Bleed Demand Trimming.

[John H Watson]

I'm trying to understand the point of the Air Supply Control and Test Unit (ASCTU) sending bleed demand signals to the EECs. Won't the EECs simply pour more fuel into the engine if any additional load (pneumatic or otherwise) is slowing down the engine?

The bleed demand signal is sent via the EIUs and FMCs, although I haven't been able to find out if this bleed demand trimming is stopped with the A/T off. We know engine equalisation trimming (when the thrust levers are slightly staggered) stops when the A/T is off.

Does bleed air demand trimming change the position of the white N1 command cursor on the EICAS (like engine equalisation trimming), so that the position of the cursor is no longer a direct indication of thrust lever position?

The bleed system also sends similar demands to the APU.

I find myself getting into chicken and egg arguments over these issues. What does N1 actually represent? Propulsive force or propulsive force and engine accessory loads or some % of engine loads?

[Hardy Heinlin]

If there is a fixed almost-maximum N1 RPM reference, then I would say N1 is an RPM percentage based on that reference. ("Almost" because it allows settings above 100%.)

If you've been inspired by that comment on the "HPSOV" in that other thread and the momentary EGT rise: This effect, by the way, occurs in PSX not because of HPSOV interactions but because there is generally an extra EGT rise whenever the engine RPM is accelerating. That extra always fades away when the RPM stabilizes.


|-|ardy

[John H Watson]

If you've been inspired by that comment on the "HPSOV" in that other thread and the momentary EGT rise: This effect, by the way, occurs in PSX not because of HPSOV interactions but because there is generally an extra EGT rise whenever the engine RPM is accelerating.

Ah, no. This is something different. It's purely about the meaning of N1 and about trimming.

If there is a fixed almost-maximum N1 RPM reference, then I would say N1 is an RPM percentage based on that reference. ("Almost" because it allows settings above 100%.)

Limits come in all shapes and sizes. There are thrust rating plugs on engines with big debates on how these operate (purely an N1 limit?). You can swap 744 CF6 engines with 767 engines, but they have different ratings on different airframes. There are engine internal pressure limits which will may limit N1 in certain conditions. There are electronic and mechanical limits on maximum rotor speeds.

According to some books, the thrust levers are calibrated to achieve maximum takeoff thrust at 48degrees of lever movement. I don't know what the relationship is between the cursor and the thrust levers at the mechanical limit (50 degrees) with the EEC in Normal mode and the thrust levers moved manually. On top of this, we have to integrate the FMCs influence on the thrust.

The pilots' manuals say that if the A/T is inoperative/off, the pilots should adjust the levers before and after takeoff to achieve the required target thrust, but don't specify what things have to be compensated for. Is pneumatic load one of those things, or does the EEC still get that data and adjusts accordingly? Obviously they have to adjust for thrust lever stagger because engine equalisation is inactive.

[Hardy Heinlin]

When you look at cruise performance tables, there are always footnotes saying that you need to increase the thrust values shown in the table by X units for certain anti-ice or pack high flow conditions. So it seems to me that the thrust indication on the EICAS doesn't integrate addditional bleed air demand. When there's more demand, the thrust indication needs to rise to maintain the cruise speed. This effect also occurs in PSX. Run any stable cruise scenario, turn off the A/T, check that the airspeed is stable, then click off all four bleed air switches. The thrust and the airspeed will slightly increase.


|-|ardy

[John H Watson]

Sorry, I'm not familiar with cruise tables. Are they for A/T off operation?

Why doesn't the EEC sense the slowing down of the engine due to the increased load on the engine and simply put it back to the same N1?

Are the GE engine EECs not relying on N1? Are they simply using TLAs to produce a target "power" (however it is defined) based on tables and non-N1 engine sensors (and ADC values)? 

[Hardy Heinlin]

Are they for A/T off operation?

I think they are for both off and on. I think it's the same database value the FMC uses for the magenta command thrust line on the EICAS that appears when the A/T is off while VNAV PTH is engaged in level flight.

