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rgamurot

744 Refuses to Decend Under VNAV

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This use to happen occasionally before but I would just click on the ALT knob and the aircraft would descend. Now though, I have to actually disable VNAV and use the VS to descend. There also aren't any Altitude Restrictions on any waypoints either. If there were, I'm still above it.Sometimes I also have trouble ascending. I know it has something to do with the altitude restrictions set on some waypoints but I usually have the same remedy; push the ALT knob and the restriction is deleted. Lately though, not even that works.Does anyone know what I'm doing wrong?Ryan GamurotLucky to live Hawai'ihttp://www.precisionmanuals.com/images/forum/supporter.jpg


Ryan Gamurot
 

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If I remember, you need at least ONE altitude restriction in your descent path for VNAV to be able to calculate a descent profile.As far as I know, using VS as a general descent mode is not a good practice as it has not speed protection. If you don't want to use VNAV, better use FLCH, idle or near idle thrust is set by the A/T, and then you can play as you want with your speed and rate of descent (also the green arc is a good helping tool).Jose Luis


signed: José Luis

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If anything, the only altitude restriction I ALWAYS have is the runway height. This has always been enough for me to get a descent path. Other than that, if I have any other altitude restrictions, I'm still well above it but the aircraft will simply level off at a random altitude and will not descend.Ryan GamurotLucky to live Hawai'ihttp://www.precisionmanuals.com/images/forum/supporter.jpg


Ryan Gamurot
 

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Ryan,And you've reset the MCP altitude as well before pushing the ALT INTV button?Hope it helps,


Mats Johansson
PMDG Flight Test Dept
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Yes. Here's exactly what happens. While I'm descending, ATC clears me to say FL170. Then, when I'm at about FL185, I'm cleared to FL110 and reset my MCP then hit ALT INTV. For some reason, the aircraft will start to level at about FL120. Come to think of it, it's because I have a speed restriction at FL110. So I level at FL120 to slow down but I stop descending, even when I complete my speed reduction.Other times, if I do have an Altitude restriction, I'll descend to that altitude before I hit that waypoint. Then level off and the aircraft refuses to descend further.Ryan GamurotLucky to live Hawai'ihttp://www.precisionmanuals.com/images/forum/supporter.jpg


Ryan Gamurot
 

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Can you provide some screenshots?


Mats Johansson
PMDG Flight Test Dept
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| Asus Z270-A | Intel i5-7600K @ 4.8 GHz OC/H2O | nVidia Geforce GTX 1070 8GB OC/O2|

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I'm not sure what you mean really.. to make the aircraft descend in VNAV for starters a descent profile has to be active within your legs page.. each waypoint with a speed and altitude restriction.. These are in a large font if you manually enter them.. but a small font if they are calculated through interpolation by the FMC..If you are flying in your cuise and you go to the PROG page.. you will see T/D.. the top of descent.. this is the point at which the FMC has calculated when you need to start you descent in order to maintain the calculated profile.. worked out by the FMC.. Once you pass that T/D point.. the descent profile indicator will show on the ND and indicate your relative position to that profile..Reasons for no descent..You are in an altitude HOLD mode on the MCP.. no matter what is indicated the aircraft will hold the current altitude... in which case the MCP overrides any profile in the FMC... The same if the MCP altitude is indicated as the cruise altitude.. in this case the FMC will caution you with RESET MCP ALT before you reach T/D.. to make sure you are aware the aircraft has a restriction.. (This is fairly common as the profile for your descent might not be compatible with ATC requirements).FYIVNAV will only follow the descent profile.. it will only use the MCP indication as a restriciton.. SO.. if you are descending from FL300 to FL250.. as instructed.. and set the MCP to FL250.. VNAV will STILL follow the descent profile in the FMC even if that means a descent to FL280.. then level flight for 30 miles before continuing.. the MCP in VNAV.. is a hard.. RESTRICTION.. not INSTRUCITON..As far as I am aware.. pressing the ALT INT.. the knob on the MCP... instructs VNAV to ignore the NEXT altitude restriction entered in the FMC.. in the descent profile.. and use it's own calculation.. for example..FL300 to FL250 again... the FMC legs are.. WAY1 .826/FL300 WAY2 .820/FL270 (hard entered by yourself.. so a restriction.. and not a calculation by the FMC.. then WAY3 0.820/FL200.. If you begin the descent by setting your MCP to FL250.. and then press.. ALT INT.. the FMC will delete the HARD RESTRICTION.. you entered.. and calulate the profile itself.. so.. your legs will go from..WAY1 .826/FL300WAY2 .820/FL270 (hard entered by us)WAY3 0.820/FL200TO.. after ALT INT.. if we assume equidistant waypoings..WAY1 .826/FL300WAY2 .820/FL250 (the hard entered value is removed and the FMC calcs an appropriate altitude for the descent profile).WAY3 0.820/FL200The FMC then recalculates the profile based on the speed and distance apart the waypoints are.. It's not a descend now instruction..EDIT:If I elaborate on that a tiny bit more.. IF.. there were to restrictions.. WAY1 .826/FL300WAY2 .820/FL300 (hard entered by us)WAY3 0.820/FL300 (hard entered by us)Then you pressed ALT INT you would get:WAY1 .826/FL300WAY2 .820/FL300 (FMC calculated level)WAY3 0.820/FL300 (hard entered by us)As you can see.. since the following restriction is still FL300.. the FMC profile will keep the aircraft level.. and ALT INT.. will NOT.. make the aircraft descend if pressed once..I hope I've not just told you things you already know.. and this is my understanding of the descent automatics.. I might have it a little wrong.. Someone I am sure will correct me if I am..I would look at your profile.. in the FMC before you start your descent.. perhaps put markers on using your FIX page.. and then you can see what is actually happening..Hope that helpsCraig


