The 6 top aircraft manufacturers in the U.S. : Boeing, Lockheed Martin, United Technologies, GE Aviation, Northrop Grumman, and Raytheon is called US aircraft primes. Thus, prime means the most valuable, the most important and the most influential.

'primes' is referring here to the top aircraft manufacturing companies. The word is derived from 'primary'. So 'US aircraft primes' = 'Top/primary US aircraft manufacturers'.