777 autothrottle unable to maintain speed during approach

[pouts 0]

Hi all,

 

I'm having a problem while flying the 772 during descents and approaches. I can see that the autothrottle cannot accurately maintain speed, even in very low wind conditions.

 

During initial descent everything is fine. I usually lower the gear at the FAF and complete the landing checklist. Once I get the flaps to 25 or 30, speed usually at VREF+5, the aircraft cannot maintain it. Suppose we have a Vref of 138kts, I will see the speed drop to 132, when the power will get increased significantly and the aircraft will accelerate to 150+ knots and then back to idle, speed dropping rapidly again. Approaches get really difficult with this behaviour.

 

Even though I am not new in the FS world at all, I am rather new in the PMDG cockpit so, experts, am I doing something wrong here???

 

Many thanks

Michael.

[scandinavian13 5,192]

 

 

Even though I am not new in the FS world at all, I am rather new in the PMDG cockpit so, experts, am I doing something wrong here???

 

How stable is your approach? Remember that, when using A/T and hand flying, the A/T is reacting to your actions. It can't predict them as if it could when it's fully in control.

 

Does this happen if you autoland?

[Nyxx 6,993]

Like Kyle said, how is your pitch if hand flying with AT on?

 

@Kyle

About time for a new you tube vid slacker   :smile:

[scandinavian13 5,192]

 

 

@Kyle
About time for a new you tube vid slacker  

 

haha - yeah, I've been tossing a few ideas around in my head. I just haven't been fired up about a particular topic recently...

 

...well...except for this one (which will have an associated video :wink:  - forgive the image, it's a screenshot blown up from my phone as my office firewall blocks Dropbox...somewhat understandably:

 

 

Yes, it's taken me since the date mentioned in the footer - and that's only when I pulled the weather data and made the initial OFP and flight. Work has been going on since July, which really emphasizes that these things are actually a ton of work.

 

Also: this is completely unofficial, and a work of my own not at all connected to my position on the Beta team or otherwise. I'm hoping the different coloring helps to emphasize this, but I figured I'd point it out here, too.

[Nyxx 6,993]

Look forward to it.

Chop chop :ph34r:

[pouts 0]

Thank you all for your answers.

 

Hand-flown approaches have imperfections obviously, but it is also visible with automated approaches with some wind.

It is noticeable even with AP engaged in straight and level flight at low altitude, for example while waiting to capture the localizer.

 

Anyone who has the time can try and reproduce; fly straight and level at 2000-3000ft for example, flaps 25, speed 160 and introduce a variable crosswind of about 10kts... my aircraft speed fluctuates from -10 to +20 knots, autothrottle struggles to keep up.

 

This is something I haven't seen on the PMDG 737NGX or any other aircraft...

 

Best regards

Michael

[LABOX 40]

Are you using ASN by any chance? If so, there's a thread about unstable approaches while using the newest version if ASN in the general forum...

 

http://forum.avsim.net/topic/454857-before-and-after-the-sp1c/?view=getnewpost

[downscc 1,327]

I turn off all turbulence, updrafts and downdraft options in ASN.  There is a pretty good section in the Introduction regarding turbulence and how 'unreal' MSFS is in this regard.

 

Also, you didn't mention if you are using MCP SPD to control your approach speeds. The B772 VNAV will not behave the same as the B737 and you'll have to start dialing in your speeds before final course.  The Tutorial explains this as well.

 

I have no troubles with A/T maintaining MCP SPD in any weather, without turbulence as mentioned.

[pouts 0]

@ Labox

thanks for replying. Indeed I am using ASN. Very good suggestion, I will check without running it. Many thanks.

 

@downscc

Yes indeed I leave VNAV-managed speed well before final course.

 

Thank you both, I will check without ASN and revert back for any other simmers who may run into the same problem.

 

Best regards

Michael

[pouts 0]

Again thank you all for replying, indeed ASN generated turbulence when enabled causes the problem, I will monitor the other thread for this subject.

 

http://forum.avsim.net/topic/454857-before-and-after-the-sp1c/?view=getnewpost

 

Michael