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.
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'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'.