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dogs53

Altitude Questions in MCP

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Hi All:I have a question on the proper use of the ALT Settings on the MCP.I am flying both the PMDG 737 and 747-400, and similar problems occur in both. I know that I am not doing something, or am doing something wrong and have searched the forums and find issues that touch on my problem, but I don't have a clear understanding of it.I have also downloaded several tutorials, and have two of Mike Ray's books, but still I don't quite get it.Here is the problem...When flying after having programmed the FMC, and after I get to top of descent.. either the aircraft doesn't descend or descends rapidly. I think that I should set the ALT to the next cleared lower ALT, but if I just set it to RW ALT, it seems to want to go down immediately, in spite of VNAV being on.Can someone explain the correct procedure to use for setting the ALT?Thanks in advance.Bob

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You are correct, set the ALT to cleared lower altitude. If you are in VNAV mode it will maintain altitude until you reach TOD. It will then descend on VNAV PTH. If you don't reduce the MCP ALT before reaching TOD the FMC will give you a reminder to CHK MCP ALT.The descent from high, say FL360+, can seem very rapid unless you are used to it. Keep in mind the air is real thin and she is descending to reduce fuel so to keep the speed up it takes 2800+ fpm descent.


Dan Downs KCRP

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>You are correct, set the ALT to cleared lower altitude. If>you are in VNAV mode it will maintain altitude until you reach>TOD. It will then descend on VNAV PTH. If you don't reduce the>MCP ALT before reaching TOD the FMC will give you a reminder>to CHK MCP ALT.>>The descent from high, say FL360+, can seem very rapid unless>you are used to it. Keep in mind the air is real thin and she>is descending to reduce fuel so to keep the speed up it takes>2800+ fpm descent.>>Thanks.. but if no ATC is up, can't I just put in the ALT for the field and then let it descend as needed? Or to put in the intercept ALT for the Outer Marker or similar. It seems that when I do this, it just descends below the Glide Slope. Is it better to just "step it down" in increments? Tks again.

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Bob,if no ATC altitude restrictions apply you can set it as low as you want yes. However NEVER set the MCP alt to a lower value than your final approach fix (FAF) before you actually start to descend from there. So if this is an ILS approach you can set your MCP alt to the G/S intercept altitude right from CRZ. The aircraft will then fly your programmed route including the STAR and all programmed altitude restrictions should you use VNAV. If you use FL CHG the autopilot will just descend to the MCP alt with the engines at idle all the way down. If you fly a published STAR using VNAV without setting MCP alt below the FAF nothing bad can happen and the airplane will not descent below the glide slope until you are allowed to descend on it according the approach procedure. You will then use the APP mode if on an ILS approach which means the next change on the MCP alt will be to set the missed approach altitude.Of course things will get more complicated depending on the STARs and approaches you fly, especially when doing non precision stuff like an NDB or VOR/DME approach.Regards,Markus


Markus Burkhard

 

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