ORIGINAL: szmike
There was information having 1-2 pilots with good skill improves gains, but I don't know if it holds true with all the changes over the years.
The number of pilots and Exp of the pilots that are required varies. The key is that the high skill pilots raise the average experience of the unit. In my screenshot above, the numbers in the parenthesis above the skill heading are group averages. In that example, all the pilots below 38 Exp will get a bonus when training. The 6 x 62 exp pilots cause the group average (38) to be greater than the rest of the pilots in the squadron (35/36). All of the 35/36 Exp pilots will get a bonus when training.
Reports that suggest adding highly skilled pilots does not help in training are likely due to the fact that most players pull their pilots in from the replacement pool and take whatever they get.
Consider a squadron with 20 pilots drawn from replacements with the following Exp:
(27, 29, 29, 30, 31, 32, 32, 32, 32, 33, 34, 34, 35, 35, 35, 35, 36, 36, 38, 39).
Group Exp = 27 + 29 + 29 + 30 + 31 + 32 + 32 + 32 + 32 + 33 + 34 + 34 + 35 + 35 + 35 + 35 + 36 + 36 + 38 + 39=664
664 / 20 = 33 (fractions round down in virtually every calculation in WitP:AE)
Therefore, in that group the pilots with Exp = 27, 29, 29, 30, 31, 32, 32, 32, 32 will get a training bonus.
If I add a 70 skill pilot,
Group Exp = 27 + 29 + 29 + 30 + 31 + 32 + 32 + 32 + 32 + 33 + 34 + 34 + 35 + 35 + 35 + 35 + 36 + 36 + 38 + 39 + 70 = 734
734 / 21 = 34
Therefore, with the extra experienced pilot, the trainees with Exp = 27, 29, 29, 30, 31, 32, 32, 32, 32, 33 will get a training bonus.
If I add 2 x 70 skill pilots,
27 + 29 + 29 + 30 + 31 + 32 + 32 + 32 + 32 + 33 + 34 + 34 + 35 + 35 + 35 + 35 + 36 + 36 + 38 + 39 + 70 + 70 = 804
804 / 22 = 36
Therefore, with two experienced pilots, the trainees with Exp = 27, 29, 29, 30, 31, 32, 32, 32, 32, 33, 34, 34, 35, 35, 35, 35 will get a training bonus.
The problem is that anyone looking for those effects will be looking at a very small change in experience gain and conclude the bonus does not exist. In the example above, one experienced pilots gets precisely 1 pilot a bonus, while in the 2 pilot example, 7 pilots will get the bonus.
This is why I stratify my pilots. Consider the same example with all 35 skill pilots:
20 x 35 = 700
700/20 = 35 group Exp
20 x 35 + 70 = 770 = 36
All pilots get bonus from one experienced (exp = 70) pilot.
The bigger the squadron, the less experienced the "good" pilots, the wider the gap between the most and least experienced trainees, the more "good" pilots you need to maximize the bonus.