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VNAV descent oddities

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Did a short hop LFMN to EGKK.

 

For some reason the auto-throttle refused to set an appropriate thrust for the descent causing excessive speed (not for the first time). On this ocasion it lead to the peculiar situation where the aircraft was simulatneously demanding more drag and complaining that the speed brake was extended (because the engines were not at idle).

 

I have set the option to ignore thrust lever position in the options (NEVER) and in any case they were in the idle position.

 

Thoughts appreciated:

 

Edit: Forgot to say that FSUIPC not used for any axes.

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OK, so I'm not crazy. This is the exact problem II keep having.

 

I still think it may be me doing something wrong.

 

 

 

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You're not crazy, or maybe we're all crazy :)

 

A few friends and I have noticed strange behaviour in VNAV descent, large oscillations in V/S in situations it should be no problems for the FMC to calculate a smooth descent.

 

Still I am far from testing this in a neutral environment so I am not pointing at anything so far, but it feels a bit weird.

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I have similar problems with vnav mode. Once it's enganged, it doesn't follow the "path". The plane decents sometimes slower or faster and the magenta indication of the vertical path goes up or down from the center position.

am I doing something wrong?

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Be aware, the behavior of the real aircraft is for the A/T to go in to hold mode during a VNAV descent at which point the throttles need to be manually moved to idle.  If you are using hardware throttles, you will need to move them to idle as well unless you have the "A/T Override" option set to never.

 

It doesn't appear this is the OP's problem, but I suspect there are more than a few people out there who may have this as the cause of their problem.

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Howard,

 

Can you give me the exact details of this route please?

- Coded route

- ZFW and fuel load

- Cruise altitude, CI etc

 

I'll try it.

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Hey Ryan, wo you are still working?? Thought you are in the caribic together with Robert and the team ;)

 

I had some issues on VNAV descend also, as if I start the descend before TD with "descend now" or just by pressing the ALT knob, it does not descend on a 1000ft/min descend as the NGX does as soon the path changes - sometimes the aircraft goes into a level flight or even tries to climb in order to get to the correct speed.. is this realistic?

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According to the fcom the descend now button on the fmc should command a vs of 1250 down but when I tried that it didn't work bit could be user error. Will try again tonight.

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I'm having the same problems. I haven't been able to do a proper VNAV descent, and disconnecting my controllers didn't help. I'm not talking about final approach, but since FL200 or higher.

 

- On my first flight I had very bad weather at descent, and the AT would go 90 N1 at every gust, and it oversped at about 6000 ft. I assumed it was because of the weather. So I reset my AS settings and decided to try later at good weather.

- On the 2nd and 3rd flights I've had the exactly same issue as the OP. So I descended with FLCH and V/S.

- On my 4th flight I started with FLCH from T/D. On the 3rd and 4th flights engine would go from idle to 45 - 60 N1 with FLCH selected even with controllers disconnected! So I had to descend with my hand on F1.

- On the 4th flight the auto throttle got me to 10 knots above selected speed (Vref+15) and slowly rising to 15. Wind was 3 knots head wind and no gusts.

- I've tried two more simple take-off and landing flights to reproduce this last item but it didn't happen again.

 

That's all I had time to fly until now. I didn't save any screenshots from the flights because at first I thought I was doing something wrong, but as soon as I have time I will try to reproduce the problems and share everything I can with PMDG.

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Similar experience on my first flight yesterday:

 

KIAD to KJFK at FL210, VNAV and LNAV engaged for most of the flight.

 

15NM before the TOD I preset altitude 11000 in the MCP. Right after that the plane initiated a descend by itself (13NM before TOD), and even though it was shallow and managed to make the 11000 altitude restriction at the next waypoint, there was about 3000 feet deviation from the VNAV path. Further I selected 3000 feet in the MCP and continued descend with LNAV and VNAV. The descend speed was 280 knots (VNAV calculated), but before arriving at 10000 feet, the plane started slow acceleration and busted the restriction at 300+ knots. At that point the FMC was showing DRAG REQUIRED, while the throttles were at about 50%. I had to change mode to FLCH and have some spoilers to get the speed management under control.

 

OPUS real weather was on but nothing major. The PMDG throttle behavior is "NEVER" and FSUIPC is disabled for the throttles input.

