VNav Path to VNav Speed

[pilotpm 0]

Hi , I see my plane in Vnav path what is automatic used at decend swinging softly up and down.Is there a way to force Vnav path to set to Vnav Speed with the FMC or the CDU ? cheersMario

[badderjet 181]

You can't "select" PTH or SPD via the FMS. Whatever mode is active depends on the actual conditions. The swinging that you see has probably nothing to do with VNAV PTH at all. I see it as well, and it occurs also in other modes, e. g. VNAV ALT or ALT HOLD. Checking the SYS page or looking at the yoke clearly shows the elevator oscillation. No weather add-on in use BTW. Happens every once in a while, dunno why, kinda annoying.

[Rocky_53 1,583]

I recently experienced this, with no obvious reason. If anyone knows why then please let us know!

[badderjet 181]

Now here's another funny one, regarding control surfaces... Have you ever noticed autopilot behavior at less than 1x sim speed? I like 1/4x the most. Beware!HDG SEL will first of all turn into the wrong direction. But it won't stop there. If you don't do anything, the AP will drive you right into some attitude calling dangerous would be understatement. Yes, this is my AP induced attitude from straight and level flight under blue skies! (In fact, I have to be honest as I get this occasionally on a cruise every once in a while even at normal sim speed. No good idea to leave the NGX unattended during prolonged cruises. She really likes to do crazy things. It just happened way too often I'd return to the computer finding myself scratching the ground at high thrust and gear up...)

[sgreen91 182]

If you want to go vnav speed in descent hit speed intervention on mvp. You will notice fma will then say vnav speed instead of oath.

[Alaaar 34]

I think its pure physics / Aerodynamics you can’t maintain a constant CR/DR through all climb or descent stages, so the plane is meant to maintain speed and not CR/DR, I think it’s simulated realistically in PMDG.

[trisager 2]

I've seen this pitch up/down a few times in VNAV PTH, looks like some unfortunate combination of descent speed and winds. If it bothers me enough I switch to LVL CHG instead (at the programmed descent speed), this cures it completely.

[badderjet 181]

Interesting, thanks for the temp fix. Although I don't recall to have had any winds then. But I'll keep an eye on it.

[ARM505 0]

If the entered winds on the descent forecast page disagree with what you're seeing in reality, then the predicted vertical path that the aircraft calculated prior to descent may be unattainable. You'll see this as a consistent deviation from the planned descent speed/Mach no. Is this happening? Or is it just wiggling around? Wiggling around I can't help you with :) You can actually choose between PATH and SPEED for a descent - on aircraft with the 'speed/alt intervene' buttons on the MCP, just push the speed intervene button twice - the 1st time the speed window will become active, the next time it will blank and VNAV SPD will annunciate on the FMA. On aircraft without the speed/alt intervene buttons, just go to the descent page and select 'SPEED' from one of the lower LSK's - the FMA will change from VNAV PATH to VNAV SPEED, and will now fly the FMC planned speeds, not the precomputed path.

[trisager 2]

Or is it just wiggling around? Wiggling around I can't help you with :)

The problem that I'm seeing on some descents is that the airplane pitches up and down repeatedly. When it happens it is typically flying 50' or so below the predicted path, slightly slower than the target airspeed. It looks like VNAV cannot make up its mind if it should pitch up to get back on profile, or down to increase airspeed. This is an idle descent, with the autothrottles in ARM mode. Adding thrust manually to help a bit doesn't solve the problem. When I hit LVL CHG the airplane immediately settles down in a rock-steady idle descent.

[badderjet 181]

Yes, it's more like a two or three second oscillation around the lateral axis. As I said, you can clearly see on the SYS page how the AP induces this motion by constantly moving the elevators up and down.

[Spin737 102]

You can't "select" PTH or SPD via the FMS. Whatever mode is active depends on the actual conditions. The swinging that you see has probably nothing to do with VNAV PTH at all.

There is (was?) an option to select VNAV SPD on the DESC page. I don't know if that still exists since "common VNAV" has been adopted where the plane will switch between PATH and SPD more automatically.

If you want to go vnav speed in descent hit speed intervention on mvp. You will notice fma will then say vnav speed instead of oath.

Agree.

If the entered winds on the descent forecast page disagree with what you're seeing in reality, then the predicted vertical path that the aircraft calculated prior to descent may be unattainable. You'll see this as a consistent deviation from the planned descent speed/Mach no. Is this happening? Or is it just wiggling around? Wiggling around I can't help you with :) You can actually choose between PATH and SPEED for a descent - on aircraft with the 'speed/alt intervene' buttons on the MCP, just push the speed intervene button twice - the 1st time the speed window will become active, the next time it will blank and VNAV SPD will annunciate on the FMA. On aircraft without the speed/alt intervene buttons, just go to the descent page and select 'SPEED' from one of the lower LSK's - the FMA will change from VNAV PATH to VNAV SPEED, and will now fly the FMC planned speeds, not the precomputed path.

I don't know about pushing the SPD INT button twice to get to SPD. Is that in the manuals anywhere? If you close the window, AFAIK, it should just go back to PATH. The plane does do some hunting in RW. The plane doesn't just lock in on a pitch and head down to 10,000'. How much oscillation are you guys seeing?

[badderjet 181]

There is (was?) an option to select VNAV SPD on the DESC page. I don't know if that still exists since "common VNAV" has been adopted where the plane will switch between PATH and SPD more automatically. I don't know about pushing the SPD INT button twice to get to SPD. Is that in the manuals anywhere? If you close the window, AFAIK, it should just go back to PATH.

No, that option on the DES page has been removed since the introduction of common VNAV. Also the 2nd push on SPD INTV, never actually heard about it. On the NGX at least, it just toggles back and forth, just like described e. g. in FCOM Vol II SP.11.17.

The plane does do some hunting in RW. The plane doesn't just lock in on a pitch and head down to 10,000'. How much oscillation are you guys seeing?

It's not horrible (not out of control), yet pretty significant. Would fill a number of barf bags at least. As I said, it's pretty short term, maybe a second or two of frequency. If I only had the means to take video.

[mikea76 184]

Now here's another funny one, regarding control surfaces... Have you ever noticed autopilot behavior at less than 1x sim speed? I like 1/4x the most. Beware!HDG SEL will first of all turn into the wrong direction. But it won't stop there. If you don't do anything, the AP will drive you right into some attitude calling dangerous would be understatement. Yes, this is my AP induced attitude from straight and level flight under blue skies! (In fact, I have to be honest as I get this occasionally on a cruise every once in a while even at normal sim speed. No good idea to leave the NGX unattended during prolonged cruises. She really likes to do crazy things. It just happened way too often I'd return to the computer finding myself scratching the ground at high thrust and gear up...)

PMDG should ask one of the pilots to test this on the real aircraft the next time they fly.. I am interested to see how the real ng reacts when you slow time down to 1/4 speed

[falcon999 24]

PMDG should ask one of the pilots to test this on the real aircraft the next time they fly.. I am interested to see how the real ng reacts when you slow time down to 1/4 speed

They did, this exactly how it reacted. :-p