VNAV decsent

[falcon999 24]

If A/T is not working properly with the FMC options set so that throttle movement has no effect while A/T engaged then something else is wrong. The A/T will manage the appropriate thrust levels to stay on path. If the path can't be followed then VNAV will disengage and you will get an FMC warning displayed (like: decent path not achievable). Throttle position, movement, calibration, etc will have absolutely no effect. The NGX was specifically programmed this way and with these options to deal with the inifinite issues that can arise due to the multitude of controller hardware combinations and the subsequent configurations.

 

So the first test is to see if things work with the throttle axis turned off during automated flight modes. If that works then you know you have a calibration problem and there are lots of threads on how to deal with that. IMHO this sounds like the problem, but...

 

If things aren't working with the throttle axis disabled during autoflight, then there is something else going on and we need more information on specific FMC programming to try to solve. Screen shots of the setup and flight plan FMC pages would be extremely helpful.

[EdubFlyer 2]

I'll do some more tests and get screenies later. The basic problem is, lets say you're cruising at FL390 with LNAV/VNAV. You dial in your new altitude before TD, at top of decent the throttles RETARD to idle. If you watch them in the virtual cockpit they go all the way to the stops then inch back up slightly. I use the good old microsoft FF2 joystick not calibrated with FSUIPC. This behavior happens even if I unplug the joystick. Then the primary display says ARM. N1 is about 45%. If I hit F1 (with or without joystick connected) you can see the throttle levers in the V/C go to their idle stops then inch back up slightly again. If nothing else is done they will remain in that position all the way down, with N1 decreasing slightly as we lose altitude but never going below 40%.

Now once you are below say FL260 or so you can hit F1 and you will see the throttles inch back to their idle stops and stay there. Or you can switch to Level Change mode and back to VNAV , or you can disarm the autothrottles and re-arm. All of these behaviors happen even with the joystick unplugged.

I don't think the issue has much to do with the FMC as it does the behavior of the throttles at high altitudes. I think the FMC thinks it already put the throttles all the way down so all it can do after that is give DRAG REQUIRED warnings. My workaround as stated above is to check the throttles from time to time all the way down the decent and correct (hey real pilots do it too, if you've ever got to sit in the jump seat, those guys are always grabbing and checking the throttle levers) as necessary.

Hope this narrows it down a bit

 

Eric Walace

[falcon999 24]

As I mentioned, if you have the axis disabled in the FMC options then any changes in N1 are being controled by the autoflight systems. Altitude has nothing to do with N1. Unplugging the joystick is not necessarly going to solve hardware related issues. You must make sure the FMC options are set. Another big variable is WX. If you are using an external WX program or FSX WX then this will influence and/or cause everything your describing. If you have the axis disabled then the A/T doesn't think it puts the throttles at idle, it does.

 

Th best way to troubleshoot this is to ensure the FMC options are set correctly and then fly the route again.

[skyhawk8 0]

What cost index are you using in the FMC during set up. Also when at cruise are you near the limit for speed?

[Spin737 102]

Two observations:

N1 (N2) idle does vary with altitude.

CI has nothing to do with this.

[nrcrate 40]

You know- this is something that I've been having to deal with for a while. I've done everything I can think of and nothing seems to work. I'm sorry, but I can only think that this is a NGX issue. I'll add my general experience:

 

Let's say I'm cruising nicely along at FL370 and I reach my top of descent. I've dialed my altitude down as desired and as I hit the T/D point, RETARD is annunciated and the throttles come back to idle, then ARM is annunciated. If I hit F1, the engines will temporarily reduce thrust slightly and then return to the proper flight idle limit that is determined by the EEC's at the time.

 

Now here is where I think the bug is. (Mind you, throttle override is disabled for me) During this whole descent, the EEC's should be gradually lowering the flight idle limit- which they do properly... But the actual current thrust does not follow this limit as it decreases. I must continue to hit F1 about two more times during the descent. This is where I think the bug lies- the current thrust is not following the idle limit, but staying at its initial limit (when it was back at FL370) until it's taken down by our intervention.

 

Phew- that was a lot for typing on a phone. That's my theory at least. I'd be interested to know if this is also describing what others are seeing.

[WhiteKnuckle 11]

Folks, not sure how relevant this is but I've found I get better\smoother decents (less instances of Drag Required) if I select Descend Now about 5nm before TOD. I think this is mentioned in Tutorial #2. I have never used F1 when decending and have AT overides off.

[Kerlix 99]

Let's say I'm cruising nicely along at FL370 and I reach my top of descent. I've dialed my altitude down as desired and as I hit the T/D point, RETARD is annunciated and the throttles come back to idle, then ARM is annunciated. If I hit F1, the engines will temporarily reduce thrust slightly and then return to the proper flight idle limit that is determined by the EEC's at the time.

 

I've noticed that I can do this too, and I also have the Override disabled.

 

not sure how relevant this is but I've found I get better\smoother decents (less instances of Drag Required) if I select Descend Now about 5nm before TOD.

 

I do this as well, much easier to capture/maintain the descent path. However, I do periodically need to add spoilers. Most likely my fault for not entering accurate descent forecast winds.

[nrcrate 40]

I've noticed that I can do this too, and I also have the Override disabled.

Yes, but I'm pretty sure this is the proper behavior- the paragraph after that is what I think is the problem here.

[EdubFlyer 2]

Nick explained it better than I, that is exactly what I have been talking about. So I set up a flight from the fsx intro screen, using fsx internal real weather, PAJN to PANC. got to top of decent , throttles RETARD, then ARM, throttles in V/C are 1/4" from their stops. I hit F1, they dip to their rests, then return to 1/4" up. Now at 30,000 feet or so they go back to their stops! Perfect that's how I think it is supposed to work.

 

I set up the same flight with AS2012 as my weather. Now, the same behavior described above occurs, TOD throttles RETARD, then ARM, throttles go all the way down to 1/4" from their stops, I hit F1, watch them dip then return to 1/4" above their rests, now it stays there all the way down the decent path causing overspeeding and DRAG REQUIRED warnings. At some point around FL280 I could hit F1 and the throttles go all the way down and the plane does a better job maintaining the planned decent.

 

Could the external weather have anything to do with this issue?

 

Eric Walace