FGD135: Almost correct.
On the B737 it's the case that one can only engage an autopilot if there is no control loading on the yoke.
It is for that reason (
on the B737 for sure) that it's the PF that selects / presses the autopilot button (
as opposed to asking the PNF to do it) as only the PF knows if any pressure is being applied (
by himself/herself) to the yoke. Indeed, the engaging of an autopilot is (
or should be) the only time that the PF ever makes a selection on the MCP when they are the PF (
i.e. during manual flight).
If the PF is applying any kind of control loading to the yoke (
e.g. because the aircraft is out-of-trim) then the autopilot will not engage.
The Boeing B737 FCTM also states: "
The airplane should be in trim, and the flight director commands should be satisfied before autopilot engagement. This prevents unwanted changes from the desired flight path during autopilot engagement."
For a prime example of f'ing up the activation of an autopilot, look no further than the
Ethiopian (ET409) B737 crash out of Beirut on 25th Jan 2010.