The Air Force doesn't have enough fighter pilots in its active duty cockpits due to force structure cuts and increased airline hiring, leading to a long-term drop in those available to train new pilots and test new aircraft, service leaders told lawmakers Thursday.

The active duty Air Force is 520 pilots short of its total manning requirement, with that projection expected to get worse in the near future due to an increase in private airline hiring luring the pilots away and continued cuts to the service's fleet, according to joint testimony to the Senate Armed Services committee by William LaPlante, the assistant secretary for acquisition; Lt. Gen. James Holmes, the deputy chief of staff for strategic plans and requirements; and Lt. Gen. Tod Wolters, the deputy chief of staff for operations.

"The shortfall evolved from force structure reductions that cut active duty fighter squadrons and fighter training squadrons to a number that cannot sustain billet requirements," the testimony states. "As a result, the Air Force is currently unable to produce and experience the required number of fighter pilots across the total force."

The service is prioritizing its overall manpower to keep operational cockpits full, causing a drop in air operations expertise during higher-level planning and a drop in fighter pilot experience for trainers and test programs.

"Without these fighter pilots, the Air Force will be very challenged to continue to provide the air supremacy upon which all our forces depend," the testimony states.

The Air Force projects that the airline industry will hire about 20,000 pilots over the next 10 years, and with changing requirements, the companies will target military aviators at an increased rate, Air Force Chief of Staff Gen. Mark Welsh told the Senate Armed Services Committee on Wednesday.

Budget restrictions have limited the total amount of flying hours available for flying, causing frustration among pilots. This frustration will return if sequestration comes back, Welsh said.

The Air Force uses big bonuses to try to keep its pilots around. The service offers Aviator Retention Pay payouts for eligible pilots who agree to serve for nine more years, at a rate of up to $225,000. Fighter pilots, other valuable pilots and combat systems officers who sign up for five more years can also get a $125,000 bonus.

About 840 airmen were eligible for the bonuses last summer, including 245 fighter pilots. In 2013, 132 fighter pilots signed up for the bonus, with 82 percent taking the nine-year contract. Another 483 total pilots took the five-year bonus.