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ICSimulations

VNAV altitude not resetting due to VNAV page ALT change mid flight.

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Greetings! I performed a flight from KBOS to EHAM. The flight was overnight and when I revisited the sim and set the MCP ALT to a lower amount so I can descend via VNAV. I noticed nothing was happening. When I went to the VNAV page I noticed VNAV Alt was set at 8000 and not FL350. All of the preparation work was done using PPFP as I am teaching myself ETOPS operations. Upon further review of the legs page I noticed three waypoints in the start with 8000 for the altitude. Any ideas of why this happened? I reset the VNAV cruise from 8000 to FL350 and then used the MCP ALT and pressed it in to start the VNAV SPD mode for descent. 

 

Thank you.

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A couple of question to get a clear picture:

_ What altitude/level did you fill in the perf init page?

_ What altitude did you enter in te MCP altitude box?

> What was the status of the aircraft when you left your computer for the night:

_ What was displayed on the FMA (SPD, LNAV, VNAV PATH/VNAV ALT)?

_ What was the altitude of the aircraft?

_ And probably not significant, but is the auto S/C selected?

 

When you say VNAV ALT on the VNAV page was set to 8000, so you mean the CRZ ALT in the ACT ECON CRZ page?

That value is a manual entry and is not modified by the FMC itself.

There are only three ways to alter that value:

_ When entering an altitude/flight level in the perf init page, it is reported automatically there.

_ Entering manually a value in the  ACT ECON CRZ page

_ Entering a new value in the MCP altitudue window and pushing the Altitude select button

 

So if you read 8000 there, it is probably because it was entered as such in the perf init page. But it that case, the aircraft would have not climbed to the FL350 if it is the case.

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