LNAV/VNAV Question

So I have a question about LNAV/VNAV. If I have my flt pln set up correct(speed and altitude set in the route/legs page) Do I need to input anything on the glare shield? For example If I am flying at FL390 and MACH.084 according to the flt pln. Do I need to enter my ALT of 39000 on the glare shield or will the plan climb to FL390 automaticly? Same question for speed and decent.

Yes, 39k will need to be entered in the altitude preselector to instruct the autopilot to climb (or descend). Same with the speed/Mach. The FMS route/legs page is simply a planning tool. The autopilot panel ultimately does all of the work in conjunction with FMS and LVAN/VNAV commands.

Cool. Thanks. Thats what I thought.

Actually, in the real aircraft you do not enter a speed on the MCP (glareshield).
If you are flying in pure LNAV/VNAV mode, the speed window should be blank.
At least, this is the case with the 737NG, which I fly in real life.

There is a SPD INTervention button on the MCP, which will open up the speed window to override the speed programmed in the FMC.
The autopilot will then pickup the new speed, but it doesn’t recalculate the new vertical path. It just flies the speed, so you might potentially deviate from your vertical planned path.

As far as altitude goes you are correct. Even if there are altitudes programmed in the FMC you still need to select one on the MCP. This also functions as a safety measure so that you only climb or descent through an altitude once ATC has cleared you and not when the aircraft thinks it needs to. :smile:

Additionally, you can enter any altitude in the MCP and that then functions as the hard limit where the aircraft will stop, still, if there are altitude constraints programmed in the FMC on the way to your altitude set on the MCP, VNAV mode will level off the aircraft there until it can resume descent.

The way VNAV works is that the FMS calculates a path from the programmed runway, via the approach and arrival routes towards the aircraft.

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@Yassy Thank you. That made sense.