Hi Clark78
The "Night Level (Free or Fixed) in the Preferences screen" must be a Tornado 1.0E addition since I have the Tornado Version 1.0 Manual and there is no mention of free or fixed options for the Night.
Night Levels - there are 4 levels of night.
If you run tornado's FLIGHT.EXE from the DOSBox command line like so:
FLIGHT /a /n1
-or-
FLIGHT /a /n2
-or-
FLIGHT /a /n3
-or-
FLIGHT /a /n4
then you will see that each level brings the night to a later (and darker time).
So I believe that Free means that if you play a mission long enough, the night sky will gradually move from one level to another level, starting from the start time of the mission.
And that Fixed means that the night sky will remain fixed at the night level of the starting time of the mission.
Regards
Frankie Kam
Update:
See this download link
https://www.scribd.com/document/276126728/Tornado-Manual-PC-En#download specifically page 65. That page reads :-
Miscellaneous preferencesWe have introduced one extra option - Night Level. When this option is “Free”,
all four night levels will occur i.e. from dusk to pitch black. In normal ambient
lighting conditions, the darker levels (3 and 4) can make the game difficult to play.
By setting this option to “Fixed”, only levels 1 and 2 will be used by the program"
I just found this out. Haha.
Of course it is difficult to play at Night Levels 3 and 4 since Tornado out of the box didn't come with the NVG (green flourescent) colour views that the modded version has with Shift+Tab (to toggle NVG on and off).
Regards
Frankie Kam