam, Axial Thrust, Roto Cam etc. a few years ago. What happened to that?
Click to expand...
"Revolutionary" engines that bankrupt their developers. It looks so simple until you go to do it.
 
If piston certified GA survives long enough- and it got some reprieve with Covid- I think the move to diesel is inevitable. It all depends on SR22 really. The extra cost of putting diesel into near 1MM plane is pretty negligible. The advantages (especially for non-US) are abound. Once they go that route, avegas IO engines days are basically numbered for anything new.
 
David Megginson said:
The Germans in the Second World War were wizards with diesel -- their diesel-powered E-Boats (Snellboots) were all metal and heavier, but could still outspeed the British light wooden MTBs and MGBs with petrol engines, and outrange the American petrol-powered PT boats. Despite all that, they didn't do much with diesel in their aircraft, whatever the reason (it wasn't a lack of capability). Maybe they never managed to deal with the vibration. (?)
Click to expand...
Compression ignition relies on the heat of compression to ignite the fuel. At altitude, where the air is thin and very cold, this doesn't work so well. You need lots of boost, and even then the cold is a problem. There's not much latent heat in really cold air.

A review from 2012: https://www.aviationconsumer.com/aircraftreviews/cessnas-new-diesel-sma-sr305s-oem-debut-2/
 
genna said:
True, but as discussed, tbo is just a suggestion. This is a new engine, they may be very conservative. This engine is also hard to hurt with improper operation.
Click to expand...
TBOs on geared engines are often lower due to the gearbox wear. On the original Thielert engine it had a 300-hour gearbox TBR (time before replacement). Went to 600 hours after a while. Don't know what Austro's version is for the gearbox.
 
Last edited:
IK04 said:
I submit that any modern automotive engine would far outlast an air-cooled aircraft engine from the mid-20th Century in the same application.
Click to expand...
It probably would, but getting it to work reliably as an aircraft engine is the problem. Homebuilders are doing it all the time, with some notable failures and few real successes. It's just not that easy.

I've been there and done it.
 
deonb said:
Or gas-electric hybrids.

Once you have a super-reliable Electric motor, you can put in an automotive-grade gasoline engine to power it, and then provide a battery backup. You just need enough battery backup to cover an engine failure at low altitude. Maybe 30 mins. Combination of those would likely yield on par or better than the safety record of existing piston aviation engines.
Click to expand...
You're adding a LOT of weight, and weight is the enemy, to say nothing of a lot more failure points. Works good in vehicles that don't fly.

Words like could and might and likely just don't get anything done. They're just signs of wishful thinking, not based on any real facts and numbers or knowledge of the issues. Real facts and numbers gave us the engines and airplanes we have now. Real facts and numbers are currently seriously hampering useful and practical electric flight.
 
I understand from discussions here and elsewhere that Rotax engines are getting more reliable, which is great. When I was doing my PPL and IFR training in 2002-2003, there was another (now defunct) flight school on the field that flew Diamond Katanas with Rotax engines -- let's just say that more than a few of their students got excellent practice with forced landings.

When they were lucky, the failure happened in the circuit with the power reduction on base and final, so they'd just glide in and not have to report anything; from time to time, it was off airport into a field, and there would be a newspaper article. As far as I know, no one was every injured -- it's mostly flat fields around here, and the DA-20 is a great glider (almost more of a powered glider than a regular powered plane).
 
Dan Thomas said:
You're adding a LOT of weight, and weight is the enemy, to say nothing of a lot more failure points. Works good in vehicles that don't fly.

Words like could and might and likely just don't get anything done. They're just signs of wishful thinking, not based on any real facts and numbers or knowledge of the issues. Real facts and numbers gave us the engines and airplanes we have now. Real facts and numbers are currently seriously hampering useful and practical electric flight.
Click to expand...
Yes, hybrid won't work for us. Batteries are heavy enough, but if we add batteries and fuel and an extra internal-combustion engine (or at least, generator), you'll be at max gross as soon as you put your flight bag in the plane.

Electric will get there. Right now, electric planes are still essentially advanced ultralights with 90-120 minutes of endurance -- OK for basic training or short sightseeing flights, but not much else. Still, that's a significant advance. If the next generation of battery tech decreases the weight:energy ratio by, say, 4x, electric planes will start to be in competition with low-end pistons like the PA-28 or C172 for training, short family flights, hamburger runs, etc.
 
Daleandee said:
I follow you well enough to know that you don't make off hand statements. I fly a Corvair and follow them closely but I can't recall that many breaks. Yeah I know Langford had three in his KR. Seems adding the fifth bearing for the prop loads has eliminated that concern. In addition to that Dan Weseman at SPA now has billet cranks for stock and stroker engines.
Click to expand...
Ron Wantajja could correct me if my number of 14 is wrong. I read it somewhere a while ago. The original crank, as implied by the aftermarket billet crank, was prone to failing even in the cars. I remember my father fighting with trying to rebuild a friend's Corvair van engine after it broke its crankshaft. Around 1966, maybe.
 