Craig Read, EGLL

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Hi Craig, I guess I should have mentioned that I use Radar Contact and they have me starting down before T/D. This does put me below my decent profile but I still descend none the less. The aircraft maintains about 1200FPM until I intercept my decent profile, then I descend at whatever rate the FMC calculated. You wouldn't know if there is a way to have RC let me descend later, like at TD, would you?When you talked about setting the MCP to FL250 from FL300 and the aircraft levelling at FL280 for awhile before continuing, that is exactly what's happening. I do hit the ALT INT but I'm guessing the Altitude Restriction doesn't reset because no matter how many times I hit ALT INT, the aircraft refuses to descend. This only happens below FL200. Also, the only restrictions I have are automatically in place from the STAR or the approach. I usually set speed restrictions myself but not altitude restrictions. Ryan GamurotLucky to live Hawai'ihttp://www.precisionmanuals.com/images/forum/supporter.jpg


Ryan Gamurot
 

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Ok..I'm not as well versed in RC4.. but I do have it.. RC4 does simulate ATC instructions much more realistically.. Often they'll ask you to start your descent early.. but quite often they'll give you the option to ask for a PD Descent.. Pilots Discression.. in which case I wouldn't intervene with the VNAV system.. I would simply reset the MCP to the altitude asked for by ATC.. and not actually initiate the descent until VNAV does this automatically..IF.. you are instructed eailier.. I would interve with a Flight Level Change FLCH.. and perhaps put a restriction on with a vertical speed.. so the aircraft doesn't descend too rapidly.. If there were a specific altitude instruction for a waypoint... In this case I would initiate the descent right away with FLCH and the requested altitude on the MCP.. Once in a stable descent, I would look to the FMC and enter the new restrictions into the flight plan on the LEGS page... at this point.. I would probably.. take a look at the calculated altitudes within the FMC.. adjust my descent to match them... then re-engage VNAV to conduct the descent on a profile for me automatically.. IF I were in the latter stages of an approach and the chart featured restrictions in altitude and speed targets.. and I were told to ensure a altitude by a certain waypoint or distance.. 30 miles.. etc.. I would either introduce a new waypoint into the plan and set the appropriate altitude.. and allow VNAV to continue the descent on a profile.. and simply observe that profile and use speedbrakes to ensure I remain on that profile..As for your ALT INT statement there.. what is happening.. is you're REMOVING a manual restriction in the FMC for the next waypoint for your altitude.. IF.. there is NOT.. a restriction entered manually into the next waypoint.. you're not actually deleting anything.. and the FMC will continue on it's descent profile.. which must mean it's got to descend slowly..Post some screenshots and we can have a better look :)CheersCraig


Craig Read, EGLL

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Guest piper99

Hi Ryan,When VNAV is doing things you don't expect or understand best is to check the LEG page of the FMC. This tells exactly what VNAV is trying to do.Also here are a couple of things you might like to check out:1) Manual page 8-15 (Automatic Flight Management System) - Explains what happens when pushing the ALT knob.2) The PMDG Type Rating Course 4 - Explains LNAV and VNAV in detail.Hope this help,Christophe.

