Changing from VNAV -> SPEED

[Guest D17S]

Richard, did you try it? Are we having fun yet!?And Q, Thanks for the feedback. You asked if I have the books or if I

[WebMaximus 372]

That's a lovely post and way of describing it Sam, thanks :-) Got the picture now in a much better way than before reading your post! One question though, how does this logic work when it comes to the 737NG? Haven't flown that one in quite some time now but if my memory serves me right there you actually were able to engage/disengage VNAV the way you wanted...but I guess disengaging VNAV automatically changed the pitch mode to another one...?

[WebMaximus 372]

This is interesting Q...because on a flight the other day in PMDG's 747 I actually managed to have the speed drop down to the area where the stick-shaker sound was heard before I managed to engage the A/T again by disarm/arm it a couple of times. Haven't enough experience/knowledge yet though to give any 100% certain reports - will have to do some more experimenting when time permits. Right now I'm busy concluding the IVAO VFR World Tour in the B1900C and there you don't have to worry about these things ;-)

[Guest D17S]

That would be interesting check out. I don't have the 737NG, so you'll have to do the test flight. Dust it off, file an XB737 flight plan over the Mohave and put it through the test flight sequence we've talked about so far. Try not to crash! (But hey, that reset button we have is one cool feature. Bet Boeing wishes they could design in one of those.) Here's the crux of the questions we've been looking at: Should it be possible for the AP to fly the airplane, or for the FD to provide vertical flight guidance without to a pitch mode designated? See how the 737NG designers handled this question.

[Tabs 2,790]

Another point of correction here:VNAV Speed Intervention really has two functions depending on which phase of flight you're in. If you're in normal descent and you press the speed knob, the plane will transition to VNAV SPD mode - it does not remain in VNAV PTH (which isn't dependent on holding speed).If you are in the approach phase (25nm from the airport) the aircraft will remain in VNAV PTH when you speed intervene and it WILL attempt to reduce to the target speed if possible rather than bringing up thrust in SPD mode to maintain the previously higher thrust setting. This is what makes VNAVing approaches possible etc...We're gonna have a Type Course lesson on the proper uses of the VNAV modes soon - look for it!

[macpac22 0]

excellent posts guys im learning a lot just reading through this topic! :) cant wait for the Type Course lesson on VNAV modes, thanks to PMDG for their continued help and support!

[WebMaximus 372]

Hi Sam,No, I haven't had the time to try these things yet but will do as soon as I find the time for it which should be in a couple of days - it will be great fun! And I agree that trying these things and having these kind of discussions is a great way of learning new stuff and how things really work...at least in this simulator and hopefully what we find here is also what we would have found in the real a/c if everything is modelled correctly which I don't doubt.

[WebMaximus 372]

Sounds great, I'm looking forward to that Type Course lesson Ryan!

[michal 44]

>We're gonna have a Type Course lesson on the proper uses of>the VNAV modes soon - look for it!That would be highly welcome probably by majority. I admit I am having trouble with Sam's writings - lots of words, certain things just don't sound right to me, the text seems out of focus in places (forgive me Sam). We need some terse description of all the VNAV stuff (with emphasis on gotcha items).Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest D17S]

No problem Michael. Some like it terse some like it loose. As you noticed I've got the boxes thinking . . . sometimes even talking! That's just the way I need to understand things. I need things simple as dirt to be able to understand 'em. Guess I'm kinda dense that way.Would be very interested to get your comments. Maybe you have different ways of testing the functionality of these AFS modes. If something doesn't seem right, would apprecite the discussion. Just book quotes won't do though . . . unless the reasonong can be fleshed out. Of course then we go test fly. Bet it's there!

[Guest D17S]

Well it seems the sim just has unlimited depth. I was experimenting with speed interventions during the cruise phase. As you suggested, there

[Qavion 19]

"Now re-engage the AT and re-establish in VNAV PATH for the overspeed test. Once you

[Jeff Nielsen 1,386]

So the practical application of using "VNav" in the approach would be for a GPS approach, right? And if flown right you'd be disco'ing long before you're over the numbers, but theoretically could be flown down to almost the ground, so to speak.

[Guest D17S]

Excellent question. If VNAV SPD engaged to arrest an overspeed caused by excessive thrust, it would have to maintain a pitch angle adequite to control the airspeed. This is the essence of a speed on pitch climb. TOGA, FLCH and VNAV SPD all use this dynamic by intentionally provoking pitch changes by increasing thrust. A ROC simply occurs, by design.Now we have the same scenerio. Excessive thrust and the use of pitch to control the excessive speed. A ROC will have to occur. The question then becomes "If VNAV SPD is used to control excessive airspeed from a VNAV PATH cruise, how shall we deal with the resulting climb? At what altitude does this revert to something else?" It will definately stop on it own at some point but how? Do we do we want to let that happen on its own? Will VNAV SPD work for speed control in Path cruise? What cha think?

[Guest D17S]

Hi Jeff, Ahh, I guess so, but I haven't really gotten into GPS approaches. Maybe you could expand a bit on that what a GPS approach is all about. But I think this is one here. The IRUs are constantly being updated by GPS (twice a millisecond, or some such interval), so the airplane is always "really" using a GPS present position (PP). All waypoints are calculated from the airlanes PP, so actually all waypoints are GPS waypoints . . or at least calculated on the basis of a GPS PP.So if you get down to it, if the use of GPS waypoints are the basis of a GPS approach, this is one. I guess!??But that's all for the Lnav side. The other magic part here is the Vertical (vnav) dimension.