According to a new internal assessment produced by the U.S. Air Force, the branch may be forced to cut their plans for a fleet of F-35 Joint Strike Fighters by more than a third due to the high cost of operating and maintaining the aircraft.

While this report does not represent anything close to a final decision regarding the total number of aircraft being ordered, it does offer a glimpse into the exorbitant costs associated with keeping what has been touted as the most advanced fighter platform in the sky. According to the estimates within the report, which was first revealed by Bloomberg News, the Air Force may soon face a decision between finding a way to cut operating costs of the F-35 program by a whopping 38%, or they’ll be forced to reduce their standing order for aircraft by 590 jets. This revelation has compounded concerns about the number of delivered F-35 platforms that have been listed as “non-operational” — which is nearly half of them.

Considering the Air Force planned to field a total of 1,763 F-35s by the program’s completion, cutting that order by 590 would not only represent a significant reduction in operational forces, but could also drive up the per unit cost of building each aircraft, effectively stymying at least some of the saving associated with reducing the order in the first place. It would seem that the Air Force may be stuck between a rock and a hard place, except that in this case, both the rock and the hard place represent billions of dollars in taxpayer money.

The Department of Defense currently plans to procure a total of 2,456 of the fifth generation fighter. Current projections place the overall cost of delivering these aircraft to be at around $406 billion, making the F-35 the most expensive weapons platform ever built, but even that massive number is currently dwarfed by cost projections for operational maintenance on the aircraft, which is estimated to be $1.1 trillion over the aircraft’s service life. That would break down to costing the Air Force alone somewhere in the neighborhood of $3.8 billion per year to keep the F-35s flying.