TwinStar's FL180 limitation

[MachFly]

I've been trying to figure out why Diamond put a FL180 limitation on the TwinStar (specifically DA42 NG & V1) and so far I wasn't able to find anything good. I realize there aren't too many people here who are intimately familiar with the AE300, however I'm sure there are plenty of people here who are familiar with diesel engines and turbochargers. Regarding the specifics of the engine, I should be able to explain all of that, so please ask if you do not have enough information.

Performance wise it should have no problem going above FL180. The NG can cruise at 175kts and climb at ~700fpm at that alt, the V1 should (I say "should" because I obviously never flown it) cruise at 190kts and climb at almost 1000fpm. So obviously they have the performance to go above FL180.

Specifically the limitation states "The maximum operating altitude is 18,000 ft (5,486 m) pressure altitude."

I've been able to come up with two possible theories why that limitation is there.
First one is the turbo obviously need to spin faster at a higher altitude, so perhaps above FL180 it will either overheat or go past it's RPM's red line. I never tested this, so this may be wrong.
Second theory is because Diamond kinda rushed the NG into production and the AE300 was still brand new at the time they added a whole bunch of useless limitations to try to keep it safe. The problem is that now the V1 is coming out and the limitation is still there.

The interesting thing is that Diamond posted a video on youtube (http://www.youtube.com/watch?v=RWLjD519L5A) which shows them tacking the V1 to FL200.
Perhaps that's just a testing video because the Vne is 188kts and the video shows them moving at 200kts IAS.

So any ideas why that limitation is there or what's going to happen if I go above that?

 

[flyingmoose]

Is it an engine limitation or airframe? Not sure I would have a good answer either way, but just first that that came to mind.

 

[MachFly]

Is it an engine limitation or airframe? Not sure I would have a good answer either way, but just first that that came to mind.

I assume it's the engines. I can't think of anything that can be wrong with the airframe for it to be limited to only 18K.

 

[Ted]

As you go up in altitude, turbos have to get significantly higher pressure ratios to maintain the same manifold pressure. So for example, a turbo-normalized plane that performs 30" at sea level uses a pressure ratio of 1:1. That same 30" takes about 2:1 at 15k, etc.

I don't know a ton about the AE300, but most modern diesels run in the range of 2:1 or 3:1 at sea level (15-30 psi boost, or 60-90" MP). That means that at 15k for the same boost you'd need 4-6:1. Modern turbos are limited by physics to around 5-6:1 pressure ratios. You also have (somewhat related) turbo speed limitations, which have typically been the cause for manifold pressure limits on turbo piston aircraft.

On the bottom end, diesels need a certain manifold pressure in order to self-ignite the fuel/air mixture. So if you get below a certain manifold pressure, the engine will simply stop running, and I don't know what altitude you'd need to get back down to for a restart, especially since you won't get any help from the turbo (turbos need exhaust gasses to spin, and won't have enough on just raw air, especially up high).

So, my guess is that above FL180 you get to a point where the turbos are working too hard and the engines may have potential flame-out concerns. It's probably realistically higher than that, but FL180 was likely an easy place to limit service ceiling and would provide some margin.

 

[gismo]

As you go up in altitude, turbos have to get significantly higher pressure ratios to maintain the same manifold pressure. So for example, a turbo-normalized plane that performs 30" at sea level uses a pressure ratio of 1:1. That same 30" takes about 2:1 at 15k, etc.

I don't know a ton about the AE300, but most modern diesels run in the range of 2:1 or 3:1 at sea level (15-30 psi boost, or 60-90" MP). That means that at 15k for the same boost you'd need 4-6:1. Modern turbos are limited by physics to around 5-6:1 pressure ratios. You also have (somewhat related) turbo speed limitations, which have typically been the cause for manifold pressure limits on turbo piston aircraft.

On the bottom end, diesels need a certain manifold pressure in order to self-ignite the fuel/air mixture. So if you get below a certain manifold pressure, the engine will simply stop running, and I don't know what altitude you'd need to get back down to for a restart, especially since you won't get any help from the turbo (turbos need exhaust gasses to spin, and won't have enough on just raw air, especially up high).

