Yes, the EPR remains stable, but if you look at N1, you will see that it increases. The thrust reduction at 400 ft is, in the GE, in order to meet the green N1 target. I haven't checked the RR/PW, but I suppose that something similar happens, hard to say why the EPR target is not met (maybe THR REF wants to meet N1 instead of EPR?). I hope PMDG is looking into the whole logic and fine tuning it in the first update...

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65 kts, HOLD --> Actual N1 starts increasing (erroneously) without a corresponding increase in the preset takeoff green N1,

James, you seem adamant that they should be the same. Do you mean under ideal theoretical conditions? i.e. in the sim world?

In the real world, the green thrust reference value is based on data inputs (and pilot entries) into the FMC. The FMC normally sends this data to the EECs and they use the same data. Up to 65kts, engine equalisation data and bleed status is also sent. However, above 65kts, this changes and the EECs go into a "lock and lapse" mode (refer to recent message thread on PPRuNe.org). Bleed status and (4) engine equalisation data is frozen, and as I understand it, actual TAT is no longer used for thrust control. Engine EEC thrust computations then seem to be based on data measured at 65kts, but modified according to standard (expected) changes in atmospheric data due to sensed changes in speed and altitude. i.e. to at least 400 feet above the runway. In an ideal sim world, yes, you might expect the engines to maintain the green target thrust.

Cheers

JHW

Ok John, but what I have seen based on the careful examination of takeoff videos (real life, Aerowinx PSX precision simulator and even the iFly 747) is that in real life, after HOLD engages at 65 kts and until lift off, there are NO changes to the green reference N1 and consequently NO changes to the actual thrust, while it's clear that our PMDG 747 has a constant increase in actual thrust (while the reference N1 does not change, so who is telling the thrust to increase??). This brings the actual thrust at lift off to be about 2% higher than it was at HOLD engagement (once again, the N1 reference has NOT changed meanwhile)!  In real life after lift off the green reference N1 may increase and, consequently, also the actual thrust, that sticks to that reference N1. At 400 ft, when THR REF engages, the A/T will not do any thrust reduction, because the thrust is already at the wanted value. In our PMDG 747 when THR REF engages at 400 ft, it finds the actual N1 higher than the expected value (that is green N1), and will consequently reduce thrust.

The issue in our 747 is that in HOLD mode, the actual thrust DOES NOT STICK to the green reference N1, as instead it should do...

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, while it's clear that our PMDG 747 has a constant increase in actual thrust (while the reference N1 does not change, so who is telling the thrust to increase??).

As one "expert" said before, James, the EEC has the potential to change the thrust. However, if you have a link to a few clear videos, I think everyone would appreciate it... including the PMDG team

By the way, PSX (as is PMDG) is going through a constant evolutionary process. There are changes being made sometimes on a more-than-daily basis, so we know that both sims haven't reached perfection yet. It is good to compare them. I've made some (rather) quick comparisons and where there were differences, I consulted the books or asked the experts how it should be. Both sims have improved as a result of this. Anyway, as you've probably read, a ticket has already been submitted to the team.

Cheers

JHW

 

2 minutes ago, Qavion2 said:

As one "expert" said before, James, the EEC has the potential to change the thrust. 

But I suppose this EEC potential should be reflected first of all into an N1 reference change, then into a consequent actual thrust change.

 

3 minutes ago, Qavion2 said:

Anyway, as you've probably read, a ticket has already been submitted to the team.

Yes, and one submitted by me, already aknowledged.

Best regards.

19 hours ago, jgoggi said:

But I suppose this EEC potential should be reflected first of all into an N1 reference change, then into a consequent actual thrust change.

Good point, James. I'm not sure. I don't know how "dynamic" this reference value is during takeoff, or indeed any stage of flight.

I just wish I'd known to ask my instructor these questions when I was completing my 400 training.

