EEC ALTN mode cannot redline?

[AzN1337c0d3r 3]

Compare it to a car if you want.

(purely hypothetical I know cars dont have PS limit protection)

 

Take a car with max 150PS. More than 150PS does damage to the engine, the bearings or whatever.

That is where your Thrust Limit Protection comes in. It prevents you from producing more than 150PS even with the peddal floored.

That is what the EEC can only do in normal mode.

 

Then there is the RPM. The engine redlines for example at 8000RPM.

Beyond that you again do damage to the engine.

Not because of 150PS but purely centrifugal forces blowing the engine apart.

That is where N1 Redline protection comes in.

This EEC prtection works in normal as well as Alternate mode.

Ok, let's take my Toyota Camry for example. The red-line on my tachometer is 6200RPM. The governor kicks in EXACTLY 6200 RPM (it's computer controlled). I can't get my engine above 6200 RPM. Not 5500 RPM.

[777simmer 117]

We agree on all the points I have quoted. What I dont understand is why that some here think there is some magical protection before 110.5% N1 in EEC ALTN mode.

Because there IS. It is called OVERSPEED PROTECTION or N1 REDLINE protection and has nothing to do with the amber thrust limit protection line.

[AzN1337c0d3r 3]

Because there IS. It is called OVERSPEED PROTECTION or N1 REDLINE protection and has nothing to do with the amber thrust limit protection line.

N1 REDLINE is 110.5% N1!!! Even in your OWN examples, you are nowhere close to 110.5% N1. Actually in one of them you aren't even over 100% N1.

[CessnaPilot 1]

First I would like to introduce a term rated thrust 

Rated Thrust is the amount of thrust that manufacturer guarantee that this engine could produce untill ISA+15. (30C at Sea level). In order to produce to same amount of thrust at ISA+15 (30C) compare to ISA(15C) the engine must throttle up from maybe 100% N1 to 106% to compensate for less mass airflow pass through engine and this is what actually happened in alternated mode. 

In alternate mode the EEC lost its air data input and hence unable to calculate the amount of N1 it require to achieve the rated thrust. So in alternate mode the EEC assume that you operate at the edge of certify envelope (around ISA+15). Hence for the same throttle position, the EEC in alternate mode will automatically increase the N1 by couple percent in which if you fly in ISA lower than +15 you will have an "Overboost" due to the increase in air density.

However, the physical limit of N1 or N2 (ie due to the bearing load as Rob point out) always limited in either normal or alternate mode by governor in EEC control logic and shall never ever be over that value.

[777simmer 117]

N1 REDLINE is 110.5% N1!!! Even in your OWN examples, you are nowhere close to 110.5% N1. Actually in one of them you aren't even over 100% N1.

Yes but how do we know how much this OVERSPEED PROTECTION limits the RPM?

Thats why I asked how much you would like...103...104...105...?

Not to annoy you but because we dont know if overspeed prtection in the real aircraft limit the RPM to 110.49999% or to 105 or 102. I have no data on that and I dont think my Captain on tomorrows flight back home will let me try to see how far I can go with EEC off. Sorry :-)

 

Ps I noticed your toyota has a hypothetical governor as well ;-)

I just mention Governor as a possible item that could fail and result in RPMs going crazy!

Normally your RPM are not supposed to go up to redline in either EEC mode.

Nothing hypothetical about it.

Had it once on a 737 simcheck on take off. TOGA...full thrust and right N1 went through the roof. Airplane veered to the left so I closed the thrust lever. Right N1 still through the roof (Governor broke so no response to thrust level position) so we had to shut down the fuel control lever quickly. Ended up in the grass anyway though.

Good lesson!

[kevinh 409]

Ok, let's take my Toyota Camry for example. The red-line on my tachometer is 6200RPM. The governor kicks in EXACTLY 6200 RPM (it's computer controlled). I can't get my engine above 6200 RPM. Not 5500 RPM.

The throttle schedules an N1. It's a moveable governor setting if you like. Even if the EEC limits are off, the scheduled RPM will still have a maximum N1. So the engine is still governed, but not limited.

 

The 6,200 rpm in your car is the user's red line, a safe limit. You can be sure the engine can safely run faster than that, possibly 7,000 rpm or more. A FADEC control on your Camry might schedule a demanded engine rpm, increasing linearly up to 6200 rpm at full throttle. But to be able to achieve 6200 rpm at that position in all conditions, the "non limited" engine controller must be able to exceed 6200 rpm at full throttle. So if you set 6200 rpm and somehow failed the limiter the engine might now run at 6,400 rpm (for the sake of argument). This is still not be anywhere near the real red line of the engine.