So, my guess is that above FL180 you get to a point where the turbos are working too hard and the engines may have potential flame-out concerns. It's probably realistically higher than that, but FL180 was likely an easy place to limit service ceiling and would provide some margin.

AFaIK, the turbos in a diesel DA42 are not designed to maintain a constant MP during climb, but rather to maintain a constant pressure ratio in the turbo(s). IOW power falls off like a NA engine with altitude.

 

[Ted]

AFaIK, the turbos in a diesel DA42 are not designed to maintain a constant MP during climb, but rather to maintain a constant pressure ratio in the turbo(s). IOW power falls off like a NA engine with altitude.

I'm not familiar with the FADEC logic in the DA42. I'd be surprised at a constant PR, I'd just expect a limit. But I didn't program it.

 

[Henning]

IIRC, that limitation was placed there because the Thielert wouldn't restart above 18,000. It didn't make a heck of a lot of sense to me at the time either. So what if you can't get it to restart above 18,000? Drift down if you're above that until you can start it.

 

[Ted]

IIRC, that limitation was placed there because the Thielert wouldn't restart above 18,000. It didn't make a heck of a lot of sense to me at the time either. So what if you can't get it to restart above 18,000? Drift down if you're above that until you can start it.

It might have to do with pulling back to idle and the engine shutting down once the turbo no longer provides boost to keep manifold pressure in the self-ignition range. Aircraft diesels would benefit from having higher compression diesels (say 20+:1 CR), whereas I think some of the more modern ones are in the 18:1 or less. My 2004.5 Cummins was 16.5.

 

[Henning]

It might have to do with pulling back to idle and the engine shutting down once the turbo no longer provides boost to keep manifold pressure in the self-ignition range. Aircraft diesels would benefit from having higher compression diesels (say 20+:1 CR), whereas I think some of the more modern ones are in the 18:1 or less. My 2004.5 Cummins was 16.5.

Could be, but still the same, drift down till you can restart. The real solution is to put a belt drive Paxton on as well.

 

[Ted]

Could be, but still the same, drift down till you can restart. The real solution is to put a belt drive Paxton on as well.

Operability requirements get in the way there. You aren't allowed to have an engine that will just flame out that way, but you also need to be able to reduce power. So simply saying "You can go above 18k, but if you do you'll be going down to 5k when you descend" isn't acceptable.

 

[MachFly]

As you go up in altitude, turbos have to get significantly higher pressure ratios to maintain the same manifold pressure. So for example, a turbo-normalized plane that performs 30" at sea level uses a pressure ratio of 1:1. That same 30" takes about 2:1 at 15k, etc.

I don't know a ton about the AE300, but most modern diesels run in the range of 2:1 or 3:1 at sea level (15-30 psi boost, or 60-90" MP). That means that at 15k for the same boost you'd need 4-6:1. Modern turbos are limited by physics to around 5-6:1 pressure ratios. You also have (somewhat related) turbo speed limitations, which have typically been the cause for manifold pressure limits on turbo piston aircraft.

On the bottom end, diesels need a certain manifold pressure in order to self-ignite the fuel/air mixture. So if you get below a certain manifold pressure, the engine will simply stop running, and I don't know what altitude you'd need to get back down to for a restart, especially since you won't get any help from the turbo (turbos need exhaust gasses to spin, and won't have enough on just raw air, especially up high).

So, my guess is that above FL180 you get to a point where the turbos are working too hard and the engines may have potential flame-out concerns. It's probably realistically higher than that, but FL180 was likely an easy place to limit service ceiling and would provide some margin.

I'm not sure what the pressure ratio is (interesting thing is that the POH & the maintenance manual do not say it), however the max turbocharger speed is 172000RPM.

The computer increases my idle speed so the engines keep running at high altitudes. I don't remember the idle power percentage at 18K but at 12K it's about 20%.

POH also says that the maximum immediate altitude for a restart is 18K. So perhaps that is the reason.

AFaIK, the turbos in a diesel DA42 are not designed to maintain a constant MP during climb, but rather to maintain a constant pressure ratio in the turbo(s). IOW power falls off like a NA engine with altitude.

Actually the power stars to fall after 14K, so as long as you are at 14K or bellow the turbos can fully compensate.