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Christophe, I'm not sure how the LEGS page shows what VNAV is doing. It only shows my waypoints, speed restrictions, and altitude restrictions.Other than that, I tried my best to reproduce the problem but it only happened at about 3500ft when I was shooting for 2500. At one point, I even started climbing slightly. I'm not sure what you wanted screen shots of so I took the DES, LEGS, PROG page 2, and one of when the aircraft started to ascend. The ascent image was too big to upload though. Basically the same information was present in the image as the others.I did stop descending at FL070 but all I got was an UNABLE NEXT ALT message. Then when I hit ALT INT, the aircraft actually did what it was supposed to do. Ryan GamurotLucky to live Hawai'ihttp://www.precisionmanuals.com/images/forum/supporter.jpg


Ryan Gamurot
 

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Ooops, there is a DISCO in your LEGS page.Jose Luis.


signed: José Luis

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Hi,All the images show you are below your descent profile, so VNAV will attempt to re-align with your descent profile..You also have a discontinuity here.. VNAV will use the flight plan and waypoints to calculate the descent profile.. If there is a discontinuity in the flight plan (I am not sure of this effect).. but I imagine it will makes things more difficult for VNAV in terms of following a profile as it's not always over waypoints or track lines.. It is still working out a profile and your FMC LEGS page shows this..BUT your FMC page also shows that the aircraft is trying to slow from 213knots to 180.. That might be why the profile is flat.. as for ascending.. I am not sure.. I'd have to see the photo.. it depends how minor the ascent is..As to the reason for your UNABLE NEXT ALT message.. it could be to do with a restriction.. perhaps a speed restriction.. were you descending through FL100 to FL070? In which case.. the profile may be calculated but the speed reduction from 300+knots to 250knots which will mean a flat portion of that descent to allow for reducing speed, this is usually denoted by two green dots on the ND flight plan.. one for the speed reduction begining.. one for when it is to complete.. When you hit ALT INT.. you're deleting the speed restriction as well as the altitude restriction.. Initially.. perhaps it was not possible to reduce speed and still hit the FL100 target in time.. as a result the aircraft may be over the profile and the FMC cautions you with UNABLE NEXT ALT.. this is not possible with the restrictions in place.. It's saying "I can't descend on that profile while maintaining your speed demands too as I can't slow down to 250 and maintain the correct ascent rate"... Pressing ALT INT.. is saying to the FMC... ignore the restrictions on the next waypoint.. and it goes.. "oh.. oh right.. yeah I can do it now that speed is not in the way.." and you interpret this as "doing what it's supposed to do".. It is in fact doing what it's supposed to do all the time.. It's following your commanded profile..In some cases of course this will not make a difference.. if there is a 250/10000 cap in the FMC for speed.. then obviously VNAV when it calculates will too.. take the speed to 250.. in which case no amount of hitting ALT INT.. will make any difference to the descent profile..When approaching a descent.. I'll open the FMC LEGS page.. and look at the restrictions for the next few waypoints.. I usually cancel FMC messages of RESET MCP ALT until I get clearance to descend... Once I get clearance to descend.. I'll set the altitude bug to the commanded altitude by ATC.. and initiate the descent in VNAV.. although we won't be on the path right now.. with application of speed brakes.. and a few other alterations.. I can usually re-capture the profile with little difficulty.. In some cases I might not even want to.. As I am not sure the path to the airfield yet.. or I know I'll be vectored around a while.. So long as I meet ATC expectations.. If I am going to conduct a VNAV descent.. when I am 1000 feet from the next ATC target.. I will ask for further descent so I do not have to interupt VNAV..Once I'm this low.. unless I'm following the specific flight plan I've usually switched to a manual mode for altitude anyway (V/S)... Unless ATC want me to follow the STAR altitude restrictions, in which case I follow the plan within that STAR.. Then VNAV has something to follow and I usually let it carry on.. This is the case in my tutorial.. as the STAR goes right to the final stages of the approach to LAX..CheersCraig


Craig Read, EGLL

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I usually am below my decent profile when ATC vectors me in from the STAR. But I don't usually have my descent problem this low. Usually it's between FL200 and 6000ft with no route discontinuity. Recently though, I've been using V/S a lot to meet ATC altitude restrictions. Regarding my ascent, I cut off the far right side of the image to make it smaller in size. http://forums.avsim.net/user_files/167623.jpgAs far as the aircraft refusing to descend at my usual FL200 to 6000, I think I figured it out based on everything you said. I think what is happening is that I cleared all of my altitude restrictions so I couldn't readjust my descent any further. At that point, the aircraft refused to descend until it returned to the descent path.As for the UNABLE NEXT ALT message, I was descending to 6000 but had a 7000 altitude restriction. The aircraft shot through 7000 and then suddenly levelled and gave me the message. Since I was suddenly below 7000, the aircraft couldn't make that restriction any more.Thanks for all your help. I think I'll stick with V/S from now on since it does actually give me more control over my decent and also my speed.Ryan GamurotLucky to live Hawai'ihttp://www.precisionmanuals.com/images/forum/supporter.jpg


Ryan Gamurot
 

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