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Guest Chief Wiggum

VNAV descent

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Guest Chief Wiggum

I posted this in another topic, but as the initial subject was completely different I decided to post again.1. After I get a FMC saying that it would be a steep descent, I got this message: "DES PTH UNCHIEVABLE". VNAV disconnected. Shouldn't it descend as quickly as possible and *try* to be at the selected altitude even if it's not possible? Is it a PMDG FMC bug or I should use VS on steep descents?2. When I have my gear down VNAV start to behave strangely. It tries to speed up using a higher descent rate and don't care about the planned altitudes. What is happening?Any help is appreciated!Thanks,

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Guest Chief Wiggum

No one can help? :(

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The same thing happens to me I have been writing TACAN approaches and all the restrictions are right off the charts but each time it begins the approach it says descent path unachievable I don't have any Idea why. I can fly it right on the numbers using V/S but not using the VNAV.

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Hi,Actually the FMC will TRY to meet any altitude restriction, but if the descend is to steep it will disconnect. Remember that the VNAV will disconnect if the path exceeds 6 degrees below 10000' and 7 above. VNAV is not always very keen on too many restrictions during descend, especially hard constraints as the FMC calculates from point to point. Restrictions to close to each other may lead to a disconnect as it need some time to descend.

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Guest Chief Wiggum

Thanks for your help, Per.When a steep descent is needed and the VNAV disconnects, the right thing is to use V/S as the descent method?What about the gear question?

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Well, I would say, it depends..., are you descending from cruise alt, are you at final app. stage.. There are several methods, you can exec a DES NOW prior to T/D and get a smoother descend until you catch up with the profile again.But in this case, you can use V/S and try to keep the point centered on the deviation guide, but take care of your speed.There several other ways, and a combination of them too, but good planning, situational awareness, thinking way ahead of the plane is a key issue to manage tricky descends. There are a lots of posts reqarding descends on this forum, take a look at them. I also recommend the B737_profiles available at fmcguide.com, and Brad Marsh' Geezer guide her:http://home.swiftdsl.com.au/~zapper/737%20Info/Geezer%20Guide%20Pacific%20Blue.doc.

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Guest Nickman

I remember once I got a little descent "guide" once in the PFD that I think would help you guide your descent but I never paid attention to it (it had 1 long bar with a ledge at the end of the bars with an arrow and some numbers). Anyone one know what triggers this or how to trigger it?Nick B.http://www.precisionmanuals.com/images/forum/800driver.jpg--AMD Athlon XP 3200+ @ 2.2 Ghz400W Power supply3x 80 mm Case FansSoyo VIA KT600 Dragon PlusnVidia GeForce FX 5200 128 mb2 x 512 PC400100 GB Western DigitalMicrosoft Sidewinder Precision 2

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Guest Bpilot

Hello, Captains,I have tried a procedure to avoid a disconnect and make the VNAV work quite smoothly, by selecting a 'dummy' runway at the landing airport, since most of the time, you are too far from ATC when commencing the descent and have no info about the RW. Don't botter to clear any DISCO or chose STAR, since it's just dummy. I think that FMC disconnects VNAV when lacking reference to a point beyond the LEG marked for 240/10000.With that, VNAV will descent quite nicely until 10,000 ft, at 315 speed and reduce it to 240 at 10,000. From 10,000 to approach altitude it's the same, but if you are using ATC instructions, anything below 10,000 could be better by using LVL CHG or V/S. V/S is better because you can control your rate of descent and avoid going too fast. LVL CHG has little control of airspeed.If you are prompted by ATC to descent before your T/D - what normally happens if you use the MS ATC - don't forget to press DES NOW on the FMC DES page.Also, don't forget to change to the specific approved runway.These are personal experiences, and I don't know if they are 'officially' approved, but it works...Regards from Brazil,BPilot

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Guest sj3

I'm not sure if V/S will provide stall (alpha?) protection. I think it doesn't, but LVLCHG does provide it. If you go through Tim's v2 tutorial, you will learn boatloads re: how to keep vnav locked and ready and also the different types of descent methods. It's worth it!Sam

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