IIRC, that limitation was placed there because the Thielert wouldn't restart above 18,000. It didn't make a heck of a lot of sense to me at the time either. So what if you can't get it to restart above 18,000? Drift down if you're above that until you can start it.

Well Thielert shouldn't have anything to do with it, it's a completely different engine.

It might have to do with pulling back to idle and the engine shutting down once the turbo no longer provides boost to keep manifold pressure in the self-ignition range.

The computer will not let that happen, it changes the idle power setting depending on the altitude.

Could be, but still the same, drift down till you can restart. The real solution is to put a belt drive Paxton on as well.

Yeah I don't see why you can't just drift down and restart at a lower altitude.
It's not like shutting down the engines above 18K is standard procedure, that would make it an emergency situation in which case just drift down. I know the engines will restart without a problem at 12K, so it's not like it's a very long way to go.

 

[Henning]

Operability requirements get in the way there. You aren't allowed to have an engine that will just flame out that way, but you also need to be able to reduce power. So simply saying "You can go above 18k, but if you do you'll be going down to 5k when you descend" isn't acceptable.

Yeah, I get it, that's why it should have a mechanical supercharger.

 

[Ted]

I'm not sure what the pressure ratio is (interesting thing is that the POH & the maintenance manual do not say it), however the max turbocharger speed is 172000RPM.

The computer increases my idle speed so the engines keep running at high altitudes. I don't remember the idle power percentage at 18K but at 12K it's about 20%.

POH also says that the maximum immediate altitude for a restart is 18K. So perhaps that is the reason.

I'd be surprised if the POH listed a max pressure ratio. That ends up being governed by physics - you can't exceed it. Meanwhile exceeding the turbo max speed is physically possible, and the result will be needing a new turbo, unless it explodes, then it might require more.

It makes sense that the FADEC limits what the min idle speed is. I'd be curious as to min speed/power at 18k, as well as max power. I also don't know about operability requirements for diesels, and they might do something different there, especially with a FADEC.

Actually the power stars to fall after 14K, so as long as you are at 14K or bellow the turbos can fully compensate.

That's pretty impressive. What's the maximum manifold pressure? They likely have a turbo more in the 6:1 range.

Well Thielert shouldn't have anything to do with it, it's a completely different engine.

But they will. There are enough similarities between the two, much like Continental and Lycoming. So expect lessons learned from Thielert by the FAA to enact regulations.

Yeah I don't see why you can't just drift down and restart at a lower altitude.
It's not like shutting down the engines above 18K is standard procedure, that would make it an emergency situation in which case just drift down. I know the engines will restart without a problem at 12K, so it's not like it's a very long way to go.

Again might get into operability. Plus a shutdown that requires a 12k restart could be bad if you're over the rockies. Remember that certified regs are very conservative typically. Someone died, or they think someone might die, so we get a new reg. Just because you find that acceptable doesn't mean the administrator will - the difference between certified and experimental.

 

[Ted]

Yeah, I get it, that's why it should have a mechanical supercharger.

That would probably help as well if it then allowed for you to have sequential forced induction. More boost at higher altitudes. But it also adds weight, so I doubt if it will happen.

 

[Henning]

That would probably help as well if it then allowed for you to have sequential forced induction. More boost at higher altitudes. But it also adds weight, so I doubt if it will happen.

Blowing blowers has been done for a long time, no problem compounding turbos on top of superchargers. With a Paxton you could even go it the other way and clutch it out at low altitude where it wasn't required.

 

[MachFly]

I'd be surprised if the POH listed a max pressure ratio. That ends up being governed by physics - you can't exceed it. Meanwhile exceeding the turbo max speed is physically possible, and the result will be needing a new turbo, unless it explodes, then it might require more.

It makes sense that the FADEC limits what the min idle speed is. I'd be curious as to min speed/power at 18k, as well as max power. I also don't know about operability requirements for diesels, and they might do something different there, especially with a FADEC.



That's pretty impressive. What's the maximum manifold pressure? They likely have a turbo more in the 6:1 range.

