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springbok204

Need some help and finer flying advice

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Oh Revered Flyers of PMDG 747-400,I need some help and finer flying advice on using VNAV. This topic has probably been beaten to death and talked out but I tried what I read before and nothing worked.So here goes...I'm in cruise at FL310 and am starting descent BEFORE the top of descent point on my ND. I dial in 25000 on the alt select, open the FMC VNAV descent page and select DES NOW. The aircraft starts to descend but then levels off at 29xxx and does NOT continue to descend. Look at this pic labelled Descent 1.http://forums.avsim.net/user_files/178820.jpgI've read about discontinuity in the LEGS pages but here's my legs page with arrival runways and other information properly selected. Look at the pic labelled Descent 2.http://forums.avsim.net/user_files/178821.jpgI've also included a pic of my VNAV descent page. Look at the pic labelled Descent 3.http://forums.avsim.net/user_files/178822.jpgThe long and short is I use Radar Contact with usually gives descent clearances BEFORE the T/D. Can I completely descend in VNAV? Would it be better to select another mode like FLCH or V/S. How's it done in the real world?I am certain I'm doing something wrong and there is that tutorial on using VNAV which I've read quite a few times. I can program altitude and speed crossing restrictions with ease and VNAV performs quite well. But the initial descent seems to be a problem. I think I understand what pushing the alt selector does but feel free to give your advice.Thanks much flyers.


Kirk Mayers

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Hi there springbok204First of all may I remind you of the forumrules saying that we shall always signing our posts with our real name. :)That said, regarding the "DES NOW" function I can't shed to much light, but if you need to descent before the T/D point, the easiest way is to just dial in the wanted altitude and press in the middle of the dial button. The FMC will then commence a 1200 f/min descent until the VNAV path is reached, and will after that follow the VNAV path until desired altitude is reached. This method can be used at any time when the plane is in VNAV mode, both ascend and descend, it's an easy way to perform the step climbs for example. :)Don't hesitate to ask if anything was unclear in my ramblings here. :)Happy flyingRegards Audun


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Audun Sjøen

 

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Please sign your posts with your real name per our forum rules from now on.Do you have the 400F? I know some stuff related to DES NOW was fixed in that... Nothing looks wrong to me in your screenshots as far as the modes etc.


Ryan Maziarz
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Hey Guys,I read this forum pretty regular but because I never posted anything, I never bothered to read the rules. Sorry!To the guys who did respond, thanks. I know about dialing in the altitude and the pressing the button. I do that too. Aircraft starts to descend at 1200 fpm and then levels off at some intermediate altitudeand does not continue to descend. Its the strangest thing. Also this is flying the pax 747-400.I guess I'll try to figure it out. I would like to know if anyone else has experienced this.Kirk Mayers


Kirk Mayers

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Here's what I do to get an approximate descent to meet an approach crossing restriction with a 737NG:Go to the FIX page.Enter your destination airport in the scratchpad and LSK1 to get the fix ID.Enter /40 in the scratchpad. LSK2 (or 3?) to get the range in. This will place a 40 nm range ring around your airport. I dial in the restriction altitude in the MCP and engage FLCH. I then switch to V/S mode noting the descent trend arcs on the ND. I adjust V/S until the descent arcs coincide with the 40 nm range ring. This will get you pretty close to the restriction with a constant descent.In RC if not initially stated by ATC "when ready descend to" there should be on the extended RC menu a pilot's discretion request. For this you would need to create a hard altitude fix either by adding one via the fix page or using an appropriate waypoint on the legs page. This will I believe adjust your TOD and you can delay your descent using VNAV to get to the crossing restriction. The real world goal is to stay high as long as you can followed by a constant speed idle descent both conditions offering the most efficient use of fuel.


