Question regarding VNAV in 737-700

[cmpbellsjc 2,960]

Hi folks. I have been flying the PMDG 737-700 for about 2 years now, but haven't until recently been using STARS's when approaching my destination airport, or for that matter using SID's/DP's when leaving. Lately I starting using NACO - Digital Terminal Procedures Search and FlightAware - Free Flight Tracker - IFR Flight Status, Tracking, History, Maps, to plan my routes and figure out what SID/STAR combo's are being used by real world airlines. I am now enjoying my flighs a lot more knowing that I am flying the real procedures.With this being said, my question is regarding the VNAV function. I never used it before. I would just adjust altitude using Altitude/VS adjustments on the MCP. However since I began flying acutal STAR's that have altitude and speed constrants I thought it would be useful to use the VNAV function to manage this portion of my route. This is where my trouble begins. The other night I was flying from KABQ to KPHH using the route LARGO2 ZUN EAGUL1. After passing the ZUN VOR the EAGUL1 STAR calls for passing the PAYSO fix at FL240 and 280 KTS and the following HOMRR fix at 12000' and 250 KTS. I set these limits in the FMC when I was programming prior to departing KABQ. There are also a few more hard constrants in the STAR after the HOMRR fix that I set in the FMC. So I take off from KABQ get to altitude FL340 set LNAV and VNAV to take care of the route from there. After I cross the ZUN VOR I see in the map display the T/D point where the AP should start it's descent to FL240/280 KTS at the PAYSO fix. I set the altitude to 24000 in the MCP windown just incase of a disconnect. As soon as I pass the T/D point everything worked fine, the aircraft arrives at PAYSO on schedule FL240 and 289 KTS. I have two more waypoints after PAYSO until I reach HOMRR which I am supposed to cross at 12000 and 250 KTS. The problem is VNAV never starts the next descent to HOMRR even though I have it programmed into the FMC as 250/12000. I had to start a mad dive since it never worked on it own to reach this fix at the right altitude and speed. The question is am I supposed to do somthing to initiate this next descent, cause the STAR I was flying requires a few more after HOMRR and I had to do these manually instead since VNAV wouldn't do it. I tried researching in Tim Metzinger's tutorial and in the manuals but can't seem to figure the cause to why VNAV ceased to do anymore descents after it completed the first on. Thanks in advance and your help would be much appreciated in helping me figure this out.Sean Campbell

[crvm 0]

You could always post a screenie with the current LEGS page showing the mentioned WPTS on the CDU. There is also a deviation scale icon on the NAVigation display that will show your vertical position in relation to any vertical path deviation. When you say that it never descends - what are you seeing that brings you to this conclusion? Do you actually pass the wpts in question before you start the manual dive or are you making a judgement based upon something else?Best,Randy J. Smith<<>>

[basa 0]

Hi Sean:Maybe a stupid question, but, have you entered a lower altitude than 12000 in the alt window of your MCP ?. If you haven't it doesn't matter what you have programmed in the FMC: VNAV will never "violate" the altitude you have set in the MCP.Hope it helps. Jose Luis.

[dave_paige 11]

The PMDG FMC has an issue dealing with multiple crossing restrictions during a descent. The solution is to wait until you just passed the most recent crossing restriction, then dial in the new altitude on the MCP, then simply deselect and reselect VNAV on the MCP and it will then continue on the VNAV descent.

[downscc 1,327]

Dave is right.. the 737 FMC has several VNAV bugs that PMDG is going to wait (until whenever) to resolve in 737NGX... you'll notice that the VSI indicator will suddenly get a magenta command bug after crossing a fix with an altitude constraint. Press the VNAV button twice to clear it. I notice it happens more often than not, seems to occur whenever the aircraft is not exactly at the constraint when crossing the fix (totally unrealistic), and forget about the double constraints (AT OR ABOVE & AT OR BELOW).Sometimes I press the ALT INTV button just before the fix to take me down to the next fix with that constraint already in the MCP ALT window, but usually I just don't use VNAV for descents relying instead on LVL CHG or V/S as appropriate.

