PMDG 737NG Help Needed!

[captain_crunch]

I know that this is the FSP forum, but I also know that there are a few PMDG users on here so I thought I might as well ask!

I decided to bite the bullet and buy this beast a few days ago. I've read (most!) of the docs, I've followed two different tutorials and
everything is fine except for one thing....

I don't know if it's me or a bug, but you know how the FMC sets the speeds for each waypoint? And how the speed bug and autothrottle
follows this during your decent? Well on mine it only works for the first two waypoints during decent!

As an example, imagine that you're about to reach the T/D point, and there are three more waypoints after that, 'way1' - 'way3', each with
altitude and speed limits set. You're still using VNAV on the MCP. When the T/D point is reached, the aircraft slows slightly and begins it's
descent. As you continue descending to way1, the FMC and autopilot behave as you would expect i.e. the speed bug falls and the
autothrottle acts accordingly, still maintaining the correct descent profile.

You then pass way2, and a quick glance at the LEGS page suggests that the aircraft should be slowing further. But it isn't, the speed bug
stays in the same place and, naturally, you keep going at the same speed. You pass way3 and, guess what? You're still going the same
speed despite the LEGS page saying that you should already be going a lot slower than you are...

And so on all the way down to the runway! Is it me or is something not working right

So far I've been switching VNAV off and using the altitude hold, speed holds, and / or LVL/CHG to fly the approaches which is ok, but it
doesn't correspond to the tutorials and surely if the waypoints have speed limits set, the FMC should at least move the speed bug to
reflect them? It seems to handle altitude OK. Just to clarify, this is NOT a case of me not slowing the plane down, the FMC seems to stop
acknowledging the speed limit restrictions attatched to waypoints.

Any thoughts? The tutorials I've used are by Fred Clausen and FS Station, are there any other ones out there? I've also flown exported
FSnavigator flight plans, same problem



Post Edited ( 05-29-06 21:17 )

[captain_crunch]

Just one bump for this, then I'll let it die!

The plot thickens... The FMC is now happily moving the speed bug and slowing the plane down to comply with the speed restriction
below 10000ft but it still ignores the speed restrictions assigned to waypoints. I don't actually want the plane to fully fly itself, it's just that
the FMC seems to be calculating the descent profiles in such a way that you can't possibly arrive at your initial approach fix with the right
altitude and speed at the same time, you have to settle for one or the other and sort the mess out when you get there using holding
patterns!

Not very reaslistic for one of FS9's most realistic planes, or at least I don't think it is? What annoys me the most about it is that I've paid
money for a plane that is supposed to have a fully functional FMC but I have to fly half of the flight using the MCP the same as I would with
the bloody default 737!

In fairness, it could well be something that I'm doing wrong. One of the tutorials hints (very vaguely) that you have to manually switch
between LVL CHG and VNAV to keep the plane on track and at the right speed. Fair enough, but another tutorial says that this switching
happens automatically and yet another one claims that the FMC should take care of the decent all by itself with VNAV on all the time!

I'm seriously confused

[vollmey]

And you are setting the Speed and Altitude in to the legs page right?? If I'm on a arrival I will usually program into the arrival the fix that I
need to cross a certain altitude and speed and then I take it from there, and that's always above 10,000ft. The PMDG FMC does a great
job getting you to that point, I will usually start down a bit early (10-15 nm before the T/D) and it sometimes ask me for some drag to
make sure I get to that fix at the altitude and speed I programed in. I never really let VNAV handle the plane below 10000 though, but I'll
give it a try and see what happens.

[captain_crunch]

Thanks for the reply

Yes, I've been setting the speeds and altitudes in the legs page, and I too get the 'drag required' message occasionally. Usually what
happens is that it'll slow to about 184kts just before the T/D and then start descending at about 2500-3000 f/s. But, say I have a speed
restriction of 240kts programmed in for 13000 ft, the plane will just continue descending past that point at 284kts (if i let it!), it doesn't
even move the speed bug to reflect the 240kts limit as I pass 13000.

I can get round it by changing the 250kts speed limit under 10000ft in the DES page to 240kts under 13000, in fact one of Fred Clausen's
tutorials suggests doing this anyway in order to give you plenty of time of slow down for 10000ft.

I'll try starting the decent a little earlier as you suggest

[vollmey]

It should not slow to 184kts right before T/D. What should happen is say you need to cross so and so waypoint at 250/11000ft, you
should descend at 2500/3000 ft/min and maybe 280-290 knts. On your EFIS you'll be showing two green circles 10-15 nm miles apart
that show you where the FMC needs to decelerate and slow it's decent to burn off that speed to make it to that checkpoint at at your
requested speed and altitude. If you start down a bit early on the DES page it helps out alot especially if you got a bit of a tailwind or a
little heavy. I'm not sure why it's slowing to 184 kts righr before descent, it should not do that unless your putting that # in. Maybe your
doing this but just try putting in reference altitude and speed aat around 250/10000 and let the FMC caluculate the rest of the way down.
Report back!!!

[captain_crunch]

Right, it kind of worked...

I've found out that the slow to 184kts before decent is caused by a setting in the DES page of the FMC, it's just the standard descent speed and you can change it to whatever you want.

I set a limit of 220kts at 10000ft to a way point near the destination. At 40nm from T/D, I started the descent and the FMC put the plane into
a 1000f/m descent. After a few minutes, I intercepted the VNAV path and the plane started decending at 2500 - 3000f/m. Everything was
fine, the speed limit below 10000ft (which I'd changed to 13000ft for the purpose of this test) kicked in ok but then VNAV disconnected itself, no message on the FMC or anything! I then switched VNAV back on and the rest of the descent went fine, the green circles appeared as you mentioned and the plane slowed down to 220kts, with the FMC informing me of drag required as you'd expect.

Thanks alot for the help I don't know what I was doing wrong before, but it seems to work now apart from VNAV disconnecting when it
feels like it!



Post Edited ( 06-01-06 16:01 )

[vollmey]

Yes, I've had the VNAV disconnect when I had a slight wind shift or it maybe around a storm. Not sure what causes that because I've
never had the PIC 733/4/5 disconnect on me.

I'm happy you got it working right, I really believe the key is to start down early. It just kind of gives the FMC a bit more time to plan the
descent.