See one thing I really hate about the way Diamond does things is they keep their pilots on the need to know basis, and their definition of "need to know" is very different from mine. Diamond didn't publish the max MP, they didn't even bother to give me a MP gauge, I don't even have a turbo boost gauge.

The max percent power I've ever seen at 18K was 89%. I've never tested for the minimum but next time I'll be sure to do that.

If your interested I'll be happy to upload the POH.

But they will. There are enough similarities between the two, much like Continental and Lycoming. So expect lessons learned from Thielert by the FAA to enact regulations.

Yeah I guess that makes sense.

Again might get into operability. Plus a shutdown that requires a 12k restart could be bad if you're over the rockies. Remember that certified regs are very conservative typically. Someone died, or they think someone might die, so we get a new reg. Just because you find that acceptable doesn't mean the administrator will - the difference between certified and experimental.

Right

 

[RussR]

Could it be a flutter concern? Flutter is dependent on TAS, not IAS, so as you go higher and TAS increases, could this be why?

 

[Henning]

Could it be a flutter concern? Flutter is dependent on TAS, not IAS, so as you go higher and TAS increases, could this be why?

Valid thought, but I don't think it applies here. I also remember something about 25,000' being a certification requirement in those regards, but not positive. This is an engine operations issue to the best of my recollection.

 

[Ted]

See one thing I really hate about the way Diamond does things is they keep their pilots on the need to know basis, and their definition of "need to know" is very different from mine. Diamond didn't publish the max MP, they didn't even bother to give me a MP gauge, I don't even have a turbo boost gauge.

The max percent power I've ever seen at 18K was 89%. I've never tested for the minimum but next time I'll be sure to do that.

If your interested I'll be happy to upload the POH.

That is interesting. They actually have mimicked the jet world there, and turboprop to some extent. Although in the POH those actually list what percents equate to. Realistically from a pilot's perspective, percent power with a speed correlation table makes things easier, and so long as the FADEC is able to detect faults (which I presume it is), that reduces workload further.

That said, as an engineer, it would annoy me as well.

Yeah I guess that makes sense.

Better than just ignoring some significant engineering defects, especially since the engines have a lot of general architecture in common.

Right

We're not happy until you're not happy.

Valid thought, but I don't think it applies here. I also remember something about 25,000' being a certification requirement in those regards, but not positive. This is an engine operations issue to the best of my recollection.

There is something different about FL250 these days, although I don't think there used to be. Hence why the 421 is certified to somewhere like FL300 (even though nobody ever flies it there), but these days you don't see pistons certified that high. I don't recall what the exact difference is, but it basically just gets stricter above that, and more effort and expense than is worth going to for capability that very few people (if any) will ever use.

Thinking about it now, there might today be something different certifying above FL180 from a DO-160 perspective (which the FADEC would have to be tested to). For a non-pressurized plane (i.e. rarely flown that high), maybe they just chose to limit it there. It's been a few years since I've had to look at DO-160, so I forget if that's the case or not.

 

[Dynasty22]

The 777 requires a driftdown to FL280 before getting enough pressure for a engine restart

 

[Ted]

The 777 requires a driftdown to FL280 before getting enough pressure for a engine restart

Engine restarts in turbine aircraft are a different animal, as well as the difference between a windmill start and an assisted restart. But it's worth noting that at FL280, you aren't going to hit anything.

 

[airheadpenguin]

Also worth noting that the 777 is capable of continuing the flight under most circumstances on 1 engine unlike light twins

 

[Ted]

Also worth noting that the 777 is capable of continuing the flight under most circumstances on 1 engine unlike light twins

While true, that is a given for its certification and is not related at all to requirements for in-air restart.

 

[Electric]

Could it have anything to do with minimum operating temperature and humidity of electronics on board? (FADEC and/or avionics)

Regarding the risk of a flame out, as far as I understand, as long as ambient pressure is higher than idle MP, the engine should keep running.

 

[MachFly]

That is interesting. They actually have mimicked the jet world there, and turboprop to some extent. Although in the POH those actually list what percents equate to. Realistically from a pilot's perspective, percent power with a speed correlation table makes things easier, and so long as the FADEC is able to detect faults (which I presume it is), that reduces workload further.

That said, as an engineer, it would annoy me as well.

