Subject to confirmation as I am not current on the machine,
I think the FCOM quote says it all. As well, while in VNAV path with the thrust levers in hold, once the speed decreases to 15 knots below command speed, the autothrottle wakes up and SPD mode is activated.
With airspeed unreliable, merely disconnecting the autothrottle is not part of the procedure(which would leave it armed). Instead the Autothrottle Arm switch itself is turned off depowering the entire system. I believe this is to prevent the wake up feature if indicated airspeed erroneously displays too low.
Last edited by JammedStab; 20th March 2016 at 18:08.