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TomCYYZ

Learn something new everyday!...FMC CRZ function

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This may be helpful to those that have not come across this type of "pilot error" before. Maybe it's already been covered in this forum. Anyway, I am currently flying from KSEA to PANC for UVA (United Virtual Airlines) in the 737-900 and when I setup the FMC I chose FL320 (which was wrong to start, s/b FL310/350/390 etc). Now, during flight my "MCP VNAV" light goes off and I keep getting a message "CHK MCP ALT". I'm scratching my head, "What's going on here?"... I set the correct MCP ALT (FL320). But, "hold on there cowboy, remember you revised your MCP altitude about 100nm back, to FL350?"..."Oh ya, doh, I have to go to the FMC "CRZ" page and change my "CRZ ALT" to FL350 from FL320". Now when I click on the "MCP VNAV" button it activates and everything is in line with the universe once again. So, I now know, whenever I change the MCP ALT to a "higher" altitude during the cruise mode of the flight, I must remember to change the "FMC CRZ" altitude on the CRZ page to keep VNAV happy.Regards,Tom Stewart

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Hi Tom,The way I read the tutorial by Timithy Metzinger is that in VNAV mode all flight levels are attempted but the alt on the MCP will always constrain it.For example consider a STAR where you have a stepped descent. If you have programmed this into the FMC so that it decends from 7000 to 5000 and finally 2500 and you only set the MCP to 5000, it wont continue to the next step down after the descent to 5000 UNTIL you set MCP to 2500.This of course is if you have VNAV on.I hope this is a) correct and :( making sense, but its the way I read it.Simon Rigby

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Thanks Simon, yes I do the same on the descents as you have described, but it's just that I have never ascended to a higher altitude before using the PMDG FMC. It was a new experience for me, and a learning one I might add...LOLRegards,Tom Stewart

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Simon and Tom,Simon's explanation is correct, but there are other ways:1 - Verify the LEGS page, and you'll see that each Leg has a speed and altitude. The altitude is fixed when you plan it, but will vary if you don't follow the right steps.2 - In a case where a Fix is at 5000 and the next at 2500, you may change altitude in the MCP to 2500. VNAV will level at 5000 and than descend to 2500 at the proper time.Neat, isn't??Regards from BrazilBPilot

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Thanks for clarifying. Indeed that was what I was trying to say but I can see where the ambiguity would be.We're all soooo helpful. :)Simon Rigby

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Hello Simon,same technique you can use for NDB approaches. Create a fix short before the treshold with 50 or 100 feet high over the runway elevation. Create other fixes corresponding to your charts. Enter in the LEGS page speed and altitude for these fixes.Ariving FAF in ALTHLD and in Landig Configuration. Stay in LNAV and use VNAV or step down using V/S. You can control path with VNAV Path indicator. Best regardsStefan

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You have another option:dial the new altitude in the mcp, and then push the "ALT INTV".BRClaudio

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