I.e. when the bleed air demand rises and you want to maintain the cruise speed, the thrust needs to increase as well. Do we agree on this part?

But if I understand your question correctly, you're just asking if the EICAS command cursor automatically rises when the thrust demand rises under these conditions -- while the levers don't move?

Would this automation just react to bleed system related valve positions, or to actual PSI fluctuations? If it just reacts to simple valve on/off positions, can it be so precise to predict the required thrust compensation in every situation so that the levers won't move at all?


|-|ardy


Do you remember the diagram with the TLA curve versus command thrust? I implemented that in PSX. It refers to the current idle minimum and the THR REF limit, which is dynamic. On GE engines, this is the reason the thrust may increase when you turn off the EEC switches as the current TLA may then command a higher thrust. As far as I recall, this curve is not related to bleed air demand directly but to the THR REF limit.

[John H Watson]

I.e. when the bleed air demand rises and you want to maintain the cruise speed, the thrust needs to increase as well. Do we agree on this part?

Yes, but that's a speed issue, not a thrust issue. I'm wondering more what happens when thrust is a fixed target (takeoff, climb, etc).

But if I understand your question correctly, you're just asking if the EICAS command cursor automatically rises when the thrust demand rises under these conditions -- while the levers don't move?

That's part of it, yes. I just don't know what is supposed to happen in reality.

Do you remember the diagram with the TLA curve versus command thrust? I implemented that in PSX. It refers to the current idle minimum and the THR REF limit, which is dynamic. On GE engines, this is the reason the thrust may increase when you turn off the EEC switches as the current TLA may then command a higher thrust. As far as I recall, this curve is not related to bleed air demand directly but to the THR REF limit.

I only recall the RB211 curves. Introducing EPR at this point is too much for my head. I have heard that GE engines also may increase thrust (N1) when switching to ALTN mode, but I haven't yet looked at the details (I would guess that it's partly based on the fact that different sensors are being used to compute thrust).

There are lots of random statements in my books regarding the GE EECs, fuel flow and N1.

The EEC's primary role "is to schedule fuel flow requirements based on TL resolver inputs".

But then continues to say that sensors are used to compute N1 command.

"The thrust rating logic uses N1 command and several EEC control systems to determine the required fuel flow".

EEC Logic Diagram

Does the EEC compute an N1 which it equates to fuel flow and then, after sending these requirements to the HMU, ignores N1? (unless a limit is exceeded)

Perhaps I need to look at HMU operation.

[Hardy Heinlin]

I would say it sets an N1 target value for the current TLA according to the current translation ratio, and it controls the fuel valve to reach and maintain that value. Not vice versa. Not the fuel flow first and then accept any N1.

[John H Watson]

On the final page of engine fuel control, it sums up what we would expect:

The ECU gathers the information and then issues command signals to control the engine as efficiently as possible. The normal command mode is the "N1 rating" mode. The ECU determines commanded N1 from the TLA input signal. It determines the actual N1 from a dedicated speed sensor and modules the metering valve position to keep the commanded and actual N1 values equal.

Unfortunately, it doesn't answer what I wanted to know:
What trim inputs are not available with the A/T switch off?
What happens to the engine under unexpected loads (and why should they differ from expected loads)? Surely the feedback circuit would take care of this.

If the airplane increases speed or gains altitude, is the N1 command changed? If so, does the N1 command cursor move even though the levers haven't? Or is this increased N1 command invisible to the pilot?

Is N1 speed a true indication of thrust? We know some percentage of the thrust is generated the core, but we also know that air going through the core is generating the power to drive the fan.

[Hardy Heinlin]

I've learned that EPR and N1 are approximate indications of the actual thrust. There is also a variable air drag generated by the air compression inside the engine. There are very complex interdependencies of net thrust and self-generated drag.

When the density altitude changes (TAT or pressure altitude), the THR REF limit automatically changes as well. And that automatically changes the TLA translation relative to that limit (within a certain tolerance), as far as I understand it. That is, the white EICAS cursor may move a little while the lever doesn't. I think this is independent of the bleed air stuff. It's just density altitude.


|-|ardy