[cmpbellsjc 2,960]

Thanks Randy, Dan, Dave, and Jose for the replies to this question I had. I didn't know this was a bug for crossing multiple fixes with restraints. Randy said "When you say that it never descends - what are you seeing that brings you to this conclusion? Do you actually pass the wpts in question before you start the manual dive or are you making a judgement based upon something else?" I was making this conclusion that after VNAV makes it's initial descent to the first waypoint. After crossing that waypoint and I am approaching the next one that have an altitude restriction in, that is a few thousand feet below where I am now, it never starts a descent. I just keep flying at the same altitude. That's when I had to deselect VNAV and descend manually. Jose, I had set my MCP altitude to the altitude restriction at the next fix just after I passed the first waypoint that I had descended to. I will try Dan and Dave's solution tonight when I try this flight again, by turning off and on VNAV so that I will start its next descent to the next fix. The STAR I was flying had 5 fixes all with altitude and speed restrictions. VNAV completed the descent and speed restriction to the first fix, but that was all she wrote after that. I did it manually from there until landing. BTW, Dan mentioned that he sometimes uses the ALT INTV button. I don't see that button on my MCP. Is that just for the people who upgraded to the 800/900 series? I am still flying the original 600/700 package. If I do have that function where is it located?Thanks for all the help. I will go fly now and report back later and let you know if this worked.Regards,Sean Campbell

[basa 0]

>Dan mentioned that he sometimes uses the>ALT INTV button. I don't see that button on my MCP. Is that>just for the people who upgraded to the 800/900 series? I am>still flying the original 600/700 package. If I do have that>function where is it located?>Yes, the PMDG 600/700 package has not an ALT INTV button, is a feature of the 800/900 upgrade.In the real world, I think that is a customer's option for the NG series.Jose Luis

[dave_paige 11]

My procedure on multiple constraint arrivals is to; just before top-of-descent, set the MCP altitude window to the lowest altitude constraint on the arrival. Then I just reset ( cycle off and on ) the VNAV button on the MCP as I pass each waypoint with a constraint. This method has worked beautifully for me. Of course, you must always honor any altitude constraints given by ATC.

[cmpbellsjc 2,960]

Thanks for all the help guys. I flew my KPHX to KABQ route again last night trying what you guys suggested and it worked like a charm. I agree Dave, setting the lowest constraint altitude in the MCP worked great so I didn't have to keep fiddling with it after I cross fixes. This makes it a little easier to get other work done when setting up for approach since I don't have a co-pilot helping out, lol. These flights are really a lot more enjoyable now that I am able to intergrate the actual flight routes and download the SID/STAR/Approach Plates off the internet and fly them as they should be flown. Thanks again.BTW, Dave, I don't honor many of FS9's ATC altitude constraints as they have flown me into a few mountains, ha ha.Regards,Sean Campbell

[Guest OctoberBaseball]

I'm also having this problem with a flight from KBWI using the JFK.INDDY2 arrival into KBOS. (http://204.108.4.16/d-tpp/0702/00058INNDY.PDF)As I near my Top of Descent, I lower the altitude on the MCP to 11,000 since it's the last altitude restricton on the STAR, which is to cross PVD @ 11,000. So now I'm at my Top of Descent and descending to cross NEWES @ FL240. The descent goes smoothly up until I cross NEWES. I cross NEWES @ FL240 and after NEWES, the plane refuses to descend to cross PVD @ 11,000. Clicking VNAV does work, but it is annoying to do that everytime I cross a waypoint.John

[dave_paige 11]

That's just the way it is. As I said it's a minor bug. Quite remarkable given the complexity of the PMDG simulation that there are not more. My guess is that even the real aircraft has a few quirks.

[Mats_J 2]

>My guess is that even the real aircraft>has a few quirks.You'd be surprised if you knew how many. And a lot of pilots have worked out work-arounds for them as we have been doing here.Cheers,

[Tabs 2,790]

Many real pilots in fact don't even use VNAV descents due to just those sort of "quirks" in the real thing... It's gotten better with the later software revisions and newer FMC models, but I know it was rather scary at times with the first 737-300 and 757/767 versions of it...