You are right when you say it is not documented but I did a lot of tests today and I realized that the only way to use it by itself is by deselecting N1, selecting and deselecting VNAV, then you can select mcp speed alone and it will control mcp speed.

Hmmm. It is documented, just not the way you want to use it. Reading between the lines, SPEED is just not a compatible AT mode for the TOGA pitch mode. If you select VNAV, then deselect VNAV you are removing the pitch mode is is probably why you can select SPEED.

then you can select mcp speed alone and it will control mcp speed.

This point I don't get. You do know that the speed of the airplane is already being controlled through pitch commands? If you want the MCP SPD FMA, just select LVL CHG or VNAV and SPD INTV (edit to say that SPD INTV does not show MCP SPD, but does the same thing). By forcing the selection of SPEED you are permitting less than full N1 thrust during a climb! If you want to achieve that just select VS, then the SPEED button will illuminate automatically and you can pitch down at less than full climb capability and thus reduce thrust for the climb.

 

You are trying to operate the AFDS in a way it was not designed too and possibly in a way that would not work in the real aircraft anyway.

 

I too am confused at what you are trying to achieve in terms of the departure. If you are trying to understand the modes better then fair enough

 

But as you can see, the FMA MCP SPD simply means the AFDS will attempt to achieve the speed in the MCP window in accordance with the selected pitch mode. With the SPEED illuminated, it gives the AT permission to adjust thrust to achieve the bugged speed. If you pitch down, the thrust levers will retard if you pitch up they will advance. So in terms of AT/PITCH mode FMAs, MCP SPD/VS and MCP SPD/ALT HOLD are a completely different AT modes than MCP SPD/TOGA or MCP SPD/MCP SPD even though the AT mode FMA is MCP SPD in all cases.

 

Another edit to say, looking at some of the above replies, it looks like I am bit late to the party

I do have a feeling this topic will be soon closed :-) and before it does..

Did any of you wacth the Ifly takeoff at the end of the video? How come I can select speed there not in the same way as in the Pmdg ng?

Who models this correct?

This is so funny!

 

 

 

David

Did any of you wacth the Ifly takeoff at the end of the video? How come I can select speed there not in the same way as in the Pmdg ng?

I say that iFly is most definitely wrong because TOGA is a pitch mode. If you removed the pitch mode, then I can see SPEED being selectable, as you showed in your previous screen shot.

 

I think that answers it

Another edit to say, looking at some of the above replies, it looks like I am bit late to the party

 

Not at all. It's always nice to have one's theoretical musings confirmed by a real-world 737 driver.

I say that iFly is most definitely wrong because TOGA is a pitch mode. If you removed the pitch mode, then I can see SPEED being selectable, as you showed in your previous screen shot.

 

I think that answers it

 

Ok thanks for your patience!:-)

 

 

David

Summing up:

 

in an airplane, you can be in two "modes of operation":

-Pitch adjusts speed, power adjusts rate (normal)

-Pitch adjusts rate, power ajusts speed (FS "normal").

 

Apparently you cannot be in both modes at the same time.

TO/GA, LVL CHG and VNAV Climb are all "mode 1" autopilot modes, and they adjust speed by setting pitch. Power is set at N1.

SPEED is "mode 2" autopilot mode. For power to be set at whatever is needed to get the speed, you need to adjust pitch to a given rate. That is done by commanding a given vertical speed (by V/S mode, VNAV PATH, ALT HOLD mode, also CWS works).

Summing up:

 

in an airplane, you can be in two "modes of operation":

-Pitch adjusts speed, power adjusts rate (normal)

-Pitch adjusts rate, power ajusts speed (FS "normal").

Unfortunately FS ATC assumes you're using that second one, and barks at you for not following their instructions quickly enough if you use the first one.