I'm new to the site and hope that this is the appropriate section of the forum for this question about engaging the FLCH SPD mode via the Boeing 747-400 MCP.

I apologise in advance if it isn't.

Let's assume the following:

1) I've leveled off at an FMC constraint altitude of 4000 ft with VNAV PTH and SPD pitch and A/T modes.

2) There are two other FMC constraints at 9000 ft and 15000 ft. 

3) ATC has cleared me to FL240.

If I dial 24000 in the Altitude window and hit the FL CH button, will it override all altitude constraints or do I have to first delete them one by one with Altitude selector knob pushes?

if the constraints are not overridden, would the FL CH mode even engage? Maybe my thinking is incorrect here, but if it does engage with the altitude constraint still in place, wouldn't that result in a sort of weird situation with airspeed control shifting from throttles to elevators, and the THR A/T mode attempting  to hold current constrained altitude.

it seems more logical to me that FL CH would eliminate all altitude constraints between the airplane and the selected MCP altitude, but I'm guessing here.

Thanks

FLCH won't do anything to your FMC and vice-versa , it just command AP to climb or descend aircraft to whatever you have set on MCP ALT window

The vertical mode that interact with FMC is VNAV, when you engage VNAV, it'll follow FMC constraints, if you want delete FMC constraints, you'll need to push ALT INVT (inner button of ALT kinb) or do it via CDU.

ALT INVT do more than delete constraints, and it's quite a long topic, I'd suggest you to read the FCOM then.

8 hours ago, C2615 said:

FLCH won't do anything to your FMC and vice-versa , it just command AP to climb or descend aircraft to whatever you have set on MCP ALT window

Thanks. I realise  that it won't modify or alter anything in the FMC, but if there are FMC altitude constraints between the current altitude and the MCP altitude, then FL CH will bypass them, will it not?

18 hours ago, Rruffpaw said:

Thanks. I realise  that it won't modify or alter anything in the FMC, but if there are FMC altitude constraints between the current altitude and the MCP altitude, then FL CH will bypass them, will it not?

Yes, it doesn't care about FMC, you can have FMC totally broken but FLCH will still work just fine.