NewGRFs » World Airliners Set

For freight planes:
use Action0 property 18 and 19;
18 \wxFFFE
19 \wx0001

for tourists, pax, mail and valu:
18 \wx000B
19 \wxFFF0

However: this will let the tourist capacity be rather low... (1/4th of pax).
If you use VarAction2 variable 47 you can set the correct ammount if linking it to Callback 15 (Remember to activate it action0 property 1E)

the combination would be something in this direction:
-1 * 0 02 0x nn 81 47 10 FF 04
\bPP 80 01 01 // Pax capacity
\bTT 80 05 05 // Tourist capacity
\bMM 80 02 02 // Mail capacity
\bAA 80 08 08 // Valuables capacity
\b00 80 // Everything else

then link nn to a callback15 in your callbacks line...
ps. If the value (PP, TT etc) is more then 255 this is no problem. Lets say you want 500 ... 500-256 = 244 ...

\b244 81 01 01 ... note the 81, the "1" means 256 ... 700 is \b188 82 (2=2*256=512, 512+188 = 700)

If that were unclean, just drop me a note on IRC or MSN :)