VNAV decsent

[EdubFlyer 2]

I know this has been touched on before, however there are so many topics of varying subject matter I will ask again. I'm cruising in LNAV/VNAV. nearing my t/d point I reset the altitude in MCP. At top of descent the throttles RETARD then show arm. As per tutorial 2 I hit F1to ensure throttles are at idle. N1 setlles at about 45%. N1 will reduce slightly as we get to 25000 ft, at this point we may be too high or too fast. Below 24000 or so I can hit F1 again and the N1 will go down into the thirties. Sometimes at around 15000 I can hit N1 once more and N1 will reduce again, perhaps to the low 30's, and I catch up to my descent path and target speed. I have the ol' MS sidewinder force feedback that is NOT calibrated in FSUIPC. Not sure if this is normal or if I have a setup issue. Any ideas?

 

Eric Walace

[ctdesing 5]

Most times on descent i have to deploy the speed brake to keep the airplane on vnav path. But if you think that your throttle axis isn't calibrated just check it against airplane throttle and see how it performs.

[ajpongress 467]

Why are you pushing F1? If autothrottle is on, it will automatically manage the proper thrust setting for your descent.

I use VNAV every time I fly on my descents and I don't have to push F1 to stay on the descent path my FMC shows me.

[AirbusCG 52]

If you enter the correct values into the FMC it'll pedict you a path you can follow without the need of doing anything on the throttles manually.

 

You should check if you have the autothrottle override in the sim on. This way your joystick may override the idle setting the the autothrottle while you're in arm mode. There an option for this in the PMDG setup in your FMC.

[Fabo 213]

If autothrottle is on, it will automatically manage the proper thrust setting for your descent.

 

Yeah well... thats cool and all... but it does not... if it did, F1 would not do a thing...

[alvin 0]

I think OP refers to the situation when AT stays in ARM mode for a longer period of time (after initial RETARD), and thus the throttle is under the direct control of the pilot.

 

I too find this behavior, and I have to press F1 every now and then to lower N1 to its current minimum. I'm using Logitech Force3D pro joystick, but I don't know whether this is hardware issue or it has something to do with the NGX or FSX itself.

[EdubFlyer 2]

Yes I do have the autothrottles set to overide. I look at the throttles in the VC and they do not go all the way to their idle stops. I hit F1 and they do, for a second then back up to where they were. Once I get below FL240 or so I hit F1 again and the throttles go all the way down. I remember reading about why they had to do it this way in fsx.

[Spin737 102]

Hmmm. There is an idle N1 that decreases as you descend. At the top it's around 40% and as you descend it goes below 30%.

 

It sounds like the throttles are not following the decreasing idle.

[rsvit 16]

Hmmm. There is an idle N1 that decreases as you descend. At the top it's around 40% and as you descend it goes below 30%.

It sounds like the throttles are not following the decreasing idle.

 

The faster you go (look on ground speed) the higher idle N1 you get.

[Spin737 102]

 

 

The faster you go (look on ground speed) the higher idle N1 you get.

 

Ground speed probably wouldn't be the best choice checking for the windmilling effect. I'd say the ram effect is relatively minor compared to the N2->N1. N1 follows the N2, which is the actual idler.

[Fabo 213]

It sounds like the throttles are not following the decreasing idle.

 

That does not sound right. Shouldnt idle always be at throttles closed, whatever idle is used (anti ice, approach, flight, altitude/speed changes...)

[Spin737 102]

That does not sound right. Shouldnt idle always be at throttles closed, whatever idle is used (anti ice, approach, flight, altitude/speed changes...)

I agree. It doesn't sound right. It sounds like the idle at the TOD is as low as the N1 (N2) goes without hitting F1. It sounds like the ARM function doesn't compensate for the decreasing idle.

[EdubFlyer 2]

That's exactly my point. I understand at high altitude and mach # the windmill effect. descending from tod the n1 does go down gradually, just never gets below 41% or so all the way down until I hit F1 around 24K. then it makes a noticable dip into the thirties which allows me to intercept VNAV path again in may cases without spoilers. One peculiar thing is the fuel flow # doesn't change at all, even when you hit F1 and the engine speed decreases and you descend faster/slower. I suspect this is part of the workaround PMDG uses to compensate for fsx jet modeling, which is why I think there is something setup related that I may be missing. Another trick I have found is to hit the flight level change button then vnav again and that will also correct the idle position sometimes.

 

Eric Walace

[lassekronborg 43]

Did anyone ever find a solution to this problem? I've experienced exactly the same symptoms. When I let VNAV manage my descent speed, it reduces N1 to the appropriate flight idle percent according to my TOD altitude. That's about 40% at FL390. During my descent I would expect the flight idle to slowly decrease until reaching somewhere around 30% at 2000ft.This doesn't happen though. If I deactivate then reactivate autothrottle I end up with the correct N1 according to altitude. This should happen automatically, but it doesn't. Hence, as I descent, I'll start to build up speed and get way too fast. VNAV descent speed targets at 260kts are easily overshot by 30kts.

 

My work around is, as mentioned, to disengage then reengage autothrottle.

 

I'm using a logitech extreme 3D joystick

 

I have Joystick override autothrust disabled

 

Everything seems to be callibrated correctly.

 

Cheers

Lasse

[EdubFlyer 2]

I never received an answer, but I did find a setting in the aircraft options that allows me to manipulate the throttles when in arm mode, much like can be done in real life. It's under A/T override, and you set it to something like override in hold or arm. What this does is allows me to blip the throttles up and down to idle briefly at about FL250, then i see the throttles in the V/C are back at their rests and I'm good to go all the way down.

 

Eric Walace