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gpbarth

A Bit of Confusion on the FMC and VNAV/LNAV

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I've been having some problems with the 737 and my flight plans. I program the plan in, make sure everything is in order, including all of the legs, including the speeds and altitudes. I then take off, activate TOGA, the LNAV and VNAV, and then the A/P, and off I go. But somehow, I was under the impression that the A/P would follow the FMC program without intervention. I set my initial altitude at 3000', due to ATC restrictions in the SID, then to (say) FL330.

 

After take-off, I have to press the Alt button to start the climb to 33K. No problem. And along the route, the A/P keeps me cruising along the route. UNTIL - I get to my first transition point, like a checkpoint where the altitude changed from FL330 to FL240. Crossing that point does nothing, and I have to enter the new altitude and press the Alt button to start the descent. As the flight progresses, I have to continually manually force the altitude changes by entering the new altitude and activating it. Even at the DECEL point, with the VNAV and the auto-throttle in action, I have to enter in a new speed manually.

 

I thought that with the A/P, VNAV, LNAV and autothrottle, all of this would be automatic. Am I doing something wrong?

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What would you like to see? The FMC screen? I can snap-shot anything you want to see - let me know.

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Am I doing something wrong?

 

I mean no offense by this, but probably...

 

I could go into a long-winded explanation here about how to properly use VNAV to fly, but it wouldn't have pictures, which would provide more clarity.

 

Instead, I'd suggest flying the two tutorials that came with the aircraft.  Those should answer all of your questions, and actually teach you a few more things.  The pictures would better illustrate the concepts of what anyone would try to explain here.

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I get to my first transition point, like a checkpoint where the altitude changed from FL330 to FL240. Crossing that point does nothing, and I have to enter the new altitude and press the Alt button to start the descent.

 

You need to reset the mcp to the lower ALT before reaching TOD, or it wont follow VNAV (As you, the pilot, are telling it to fly at FL330, as thats what set in the MCP)

 

If your next waypiont was lower that FL240, you would need to set the lower ALT before the aircraft got to FL240.

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I mean no offense by this, but probably...

 

I could go into a long-winded explanation here about how to properly use VNAV to fly, but it wouldn't have pictures, which would provide more clarity.

 

Instead, I'd suggest flying the two tutorials that came with the aircraft.  Those should answer all of your questions, and actually teach you a few more things.  The pictures would better illustrate the concepts of what anyone would try to explain here.

No offense taken. I have started the 1st tutorial several times, and somehow never gotten around to finishing it. And so, in the spirit of cooperation, I will go back and restart it again. I think I have gotten a bit of overload what with all of the YouTube tutorials and such. In my (pitiful) defense, I am a bit of ADD, in that I want to get this stuff all done at once, and as a result, I end up not concentrating on ONE good focus point. I do recall that the tutorial went through much of this, and so I'm on my way back there now.

 

Thank you for that subtle suggestion. I really do want to master this aircraft, as I did the Coolsky MD-80. The FMC in this thing is very powerful, and I need to figure it out fully.

 

You need to reset the mcp to the lower ALT before reaching TOD, or it wont follow VNAV (As you, the pilot, are telling it to fly at FL330, as thats what set in the MCP)

 

If your next waypiont was lower that FL240, you would need to set the lower ALT before the aircraft got to FL240.

I thought I was doing that. If I'm at FL330 and the next waypoint calls for FL240, I set the MCP for 240 prior to reaching it. But for some reason it doesn't seem to start the descent when I cross it. But if what you are saying is correct, then I still have to monitor the FMC and manually set the MCP for each altitude prior to reaching that waypoint. For some reason, I thought that the VNAV function did that automatically. If not, no problem...I was just wrong in my assumption.

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I'm sure there's a much more scientific explanation than this, but basically MCP overrides FMC (for safety??) so if you expect to descend and your MCP alt is higher than the FMC alt it will stop you descending past the MCP alt and vice versa.

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No offense taken. I have started the 1st tutorial several times, and somehow never gotten around to finishing it. And so, in the spirit of cooperation, I will go back and restart it again. I think I have gotten a bit of overload what with all of the YouTube tutorials and such. In my (pitiful) defense, I am a bit of ADD, in that I want to get this stuff all done at once, and as a result, I end up not concentrating on ONE good focus point. I do recall that the tutorial went through much of this, and so I'm on my way back there now.
 
Thank you for that subtle suggestion. I really do want to master this aircraft, as I did the Coolsky MD-80. The FMC in this thing is very powerful, and I need to figure it out fully.

 

Luckily aviation is exempt when it comes to my attentive shortcomings (which affect just about every other written subject).  It's somewhat understandable.  If you can just push through the two tutorials, you should have yourself sorted out.  I have a feeling that the problem is with your commands in the descent, which are explained in the tutorials.

