I have been flying the PMDG 737 NGX since release and have never had any real issues until today. As usual completed all the relevant set up took off from EGPF runaway 23 and the autopilot would not engage. I have had situations when it has taken a bit of time for the CMD to "light up" however today I did not want to know, after 15min I gave up.

 

Has anybody got any ideas what the problem was?

 

Marshall Priceman (BAW 073)

A number of things can cause autopilot not to engage. Did you skip a step in the cockpit preparation? Did you turn on the flight directors. Are all of the transfer switches (between yaw damper and fuel pumps) in the center position? How did you last disengage the autopilot? If you pulled on the stick too hard instead of using the switch, you might have broken the servo conector pins ( I don't know the real name for them ,someone will corect me). Check the fmc maintenance page.

Plane in trim and GENs online?

The autopilot can be engaged also with both FDs OFF.

Now, You need to verify some things (in order of common user mistakes)

-Check the joystick, it must be centered (as spin737 said, aircraft trimmed)

-Check FMC for failures about autopilot, it could be possible that you broke the shear pins overforcing the AP when it was used the last time.

-Check IRS for failure or switch position (NAV and already aligned)

-Aircraft power, you need generators online.

 

Other things to check are the hydraulic systems, both on and working or at least the one of the selected AP working.

Thanks for all the support did a trip today and everything worked well must have made a "school boy error" last time however with the detail you guys have supplied should be able to highlight the problem