From HTC's scoring page:
Air Categories
Kills per Death = TotalKills / ((Discos * 0.5) + Bails + Captures + Deaths + 1)
Kills per Sortie = TotalKills / TotalTourSorties
Kills per Time = TotalKills / TotalTourSortieTime
Hit percentage = TotalTourBullet**** / TotalTourBulletsUsed
Air points = PlaneDamageScore + KillScore
Ground Categories
Damage per Death = TotalDamage / ((Discos * 0.5) + Bails + Captures + Deaths + 1)
Damage per Sortie = TotalDamage / TotalTourSorties
Bombing percentage =
(TotalTourBomb**** + TotalTourRocket**** + TotalTourTorpedo****) /
(TotalTourBombsUsed + TotalTourRocketsUsed + TotalTourTorpedosUsed)
Ground points =
(DamagePointsScoredOnObjectsTh isSortie * DeathMult) +
TotalTourDamagePointsScoredOn Objects +
(DestroyedPointsScoredOnObject sThisSortie * DeathMult) +
TotalTourDestroyedPointsScore dOnObjects
Field captures = FieldCapturesThisSortie + TotalTourFieldCaptures
TotalKills are just air-to-air kills for fighter missions, air-to-air + air-to-ground kills for all other mission types.
PlaneDamageScore =
(DamagePointsScoredOnEnemiesTh isSortie * DeathMult) +
TotalTourPlaneDamageScoredOnE nemies
KillScore =
((AirKillsThisSortie * 1.0) + (AirAssistsThisSortie * 0.25) +
(GroundKillsThisSortie * 1.0) + (GroundAssistsThisSortie * 0.25) * DeathMult) +
TotalTourKillScore
TotalDamage =
(DamagePointsScoredOnObjectsTh isSortie * DeathMult) +
TotalTourDamagePointsScoredOn Objects +
(DestroyedPointsScoredOnObject sThisSortie * DeathMult) +
TotalTourDestroyedPointsScore dOnObjects
Death Multipliers
Landed
Discoed
Bailed
Ditched
Captured
Killed
Crashed
1.0
0.25
0.5
0.75
0.4
0.25
0.25
Once you get past a certain K/D and K/S, you are pretty much on a flattish curve. You can have a drastic improvement in either category for not much gain in "rank" in that category. K/T is a little less stratified, but not much. Hit % is much the same.. once you get past ~15% you can go all the way to 30% and maybe only pick up 10 "spots".
Figure two pilots, A & B.
Pilot A has a K/D of 20 and is ranked 2nd, a K/S of 4 and is ranked 10th, a K/T of 12 k/h and is ranked 50th, a Hit % of 15% and is ranked 20th. I'll cover points later. Pilot A's "overall" rank will be the addition of all the "sub-ranks".
So 2 + 10 + 50 +20 = 82. So if you don't count the points category he has 82 "rank points" for lack of a better word.
Pilot B has a K/D of 6 and is ranked 15th, a K/S of 5 and is ranked 4th, a K/T of 15 k/h and is ranked 2nd, and a hit % of 13% which is good for 65th.
So Pilot B has 15 + 4 + 2 + 65 = 86 "rank points".
Pilot A would be 4 points ahead of Pilot B, and probably be 1 or 2 ranks higher in the fighter category.
But now lets add in the points category. Lets say Pilot A has only flown 5 sorties over the course of an hour and 40 minutes. Pilot B has flown 200 sorties with lots and lots of time. Pilot B would have many more points than Pilot A, and thus be ranked higher.
Second case, lets say Pilot A and Pilot B both fly one sortie. They both get 5 kills. Pilot A lands, Pilot B runs out of gas and ditches (in friendly territory). Overall K/D and K/S are the same, but the points (even if they literally did the exact same thing overwise, down to damaged planes and assists) would be 25% higher for Pilot A. In this way, "flying to live" is rewarded.
The way I look at it is you get 1X points for dying or crashing, ~2X for bailing or being captured (.4 for capture, close enough), 3X for ditching, and 4X for landing.