Thrust slowly increasing beyond takeoff thrust after 80 knots

[jgoggi 65]

Hi, let's say I have a takeoff thrust of 93.4% N1. I advance throttles to 60%, then press TOGA and the thrust is set at 93.4. After reaching 80 knots (HOLD), the thrust slowly increases, 93.5.... 93.6.... 93.7.... 93.8....more or less 0.1 every 1-2 seconds.  This is not normal, isn't it? Ah, this can't be a throttle calibration problem, because it also happens with the keyboard, no joysticks at all! Anyone encountered this problem? Thanks.

 

UPDATE: I think it is a bug to be corrected, the first takeoff video I found on Youtube has this problem. From a takeoff thrust of 89.7 there is a constant thrust increase to 90.0 and above. See from about 0:50:

 

www.youtube.com/watch?v=3mD9ITG4kgU

[downscc 1,327]

I recommend you refer to FCTM 3.15, which is a discussion on reduced thrust and derated thrust takeoff limits. I believe, but am not 100% sure, what you are observing is a verrrry slight increase in fan speed as you accelerate and if so then another example of how well this engine is modelled. Keep in mind that these fans are as big around as the 737 fuselage.

[tcas_climb 45]

Yep it appears that as the aircraft accelerates, ram air hits the fan blades and cause them to spin faster

[jgoggi 65]

Thank you for your replies. In fact I did not find anything concerning this feature on the FCTM. Can a real world 777 pilot confirm that?

 

Look at this real world video, from min 3:45. The takeoff thrust remain absolutely stuck on 89.0 N1, you can see it at 3:58, 4:07 and 4:26 (at 4:26 it is 89.2, but in the PMDG 777 at rotation it would be at least 90 or 91):

 

[whosyourcaddy 0]

Takeoff power should be set before 80 KIAS. What the N1 does after 80 KIAS is a non-issue as long as you don't redline the engines. It will climb slightly as the fan blades' angle of attack decreases as the airplane accelerates. To me, a 1% increase in N1 is acceptable. I would be monitoring all the other systems instead of concentrating on N1.

[scandinavian13 5,192]

 

 

UPDATE: I think it is a bug to be corrected

 

Then you've submitted a ticket to let them know, correct?

[jgoggi 65]

Then you've submitted a ticket to let them know, correct?

 

No, I said "I think", but if someone (possibly pilots) can clearly state that it's like that in real life, then no bug and no ticket... 

[willywonka 36]

No, I said "I think", but if someone (possibly pilots) can clearly state that it's like that in real life, then no bug and no ticket... 

You can submit it anyway. The team, which includes pilots, will sort it out if it's a bug or not.

[Tabs 2,790]

Takeoff power should be set before 80 KIAS. What the N1 does after 80 KIAS is a non-issue as long as you don't redline the engines. It will climb slightly as the fan blades' angle of attack decreases as the airplane accelerates. To me, a 1% increase in N1 is acceptable. I would be monitoring all the other systems instead of concentrating on N1.

 

To add to this - once the thrust mode on the FMA goes to HOLD - *you* are in direct control of what thrust is being commanded. If you see it getting too high for whatever reason, pull it back slightly - this is what the throttle override option is there for - by default it gives you manual control in HOLD mode, just as the real thing does. Use that in conjunction with the show thrust lever position option.

[jgoggi 65]

Thanks, Ryan. Can you confirm the theory that in HOLD moed, thus with N1 left free to rotate (unless the pilot controls it manually) the fan speed may increase due to the change of the blades angle of attack? That sounds a bit strange because it's the first time I experience it, and, after lift off, at times N1 is even 2-3% higher than the takeoff N1,  

[mc64938 0]

I've had no problem with take offs . Works fine for me

Yes ryan is right its like in real life.

[jgoggi 65]

after lift off, at times N1 is even 0.2-0.3% higher than the takeoff N1,  

 

Sorry, I meant 2-3% higher...