EEC ALTN mode cannot redline?

[AzN1337c0d3r 3]

I don't know what you mean by "engine rating", but the picture below shows everything you want I think. It's -30C SAT, sea level, 29.92 in, EEC in hard alternate. As you can see, it can only command slightly above yellow line, but it's nowhere close to even 100% N1.

[CessnaPilot 1]

My best guess would be the other protection in the EEC logic that I don't know (ie Combustion chamber pressure limit) limit the N1 RPM since I don't have FADEC manual with me I couldn't really point out what other protection it might have. 

At this point you might try submit a ticket and seek a better explanation from PMDG staff.
Please share PMDG answer to your ticket with us.

[777simmer 117]

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.

 

Had to think about that for a little while.

Air data is only one of many things the EEC requires to operate correctly.

But yes without air data it can not calculate thrust parameters.

I am not sure it will assume anything or that it will automatically add some % N1.

 

The maintenance manual I have (It is not the complete manual, that would be a big pile of books!) has only this to add:

1) When the EEC is in the soft or hard reversionary mode, the thrust calculations are less accurate.

2) In alternate mode with the thrust levers at a specific position, the thrust changes as the flight condition changes. This requires adjustment of the thrust levers as the airplane flies in order to keep the correct thrust

 

That info together with the FCOM that states that alternate thrust is higher than normal EEC thrust at the same thrust lever position lead me to the following:

 

The Alternate mode still only gives you maximum rated thrust, but since it uses a different program (one that never gives less thrust than the normal program but equal or more at the same TL position), it will overboost at this alternate limit when compared to the normal mode. That is probably a good thing, because if you had that BUG the OP mentioned at the beginning, where amber limit being calculated by the normal EEC mode is assumed to be wrong, then at least you can still set adequate thrust in the alternate mode.

This is something I have never had or seen simulated though, so it is just what I think what would happen.

So the alternate mode thrust protection limit is just slightly higher than normal mode (amber bar) because of the different program used.. Cant prove it, cant try though.

I take that as the reason for the smal (2%) overboost and the reason that there still is a limit.

It is less precise....it is not totally gone.....but less precise and can cause overboost if you dont watch it.

[777simmer 117]

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?

Quite simple, normal mode EEC and Alternate mode use different sensors.

 

But Kevin is right, you need to change your tone!

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 realy need to change that tone!

 

Good luck with approaching BOEING, GE or PMDG like that.

 

Still dont get it that N1 redline protection and overspeed protection are the same thing do you?

Overboost limit....I dont know where or in what context I used those words and what the problem with it is, but it is not a distinct system.

It is a parameter calculated by the EEC.

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.

Thanks for trying to calm things down Kevin.

I am not sure if the OP wanted to find out how things work in a polite discussion or that he just wants to be right.....but I have my suspicion.

[CessnaPilot 1]

Had to think about that for a little while.

Air data is only one of many things the EEC requires to operate correctly.

But yes without air data it can not calculate thrust parameters.

I am not sure it will assume anything or that it will automatically add some % N1.

 

The maintenance manual I have (It is not the complete manual, that would be a big pile of books!) has only this to add:

1) When the EEC is in the soft or hard reversionary mode, the thrust calculations are less accurate.

2) In alternate mode with the thrust levers at a specific position, the thrust changes as the flight condition changes. This requires adjustment of the thrust levers as the airplane flies in order to keep the correct thrust

 

That info together with the FCOM that states that alternate thrust is higher than normal EEC thrust at the same thrust lever position lead me to the following:

 

The Alternate mode still only gives you maximum rated thrust, but since it uses a different program (one that never gives less thrust than the normal program but equal or more at the same TL position), it will overboost at this alternate limit when compared to the normal mode. That is probably a good thing, because if you had that BUG the OP mentioned at the beginning, where amber limit being calculated by the normal EEC mode is assumed to be wrong, then at least you can still set adequate thrust in the alternate mode.

This is something I have never had or seen simulated though, so it is just what I think what would happen.

So the alternate mode thrust protection limit is just slightly higher than normal mode (amber bar) because of the different program used.. Cant prove it, cant try though.

I take that as the reason for the smal (2%) overboost and the reason that there still is a limit.

It is less precise....it is not totally gone.....but less precise and can cause overboost if you dont watch it.

After I do some digging in AMM of B737NG (since B777 AMM doesn't really give me any thing) I finally found something. And I think maybe you are right.

 

"In the hard alternate mode, the EEC uses static pressure (P0) to

get an assumed Mach number. To make sure the airplane will

have enough thrust for satisfactory airplane performance in all

conditions, the EEC assumes the outside air temperature with

the highest thrust requirement. In this mode, large maximum

thrust rating exceedances are possible during hot day

conditions. This can cause EGT exceedances during hot day

conditions"

 

So yes, this is also possible in B777 since EEC will receive an airdata from AIMS too and hence EEC should still calculated an N1 value not too far from what it should calculated in Normal mode.