Actually I also have a chart depicting what power percentage equals to what RPMs.

Better than just ignoring some significant engineering defects, especially since the engines have a lot of general architecture in common.

Well that would be very general. The only similarities the Austro and the Thielert have is they are both turbodiesel, both have a gearbox, and both have 4 cylinders. The defects of the Thielert really should not effect the Austro.

We're not happy until you're not happy.

That never gets old...

There is something different about FL250 these days, although I don't think there used to be. Hence why the 421 is certified to somewhere like FL300 (even though nobody ever flies it there), but these days you don't see pistons certified that high. I don't recall what the exact difference is, but it basically just gets stricter above that, and more effort and expense than is worth going to for capability that very few people (if any) will ever use.

Thinking about it now, there might today be something different certifying above FL180 from a DO-160 perspective (which the FADEC would have to be tested to). For a non-pressurized plane (i.e. rarely flown that high), maybe they just chose to limit it there. It's been a few years since I've had to look at DO-160, so I forget if that's the case or not.

What about the SR22T? That can go to like 28K and it's essentially as new as the TwinStar.

 

[MachFly]

Plus a shutdown that requires a 12k restart could be bad if you're over the rockies.

Lets say you accidently shut down both engines somewhere in the flight levels perfectly in the center of the rockies. By the time you glide down to an altitude where you can restart the engines the rockies will be far behind you and you will be able to safely get bellow 10K.

According the the POH the maximum restart altitude after a prolonged period with a shut down engine is 10K.

Also worth noting that the 777 is capable of continuing the flight under most circumstances on 1 engine unlike light twins

So is the TwinStar, therefore it can't be a factor here.

 

[MachFly]

Could it have anything to do with minimum operating temperature and humidity of electronics on board? (FADEC and/or avionics)

Regarding the risk of a flame out, as far as I understand, as long as ambient pressure is higher than idle MP, the engine should keep running.

The temperature actually might have something to do with it. My fuel heating system is depended on at least one engine running. If you loose both engines and end up gliding long enough in cold temperature the fuel will freeze. On the other hand Jet A freezes at -40C, standard temp at 18K is -25C...maybe they added a large buffer or something. But then my max altitude should increase if I use Jet A1 as it's freezing point is lower.

Also the G1000's minimum temperature is -25C....maybe they thought I wouldn't be able to fly the plane without a G1000?

 

[Ted]

Actually I also have a chart depicting what power percentage equals to what RPMs.

Manifold pressure/fuel flow is the other factor. I'm sure they give you fuel flow, but hide the manifold pressure.

Well that would be very general. The only similarities the Austro and the Thielert have is they are both turbodiesel, both have a gearbox, and both have 4 cylinders. The defects of the Thielert really should not effect the Austro.

That's a decent number of similarities from the FAA's perspective. Remember, the only similarities between Lycomings and Continentals is that they are both spark ignition, large bore, pushrod, air-cooled, and have magnetos.

What about the SR22T? That can go to like 28K and it's essentially as new as the TwinStar.

Good question. I've never had to deal with those. I don't recall the specifics of what made 25K special, but the secret may lie in there.

Lets say you accidently shut down both engines somewhere in the flight levels perfectly in the center of the rockies. By the time you glide down to an altitude where you can restart the engines the rockies will be far behind you and you will be able to safely get bellow 10K.

According the the POH the maximum restart altitude after a prolonged period with a shut down engine is 10K.

So is the TwinStar, therefore it can't be a factor here.

You're trying to apply airframe-specific logic to a certification effort, which isn't how the FAA works. The first giveaway should be "logic".

Thinking about it, the most likely reason has to do with certification, potentially of the FADEC. It may have had to do with DO-160 there. And again, they probably figure most people wouldn't bother flying up there without pressurization.

 

[MachFly]

Manifold pressure/fuel flow is the other factor. I'm sure they give you fuel flow, but hide the manifold pressure.

Oh yes I do have a fuel flow gauge. And the computer knows MP, there is just no gauge to tell me what it is.

That's a decent number of similarities from the FAA's perspective. Remember, the only similarities between Lycomings and Continentals is that they are both spark ignition, large bore, pushrod, air-cooled, and have magnetos.

