So, I've watched a few videos but I'm still a bit unsure about what the preset speed option on the front panel controls. When should I be using this option? 

1. This should be set to V2 for take off, always

2. You would use it while flying in level change (LVL CHG button) mode.  Level change maintains a constant speed during either a clime or descent.  For example, during a descent it will pitch the plane down to maintain whatever vertical speed needed to match the selected speed in the MCU.  Now you can do one of two things.  If you make the selected speed higher, your plane will descend faster and if you make the selected speed lower it will descend slower.  You can also use the throttles to make changes to vertical speed in this mode, without changing the IAS/Mach.  For example, let's say you set LVL CHG to move from FL360 to 10,000 feet and your speed is set to 300 kts.  During the descent if you add thrust, the vertical speed will adjust to maintain 300 kts descending, so the vertical speed will become lower.  If you reduce thrust, the opposit happens.

3. You can also use the speed setting to control auto thrust commanded speed on final approach.  

Think of it as automation that you directly control.  

Appreciate it, thank you!

One other thing pushing this knob will change from IAS speed to Mach speed. 

Edited January 15, 20242 yr by Crabby