Scores of US-owned Lockheed Martin F-35s would remain in the fleet with a software operating system rated below combat-grade under one of several cost-saving proposals under review by the Joint Programme Office.

Delays during the development stage caused Lockheed to deliver more than 108 aircraft with Block 2B software. Each fighter requires 150-160 modifications to be raised to the combat-rated Block 3 standard, says Vice Adm Matt Winter, the F-35’s programme executive.

The looming modification bills are threatening to suck resources from a looming production ramp-up with more than 900 aircraft projected for delivery over the next five years, Winter says.

“We’re looking at solution spaces to give our warfighters options,” Winter says.

One of those options is to keep a subset of the F-35 fleet at the Block 2B software standard. It would follow a practice used on the Lockheed F-22 programme, which has about 30 fighters maintained at Block 20 for training missions and about 150 fighters using the go-to-war Block 30/35 standard.