Dear Peter, Alex, Gerry and Alastair, Thanks for coming back to me, but I must have asked the question about the inter related AP buttons very badly as nobody answered, perhaps I should rephrase in context - I am trying to put together an autopilot gauge, and part of the process is to send messages to FS whenever an appropriate mouse button is clicked. Sometimes the process is just switching something on or off, but at other times more than one thing needs to be done. For example, when we switch the AP on, we also need to switch on the Wing Leveller and the Yaw Damper. If we switch the HDG on, we need to switch the Wing Leveller off. My question was are there any other inter relationships like this within the AP that I need to be aware of? Sorry I didn't ask very clearly. Alex, thanks very much for your other answers, I now understand what the Yaw Damper does. Regarding the engine gauge questions, I think I need to do a bit more research as there is a wide variation between aircraft types. Reference the braking mechanisms, can I ask one further question please? I understand the autobrakes are armed before landing, and then automatically deployed on landing as you describe. Am I right in assuming that spoilers and airbrakes may be deployed to reduce speed while in the air, but can also be set up to auto deploy on landing, if the aircraft is so equipped? If so, is there any mechanism to prevent the airspeed dropping too low? Gerry, I understand your point about needing the reference to the throttle setting on exit from AT, I have also done a throttle % gauge. The other one I find useful is a pitch trim gauge, on exit from the AP. Alastair, thanks for the reference, I'm off for a quick shufti. Thank you all very much for your help Frank T.