[AzN1337c0d3r 3]

The throttle schedules an N1. It's a moveable governor setting if you like. Even if the EEC limits are off, the scheduled RPM will still have a maximum N1. So the engine is still governed, but not limited.

 

The 6,200 rpm in your car is the user's red line, a safe limit. You can be sure the engine can safely run faster than that, possibly 7,000 rpm or more. A FADEC control on your Camry might schedule a demanded engine rpm, increasing linearly up to 6200 rpm at full throttle. But to be able to achieve 6200 rpm at that position in all conditions, the "non limited" engine controller must be able to exceed 6200 rpm at full throttle. So if you set 6200 rpm and somehow failed the limiter the engine might now run at 6,400 rpm (for the sake of argument). This is still not be anywhere near the real red line of the engine.

Yes, but the red-line on the aircraft engines, is also a "safe user limit". It's tested to run 60 hours at 110.5%N1. No one even really knows (except GE and Boeing maybe) how fast you can run the N1 fan above 110.5% N1 for how long before it actually disintegrates (and likely there is some distribution of failure points across individual engines...)

 

You can be sure that the GE90-115B N1 fan doesn't disintegrate when you hit exactly 110.5% N1, it still takes at least 10 runs of 6 hours (per the certification test). Obviously they dont want to rate it that high and build in some safety margins so the engines aren't run at triple redline on every flight, but why not make this available to pilots in an "oh crap" situation that they can select it if they want. Doesn't this go against the Boeing design philosophy that the pilots can override the computers if they deem that is the prudent course to do so?

 

Yes but how do we know how much this OVERSPEED PROTECTION limits the RPM?

Thats why I asked how much you would like...103...104...105...?

Not to annoy you but because we dont know if overspeed prtection in the real aircraft limit the RPM to 110.49999% or to 105 or 102. I have no data on that and I dont think my Captain on tomorrows flight back home will let me try to see how far I can go with EEC off. Sorry :-)

 

Ps I noticed your toyota has a hypothetical governor as well ;-)

I just mention Governor as a possible item that could fail and result in RPMs going crazy!

Normally your RPM are not supposed to go up to redline in either EEC mode.

Nothing hypothetical about it.

Had it once on a 737 simcheck on take off. TOGA...full thrust and right N1 went through the roof. Airplane veered to the left so I closed the thrust lever. Right N1 still through the roof (Governor broke so no response to thrust level position) so we had to shut down the fuel control lever quickly. Ended up in the grass anyway though.

Good lesson!

It doesn't make sense to have the OVERSPEED PROTECTION at anything less than 110.5% N1. The engines can do (as in a batch of engines have physically been run) 10 runs of 6 hours at 110.5% N1 (as well as max N2 and EGT) and show no signs of distress. They sure aren't going to blow up right at 110.5% N1.

[777simmer 117]

Thx Kevin for your explanations.

 

I have to call it a day.

Good luck :-)

[AzN1337c0d3r 3]

First I would like to introduce a term rated thrust 

 

Rated Thrust is the amount of thrust that manufacturer guarantee that this engine could produce untill ISA+15. (30C at Sea level). In order to produce to same amount of thrust at ISA+15 (30C) compare to ISA(15C) the engine must throttle up from maybe 100% N1 to 106% to compensate for less mass airflow pass through engine and this is what actually happened in alternated mode. 

 

In alternate mode the EEC lost its air data input and hence unable to calculate the amount of N1 it require to achieve the rated thrust. So in alternate mode the EEC assume that you operate at the edge of certify envelope (around ISA+15). Hence for the same throttle position, the EEC in alternate mode will automatically increase the N1 by couple percent in which if you fly in ISA lower than +15 you will have an "Overboost" due to the increase in air density.

 

However, the physical limit of N1 or N2 (ie due to the bearing load as Rob point out) always limited in either normal or alternate mode by governor in EEC control logic and shall never ever be over that value.

Yet another good point. OVERSPEED PROTECTION with EEC failed means that there is no input about the air density, temperature, etc. Thus, it shouldn't change with regards to air temperature. Meanwhile, in the PMDG if you set the outside air temperature really cold, (say -30C) you can't even command 90% N1. How do you explain that 777simmer and kevinh?

