@mrdetonator
actually, my intention in posting this is not to show that you are wrong... just to clarify the story about those aircrafts. Basicaly, I'm very grateful to you, for clarifying a lot of things, here as well as on some other forums that I'm following. I was just try to share this proper information with you and for your record. And by the way, just to make things clear:
MiG-21U (YAF designation NL-12) received from 1965. do 1967. in quantity of 18 aircrafts (evidence numbers from 22901 to 22918)
MiG-21US (YAF designation NL-14) received from 1970. do 1971. in quantity of 7 aircrafts (evidence numbers from 22951 to 22957)
MiG-21UM (YAF designation NL-16) received from 1977. do 1986. in quantity of 25 aircrafts (evidence numbers from 16151 to 16160 and from 16171 to 16185)
In Yugoslav Air Force meaning of local designations are as follows:
L - Lovac (Fighter)
NL - Nastavni Lovac (Trainer-Fighter)
N - Nastavni (Trainer)
J - Jurišnik (Attack)
NJ - Nastavni Jurišnik (Trainer-Attack)
T - Transportni (Cargo)
@babcia131
So, I suppose that you are the "Guest" on sgavia, that bring us the last minute help? Actually, you gave me an idea to check the diagrams and to step out with idea of additional cockpit air-conditioning. But still, some of the guys have an idea, that this could be for better cooling some of the components, or more interesting, that it's possible that this could be the system filled with freon, as a classical air conditioner, which can provide the the cooling even if the plane is static.