Ok, I try out a fighter test.
Test 1
Both Ai and I have 104 unpromoted Fighters. He put all on intercept mode and I strike him. 53 of my fighters got shot down, while 51 made through and shot down the Ai's fighters. Of the 51 that survived, 27 were injured 50% and 24 were unscratched.
My opponent lost 51 fighters and 53 remained. Of the 53 that survived, 25 were injured 50% and 28 were unscratched.
My invading fighters:
Probability: 53/104 = 51% shot down, 27/104 = 26% survived and suffered 50% damage, 24/104 = 23% survived at full health.
Ais defending interceptors:
Probability: 51/104 = 49% shot down, 25/104 = 24% survived and suffered 50% damage, 28/104 = 27% survived at full health.
Conclusion: In BTS, invading fighters and defending fighters have equal chance, because they have equal strength. In warlord and vanilla, I think it was that interceptors always have the edge against invaders. Also, a full health fighter striking an injured (50%) interceptor seems to always win. Finally, injured interceptors will sometime not intercept (perhaps the units strength influences the interception chance)
Test 2
The AI defends with 104 unpromoted Fighters and I have 104 unpromoted Jet Fighters. I lost 22 jet fighters, and 82 survived. Of those 82, 35 were injured 50%, and 47 were at full health. The Ai lost 82 fighters, and 22 remained. Of those 22, 10 were injured 50% and 12 were at full health.
My invading fighters:
Probability: 22/104 = 21% shot down, 35/104 = 34% survived and suffered 50% damage, 47/104 = 45% survived at full health.
Ais defending interceptors:
Probability: 82/104 = 79% shot down, 10/104 = 9.5% survived and suffered 50% damage, 12/104 = 10.5% survived at full health.
Conclusion: Even though Jet Fighters have twice the strength then Fighters, you should expect to lose at least 20% of air units when striking enemy interceptors. It is strange to that in all these air combats, one side will always lose a fighter.