jgoggi

Throttle HOLD does NOT hold N1 but increases in time

Recommended Posts

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.

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

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.

 

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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

 

 

 

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

See this video:

 

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

Share this post


Link to post
Share on other sites

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

Edited by Budbud

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now