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alexduskthorn

FMC question about routes

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The FMC cannot override the MCP set altitude. This is a safety measure so that the FMC can't start to descend the plane without ATC clearance and pilot intervention.

 

If you reach top of the last step climb and set your altitude window in the MCP (Autopilot thingy on the glareshield) to say, 14000ft, then the VNAV will automatically descend when it reaches calculated top of descent. Until it reaches 14000ft, then it will level off, even if the optimum profile continues descending. It will then fly at 14,000ft till either the pilot changes the MCP altitude setting below 14,000ft and re-engages VNAV or another mode, or till the aircraft runs out of fuel.

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The FMC cannot override the MCP set altitude. This is a safety measure so that the FMC can't start to descend the plane without ATC clearance and pilot intervention.

 

If you reach top of the last step climb and set your altitude window in the MCP (Autopilot thingy on the glareshield) to say, 14000ft, then the VNAV will automatically descend when it reaches calculated top of descent. Until it reaches 14000ft, then it will level off, even if the optimum profile continues descending. It will then fly at 14,000ft till either the pilot changes the MCP altitude setting below 14,000ft and re-engages VNAV or another mode, or till the aircraft runs out of fuel.

So once you reach the T/D point, with VNAV and LNAV on, it will descent its self?

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yes if you set  the mcp alt to a lower  one  that you have already set

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yes if you set  the mcp alt to a lower  one  that you have already set

Right now I just have my altitude set to FL240 and cruising. Its set in my FMC per waypoint and my A/P while on VNav and LNav, is that enough?

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yes  as long as you set the mcp before the t/d to the whatever you got in your fmc, the plane will descend  to that level you set in the fmc 

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No, if you are cruising at FL240...and you need to be at 11,000 by the next waypiont...altho it may say a lower alt in the FMC, unless you, as the pilot, set the lower alt in the alt window on the autopilot the plane will not decend....

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Can anybody confirm one or two of these "theories" so I know which one I can go by? (Someone other than these two fine people)

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The best answer is given by Trent in the second post, if you  want to find  more precise answer go to your fcoms  and it be in there in more detail

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The best answer is given by Trent in the second post, if you  want to find  more precise answer go to your fcoms  and it be in there in more detail

Where are the fcoms?

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To sum up:

 

If you don't touch anything, VNAV cannot override MCP ALT

Therefore if you are cruising on 240 and have 24000 in the MCP, it will not descend by itself.

 

It can descent by itself, if you dial in a lower altitude before ToD.

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