jgoggi

Throttle HOLD does NOT hold N1 but increases in time

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Hi, when the speed mode goes into HOLD at 70 kts during takeoff, the N1 thrust does not remain at the selected value but slightly and constantly increases, until THRUST REF kicks in after lift off. This is NOT due to physical throttle calibration, because it also happens if I don't use any hardware throttle, only keyboard. It's an issue to be looked into. Thanks.

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Hi,

I don't think this is a bug. Autothrottle in HOLD mode only means that "the thrust lever autothrottle servos are inhibited. The pilot can set the thrust levers manually" (cf. FCOM page 4.20.4).

Therefore the autothrottle no longer regulates the thrust.

 

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Of course, but if I DON'T touch the throttle at all, N1 should NOT change (in this case constantly increase, without stopping, as if I was slowly and constantly pushing the lever forward) after HOLD mode engages.This happens always, I tried 100 times, so it's clearly a bug. Further, same happened in the 777 and was fixed.

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There is a interesting message thread on the PPRuNe.org website (Tech Log section) on the behaviour of the RB211's thrust during HOLD. Even though the A/T is inactive, the FAFC (RB211 EEC) varies the thrust according to airspeed and altitude and expected TAT variation.  (i.e. apparently not actual TAT in the air, per se, but based on TAT at thrust hold, and interpolated from there.... I think). 

http://www.pprune.org/tech-log/591278-747-400-rb211-thrust-control-thrust-hold-65kts.html

 

Good luck making sense of it ;)

Cheers

JHW

 

 

 

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I  notice the same  behavior. In both the RRs and GEs (haven't flown a PW livery yet), the N1 begins to slowly increase on the TO roll after passing about 70kts, and continues slowly creeping up until after TO and the A/T kicks back in

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Thanks. So what do the developers say about this issue? I would like to highlight once again that this is not related to throttle calibration/FSUIPC, because it happens with the keyboard as well. 

EDIT: take a look here:

The takeoff N1 is 97.4 and at lift off the N1 is still 97.4, so the N1 creeping forward is an issue.

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Based on basic turbine engine theory the faster you get, the more thrust your engines produce with the same thrust lever position - unless some computer interferes with it.

I can confirm the thrust increasing in PMDGs CF-6 engines. I haven't flown other liveries yet.

Best someone raises a ticket about it with PMDG to have the developers look into it.

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See this video:

 

Takeoff N1 increases from 91.4 when HOLD engages, to 93.6 when THR REF engages at 400 ft...

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Just watched your video. I haven't observed that on my rig.

Edited by Budbud

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Try again ,please and look at N1 between HOLD and THR REF at takeoff...

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I will do two flights tonight with the 744F GE variant and will monitor closely the N1 during takeoff. I'll post the result here after.

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Thanks. Anyway you can check on Youtube, all takeoffs have this issue of the increasing thrust. On the other hand, I saw a video made with the old PMDG 747 and there it was correctly modelled, the thrust remained at the takeoff N1 after HOLD engagement...

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Quote

Takeoff N1 increases from 91.4 when HOLD engages, to 93.6 when THR REF engages at 400 ft.

Just to clarify... You're talking about the thrust limit changing above 400', not the actual thrust below 400'? Sorry, I can't see any values in that video. On my computer it's just too blurry.

Quote

Charts and tables have been developed which identify the specific N1
rpm required to produce a specific thrust level for a given ambient
air temperature and pressure altitude. These engine model N1
characteristics are incorporated into the data storage of the aircraft
thrust computer and the engine power management control.

Above 400', the A/T comes alive. What is to stop the EEC or FMC adjusting target N1 based on TAT and pressure altitude? Sorry, again, I can't see anything in that video.

When you derate an engine via the assumed temperature method, you tell the aircraft that the temperature has changed and the target thrust changes accordingly. When the aircraft is flying, the engine and aircraft sensors tell the EECs/FMC that the conditions have changed. Unfortunately, most of here don't have any experience with real world values in flight. Even Level D sims probably don't provide the randomness of atmospherics in the real world. Most of us assume "correctness" based on what we see in simulators (whatever their price).