 

Focusing on one issue at a time is one of the many learning styles.  It's actually a good style for forcing someone to address an issue that needs to be corrected.  In adult learning theory, the example is:

If you want to start a fire with a magnifying glass, you have to leave it in one spot long enough for it to catch.  From there, it spreads quickly.  Trying to move the glass and start multiple fires at once is impossible.  Similar can be said for trying to learn too much at the same time.

 

...and now you know more about adult learning theory then you never did. Ha!

 

If you have any questions, or none of it makes sense, I'm sure everyone here would be glad to help.  I just figured the pictorial explanations would be easier to understand.

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I just flew the first tutorial and had several small problems - first the derate 2 resulted in no reference speeds...I had to go to derate 1 to get them. Then it seems like I was too high almost the entire descent, and kept getting the message "Destination not reachable".  And I was going too fast, too. In the end, as I turned on what should have been a long final, I was too high, too fast, and worst of all, although the display showed me to be right down the wire, I looked out the window and the runway was about 5 miles to my left, parallel to my flight track. Even if I had been slow enough and low enough, the localizer wasn't even close.

 

I'm going to try it again, and stay on top of things a bit closer. I am concerned about the loc/glideslope thing. I know FSX has a problem with some runways - I hope this isn't one of them. The one thing I did learn was to watch the FMC, set the altitude and push the Altitude Intervention as needed.  Don't know where I got the idea that it was handled by the A/P, but that is fixed, at least. I'll report back when/if I ever fly this tutorial correctly.

 

Thanks for all of the input. I'm sure I'll be back, picked your brains some more.

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I flew the first tutorial again, with absolutely NO problems! Greased it in on the landing and didn't have one problem. I think I'm getting the hang of it. The one thing that puzzles me is the setting of 2000' on the descent - I dialed the altitude in, and didn't have to touch anything else...the aircraft seemed to descend at the right speed and at the right time, instead of dropping like a rock to 2000' as soon as the descent started. Why was that? I thought going from FL250 to 2000' in one dial-in was kind of strange, but it worked flawlessly.

 

I'm finding that I really have to stay on top of speeds and flap settings. I hope the addition of FS2Crew will take some of the work out of it. But I flew the whole thing by myself, and was quite pleased at the results. So it's on to Tutorial #2. I have TOPCAT, so now I'm wondering if the FSUIPC export will send the info to the 737's FMC, or if I'll have to do it manually. Any input on that?

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I flew the first tutorial again, with absolutely NO problems! Greased it in on the landing and didn't have one problem. I think I'm getting the hang of it. The one thing that puzzles me is the setting of 2000' on the descent - I dialed the altitude in, and didn't have to touch anything else...the aircraft seemed to descend at the right speed and at the right time, instead of dropping like a rock to 2000' as soon as the descent started. Why was that? I thought going from FL250 to 2000' in one dial-in was kind of strange, but it worked flawlessly.

 

I'm finding that I really have to stay on top of speeds and flap settings. I hope the addition of FS2Crew will take some of the work out of it. But I flew the whole thing by myself, and was quite pleased at the results. So it's on to Tutorial #2. I have TOPCAT, so now I'm wondering if the FSUIPC export will send the info to the 737's FMC, or if I'll have to do it manually. Any input on that?

 

 

Just keep at it Gary. You will get it. Sounds as though you're on the right path now. And as Kyle pointed out, forcing your way through the 2 tutorials plus the fact that everyone here is happy to point you in the right direction, will help you succeed with the NGX. FS2 Crew is a brilliant add-on program which will further strengthen & consolidate your new piloting skills. When you're ready, you may want to stretch the envelope with Emergency NGX (FS2 Crew) and learn how RW pilots deal with varying emergencies & failures. But all in due time of course! Good luck with it! Go hard!

 

Rgds,

 

Steve

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2000 would be the minimum vnav would descend to. The autopilot will follow the altitude restrictions set at each waypoint in the descent. Even if you were not on vnav, you wouldn't go from 25000 to 2000 really fast unless you also set flc( flight level change to descend at a certain speed) or vertical speed really fast. Also, if you were to change the next waypoint to 2000, the autopilot will not perform a 6000 fpm dive to make that altitude. It will abide by the speed restriction and give you an unable descent path error

 

The altitude setting is like a target. How fast you get there depends on the restrictions in the fmc plan or the flc or v speed.

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If you ever plan on flying on vatsim, get some practice with these other methods of descent. Atc does not know what your vnav profile is. They may give you alternate speeds and altitudes. It's your job to comply with their instructions. Flc and vertical speed may be necessary to do this. If they want you to descend but maintain 230 knots, flc at 230! Of course if you're flying a certain star, they will expect you to meet those restrictions, so you can follow the vnav. But be prepared and capable of using the other vertical modes.