[AzN1337c0d3r 3]

Still dont get it that N1 redline protection and overspeed protection are the same thing do you?

Still don't get that 90% N1 isn't anywhere near redline do you?

 

 

After I do some digging in AMM of B737NG (since B777 AMM doesn't really give me any thing) I finally found something.

"In the hard alternate mode, the EEC uses static pressure (P0) to

get an assumed Mach number. To make sure the airplane will

have enough thrust for satisfactory airplane performance in all

conditions, the EEC assumes the outside air temperature with

the highest thrust requirement. In this mode, large maximum

thrust rating exceedances are possible during hot day

conditions. This can cause EGT exceedances during hot day

conditions"

 

This however does not correspond to what PMDG has programmed into the sim then, because in hot day conditions, again you only get small thrust exceedance (if we can we say it is in exceedance).

 

Do we know if alternate mode EEC calculated limit is actually over boost? Could it be possible that simply the normal mode EEC calculations have a small buffer built in?

 

Anyways I have opened ticket with PMDG to see what the actual behavior of the plane is supposed to be, although I doubt we'll actually get a technical answer.

[CessnaPilot 1]

Still don't get that 90% N1 isn't anywhere near redline do you?

 

 

 

This however does not correspond to what PMDG has programmed into the sim then, because in hot day conditions, again you only get small thrust exceedance (if we can we say it is in exceedance).

 

Do we know if alternate mode EEC calculated limit is actually over boost? Could it be possible that simply the normal mode EEC calculations have a small buffer built in?

 

Anyways I have opened ticket with PMDG to see what the actual behavior of the plane is supposed to be, although I doubt we'll actually get a technical answer.

Could the manual mean the range where temperature is 40-50C? Since the EEC in normal mode would wind the N1 down above ISA+15 to maintain the T/O EGT at roughly around or below the redline (ie 98% N1 for 50C at sea level)  while in hard alternate the EEC should maintain a higher N1 value (ie matbe around 102.7% N1 ).

[AzN1337c0d3r 3]

Could the manual mean the range where temperature is 40-50C? Since the EEC in normal mode would wind the N1 down above ISA+15 to maintain the T/O EGT at roughly around or below the redline (ie 98% N1 for 50C at sea level)  while in hard alternate the EEC should maintain a higher N1 value (ie matbe around 102.7% N1 ).

I don't know if I parsed what you are saying correctly. Are you saying that at 50C, the engines N1 would be limited to 102.7% N1 because of EGT limit?

 

The engines don't reach close to 102.7% N1 (see picture below). Also note that the similarity of the EGT.

 

The sim here is also weird with respect to the EGT limits. If you fudge around with pressing F4 to try the override the EEC, you can see that the whole dial turns red at 110.5% N1 and 121% N2 as expected from the engine's type certificate. However, the EGT rated for this engine is 1090C (5 minute T/O), or 1050C (Max Cont). The sim seems to trip at 980C instead.

 

Another interesting thing to note is that the EGT of both engines are basically the same, despite the difference in N1 %.

 

Edit: Wrong picture. I'll upload the correct one in a minute.

 

Edit2: Correct picture uploaded.

 

[AzN1337c0d3r 3]

The above picture has got me thinking.

The similarity in EGT and N2 despite the difference in N1 when the EEC is in hard alternate mode seems to suggest that all that is going on in the background maybe that the sim is basically just indicating +2% to N1 gauges.

I'll do more tests after my next VA flight is done.

[CessnaPilot 1]

I don't know if I parsed what you are saying correctly. Are you saying that at 50C, the engines N1 would be limited to 102.7% N1 because of EGT limit?

 

The engines don't reach close to 102.7% N1 (see picture below). Also note that the similarity of the EGT.

 

The sim here is also weird with respect to the EGT limits. If you fudge around with pressing F4 to try the override the EEC, you can see that the whole dial turns red at 110.5% N1 and 121% N2 as expected from the engine's type certificate. However, the EGT rated for this engine is 1090C (5 minute T/O), or 1050C (Max Cont). The sim seems to trip at 980C instead.

 

Another interesting thing to note is that the EGT of both engines are basically the same, despite the difference in N1 %.

 

Edit: Wrong picture. I'll upload the correct one in a minute.

 

Edit2: Correct picture uploaded.

 

 

In normal mode, yes and the N1 will be limit as follow in your pic but I don't know why N1 in hard alternate still wind the N1 down and not archived highest N1 for that altitude (ie N1 at 102.7%) and also the EGT which is quite low compare to the real world video.