 

I thought the above experience is a direct function of not reading the T7 manual and applying too much 737NG tricks while flying the new plane. And yet, the MCP logic is pretty much straight forward. Also I see similar stories above. 

 

There will be more flying, I will post back...

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I have same problems with VNAV - erratic descent rate and nearly always arriving 3000 to 4000ft above selected waypoint altitudes.

I'm also confused over PMDG set up "AT not in hold/arm mode only/always etc".

I would be really grateful if someone could explain in plain English what these settings mean and how they should be applied.

Apart from this I love the B777 which is a joy to hand fly.

Thanks

Frank

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Hi Ryan/George.

 

I made lots of saves along the route so I can upload them via the support site if you prefer. RSVP.

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I was thinking "im doing wrong, im doing wrong" but no... im not alone to got this problem with VNAV.

 

Same problem with VNAV descent, to slow the airplane and not stay highter i have to re-calculate all the way between my STAR and the others waypoints before the runway.I put my STAR 2000ft below the FMC calculate it, doing that i have time to slow the airplane without spoilers.

 

 

Im just wondering if the real airplane is like that and i need advice too, below FL100 better to stay on VNAV or switch to FLCH ans V/S mode?

Im little confusing with this two mode.

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By the way since I switched the throttle override mode to "HOLD" I haven't seen more serious VNAV overspeed situations. The system works as advertised. I only make sure the hardware throttles are at idle when they have to be.

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I just experienced the same problem too. The Trple seven was behaving very erratically and crazy, I might say. I was flying from LOWW (Vienna) to LGAV (Athens) and I flew the cruise part of the flight with the 2X simulation speed option. Just before the top of descent I reset the altitude knob from FL 350 to 2000 ft. Passing the TD my engines went to idle and the speed showing on the primary flight display was 314 knots.

 

The vertical descent path was showing and the 777 seemed to follow it nicely. All of a sudden the speed went up to 340+ with full thrust and off course the overspeed alarm went off and the FMC demanded drag. Instead of descending the plane started to climb with about 5-6000ft. Setting the speed manually didn't help either and I was forced to end the flight. I have never experienced this with a PMDG 777.

 

The only thing that I should mention is that I have FSPS fiber accelerator installed, but in previous flights this did not present a problem.

 

Cheers, Marc

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VNAV descents work perfectly for me, if I don't use DES NOW then plane will start the descent and the FMA thrust mode will be IDLE, after a brief period it will announce HOLD. At this point I make sure that my hardware throttles are at idle and as an extra precaution I press F1 so that the thrust does not increase while in HOLD mode.

 

However if you are using DES NOW, once you press DES NOW the plane will enter a -1250 ft/min descent and autothrottle will adjust the thrust accordingly then enter HOLD mode, after which you have manual control of the thrust, so a change in wind speed or direction without adjusting the throttle will cause the speed to increase or decrease and VNAV will adjust the vertical speed to maintain the descent speed.

 

 

Also keep in mind that if you use FLCH and set a Mach number in the MCP then the V/S will increase as the plane descends.

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The vertical descent path was showing and the 777 seemed to follow it nicely. All of a sudden the speed went up to 340+ with full thrust and off course the overspeed alarm went off and the FMC demanded drag. Instead of descending the plane started to climb with about 5-6000ft. Setting the speed manually didn't help either and I was forced to end the flight. I have never experienced this with a PMDG 777.

 

Based on this, I'm guessing you have hardware throttles and forgot to place them at the idle stop before the descent began?

 

The default A/T option for the 777 is that it will override the hardware throttle in all modes except HOLD. Descents are made in HOLD, after the A/T commands IDLE. The problem is, unless you follow along with your hardware throttle when it does that, you're putting yourself at risk of the simulator listening to your hardware's position (usually at the forwards stops since takeoff), which is out of sync with where it is in the sim (IDLE). In some cases, if the hardware never spikes (that is to say, "the position the hardware is sending never changes from when it was being ignored"), the sim might not even pick up on the fact that the hardware is at the forward stop, and the descent is "normal," while in reality, you're just getting lucky.

 

You can either remember to set the hardware to IDLE, or change your hardware override setting in PMDG SETUP > SIMULATION to NEVER ("never allow the hardware to override the A/T") to avoid this happening in the future.

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Thank you, Kyle,

 

Your response is very helpful. I will give it a go and use the settings you suggested. The only thing I find confusing is that I never experienced this problem before. 

