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747-400 RB211 Thrust Control From Thrust Hold (65kts)

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747-400 RB211 Thrust Control From Thrust Hold (65kts)

Old 22nd Feb 2017, 00:22
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747-400 RB211 Thrust Control From Thrust Hold (65kts)

Fellow PPRuNeRs..

It's not clear from my (engineering) training manuals what happens to EPR target and actual values after engine thrust is set on the runway and the A/T mode goes to HOLD .. until the aircraft reaches 400'.

I understand EPR can be affected by TAT increases and temperature/pressure changes due to altitude changes. My manuals do have broad statements saying...

"The desired / Commanded EPR is established by the FAFC based on
Thrust Lever Angle (TLA), Pressure Altitude (PAMB), Total Pressure
(Pt), and Total Air Temperature (TAT).
The FAFC calculated commanded EPR is compared with the actual
EPR sensed by the FAFC. The control logic in the FAFC varies fuel
flow in a manner to make actual EPR match the EPR command."

My manuals say that trimming/thrust equalisation is locked between 65kts to 400 feet. Does this include all thrust adjustments during takeoff and initial climb... or does the actual thrust stay glued to the takeoff target thrust set during preflight?

I realise that a pilot's eyes may not be glued to engine instrumentation during this period

Thanks

P.S. I understand that the FAFC has some kind of lock in it's circuits which requires a 3 degree movement of thrust levers above 400' to unlock trimming.
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Old 22nd Feb 2017, 00:35
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P.S. From what I've read from previous posts, it is important for actual thrust to reach target thrust prior to reaching thrust hold. Would this suggest that if engine thrust does not reach the target thrust value shown above the EPR tape that the FAFC will not try to achieve that target thrust (by itself) after 65kts .... or is restricted to a limited trimming range until 400'?
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Old 22nd Feb 2017, 01:35
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At 65 knots, the autothrottle enters a hold mode ('goes to sleep') - the throttle position will remain fixed, the EPR trim (small adjustments to EPR command intended to line up EPR across the wing to account for slight throttle stagger), and the bleed debits are held constant - this is done to prevent a single failure (the FMC/Autothrottle) from affecting the thrust on multiple engines during a critical flight phase.
What the FAFC does is fundamentally different - it enters a "lock and lapse" mode, meaning it follows a fixed EPR lapse rate with speed and altitude (regardless of changes to TAT). It will continued to close on EPR Command, even if EPR actual has not lined up with EPR Command when it enters 'lock and lapse".
If you move the throttle by less than 2 degrees (memory says it's 2 degrees, not 3 degrees, that unlocks), the 'baseline' EPR command will change, to reflect the new throttle position, but it will retain the lock and lapse characteristics - just from a different baseline EPR. IF you move the throttle by more than that, the FAFC will exit lock and lapse and just control normally based on the throttle and ambient conditions.


Clear?
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Old 22nd Feb 2017, 02:57
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Clear?
Still trying to absorb all this excellent information... It's the "lapse" rate I'm still trying to get my head around. "Lapse" I usually associate with "fall behind" not keep up with.

Does an engine's EPR naturally tend to decrease or increase with rising airspeed during the takeoff roll and up to 400' (higher pressure at the front means a lower EPR?) or does more air into the engine improve the efficiency of the engine (= more thrust). My only experience is from engine ground test runs. Some adjustments cannot be carried out on the engine if the windspeed is above a certain value.

It will continued to close on EPR Command, even if EPR actual has not lined up with EPR Command when it enters 'lock and lapse".
I assume you mean the thrust lever position EPR command (not the FMC-generated limit set during the preflight).

Anyway, much appreciated. I'm always surprised at the depth of knowledge here.

Cheers.
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Old 22nd Feb 2017, 07:18
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Does an engine's EPR naturally tend to decrease or increase with rising airspeed during the takeoff roll and up to 400' (higher pressure at the front means a lower EPR?) or does more air into the engine improve the efficiency of the engine (= more thrust). My only experience is from engine ground test runs. Some adjustments cannot be carried out on the engine if the windspeed is above a certain value.

Short answer is "it depends". Some engines will increase EPR (or N1) with increasing speed, some decrease (the old hydromechanical controlled engines nearly always decreased, hence the term "lapse"). I know the PW4062 EPR would increase between 80 knots and 250 knots, I don't recall about the RB211. The nice thing about "lock and lapse" is it's consistent, so the performance is easily predicted.


I assume you mean the thrust lever position EPR command (not the FMC-generated limit set during the preflight).
Yes... The FMC 'bug' on the EPR tape is usually referred to as the EPR target (you'll also get an EPR limit - the max rated EPR for the conditions). EPR command is the line that moves immediately with throttle movement, then the tape 'fills in' as the actual EPR closes on the EPR command. EPR command is always dependent on the throttle position (with the exception of the small FMC 'trimmer' signal I referred to earlier), and the FAFC/FADEC will do it's best to close loop on the EPR command (note, near idle, the engine is being controlled to N2/N3, not EPR, so EPR command is automatically set to EPR actual since EPR command is basically meaningless when the engine is being controlled to a rotor speed).
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Old 22nd Feb 2017, 10:21
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wowww!!!! Thank you for these interesting infos, appreciated!
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Old 22nd Feb 2017, 21:59
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The nice thing about "lock and lapse" is it's consistent, so the performance is easily predicted.
"Lock" does seem somewhat of a misnomer if the FAFC/ECC is still free to chase the target. All that seems to have changed between normal operation and takeoff operation is (perhaps) the rate of adjustment. With A/T HOLD, the A/T is completely removed from the equation. I'm wondering if lapse rate is more like the minute hand on a watch rather than a second hand.

What the FAFC does is fundamentally different - it enters a "lock and lapse" mode, meaning it follows a fixed EPR lapse rate with speed and altitude (regardless of changes to TAT).
TAT is also related to speed and altitude. I may be having trouble understanding the subtle differences here. I think if I dig any deeper I will probably be getting into engine manufacturer proprietary information

Anyway, thanks again for your responses.
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Old 23rd Feb 2017, 01:17
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"Lock" does seem somewhat of a misnomer if the FAFC/ECC is still free to chase the target.
Lock applies to certain inputs (TMC trim, bleed debits, OAT), but not to throttle position or closing on EPR (or N1) Command.

TAT is also related to speed and altitude.
The difference is the real world temperature doesn't react consistently. Although the relationship between OAT and TAT with increasing speed is well defined, the OAT may change, and while there is an ideal relationship of temp with altitude, the real world doesn't necessarily match that (e.g. temperature inversions). In a highly simplified sense, lock and lapse assumes a consistent OAT, regardless of what the real OAT does.
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Old 25th Feb 2017, 21:03
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Thanks!.... I think it's starting to make sense.

My interpretation: The actual OAT/TAT is locked in at HOLD and actual airspeed and altitude is used to compute theoretical changes to TAT. i.e. real world OAT/TAT may vary from the theoretical ideal.
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