Edmund. You can do this by manually editing fs9.cfg. The location of this file will depend on your OS. Look in Readme.rtf in the FS9 main directory folder for more details.

Find fs9.cfg, open it in Notepad & look for the [FrameCallNoWarn] header. Your aircraft should be listed under there like this.

[FrameCallNoWarn]
P51B_m=0

Change the value after the = sign from 0 to to 1. Exit Notepad saving changes. That should do it.

PS. I just realised this is in the software section. I assumed you're asking about FS2004 (FS9).

There are several versions before FS95. My first version was SubLogic Flight Simulator on the C= 64. It had very basic graphics and a choice of a Lear, a Cessna and a Sopwith Camel.

Sometimes you might also see the CFS titles given half-version numbers, ie FS7.5, but ths has mostly gone out of practice.