Ron Ginsberg
KMSP Minnesota, Land of 10,000 Puddles
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"I'm in cruise at FL310 and am starting descent BEFORE the top of descent point on my ND."This problem is probably caused by the selection of DES NOW so far from T/D. Even the real FMC has problems with this (and I've seen warnings about this in manuals, but can't recall the reason). If you want to descend so early, use a cruise step descent.Note also, that if you descend too early, the aircraft may not be able to intercept the normal descent path.Cheers.Q>

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Just an observation, the vertical mod is VNAV PTH, shouldn


/Tord Hoppe, Sweden

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Thanks for your suggestions Tord Hoppe,I thought I was pretty good with flying the 747-400. It has inspired me to read just about anything technical to do with the 747, especially flying it.But from this discussion, I realise there is always more to learn. I've never even considered that the VNAV annunciations should be any different in an early descent. I'll be looking now to see if they are. Also, since I fly using Radar Contact, I'll also try getting a descent at pilot's discretion, so I can descend later. I think from the suggestions given here, if one starts descending early, it might be better so use another mode like FLCH. Perhaps someone has an idea how far before T/D a VNAV descent would be effective or if it would be best to use another mode.What's also interesting is that every tutorial on the 747 I've read and even the PMDG type-ratings, everyone discusses a descent after T/D but no one discusses a descent before T/D. Perhaps the more experienced 747 flyers have suggestions (on proper mode selection for an early descent) or even a real 747 driver?Thanks again guys,Kirk Mayers


Kirk Mayers

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Kirk, in the book/manual I mention which describes various FMC models for the 757/767, 747 and 777 it doesn


/Tord Hoppe, Sweden

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Guest tomahawk_pa38

I have been flying the 737 now for about 2 years andwhen I reflect I come to the conclusion that when descending - I don't really know what I'm doing ! This explains why sometimes it works perfectly and sometimes it doesen't. I've read all the posts on NVAV etc and I've tried using Lvl Change instead. But still I end up with a 'Steep Descent After XX' message or more usually, I can't control my speed even if I've input forecast winds. I change my VS so that the green arc crosses my path at the right place but invariably I am too fast. Any body got any tips please ?

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Guest knack

Hi Kirk,I have never had that problem with the 744 and I use RC4 all the time and it (RC4) will always start me on a descent a fair distance before the TOD calculated by the FMC. What I do is this (and it may not be recommended procedure) but it works for me every time.When instructed to descend I dial in the new, lower altitude on the MCP, click the centre of the MCP altitude knob and make sure that it (the new altitude) is annunciated in both the PFD _and_ the FMC VNAV page2. If, perchance, the aircraft does not begin it's descent then clicking on the VNAV button (even though it already IS illuminated) will invariably start the descent and as noted by previous posts it usually is a gentle 1200fpm glide down to the new altitude. Failing that, the LvlChg switch WILL work albeit in a very hamfisted manner or you could utilise your Vertical Speed dial for a more controlled descent. More than one way to skin a cat, hey? ;-) Cheers!Tony

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Thanks Tord,My situation with the 747 not descending in VNAV happens so often that I have been able to reproduce it, hence the screen shots. It happens often enough that I think I'm doing something wrong. Its not limited by any other factors except descending before T/D.Thanks to you too Tony,I will try your suggestions when I fly again. I've actually on a few occasions ended up doing everything you mentioned except pressing the already-illuminated VNAV. That is interesting. The question is why would I have to press it again if its already on? Might that be a software glitch? Please don't take that the wrong way PMDG and other readers. Too much bashing goes on with these forums sometimes. For the record I love the 747 and PMDG and support you guys whatever you do. I've also considered that my PMDG setup may be corrupted and I may need to uninstall and then reinstall but that is a last resort. Just trying to understand the VNAV beast.Kirk Mayers