[kevinh 409]

Yet another good point. OVERSPEED PROTECTION with EEC failed means that there is no input about the air density, temperature, etc. Thus, it shouldn't change with regards to air temperature. Meanwhile, in the PMDG if you set the outside air temperature really cold, (say -30C) you can't even command 90% N1. How do you explain that 777simmer and kevinh?

How about you being less challenging of other people and actually listen to what they say? Oh, and how about you start signing your posts? You know, how we are all supposed to do in this forum. It helps keep things civil. Leaving aside your conclusions (right or wrong) about how the real GE90 should behave, you refuse to accept the most obvious explanation of the 777X's behaviour (FSX engine simulation model limitations).

 

Nothing in CessnaPilot's explanation contradicts anything Rob and I have been saying. He's just explaining the difference between limiting thrust and governing RPM. I wouldn't disagree with a word of it.

 

Without a full set of engine and FADEC data from GE I can't possibly answer your question. The FCOM certainly doesn't answer it. However I can suggest that the PMDG model might not react exactly as the real engine does in very cold or very hot temperatures. One thing I can say is that on a cold day the air is more dense, so for the same N1 the engine will produce much more thrust. Another thing you should understand is that alternate mode is not off.  The EEC is still running, it's just in a degraded mode using more basic information. So it's quite possible that full throttle does not produce red line N1.

 

Whether or not the real engine will red line in alternate mode is not relevant if FSX limitations have forced PMDG to compromise on this. Please bear in mind PMDG got a lot of 777 information from access to a full flight simulator, so it's highly likely they tried operating the sim's engines in ALTN mode to see what happens.

 

I am not willing to spend time debating this with you when you clearly aren't interested in some of the points made to you. You can't be bothered to follow simple forum rules. I can only suggest, very strongly, that you open a support ticket with PMDG for this "bug" and get the answer direct from them. I cannot speak for them, I can only tell you what they told me on a related matter. 

[AzN1337c0d3r 3]

How about you being less challenging of other people and actually listen to what they say? Oh, and how about you start signing your posts? You know, how we are all supposed to do in this forum. It helps keep things civil. Leaving aside your conclusions (right or wrong) about how the real GE90 should behave, you refuse to accept the most obvious explanation of the 777X's behaviour (FSX engine simulation model limitations).

 

Nothing in CessnaPilot's explanation contradicts anything Rob and I have been saying. He's just explaining the difference between limiting thrust and governing RPM. I wouldn't disagree with a word of it.

 

Without a full set of engine and FADEC data from GE I can't possibly answer your question. The FCOM certainly doesn't answer it. However I can suggest that the PMDG model might not react exactly as the real engine does in very cold or very hot temperatures. One thing I can say is that on a cold day the air is more dense, so for the same N1 the engine will produce much more thrust. Another thing you should understand is that alternate mode is not off.  The EEC is still running, it's just in a degraded mode using more basic information. So it's quite possible that full throttle does not produce red line N1.

 

Whether or not the real engine will red line in alternate mode is not relevant if FSX limitations have forced PMDG to compromise on this. Please bear in mind PMDG got a lot of 777 information from access to a full flight simulator, so it's highly likely they tried operating the sim's engines in ALTN mode to see what happens.

 

I am not willing to spend time debating this with you when you clearly aren't interested in some of the points made to you. You can't be bothered to follow simple forum rules. I can only suggest, very strongly, that you open a support ticket with PMDG for this "bug" and get the answer direct from them. I cannot speak for them, I can only tell you what they told me on a related matter. 

No, you and others just don't seem to accept that my explanation maybe correct. 

 

I'd be happy to discuss any counter-arguments as to why my theory "thrust overboost limitation is active in alternate EEC mode" is wrong, which was what I was trying to do with 777simmer.

 

However, 777simmer keeps moving the goal-posts. Initially suggested an "overboost limit", and then to "N1 redline protection", and now "overspeed protection", all of which are distinct systems. Which is it?

 

You just seem to keep specially plead "FSX limitations" and declare it as the simplest explanation, when it is not an explanation at all. It is a complete copout. You are saying "I dont know why the sim doesn't act correctly, I'll just blame it on FSX limitations". A real explanation would be if you characterized the FSX behavior as it pertains to the throttle, created a modeled for this behavior and then provided a reproducible result that exercises your model.

