I saw a very interesting simple mission that I downloaded.
This is the exact description listed here in the download section:


NOTES:
I have created the next in, The Most Dangerous Airport Mission Series: namely Ronald Reagan.  This is a mission was real scheduled from Airtran and this particular approaches very unusual.  The Ronald Reagan Visual Runway 19 approach is rated as a dangerous approach.

The runway heading is in line with the White House Grounds and flying over these grounds is restricted.  So what now.  Simple, approaches from the west.  No, not at all.  One starts upstream follow the river and then make a right hand turn very close to the airport.  Good luck.


The above is the full description at Simviation Missions. But, after downloading, this notation is in the read me file:

The following zip files for this mission needs be found on the net and installed:
fsxat378.zip this is a addon for the default Boeing 737-800.Thank you Lutz Krieg for releasing this addon.

I looked through over 150 pages of civil aircraft downloads and found nothing and Simviation SEARCH found nothing.
So what do I do now? Why does this issue happen?

@ Pete and From_Above

I'm fairly certain that if you change the aircraft from the default one for the missions then you are not given the awards and medals related to that flight,


regards

Steve