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hydraulics45

PMDG 737 NGX Vertical speed hold problem

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Hey guys! i'm new to the pmdg 737 and i've done a couple of flights with it with no problems whatsoever.

 

Recently during a flight, just after takeoff, i turned on heading select, speed hold and vertical speed hold but after turning on

the master autopilot, the plane's nose starts dropping and i start losing altitude, so then i have no option but to manually bring the plane back up. this has been going on for some time and it's really irritating. 

 

any help would be greatly appreciated. thanks !

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...but after turning on the master autopilot, the plane's nose starts dropping and i start losing altitude, so then i have no option but to manually bring the plane back up. this has been going on for some time and it's really irritating.

 

Sounds like your aircraft wasn't properly trimmed.  After taking off, you need to trim the aircraft appropriately before trying to engage the autopilot.

 

You can also turn off "REALISTIC A/P ENGAGEMENT" in the PMDG SETUP> menu of the FMC.  This will allow the AP to take control of the aircraft regardless of how you've left it.

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im pretty sure i set takeoff trim properly.

 

Takeoff trim isn't perfect.  It's only meant to be a cursory value to get you pointed in the right direction.  After taking off, you will likely have to adjust the trim further.  Remember that your weight and balance sheet uses standard weights (that's where your CG -> TRIM value come from), and not everyone weighs what those standard weights are.  That variance will throw the calculated value off of what's actually necessary.

 

Because of this, trim needs to be adjusted off of the calculated value based on actual flight performance.

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aside from the trim, any other ideas why the aircraft is dipping? the VS hold is set to 1800+ and as soon as i turn on CMD A the nose starts dropping. i do a manual override and bring the nose back up but when i engage VS again, the plane lifts its nose and goes into a stall.

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any other ideas why the aircraft is dipping? the VS hold is set to 1800+ and as soon as i turn on CMD A the nose starts dropping. i do a manual override and bring the nose back up but when i engage VS again, the plane lifts its nose and goes into a stall.

 

Hardware interference issues would be my only other guess.  You can test for this by removing your hardware from your computer and trying the modes that way.  Keyboard arrow keys can be used to fly until the testing is complete.

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Look at the Flight Mode Annunciators on the top of the Primary Flight Display and see what they say.

 

You may know what you want the plane to do, but you might not be speaking its language. . .

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CMD V/S SPD

If there is an "FD" anywhere on the PFD screen, then your autopilot isn't doing anything.

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hopskip FD does show on the PFD. could you elaborate on what you were saying?

I'm not Trent, but FD indicates flight director, which is basically the little crosshair that shows you where you should be pointing the aircraft based on the active settings on the autopilot. It's there for reference purposes. You can hand-fly the aircraft and use the FD to aid you in figuring out where it is you need to go. BUT....if FD shows up on the PFD, that means the autopilot isn't actually doing anything. You want it to read CMD rather than FD.

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It actually does read CMD when I switch on the autopilot. But for some reason, even though VS hold is activated and it shows on the PFD, the aircraft goes into either a dive or a stall.

It actually does read CMD when I switch on the autopilot. But for some reason, even though VS hold is activated and it shows on the PFD, the aircraft goes into either a dive or a stall.

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It actually does read CMD when I switch on the autopilot. But for some reason, even though VS hold is activated and it shows on the PFD, the aircraft goes into either a dive or a stall.

It actually does read CMD when I switch on the autopilot. But for some reason, even though VS hold is activated and it shows on the PFD, the aircraft goes into either a dive or a stall.

Hi Hydraulics45!

 

... just an idea and i might be wrong, but:

In V/S it is absolutely mandatory to monitor Your speeds - and correct them if needed by adding more thrust to gain speed or minoring thrust if getting to fast.

 

If You are climbing in V/S then You have to take very good care that You do not slow down to much and get to close to a stall.

While descending on the other hand You have to be very careful not to overspeed!

 

That's why it is highly recommended to either use VNAV or LVL CHG while take off and climb rather than V/S.

 

Only my 2 cents here though and i stand corrected any time.

Cheers, Christoph

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Thanks for the help Christoph but after takeoff I usually turn on speed hold so my speed is automatically controlled. I do monitor my speed to make sure the auto throttle is working fine. I'm not entirely sure on how to properly use VNAV so I just stick to V/S

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You are better off with LVL CHG for the time being. That will do you have appropriate thrust setting (full climb rating for climb, or idle for descent), hold the speed and adjust the rate to fit.

 

 

Still, your method should work. Could you make a video?

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Thanks for the help Christoph but after takeoff I usually turn on speed hold so my speed is automatically controlled. I do monitor my speed to make sure the auto throttle is working fine. I'm not entirely sure on how to properly use VNAV so I just stick to V/S

Hi Hydraulics45!

 

But the thing might be - only might be, because i can't tell for sure - that the speeds in use are not sufficient for certain amounts of V/S climbs.

Just a fictionary example, but lets say 240 knots IAS is sufficient for a V/S climb at lets say - again just fictionary numbers - 2200 feet/minute, but(!) not enough for anything above 2200 feet/minute such as let's say 2500 feet/minute.

And so, although 240 IAS is used constantly, the problem now is that this amount of speed just is not enough for any V/S climb above a certain value and then troubles occure.

 

Again - i might be wrong and this is just some idea - but sure thing is that if V/S is being used while climbing all needs to be monitored very carefully and needs lot of attention - which is one of the main reasons why VNAV or LVL CHG should be used primarily while climbing.

 

No need to worry about using VNAV - really give it a try (and as it is "just" a sim we can do lots of trials an errors anyways :wink:) :

VNAV uses N1 thrust for climbing to the assigned altitude, whichever is dialed into the "altitude window" at the MCP.

More detailed info can be found in the various PMDG NGX manuals and/or the tutorial flights, which i found very helpful and informative.

Maybe also have a look at this thread here:

http://forum.avsim.net/topic/342469-autopilot-dont-vertical-speed/

This is again just some basic info from my side, but i hope nonetheless use- and helpful a bit!

Maybe also have a look at the great free (!) tutorial by Tom Risager (just browse the web for Tom Risager and NGX or so and You will find it - it is certainly worth reading and offers a real lot of great and detailed info also regarding climbing in various modes!)

Cheers, Christoph

 

 

You are better off with LVL CHG for the time being. That will do you have appropriate thrust setting (full climb rating for climb, or idle for descent), hold the speed and adjust the rate to fit.

 

 

Still, your method should work. Could you make a video?

+1

 ... or maybe post some screenshots?

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A few things:

 

Don't use V/S unless you have a good reason. It sounds like LVL CHG is the mode you need. None of this "well I like to..." No, that's not how the 737 is flown.

 

Don't touch the SPEED button. Go straight from TOGA to LVL CHG once the flaps are up.

 

Take some screen shots on your next flight.

 

And lastly, Don't touch the SPEED button. ;)

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Thanks everyone for your help! Really appreciate it !

Just one thing I want to share. When I tried taking the plane for a flight again I ended up pressing ALT Hold by mistake. I turned it off and pressed VS hold and just like that it started working again!

So I tried this a couple more times and it turns out that If I press VS hold after ALT hold, the plane starts ascending and descending according to the vertical speed I input. No problems at all! but I'm really starting to think that VNAV or LVL CHG is a better option.

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No problems at all! but I'm really starting to think that VNAV or LVL CHG is a better option.

 

It definitely is.  This isn't your old default plane anymore.

 

There are two tutorials that came with the plane that will definitely teach you a ton about it.

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