 

If someone showed us 100 real world videos in all conditions (including temperature inversions), where the thrust reference does not change above 400 feet, then we could say that that PMDG is not correct.

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Hi John, I am talking about the actual thrust, not the thrust limit. The takeoff N1 green value is 91.4 and this value is set as soon as TOGA is pressed. Then, when HOLD engages, you see the actual thrust increasing from 91.4 to 93.6 (while the green takeoff value above remains at 91.4). The airplane has by then reached 400 ft and THR REF takes control...

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Ok, I did two flights tonight with the 400F GE variant and I actually noticed the increase of N1 during the takeoff roll. I took a video on the first flight but it is the same as the one posted so I don't feel the need to post as well.

The human mind is curious, I would have sworn that I didn't have this issue.

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8 hours ago, jgoggi said:

Hi John, I am talking about the actual thrust, not the thrust limit. The takeoff N1 green value is 91.4 and this value is set as soon as TOGA is pressed. Then, when HOLD engages, you see the actual thrust increasing from 91.4 to 93.6 (while the green takeoff value above remains at 91.4). The airplane has by then reached 400 ft and THR REF takes control...

ah.. ok.

Are you using FSX? If the GE CF6 EEC works in a similar manner to the RB211's FAFC, the variations may be due to the FSX atmosphere not being "normal" (it isn't, I hear).

In the real world, the autothrottle is locked at 65kts, but the FAFC is not fully locked. It still applies some standard atmospheric adjustments to try to keep the thrust constant during takeoff and initial climb. However, if the FSX atmosphere is not standard or the engine bleed load changes after 65kts for some reason, then you may get the EEC not applying the correct adjustments and there will be a variation between the green value and the actual thrust.

Of course, all this assumes v3 has this level of sophistication in engine management. ;)

Anyway, I'm still trying to understand what actually goes on on the real aircraft. I don't think many people, including pilots and engineers, really know what is going on. Some things are locked in the EEC, some things aren't.

Cheers

JHW

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John,

I am in P3Dv3 and I am seeing this issue. And I dont think it has anything to do with atmosphere because this issue starts on the TO roll on the ground. And it is not just the GE, the RRs do it as well. If you look the N1 readout for the RR you will see it increase a little bit during the TO roll

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Those seeing this: are you taking off with packs "on" or "off"? I'm curious if that makes any difference. I believe BAW r/w SOP is to turn packs off at takeoff weights above 300 tons, and to leave them on below that weight. As John said, bleed load variations can affect thrust. Also, does engine derate configuration make a difference? Are you using assumed temperature + a fixed derate, or just fixed derate, or none?

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1 hour ago, Qavion2 said:

Are you using FSX?

P3D v3.4 HF3.

 

55 minutes ago, JRBarrett said:

Those seeing this: are you taking off with packs "on" or "off"? I'm curious if that makes any difference. I believe BAW r/w SOP is to turn packs off at takeoff weights above 300 tons, and to leave them on below that weight. As John said, bleed load variations can affect thrust. Also, does engine derate configuration make a difference? Are you using assumed temperature + a fixed derate, or just fixed derate, or none?

I did not check this, will try tonight at home. Up to now I have done very light-weighted tests, with weights well below 300 tons, packs on and fixed derate only (TO2/CLB2).

 

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2 minutes ago, jgoggi said:

P3D v3.4 HF3.

 

I did not check this, will try tonight at home. Up to now I have done very light-weighted tests, with weights well below 300 tons, packs on and fixed derate only (TO2/CLB2).

 

I'm pretty sure that PMDG uses their own custom engine model, rather than relying on the default FSX/P3D turbine engine emulation - but there may be limits on how much precise control they can implement - or the degree to which external factors like bleed air loads, atmospheric conditions etc. affect engine performance in the sim.

On the other hand, data like this from end users can certainly help the developers in fine-tuning the model as much as possible. For an initial release, the QOS2 is remarkably compete and trouble free - but there are always improvements that can be made as the product matures.

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I tried with packs off and different derates but nothing changes.

See also my reply in the topic "Thrust decrease after takeoff", that issue comes from this one.

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