Scenerio 1: Flight begins at the gate in Turnaround mode. All checklists and procedures are meticulously followed. After T/O,

 

SPD and NAV or HDG mode are engaged, but AP will not (no AP <- symbol on the PFD)

 

Scenerio 2: Flight begins on the runway  in Ready for Taxi mode. All checklists and procedures are meticulously followed. After T/O, SPD and NAV or HDG mode are engaged. Then AP successfully engages (AP <- symbol on the PFD) and A/C responds as expected.

 

What could I possibly be doing wrong?

 

Thank you

Make sure your yaw dampers are turned on. They need to be turned on to engage the autopilot, and they are not turned on in Turnaround mode.

To activate the autopilot the following requirements need to be fulfilled:
• All flight control computers (FCC) are operative
• At least one channel of horizontal trim is available
• At least one yaw damper is engaged
• At least one IRS system is available
• At least one air data computer (ADC) is available
• No significant instability of the aircraft

Manuals.... Manuals.... Its all there.