CF6 ECU ALTN

[John H Watson]

Before the EEC, would the flight engineer do magic on their panel to increase engine thrust a bit, or would they reach forward to push the throttles up?

It was the F/E's job to make small adjustments to the thrust levers (for thrust equalisation) in cruise and on takeoff.

[John H Watson]

So, what N1 will the EEC command now?

(a) A higher N1 because the EEC thinks the Mach number has dropped by approximately 0.1.
(b) A lower N1 because the EEC thinks the airspeed rises due to the higher TAT, while the ISA deviation is still -7°C.
(c) No N1 change because the EEC ignores the TAT and just refers to the 7°C ISA deviation and the unchanged pressure altitude.
(d) ...


I think it's (a). That will also pretty much agree with the N1 command of the normal EEC mode.
This way the difference between modes will be small.
If it were (b), the difference would be drastic. But maybe it's (c) ...

I'm with Jeroen on this one. No idea 

[Jeroen Hoppenbrouwers]

(one single throttle servo)

Yes, I get that, and that is why I see the EEC need (limited) thrust authority among each other. But I would still intuitively think that the overall thrust comes from the throttles. I know reality is stubborn and complicated. Let's try.

When you depart from a hot or high airfield, you predict that you will need more thrust to get off the runway safely, so you set the throttles higher. How much of the calculations for this are airframe-based (wings don't work as well when hot and high) and how much is engine-based (especially with N1-based engines that don't directly measure the oompf they produce)?

Just trying to understand why the mach correction has been moved into the EEC instead of for all engines together in the autothrottle/autothrust system.

It could be, for example, because there is one autothrust computer that is the same for all engine models (GE, P&W, RR) , while each engine model has its own EEC to smooth out the differences between engine models. But given all other changes to the aircraft when you swap engine models, I doubt this.

Hoppie

[Hardy Heinlin]

I think the Mach factor is used because it nicely unifies subfactors like air compression, temperature, ambient pressure etc. -- they are used to determine the maximum N1 limit (amber line). The sensed airspeed, in this context, is relevant for engine airflow control (e.g. compressor stall prevention), not relevant for aircraft airspeed control. You can set the TLA to the max and you'll get max thrust for the current ambient conditions -- which may change. For this limit function you don't even need the FMC which calculates the THR REF values (lower than the amber line). Minimum idle is also computed (based on anti-ice and flight phase etc.). The TLA scale between min idle and max thrust is accordingly interpolated, i.e. a TLA somewhere in the middle can also vary the command N1 when the EEC varies the thrust limit.


|-|ardy

[simonijs]

We know, when the OAT increases, the Mach number increases too.

At the same time, the aircraft performance decreases due to the warmer air; the airspeed decreases, the pitch increases.

If OAT increases, so will the speed of sound (dependent of temperature only). For the same TAS, the Mach number will then decrease.

If OAT increases, air density will decrease. Since air density is in the denominator of the TAS formula, a reduction of air density will result in an increase of TAS.

Example: aircraft (340000 kg) at FL350  OAT = -61 C  CL = 0,504  TAS = 488  M0,86  TAT = -29,6
                                                               OAT = -54 C  CL = 0,504  TAS = 496  M0,86  TAT = -21,6

Keeping TAS at 488 in OAT = -54 C:  CL = 0,52  M0,847  TAT = -22,6

I have a feeling that "increase/decrease" somehow got mixed up...

Regards,
Simon

[Hardy Heinlin]

We know, when the OAT increases, the Mach number increases too.

Sorry, I meant to write: "When the OAT increases, the speed of sound increases too."

(Of course, you're right, Simon, when this happens while the TAS is constant, the Mach number will decrease, not increase.)


Thanks,

|-|ardy

[Jeroen Hoppenbrouwers]

Your idea about engine limits and airflow control makes perfect sense. The experienced "Mach thrust stabilization" may be a side effect.

[ASCTU744]

Pressure altitude, TAT, and 7°C ISA deviation

When switched to ALTN (soft or hard), only P0, and last AMB T (or ISA dev). So TAT shouldn't be available at all according to my source.

I wonder how my chart says something different then yours, if we follow your chart TAT data should still be available.

So I think 'C'.

[Hardy Heinlin]

According to my training books, TAT inputs are provided by: T2 LADC, T2 RADC, T12 CHA, and T12 CHB. The latter two are sensors on the engine.

Also on the schematic you can see T12 inputs going into all three EEC modes.

In case of a total ADC failure, the only inputs you lose are the "PT" type sensors (total pressure).


Some quotes from my source:

"Each EEC channel compares: Total air temperature inputs (T2 LADC, T2 RADC, T12 CHA, and T12 CH B) to select a T2 value for calculating N1 command."

So the EEC can use its T12 to select a T2 value. That is, the EEC always has a T2.

In soft mode:
"N1 command is calculated using the assumed values for Mn, Q, TAMB, and DTAMB, and the P0, T2, TLA and bleed values."


Your KLM text is not wrong; it's just incomplete. It doesn't tell the details.

[ASCTU744]

That makes a lot of things clear, thx! Does the AMM cover this in detail? I can't find any chapter doing so.

[Hardy Heinlin]

The EEC modifications and the new EEC malfunctions are now available in PSX update 10.168:

https://aerowinx.com/board/index.php/topic,4191.0.html


|-|ardy