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abh_jc_03

Aircraft will not descend with VNAV

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I have VNAV light on and on my PFD is shows VNAV Path but the aircraft is not decending. It shows my E/D at 187 miles which is how far I am from my airport so I can't get a DES NOW, I only get a DES DIR. The times I am given in ZULU for each waypoint is the exact time. E.G. Waypoint KOBI ATA 1003Z and which is the time it is at that time yet I am still 90 miles away. I don't know if that is too confusing but pretty much my times that I should be passing waypoints is just the time the clock shows instead of 30 minutes later etc. I am thinking I cant get the aircraft to decend because it shows the E/D at the aiport so the FMC doesn't want to decend. Only way around it would be manually V/S correct?

 

2012_7_14_10_42_3_298.jpg


Josh Scholl

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Have you set the desired alt in the MCP? It will not decend otherwise.


\Robert Hamlich/

 

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Altitude was set in the MCP. I would have to hit FL change each time and the aircraft would go into a 9000-10000 fpm decent. Obviously unacceptable. I have heard the PMDG 747 is a very precise aircraft and the VNAV is suppose to do wonders.


Josh Scholl

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Something's telling me that you haven't read the manuals/tutorials (like the fact you have a signature without the required first and last name).

 

You have to roll your desired altitude in before reaching the top of descent (T/D). If you had, it would begin a descent on its own. If you fly past it, you must roll the MCP altitude down, and then confirm it by pressing on the MCP altitude wheel.

 

Furthermore, if it's initiating a 9000-10,0000 FPM descent using FL CH, then it's attempting to fly an insanely high speed at idle thrust. It seems as if you've set the FMC incorrectly as well. If the FMC is expecting weights KGs and you're giving it LBS, you're going to have problems similar to this.


Kyle Rodgers

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Perhaps we need to see a screen shot of the MCP and the PFD. This will show what the settings of the autopilot are and what mode it is in. The PMDG autopilot operates correctly according to the books. However, there is a basic need to understand the logic of it all. This is where the manual comes in. However, here is a crash course when it comes to decent. First, as you have not reported issues with course or heading, just forget the lateral modes (LNAV, HDG,..) So, you are flying at cruise and approaching your top of decent. You need to decide how you wish to execute the decent. First step is to determine the first crossing altitude. This may be on a chart, provided by ATC, or just chosen by you. Enter this altitude into the MCP. You will notice, the jet doesn't move. This is because you haven't actually changed the Autopilot's mode. All you have done is said how far the bird can descend. Now you must decide how you want to get down. There are for options. 1. Manual, pilot controls the rate of decent using the yoke. 2. Flight Change Mode FLCH: Pilot controls the rate of decent through the use of throttle and the set MCP speed. 3. Verticle Speed VS: Pilot controls the rate of decent to a specific FPM. 4: VNAV: Decent is controlled via a pre determined flight path within the flight management system.

 

So as you can see, there are a lot of options you can choose for decent. In real world ops, FLCH and VNAV are the ones which are typically used. This is because they both have built in overspeed protection where the other modes do not.


Branton Turner

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I have not flown the tutorial flights because I don't have the time to take international flights. The flight I am doing now is from REX Essentials Flightaware which had shown a 737 as the aircraft but I like PMDG and wanted to fly the 747. For some reason minutes after I sent the previous post my "time and mileage for T/D" appeared on the DES page.


Josh Scholl

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Something's telling me that you haven't read the manuals/tutorials

 

Can you explain what tutorial you are referring to? Tutorials for the 747-400X did not come with the download.


Dmytri Eck

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