Hi All,

 

When flying a FMC flight with vnav and a certain altitude is set, say going from 21000 to 17000, does the ALT SEL have to be set to descend to 17000 or does the FMC control the descent without the ALT SET having to be set in the autopilot.  In other words, does one take priority over the other?  Have always been a little fuzzy on this.  Thanks for any guidance on this.

 

Jim

You didn't mention what plane you're flying, but the autopilot mode control panel always takes presidence over the fmc. You need the altitude on the mcp to be below current altitude for any vnav decent. The mcp is the hard limit set by pilots who are aware of the reality of the situation (traffic, atc clearances) and the fmc is fixated on the most efficient ideal flight path. The pilot and mcp always overrule it.

 

Some planes automatically set an altitude hold point when the altitude is changed, others you need to arm alt sel. Same outcome though.

Hi Ethan

 

Thanks for clearing that up for me.  I kinda thought that was how it worked but wasn't sure.  I wasn't referring to any particular plane.  Just the few I have that are Vnav capable.

 

Jim