I often find that swopping downloaded planes during flight causes the program to crash, necessitating a re-start...
...but, as previously stated, if I temporally switch to a default Cessna 172 first, then select my downloaded plane after, it sometimes cures the problem...!
FS9 doesn't seem to like swopping certain planes while the program is running...

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Cheers...!
Paul...

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Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.