Strange problem with vnav

in boeing 747-800 intercontinental everything is setup for for a vnav/lnav flight but the autopilot after vnav spd show vnav pth but the aircraft neither ascent neither descent neither change speed is there some thing wrong here?

thank you

sarantopoulos nikos

Are you preselecting a lower or higher flight level on the MCP? if you dont change the alt vnav will just hold u where u last set it.

everything is set correctly on the mcp in the init ref pref init the cruising set to 10000 where it should be can you explain me better what i must do beside this steps

MCP is the Mode Control Panel. Its on the glare shield. It’s what programs your auto pilot. If you dont pre select an altitude higher or lower than your current altitude, VNAV won’t do anything.

please explain me with details because i cannot understand mcp you mean the fmc flight managment computer or flight managment system?, if it is that one please tell me where and what to press to engage vnav function as i said it shows vnav pth after vnav spd but it does nothing sorry for the missundestanding but i am confused a tutorial about that?

thank you

nikos sarantopoulos

show me if you can in i image what i must press if the altitude in the mcp is 10000 and crusing alt in init pref of fmc it will work the vnav?



where to pre select it?
in fmc or mcp?

Set the altitude in the MCP to the altitude you want to obtain.

Mcp is big panel all the hardware buttons on it in front of you, vnav, lnav, hdg, alt, cmd etc. Altitude is the pilots override for all other systems when ap is on.
Whatever is set in fmc, flight computer, the following will happen
If you are climbing and alt set is higher than current alt it will level off at that altitude.
If you are descending and alt set is lower than current alt it will stop descending and level off at that alt.
If you are cruising at same altitude that alt is set at on mcp, the aircraft will neither climb or descend until you change it manually.
This is where it gets complicated! If you don’t turn that knob BEFORE the descent or climb is programmed to occur in flight computer then it won’t move even if you change it. You can fix this by pressing level change lvl ch. It’s not ideal but it should work. The better way to do it is alt int tiny button on 737 but I don’t know if 747 has it.
To be safe, set that dial well in advance and you will be fine!
For simplicity, before take off set it to 35000 or cruise you want then a safe time before you descend set it to 1000 or decision height for the airport. You don’t want the plane to fly into the ground!