So, you want to fly a route that doesn't show your plane.
Say a route that is set up for a class 4 plane.
Bid on the route in the schedule then go to PIREP and "Get Flight Info".
The route shows a class 4 aircraft, say the Boeing 767.
But you want to fly the 737-800, a class 3 plane.
Just below the aircraft description there is a blank drop down tool.
Click on the drop down pointer and you will be shown a list of aircraft.
Click on the Boeing 737-800 and click the "Select" button just to the right.
You now have the aircraft you want.
Next make sure you set the appropriate Pax number in the Pax field.
You now have the aircraft you want flying the route you want without having to create a whole new flight plan.
