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I have also done 50hours of co-op/multiplayer and 50+ multiplayer kills, so these achievement *may* unlock as well, if you play multiplayer and get 1 kill. I don't know - if someone tries and report back, I would be grateful. Ace of Aces is the last achievement I got for this game, so if you haven't gotten all the campaign completion achievements, you should get those first, as my save file may unlock achievements out of order. The save file has bunch of skills unlocked already, and when you play a Free Mission, it will unlock a whole bunch of new skills as well as getting you the achievement.

Now, I would like to discuss the save file hex structure that I discovered. If some one is interested in making an editor using my findings that would be great, as long as I get a mention :) If you are only interested in using the save file to unlock the achievement, you don't need to read the rest of this post. You may want to read the following to edit your own save file and not use my attached save file, and that would be fine as well.

First, extract the save.dat from the savedata CON file. You don't need to, but it makes offset addresses easier, I think.
Now search for 00 00 00 24 00 00 00 01 in save.dat. This should occur only once in the file, and for me it's at offset #0820
Each Free Mission data has a size of 44 (2C) bytes.

Grab 44 bytes starting with 00 00 00 01 (offset #0824 for me). It looks like this:
00 00 00 01 00 00 00 04 00 00 00 03 00 00 07 6C 00 01 00 01 00 00 00 03 00 00 00 00 00 00 00 00 00 00 01 2C 00 00 01 7C 00 00 00 01

I will divide this up and look at the individual components

00 00 00 01
is the mission ID/number
01=Nightmare
(there is a mission in campaign called Shockwave, which is the 2nd mission, but is not part of Free Missions so 02 is skipped)
03=Inferno
04=Red Moon
05=Spooky
06=Blue on Blue
07=Power Play
08=Lock and Load
09=Pipeline
0A=Siege
0B=Hostile Fleet
0C=Launch
(0D is not used)
0E=Motherland
0F=Aftermath
(10 is not used)
11=Homefront
12=Hurricane
13=Akula


00 00 00 04
is the number of times you have completed this mission

00 00 00 03
is the mission ranking
03=A
02=B
01=C
00=D

00 00 07 6C
is the mission score

00 01
?maybe this is the aircraft ID that you completed the mission with
07=PAK
1B=Cobra
1C=Hind
15=A-10

00 01
it's either 01 or 02, no idea what it is

00 00 00 03
highest difficulty you have completed the mission
03=Ace
02=Elite
01=Pilot
00=Rookie

00 00 00 00
00 00 00 00
these are always 00

00 00 01 2C
cumulative machine gun bonus
the following missions do not have machine guns, so are always 00 00 00 00
Red Moon (04)
Spooky (05)
Lock and Load (08)
Motherland (0E)
when you get 200 or more machine gun bonus in every mission,
you unlock "Gun Enthusiast" skill which increase MG damage and reduce overheat
this number is cumulative. So if you play Blue on Blue twice,
first time you get 50, 2nd time you get 70, this number will be 120 for this mission


00 00 01 7C
highest armor bonus scored
when you get an armor bonus for every mission, you unlock
"Untouchable" skill which increases firepower of all aircraft

00 00 00 01
mission type
00=Gunship/Door Gunner
01=Fighter or Fighter/Multi or Fighter/Multi/Attack
02=Multi
03=Attack
04=Heli