I think it's very sad that Thielert's screw up can be effecting Austro's certification.

You're trying to apply airframe-specific logic to a certification effort, which isn't how the FAA works. The first giveaway should be "logic".

Thinking about it, the most likely reason has to do with certification, potentially of the FADEC. It may have had to do with DO-160 there. And again, they probably figure most people wouldn't bother flying up there without pressurization.

Would you happen to know where I can get a copy of DO-160?
I'd like to read the specifics.

 

[Ted]

Oh yes I do have a fuel flow gauge. And the computer knows MP, there is just no gauge to tell me what it is.

Yeah, the FADEC will have self-diagnostics for all kinds of failures. Just wait for the "CHECK ENGINE" light to come on.

I think it's very sad that Thielert's screw up can be effecting Austro's certification.

Not being involved with either I can only suspect that's the case. In aviation, that's how certification works. Think what Airbus will have to go through for certifying lithium ion batteries after Boeing's screw-up!

Would you happen to know where I can get a copy of DO-160?
I'd like to read the specifics.

RTCA (who publishes it) sells copies, which aren't cheap and have clauses limiting who can use them.

 

[MachFly]

Yeah, the FADEC will have self-diagnostics for all kinds of failures. Just wait for the "CHECK ENGINE" light to come on.

Thankfully it's usually a bit more specific than "check engine" LMAO.

RTCA (who publishes it) sells copies, which aren't cheap and have clauses limiting who can use them.

Thanks

 

[Ted]

Thankfully it's usually a bit more specific than "check engine" LMAO.

Curious - what sort of faults does it present? It's been a while since I played with the iE2, but my recollection was that it had either one or two lights, and that was it. Maybe the G1000 would display a few other warnings.

 

[MachFly]

Curious - what sort of faults does it present? It's been a while since I played with the iE2, but my recollection was that it had either one or two lights, and that was it. Maybe the G1000 would display a few other warnings.

It's a long list, I'll just copy and paste it from the manual.

Then there is a much longer list of annunciatiors regarding the G1000, GPS, autopilot, SVT, ect.... I doubt you want that list, but let me know if you do.

 

[Henning]

It has a stick limiter? What the purpose?

 

[MachFly]

It has a stick limiter? What the purpose?

This is going to be a long discussion...

Yes, it does have a stick limiter and a yaw damper.
The reason behind this is the aircraft has very violent stall characteristics, (I was told that swept wing jets have very similar stall characterless, but I can not confirm that from personal experience). In addition it gives you almost no warning of the upcoming stall, the airframe does not buffet. Well technically it does buffet, but it's such a minor buffet that in order to notice it you need to have a lot of time in the aircraft and be specifically looking for that buffet.
So what the computer does is it limits the stick movement in relation to the power setting, therefore either not allowing you to stall at all or making the stall very smooth and then recovering from it.
For example if I want to do a power on stall with full power, the computer simply won't let me do it, the aircraft will be climbing with maximum back pressure. In order to stall it with power on I would use about 60-65% power, in which case the aircraft will still get to atleast 11K before it stalls.
Of course you can turn the stick limiter off, by pulling the circuit breaker. However if you are going to do that you need to make sure that you have plenty of altitude to recover and you really want to make sure that you won't be doing accelerated power on stalls.

What I said above is only about the Austro models, I simply don't know enough about the Thielert and Lycoming models to make any comments (never flown them).

 

[Henning]

Interesting, thanks.

 

[Ted]

It's a long list, I'll just copy and paste it from the manual.

Actually, that list is more or less what I would expect to see for engine-related items, because those are the things that I suspect you may be able to do something about or that might influence your decision making process.

For the "Check Engine" light I was referring to all the engine-specific items that have to do with things like overboost, low manifold pressure, sensor failure, etc. Those are things that you can't do anything about, and really not a lot to do with your decision making process.

In the turbine world, this is more common. On one jet I deal with, if a master chip detector is tripped, it won't alert the pilot until after landing (weight on wheels). The pilot can manually choose to see if it's tripped or not by switching to the appropriate screen, but the intent is to not distract the pilot with something that doesn't typically indicate imminent failure.