Originally Posted by
Dan64456
I
Anyway the bug on the tape is the reference for the autopilot. When you select mach .80 or something on the autopilot and you are at fl350, it will 'reference' 272 kias on the airspeed tape. (Airspeed Tape is the left strip that tells your KIAS on a PFD in a glass cockpit)
The Speed knob on the bus has a mach/ias selection. When in Mach the airplane attempt to maintain that mach. This works basically above about FL310 where the transition on the way down is from Mach to IAS. If you set .82 in the speed window the bug should follow on the airspeed scale. As you descend the bug will move on the scale reference to where .82 is and at around FL310 it will stop moving and be at the edge of the red on the speed tape.
Often in flight if you are flying at MMO (.82) and hit a bump in turbulence you can trigger an overspped warning. When I was flying 320's we used to use .80 as our planned speed. I understand they have backed off a bit now to save fuel.
L