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I'm finding that I really have to stay on top of speeds and flap settings. I hope the addition of FS2Crew will take some of the work out of it. But I flew the whole thing by myself, and was quite pleased at the results. So it's on to Tutorial #2. I have TOPCAT, so now I'm wondering if the FSUIPC export will send the info to the 737's FMC, or if I'll have to do it manually. Any input on that?

 

Glad the tutorial helped.  Hopefully #2 will help even more, though it's slightly more challenging.

 

I don't think TOPCAT's profile matches the new PMDG 737s (the profile in there is from their FS9 version, if I remember correctly), so I wouldn't tell it to export weights at all.

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Once again, thanks to all you'z guys. As the old saw goes, when all else fails, RTFM! :rolleyes:  I am a "real-life pilot" who was working on, but never got, my IFR rating in SEL aircraft. I'm used to ATC, and can run a 3-axis A/P like you wouldn't believe. I can fly a flight plan and shoot an approach in the Mooney or Lear with no problems. It seems natural to me to set altitudes and heading bugs and tell the A/P what to do, while listening to ATC tell ME what to do.

 

All of this new stuff with LNAV and VNAV and TOGA and TOC and TOD and full control right up to auto-land is what's freakin' me out. I've been using Radar Contact fort a long time now, and with it (and the FSX poor excuse), you can't fly a full flight plan on the FMC. Especially when the descent starts and the approach phase cuts in. I can use the speed brakes/spoilers (and even the gear sometimes) to bleed off that airspeed and get down to the requested altitude. About the only thing you'll hear that helps is "assume own navigation!"

 

So you either have to put in that plan and let the thing fly itself, or do the "real world" stuff and alter the profile enroute to make ATC happy. I can change altitudes, speeds, descent rates and headings just by turning off LNAV and VNAV - I think that stuff just makes us lazy. As for NGX Emergency and the 737's own failure modes, no way. Anyone here have any of A2A's stuff, like the P-51? If I don't fly that thing for a few weeks, the battery is dead and I have to put it in the hangar and "repair" it. Part of FSX is the enjoyment of flying, and I'm 66 years old...I don't need an in-flight emergency when I already have SDK errors and CTDs, if ya know what I mean. Flying these new airliners is enough work for me without having to deal with a bus failure and a circuit breaker check on the back wall.

 

2000 would be the minimum vnav would descend to. The autopilot will follow the altitude restrictions set at each waypoint in the descent. Even if you were not on vnav, you wouldn't go from 25000 to 2000 really fast unless you also set flc( flight level change to descend at a certain speed) or vertical speed really fast. Also, if you were to change the next waypoint to 2000, the autopilot will not perform a 6000 fpm dive to make that altitude. It will abide by the speed restriction and give you an unable descent path error

This is what I think I expected with the "automatic" part of the descent - just set it at the approach (or close to it) altitude, and the FMC would manage the descent. It seems weird to keep changing the altitude and pushing the Alt Int" button, when you could set the altitude for, say, a crossing altitude restriction in the plan along the STAR, and let the FMC do all the work.

 

I'm learning, I'm learning. The heavy iron stuff is new to me, and there IS a learning curve, but I'm getting it. The Coolsky MD-80 got me going, the 737NGX is taking me over the coals, and the up-coming 777 is really going to be fun! But I think Vatsim is the way to go. Radar Contact is fairly good but it has it's failings (vectors screw it up), and I DO like the realism of ATC. I've joined up, have a pilot number, and have Squawkbox installed - I just have to get up the nerve to get my feet wet. And I want to know how to fly these things before making a total fool of myself.

 

Again, thanks all!

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I don't think TOPCAT's profile matches the new PMDG 737s (the profile in there is from their FS9 version, if I remember correctly), so I wouldn't tell it to export weights at all.

I wasn't worried too much about the weights so much as I was about exporting ANYTHING from TOPCAT to the 737. TOPCAT does look at the aircraft.cfg file (as far as that may go).

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I wasn't worried too much about the weights so much as I was about exporting ANYTHING from TOPCAT to the 737. TOPCAT does look at the aircraft.cfg file (as far as that may go).

 

Sorry, that's what I was referring to.  Fuel, passengers and cargo are all weights.  I don't recall any benefits from the linked config other than exporting the weights.  With the MD-11, because the profile in TOPCAT was built from the current PMDG version, you could avoid the PMDG load manager entirely and just use TOPCAT for that.