David Megginson said:
there was another (now defunct) flight school on the field that flew Diamond Katanas with Rotax engines
Click to expand...
I assume the reason they went defunct was that they were skimping on maintenance on both the engine and airframe. Not the first operation to have that problem - I know of a former school that lost 100% of their aircraft to mechanical issues with their Lycomings.
 
genna said:
If piston certified GA survives long enough- and it got some reprieve with Covid- I think the move to diesel is inevitable. It all depends on SR22 really. The extra cost of putting diesel into near 1MM plane is pretty negligible. The advantages (especially for non-US) are abound. Once they go that route, avegas IO engines days are basically numbered for anything new.
Click to expand...
When Cessna was developing their diesel 182, the upcharge was 17% over the avgas model. For a million-dollar airplane that would be $170K, which I would consider a bit more than negligible. In the 182 it was probably $80K or so, still a big hit.
 
WDD said:
Rotax engines (912) are built using new technology. It has a TBO of 1500 hours, similar to the death trap Continentals mentioned above. This tells me it is more of the demand placed on the engine for the weight and size than simply bad engineering and bad construction.
Click to expand...
The 912 is a rather old design. But still, the newer boosted 915 has a 2K hour TBO.
 
Dan Thomas said:
It probably would, but getting it to work reliably as an aircraft engine is the problem. Homebuilders are doing it all the time, with some notable failures and few real successes. It's just not that easy.

I've been there and done it.
Click to expand...
Yeah,I've seen and heard of a number of Subaru/Eggenfelter RV owners pulling the auto engine and replacing it with the good ol' Lycosaurus. Tough to design a reduction gearbox on a shoestring budget (relatively speaking) that's not immune to bad things happening from long-haul harmonic vibration. IIRC, the Eggenfelter box had issues with spline wear which caused some nasty failures.

I could've put any engine into my RV, but a nice direct-drive crate Lycoming with proven ignition and fuel systems gives me the most confidence. And FADEC is overrated...fiddling with the red knob gives me pilot cred, and something to do!!
 
Dan Thomas said:
getting it to work reliably as an aircraft engine is the problem.
Click to expand...
That’s precisely the issue. Auto conversions don’t have a very good track record and personally, I wouldn’t fly behind any of them.

People complain about the legacy Continental and Lyco engines being old and outdated, but they really do the job well.
 
Dan Thomas said:
You can have an SMA diesel conversion for $80K. It's an opposed four-cylinder 230-hp turbo diesel with a big intercooler and a huge oil cooler, since the heads are air- and oil-cooled, cylinders air-cooled. Comes with a three-composite-bladed constant-speed prop, FADEC and all the rest. STC'd for the 182Q. It's available now. Has been for 22 years now. Few buy it, of course, just as few buy Lycoming's ie2 engines. SMA, when I was working on fixing one of their engines ten years ago, had spent $1 Billion dollars on the program and had 50 of them flying worldwide at the time. A lesson in aviation economics.

Owners are full of ideas about how other people should spend great buckets of money to develop and sell engines for the same price as a car engine.
Click to expand...
I don't think anyone was suggesting or should be as cheap as a car engine. However, making it comparable to what it costs for a new Lycosaurus shouldn't be earth-shattering. Again, the financials don't support anyone taking a venture like a clean-sheet piston GA engine, especially given that STCs would have to be done for dozens of aircraft to even make it attractive to owners. However, that doesn't mean that there wouldn't be large improvements if such a venture were to be undertaken.
 
RyanB said:
People complain about the legacy Continental and Lyco engines being old and outdated, but they really do the job well.
Click to expand...

Agreed. They're like paperclips that way. :)
 
SoonerAviator said:
I don't think anyone was suggesting or should be as cheap as a car engine. However, making it comparable to what it costs for a new Lycosaurus shouldn't be earth-shattering. Again, the financials don't support anyone taking a venture like a clean-sheet piston GA engine, especially given that STCs would have to be done for dozens of aircraft to even make it attractive to owners. However, that doesn't mean that there wouldn't be large improvements if such a venture were to be undertaken.
Click to expand...
The STC costs might be as much as the Lycosaur's cost, with the engine's cost on top of that. An STC for an engine/prop/exhaust system/fuel system/FADEC and associated controls is going to be way more complicated and expensive than an STC for Rosen sun visors. Insurance on such a venture is also going to be monstrous.

Again, words like shouldn't just reveal wishful thinking, not reality.
 
RyanB said:
That’s precisely the issue. Auto conversions don’t have a very good track record and personally, I wouldn’t fly behind any of them.
Click to expand...
I test-flew that Subaru conversion for 15 or 20 hours, I think, but it was over open prairie the whole time. With airframe-electrically-powered ignition and fuel pumps I wasn't going to fly it through the nearby mountains. On the other hand, I flew my 67-year-old A-65 though the mountains without much worry.
 
