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?

A couple of things:

In spite of erroneously using the term 'alpha floor', sounds like a 737 so...

VNAV selection uses FMS speed unless you have speed intervention capability. Using FM speed in a VNAV Path descent can at times result in A/T idle thrust requirement. When the A/T speed mode enters ARM in idle, the auto-throttle system will allow the speed to drop 10 or 15 (don't recall which) knots below the FM speed (if necessary to hold the path) before 'waking up' and reverting to FM speed.

In VNAV, you can't do anything about this with the speed button, but you can select VNAV SPD on the FMS VNAV page if you don't care about the Path, or move the throttles up yourself if you do. A/T system will also 'wake up' with the first flap extension.

System does not have 'alpha floor', that's Airbus, but does have an alpha alert function which will flash the numbers in the speed window and can command A/T thrust. In VNAV you will never see this unless you're using speed intervention and you dial an ill-advised speed.

Now if you tell me it is actually a Bus, which doesn't have a 'VNAV' button as such, I'll start over...:)

additional edit: Sounds like you're using the term 'disconnect' for effecting a 'mode change'. Two different things.

(I stand to be corrected on some of this by a current 737 guy since it's from an aged memory. They're probably all out actually flying.) :}

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.