Your default flight needs to be in a Cold-n-Dark state when you start FSX. I use the default C172 with everything (EXCEPT THE AVIONICS) turned off as my default FSX flight. Here is how (in case you don't know ;)):

Start FSX, select the C172 ans any airport of your choice and press Fly Now.
Once FSX starts, shut down the engine and turn everything, except the AVIONICS SWITCH, off.
Save the flight and place a TICK (checkmark) in the box beside "Make this the default flight".
That's it. It is now the default flight for FSX and every flight will start as Cold-n-Dark with the AVIONICS on.

The reason to leave the AVIONICS switch ON is so that they work from the start because there is no switch in the 727 to turn them ON or OFF.

Mark