Kirk Mayers

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Kirk-A couple of observations on your original post:1) If an change in altitude from the VNAV programmed cruise altitude is desired, you can do this by updating the VNAV altitude in the CDU, or (the more common approach) you can set the altitude in the MCP and press FLCH.2) I don't get enough information from your images to tell you precisely how you went wrong on your early descent- but a few visual cues that you might have missed include the vertical path indicator on the right side of the ND. Notice that it is showing the path as being 1188 feet above you? This is an indication that VNAV is computing your optimal descent path above your current altitude- which would make sense given what you described. (Another view of it: VNAV is always going to try and give you an idle power descent to any down-line altitude restruction.... a very rough guess based on the fact that you are 74nm from your next restriction of 7000ft tells me that you need to descend 22,000 feet, at, say, 3,000fpm- so your descent profile would be expected to start around (22,000/3,000 = 7.3 minutes * 6 miles a minute (roughly) or 44 miles...roughly...)As such- since you are still beyond that point- VNAV doesn't want to go down yet....3) Since you said you updated the VNAV alt in the CDU, VNAV feels that you should remain at FL290'ish until further along the flight path. Since you are still on the VNAV programmed path, you are seeing VNAV PTH on the PFD. ;-)4) The airplane will never leave it's altitude unless you SPECIFICALLY TELL IT TO DO SO. After you set FL250, you would need to alter the vertical mode to tell the airplane to descend... just updating the MCP or CDU isn't enough. You have to tell the airplane HOW to get to the new altitude. (FLCH)5) One other item i noticed- is that you are using the term EARLY DESCENT to signify "i want to descent before ToD. The airplane's definition of EARLY DESCENT is actually a technically defined criteria that is set forth in the CDU. As you approach ToD the airplane will check to see if you have set the MCP ALT to a lower altitude. If you have- the airplane will enter a 1200fpm descent UNTIL IT INTERCEPTS THE ORIGINALLY CALCULATED DESCENT PATH BELOW THE ORIGINAL ToD POINT. (This is explained to some extent in the manual...)If you want to come down early as a result of ATC or personal preference, the best procedure is to set the MCP ALT and press FLCH. Hope that helps! Sorry I can't give you an in depth lesson on VNAV- we'd be here for a few weeks. :-p (Don't fret- many professional pilots struggle to understand the varagies of VNAV the first time they fly an airplane that uses it!)


Robert S. Randazzo coolcap.gif

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Thanks sooooo much for your input Robert!!!!I was able to clarify quite a few things from your information. First, there's nothing wrong with my PMDG installation. That's a huge relief. Your statement about the aircraft wanting to stay at altitude until further along makes completely sense. Which is why I started to think that descending at the point that I usually descend, it might be better to use another mode. I will now try using FLCH or even VS and set it to 1200fpm. I know no speed protection in VS. I think for the initial descent from cruise, I could use FLCH and engage VNAV lower down, especially if I've programmed altitude or speed restrictions.And finally, it is good to know that professional pilots take time to grasp the concepts of VNAV. Perhaps I understand now how it would be converting from a non-glass airplane to a glass cockpit. Even shows how truly complex the PMDG 747-400 software is.Thanks again Robert, I'm going back to read the tutorial on VNAV. Maybe I'll pick up something else minor that may improve my ability to manage VNAV.Kirk Mayers


Kirk Mayers

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hi,Quoted in Mike Ray, The unofficial Boeing 747-400 simulator and checkride procedure manual."The VNAV function is so far too complex for a mere human airline pilot to understand so let's accept the fact that we will NEVER fully understand VNAV. We must be constantly aware of what it is doing and confirm that it complies with what we want it to be doing...""The complex mode is simply NOT INTUITIVE and so it is confusing and difficult to operate. If you get in that position, revert the simple FL CH until you sort things out. I would say 63% of the times when you ask,"What is it doing?? It is because of the VNAV""The VNAV function is the most disliked feature of automated cockpit system.""..73% of pilots used VNAV in the climb phase, while only 20% used the function in descent and 5% use the function in approach."The way he comment the VNAV is quite entertaining and educating, I bought the book and I just want to share with you...ben

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