VNAV speed control disengages under specific conditions while on ground, during or shortly after takeoff

[intelfx]

If you arm NAV and VNAV and subsequently give speed control to FMC (magenta speed bug) while on the ground, OR in the air if the magenta speed bug is limited by Vfe, then doing any of the following:

• turning the HDG preselector
• turning the ALT preselector
• taking off

...disengages VNAV speed control and reverts the speed bug to blue.

Edited April 29, 2022 by intelfx

[ois650]

Turning the HDG selector and taking off - sounds like correct behaviour. Changing selected altitude shouldn't kill managed speed. Will take a look at this

[intelfx]

6 hours ago, oisin650 said:

Turning the HDG selector and taking off - sounds like correct behaviour.

Is it described in maintenance documentation or something else that I don't have access to? The FMS Operator's Guide only says this:

Quote

The PFD reference airspeed bug also returns to the manual setting when the FLC flight director mode is selected by the crew or by the FMS. Other conditions, such as overspeed, can also return the airspeed bug to a manual value.