 

The throttle scheduling maxing out at less than N1 redline limit is a good idea, except that it seems to go against Boeing philsophy, and the observation is that in the PMDG777 at least, the scheduling shouldn't change because there's no way the engine can even know the atmospheric conditions it is operating in.

 

[kevinh 409]

No, you and others just don't seem to accept that my explanation maybe correct. 

 

I'd be happy to discuss any counter-arguments as to why my theory "thrust overboost limitation is active in alternate EEC mode" is wrong, which was what I was trying to do with 777simmer.

 

However, 777simmer keeps moving the goal-posts. Initially suggested an "overboost limit", and then to "N1 redline protection", and now "overspeed protection", all of which are distinct systems. Which is it?

 

You just seem to keep specially plead "FSX limitations" and declare it as the simplest explanation, when it is not an explanation at all. It is a complete copout. You are saying "I dont know why the sim doesn't act correctly, I'll just blame it on FSX limitations". A real explanation would be if you characterized the FSX behavior as it pertains to the throttle, created a modeled for this behavior and then provided a reproducible result that exercises your model.

 

The throttle scheduling maxing out at less than N1 redline limit is a good idea, except that it seems to go against Boeing philsophy, and the observation is that in the PMDG777 at least, the scheduling shouldn't change because there's no way the engine can even know the atmospheric conditions it is operating in.

 

OK, let me get some things straight.

 

1. We are disagreeing about how much additional N1 you might get in ALTN mode. You have assumed it must be the maximum possible. I don't think this is necessarily correct, but I am not saying that you are definitely wrong either. You may be right, but the FCOM does not explicitly back you up. It only talks vaguely about overboost being possible. You have drawn a firm conclusion about this which I don't think is the right thing to do on the basis of the FCOM alone. So you really ought to accept the possibility you might not be right.

 

2. Whatever you think of Rob's posts, his knowledge is good and real world based, so please at least accept he might know what he is talking about. In the heat of an argument things can get confused and false impressions given however. I don't think Rob is moving the goalposts at all.

 

3. My own experience of this shows that EEC ALTN mode does not allow the CF6-80 engine to get to the red line at max throttle angle. As it happens I had to verify that simulation to Boeing data. The GE90 may well behave differently, but how different I can't say without better data than the FCOM gives (which is descriptive only).

 

4. I mentioned that PMDG had told me the EEC limits are always there in their model. I assume this is to do with how they have had to model the EEC using the FSX engine model as a basis. Hence my suggestion that if it isn't behaving as aircraft then it might be an FSX limitation. This is not a cop out, and in my experience the simplest explanation is usually the most likely.

 

5. You seem to think that if it's in the FCOM it must be in the 777X. That is not always going to be the case. This is a desktop simulator running in FSX and there are limits to what can be achieved, even by PMDG. So even if you are right about the way the GE90 operates in ALTN mode, PMDG may not be able to simulate it exactly in FSX without adversely affecting something else. Please accept that.

 

6. In summary I don't know for sure you are wrong, but I do know that the FCOM does not contain enough detail for you to be sure you are right.

 

If you are convinced this is a bug then please open a support ticket with PMDG and get the answer from them. They are very helpful if you ask politely. There is no point in arguing further about this here purely on the basis of what you assume the FCOM means.

[CessnaPilot 1]

Yet another good point. OVERSPEED PROTECTION with EEC failed means that there is no input about the air density, temperature, etc. Thus, it shouldn't change with regards to air temperature. Meanwhile, in the PMDG if you set the outside air temperature really cold, (say -30C) you can't even command 90% N1. How do you explain that 777simmer and kevinh?

Is that in Normal mode or soft alternate mode or hard alternate mode?

[AzN1337c0d3r 3]

Is that in Normal mode or soft alternate mode or hard alternate mode?

All 3. (If you mean soft alternate as failing via failures options)

[CessnaPilot 1]

All 3. (If you mean soft alternate as failing via failures options)

For Normal mode and soft alternate, it is normal that EEC will not command high N1 since the cold air mean more air density and hence more mass airflow and more thrust so less N1 it need to achieve rated thrust. But for hard alternate that's kinda weird since it should provide a fix N1 vs TLA schedule at (ISA+15) and hence should easily achieved N1 100% or more. May I ask what is your environment condition (ie pressure altitude, temp, engine rating, IAS)