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One of the big lessons I took away from the first tutorial was that the navdata in the FMC doesn't necessarily match the charts.  Whenever I enter plan and the PERF INIT info, only to see a "NO DES PATH AFTER xxx" I always look at the altitude restrictions in the FMC and compare to the current FAA chart.  For example, yesterday I shot the 25L ILS to KLAS.  The FMC (using Navigraph 1305) gave me "NO DES PATH AFTER LARRE".  It showed

PRINO   8000A

LARRE  6500A

SHAND  3800

RELIN    3800

RW25L  2098

 

The current chart from the aeronav.faa.gov site has

PRINO   8000A

LARRE  6500A

SHAND  4900A

RELIN    3800

RW25L  2098

 
When I changed SHAND to 4900A in the FMC then the message went away.
 
Speed restrictions can cause similar messages.  Often there's a descent restriction like 280/12000, which will throw a message because the default descent speed is 261 on the DES page or, occasionally,  a 250/10000 restriction which trips up the default 240/10000.  
 
So many really cool details.  I've been using this aircraft since Sept 2011 and it has become my hobby.  There is so much to explore and learn, and I haven't even tried the Service-based Failures yet.

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Whenever I enter plan and the PERF INIT info, only to see a "NO DES PATH AFTER xxx" I always look at the altitude restrictions in the FMC and compare to the current FAA chart. For example, yesterday I shot the 25L ILS to KLAS.

 

 

Larry

Once the STAR and ILS at KLAX (24L, 24R, 25L or 25R) are entered into the FMC and executed I always receive  "NO DES PATH AFTER xxx". This particular one was SEAVU and ILS 25L. Most of the time it is RIVIR and ILS 24L or ILS 24R. I have confirmed that all altitudes agree with the FAA charts. I even added the last fix LADLE because it was on the chart and not in the approach per Navigraph  FMS Data. The 727 altitude was calculated by the FMC. 700 was an "INVALID ENTRY" when tried to enter it.

 

This is not a problem since I go to V/S mode around 13000 feet but I get tired of looking at two or three times a week for hours on end.

 

Got any ideas? Just left click the images so you can see the CDU.

 

Thank you

Michael Cubine

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Got any ideas?

 

The system is getting huffy because it knows that you're going to have to decelerate that whole time, while also descending the whole time as well.  It's probably something the real world pilots have their own tricks and work arounds for as well (setting a particular speed on a waypoint, ignoring it and just using speedbrakes to keep the path, modifying the flap schedule, etc - I don't fly/work much over there, so I couldn't tell you what's "normal").

 

 

 


...but I get tired of looking at two or three times a week for hours on end.

 

The "for hours on end" can be avoided simply by leaving off the arrival runway until you start your descent (where you'd get the runway assignment in the real world anyway).  The message may still pop up, but having the runway in there the whole flight is neither realistic, nor is it gaining you any advantage.  Keeping your sanity for the hours leading up to the descent/approach is ideal.

 

 

 


I even added the last fix LADLE because it was on the chart and not in the approach per Navigraph  FMS Data. The 727 altitude was calculated by the FMC. 700 was an "INVALID ENTRY" when tried to enter it.

 

For what it's worth, LADLE is a fix after your glideslope intercept.  As such, while you need to be at or above that fix on the approach, the glideslope will help you meet that.  While adding it could add to your situational awareness, it doesn't add much.  The altitude indicated at 727 only confirms that the calculated path meets the charted restriction (700A).

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The 727 altitude was calculated by the FMC. 700 was an "INVALID ENTRY" when tried to enter it.

 

try with a zero in front (0700). if you enter 3 digit numbers i think the fmc interprets it as flight levels, in this case FL700. that's why you get the "INVALID ENTRY" message.

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try with a zero in front (0700). if you enter 3 digit numbers i think the fmc interprets it as flight levels, in this case FL700. that's why you get the "INVALID ENTRY" message.

Or how about just /7.

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Or how about just /7.

hmm, haven't tried that but i like the short entries. i always enter the altitude restrictions that are below transition level in FL format, e.g. 30 for 3000ft. now that i'm thinking about it what if you just enter 7 or 07? is the slash obligatory for altitudes below 1000ft?

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now that i'm thinking about it what if you just enter 7 or 07? is the slash obligatory for altitudes below 1000ft?
Yep, just "7" should work, too.

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You really don't wanna be bumbling around in VNAV at 700ft unless it's an actual RNAV approach, in which case you don't want to be modifying entries.

 

also 727ft is above 700ft (by 27ft no less). So "Above 700ft" and "At 727 ft" is ... the same thing. No need to change it.

 

Besides you're going to be on APP (Glideslope) mode there anyway, so the VNAV is pretty irrelevant (except for altitude check on slope).

 

If the FMS said "400ft" at that point, then there might be cause for concern (VNAV wants to bust minimums... why? And how do we make it stop?)

 

But in this case, I'd do ... nothing.

Actually no, I'd go into the INIT REF page and enter the expected Gross Weight for my arrival and select a flap/vref speed, which you havn't done (The touchdown speed in the legs page is "---" unselected).

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