VNAV approach
 

Hi there
I was flying a LNAV/VNAV Approach and when disconnecting A/T using speed switch it refused,anybody to explain me why is that?

Might need more info.

What kind of equip?

Are there no A/T disconnect switches on the thrust levers?

Sounds like a 737 question.

You were trying to directly go into the ARM mode? Maybe that isn't compatible with VNAV.

There's an A/T disconnect switch but the entire approach was carried out in LNAV/VNAV,by pressing the speed switch i was trying to disconnect A/T on Arm mode just for the alpha floor protection,my question then is:why is this imcompatible with VNAV?

As VNAV controls both pitch and thrust, I guess the SPD selection is incompatible as it doesn't know what you want to do with the pitch mode.

Minimum speed reversion is available in VNAV, so there's no need to try to change mode.

Minimum speed reversion in while in VNAV will revert to a FLT CHG mode on the classic. Where the TL go to is dependent on the MCP ALT setting.