B 737 NG – Does a Ground – Air SW failure affects Cross Bar behavior?
 

Hello,

On our simulator, if we arm a Ground – Air SW failure and then take off (with LNAV armed), the navigation display (magenta line) is correct but the cross bar maintains present heading and does not follow the magenta line (LNAV is green). Is this so or is this a failure of the simulator software?

Thanks!

I'd imagine it would screw it up, especially VNAV

VNAV ... yes ... but LNAV?

Obviously the Magenta line would be correct on the ND as the position info would be ok. However the Aircraft thinks it's on the ground so it would not engage LNAV. Therefore the Roll FD bar would not be correct.

LNAV arming criteria on the ground:
• origin runway in flight plan
• active route entered in FMC
• track of first leg within 5 degrees of runway heading
• LNAV selected prior to TO/GA.
• LNAV guidance becomes active at 50 feet AGL

Pressing TOGA gives wings level/ HDG and after rotation at 50 feet LNAV guidance would become active if the above criteria is met.
If the Air ground sensor fails ( indicated by take off page not changing to climb) the A/C thinks its still on the ground and LNAV would not be available automatically, you would need to select the active waypoint and execute in the legs page ( above 400 feet) .

VNAV climb profile would not be available unless the A/C knows the active waypoint which determines the altitude and speed profiles.

These types of failures rarely look like the sim anyway. They are normally partial with systems not behaving as usual and take a while to diagnose.
The only real way to be prepared is to have good systems knowledge.

Thanks to everybody for your fast answers!