You make the cockpit glass both inside and outside type 6 elements in the 3dz, that makes their transparency dependent upon the colour you skin them with in the tra.pcx file. From light to dark in the tra.pcx has the result of transparent to opaque on the element on the plane.
Ah, ok. Right after I posted that, I think someone told me to use the gray colors instead of just black or white from the transparency skin palette. I didn't know you could do that, ROFL.