Window closed the BUG can go anywhere between MERS and -5 (VMO/MMO/VFE) the flap bugs mean nothing.
Ie, you programme 180kts the BUG will go straight there if it's above MERS and below -5 (etc) if not it will rest on the MERS stick or sit at -5 and go to 180 when it's allowed.
A simple way to show it is to pull the speed brake while descending at highish altitude with the BUG near MERS (ATC slow down and go down) as MERS increases with speed brake extension it will carry the BUG with it = safe!
Window open gives you "airspeed low" if fitted as MERS goes above the BUG.
You see the -5 VFE example every departure as the aircraft tries to accelerate to 250kts or +100
Window open can go outside MERS and VMO but the autopilot will try not to obviously.
Last edited by Avinthenews; 23rd Sep 2015 at 17:29.