PMDG's last reply to my ticket is that hopefully the fix will be in the first service update. I really hope it will be, this is something that compromises the 'precision' of this simulation and in the extreme situation when you make a very shallow takeoff and keep a vertical speed close to zero, when you are a few feet above the ground the N1 is almost 2-3% higher than when HOLD engaged, so that when THR REF engages it needs to reduce power a lot... I am a bit concerned that in yesterday update of the Issue Tracking Thread this issue does not appear...

By the way, this is the best video I could find clearly showing how the actual N1 thrust remains around the green N1 reference (in this case 95.7) after HOLD engagement and until lift off (see from minute 5:30 on):

In the PMDG 747, it would constantly increase from 95.7 until THR REF engages at 400 ft.

Hello,

I think PMDG is more than aware of the issue. I also submitted a ticket last week about this abnormal N1 increase. Initially, I thought it concerned only the GE powered variants, but I also see this increase with both PW and RR variants of the plane. That being said, I think it's useless to insist. PMDG knows the issue exists, they will probably fix it for the first or second update. Yes it's a little annoying, but all we can do now is waiting calmly for this update to come out ;)

If you can draw something conclusive from that video, James, I applaud you

What I saw:

When taxying to the runway, I noticed the TAT was 29C , derate was 52C, packs were all on, target N1 was 95.8%

About 70kts groundspeed (couldn't see the airspeed), the TAT was still 29C, packs still on, but the target had dropped to N1 95.7%

Towards the end of the takeoff roll, the TAT increased by 2 degrees, the target N1 remained at 95.7%, but the actual thrust was all over the place (varying between 95.7 to 96.0) during the roll i.e. some engines at various points were higher than target.

From about 50' above the ground, I couldn't see any more clear values until the rad alt was 1300'. By that time, the target N1 had reduced to 95.4% (and I only saw one engine value (95.4%).

From my real world experience, I notice that you can sometimes put a couple of extra degrees of assumed temperature derate and the target thrust won't necessarily change, but if you add a few more, it does change. This could mean that the FMC thrust tables change value at specific temperature points. i.e. if the TAT was 31C at 65kts, there may have been a target thrust change at TAT 33C at the end of of the roll. 

Hopefully we can find some more videos of this nature. With only one video, it's like trying to draw a line between two points when you only have one point

Anyway, thanks for the video

Cheers

JHW

 

John, with that video I just wanted to point out that the target N1 of 95.7 is respected by the actual thrust (at least until lift off), that yes, varies between 95.7 and 96.0, but is kept within those values (that's normal thrust levers position tolerance). Had it been "our" 747, without touching the levers, between HOLD engagement and lift off the actual N1 would have increased continuoulsy... 95.7.... 95.8...95.9....96.0....96.1... 96.2....96.3....96.4.... If you had kept the airplane climbing very slowly (say 200 ft/min), when it had achieved 400 ft the N1 would have been say 102 or 103, then THR REF engagement would have cut the thrust back to 95.7...   It's like, when in HOLD mode, there is an invisible hand that slowly advances the thrust levers and if this hand is not stopped, it will slowly advance them until max thrust (but before reaching max thrust it will be stopped by the A/T engagement at 400 ft). The problem should be fixed by removing that invisible hand, that's all.

 I think these 2 screenshots illustrate quite well what James is talking about. As you can see, when HOLD mode first kicks in, N1 is right on the green line(93.9), however as the take off roll continues, we can observe an increase of N1,on the second screenshot N1 is at 95.5 whilst green line is 94.

Yes, exactly, so in fact the airplane lifts off with an N1 of 95.5 intead of 94.

Note that there is a 4degC increase in TAT in those screenshots (perhaps closer to the theoretical norm than James' video).

We still haven't established if the reference thrust should change during this period. Do you have any more videos in which the ref thrust is visible below 400 feet (even though the actual thrust values may not be visible)

I'm seeing this increase in N1 on the ERF also.
Here is a screenshot. Above is before V1 and below is after liftoff. Notice N1 increase of 2.4% above green bug.

Untouched hardware throttles.