 

 

Cheers, Marc

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Your response is very helpful. I will give it a go and use the settings you suggested. The only thing I find confusing is that I never experienced this problem before. 

 

You're welcome. Hope it helps.

 

As for the last comment (you've never seen it before), I'll re-state this part:

In some cases, if the hardware never spikes (that is to say, "the position the hardware is sending never changes from when it was being ignored"), the sim might not even pick up on the fact that the hardware is at the forward stop, and the descent is "normal," while in reality, you're just getting lucky.

 

Some hardware is pretty rock solid, and sends very precise info to the computer. The sim (FSX in this case - the PMDG 777 overrides the FSX function to a certain degree, but when it drops to HOLD mode, the basic FSX A/T is "off") stops paying attention to it while A/T is on and doesn't start paying attention to it until it's moved again. A hardware spike (errant signal), or a button press can "wake the sim up" to the hardware position, and then, well, you've seen the result.

 

My X-55 (thanks YouTube people), as long as I calibrate it before using the sim, is pretty solid, and rarely ever spikes, so it would likely go unnoticed in my sim, too, until I accidentally hit the throttle, or pressed a button on the controller.

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Hello Kyle,

 

 

I made a flight yesterday from Nice (LFMN) to Menorca (LEMH) and adjusted the settings as you suggested: PMDG SETUP > SIMULATION to NEVER ("never allow the hardware to override the A/T") 

 

The flight was smooth and the VNAV descent was exactly as it should be. I also made sure that my hardware throttle was in the idle position after takeoff. So, at least this time I did not encounter any erratic behaviour. I will give you an update after about ten more flights to see if everything is working accordingly now.

 

Marc

 

2014-11-7_16-3-55-858_zps3e3da974.png2014-11-7_16-4-30-801_zps82da33da.png

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The vertical descent path was showing and the 777 seemed to follow it nicely. All of a sudden the speed went up to 340+ with full thrust and off course the overspeed alarm went off and the FMC demanded drag. Instead of descending the plane started to climb with about 5-6000ft. Setting the speed manually didn't help either and I was forced to end the flight. I have never experienced this with a PMDG 777.

 

I had the same issue on a flight in LAX last night. I was following the KEACH1 STAR (FENUK) transition.  VNAV was operating and commanded IDLE then HOLD at TOD.  I pulled my hardware throttles (Goflight TQ6) back to idle and confirmed idle thrust. 

 

Descent proceeded normally toward TEVTE and the 23000B altitude constraint.  Approaching TEVTE, I noted that VNAV had not changed the target speed to hit the 280 kt constraint, so I hit speed intervene and dialed in 280 on the MCP.  I passed the waypoint a little fast, but nothing major.  Next active waypoint was VTU with hard constraints of 250/11000 active in the LEGS page.  I hit the MCP speed knob again to get out of speed intervene.  VNAV was still commanding 280 knots, which made sense.  I used a little speed brake to slow down to 280 as I was still fast. 

 

Approaching VTU, I had 11000 set in altitude and IDLE/LNAV/VNAV PATH on the FMA.  Again VNAV was going to hit the altitude constraint, but it was going to reach 11000 right at the waypoint (green arc) and there were no green circles indicating a deceleration.  I hit speed intervene again and set 250 kt and used the speedbrakes to hit the constraint at VTU. 

 

Shortly before reaching the waypoint, I hit the altitude knob to remove the active constraint at VTU (so it wouldn't level off there).  This made KEACH the active waypoint, with SADDE right after it (250/10000A).  At this point FMA was still showing IDLE/LNAV/VNAV (can't remember SPD or PATH).  I released speed intervene again and VNAV went nearly level,as expected since it was at speed (250 kt) and had 15 miles to descend only 1000 feet by SADDE. 

 

At this point I got distracted by ATC telling me to switch frequencies.  Once, I'd switched, I found that the A/T was commanding full thrust and the plane was climbing at ~4000 fpm.  Hardware throttles were still at idle.  VNAV was active and the altitude was set to an altitude below the current altitude.  Although I have the A/T override to "only in HOLD" my first response was to move the throttles forward and then back to idle, but that didn't have any effect (as it shouldn't have).  I ended up having to disconnect the A/T completely to regain control over thrust.

 

Any thoughts?

 

Bill Rowe

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