SoCal RV Flyer said:
IIRC, the Eggenfelter box had issues with spline wear which caused some nasty failures.
Click to expand...
Splines don't do well with reciprocating loads. Way back in the '80s when I was a machine shop foreman in an air brake component remanufacturing plant, we had numerous compressor cores come in with their drive splines torn off. This was in Caterpillar truck engines. The compressor manufacturers designed a compressor to fit the drive pad and gear on the engine that Caterpillar had designed to drive a hydraulic pump. For that pump they worked perfectly. For the compressor they did not. When a compressor's piston reaches TDC it has squeezed out all possible air from the cylinder, but there's still air in the thin gap between the piston and head and in the inlet and discharge valve cavities, and after TDC that air pushes the piston down hard, driving the crankshaft splines the other way. It happens every revolution (twice per rev for a two-cylinder compressor) and hammers the splines mercilessly. It gradually pounds them down until the remaining metal just tears off. I machined off a lot of those cranks and built up the surface with 120ksi weld metal and machined new splines. Worked OK if the customer also replaced the engine's gear with its worn splines, and didn't work well if they just bolted the compressor back on and went back to work.

It's the reciprocating forces that destroy engine gear reduction units. The prop is the flywheel for the engine, and tries to keep the engine turning at a steady rate, while the engine's rotation rate is lurching back and forth with every compression and power stroke on every cylinder. At certain RPMs a resonance sets in that increases the loads on the gearing enough to destroy it, and designing that gearing to avoid or handle or damp the vibrations is the tough part. And it's a lot tougher than uninitiated people think it is. Wishful thinking doesn't make it go away. Physics and math are hard.

The other problem with putting gearing on an auto engine is that the bearing at that end of the crankshaft was designed to take half the axial load from the piston next to that bearing, and the crankshaft's power was transmitted as simple torque to the transmission. No bearing load. Sticking a gear on that crank introduces huge axial loads caused by the side-thrust of the gear, and the bearing doesn't like that and the load bends the crankshaft constantly as it rotates, fatiguing and breaking it. Belts and chains do the same thing. That crank and bearing need the supplemental support of another bearing outboard of the gear, and that bearing has to be perfectly placed and held there absolutely rigidly to do any good. Think of the crankshaft/bearing clearance of less than .001" and then think of trying to design a unit with enough rigidity and precision to keep that crank centered in that bearing. Anyone with any mechanical engineering experience is going to instantly see a problem.
 
Last edited:
RyanB said:
That’s precisely the issue. Auto conversions don’t have a very good track record and personally, I wouldn’t fly behind any of them.
Click to expand...

Most auto conversions are doomed before they start because they’re pursued under the guise of being less expensive than the comparable aircraft engine. Being on a budget also blows any chance of doing any real durability testing before flying the conversion or offering it for sale to others. At that point, the pilots are performing the testing, and most pilots I know do not have a background in engine development.

Would I personally pursue an auto conversion? Perhaps, but I have a lot of resources and experience that the average person doing this doesn’t have. I’m of the opinion that it is best to use an engine that is designed for a specific job in that specific job, and not try and pound a square peg into a round hole.
 
Here is the take from Van's Aircraft

https://www.vansaircraft.com/powerplants/


xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Other Engines
We are often asked about using non-aircraft engine conversions. We’d like to pass along a quote from a colleague in the home-built airplane business:

“The best conversion I know is to take $8000 and convert it into a good, used Lycoming.”

That may sound a bit narrow-minded, but it reflects the basic truth: No non-aircraft engine has yet proven to be as reliable, maintainable, available and inexpensive (everything considered) as a traditional aircraft engine.

It seems magazines are always printing stories about automobile engines bought for junkyard prices, mated to inexpensive reduction drives and flown off into the sunset. It simply doesn’t work like that in the real world. The reliability we’ve all come to expect from aircraft engines is the result of years of development and refinement of engines designed specifically for the task. Automobile engines function well in their intended application: delivering low cruising power in vehicles with well-designed transmissions and power trains. Using them successfully in an airplane requires continuous high power outputs and reduction systems coupled to the propeller. This is completely foreign to their design intent. You can imagine the car engine designer banging his head slowly against his desk…”No, no, no… If I’d known you wanted to do that with it, I would have designed something different…” In fact, we’ve had those very conversations with auto engine designers!
 
If Car engines did a good job in planes, we would see car engines in planes. They don’t work, they don’t hold up, they can’t handle the stresses. Perhaps the closest is the Austro diesels, heavily modified, but they are heavy, while still being puny.
 
There's a good reason auto engines aren't popular in aircraft.....even experimental aircraft.
 
Dan Thomas said:
When Cessna was developing their diesel 182, the upcharge was 17% over the avgas model. For a million-dollar airplane that would be $170K, which I would consider a bit more than negligible. In the 182 it was probably $80K or so, still a big hit.
Click to expand...

i doubt it’s be that high. That was 17% on a sub 500k plane. Diamond pretty much already converted to diesels. I don’t know if cirrus will do it, but seems logical to me
 
You must log in or register to reply here.
Back
Top