The US Air Force and Lockheed Martin have identified a fix for fatigue cracks found in more than 80 of its two-seated F-16D fighter jets.

The air force in August grounded a portion of the 969-aircraft fleet when an immediate time action compliance technical order (TCTO) found structural cracks in the canopy sill longerons between the front and rear pilot seats.

The cracks were found exclusively in the F-16D variant, of which 82 were eventually grounded until repairs could be made. The anticipated permanent repair will involve installing a strap over the cracked longeron, rather than replacing them, according to Sue Murphy, a spokeswoman for Air Force Materiel Command.

“The scope of work will involve removal of cockpit equipment to facilitate repairs, installation of the repairs, and reinstallation of the cockpit equipment,” she says in an email to Flightglobal. “The longerons will not be replaced unless needed for other reasons (which at this point it not anticipated to apply to most jets).”

Two groups of US F-16s were found to have cracks, 39 which are on average 27 years old with 6,455 actual flight hours (AFH) and 7,016 equivalent flight hours (EFH). AFH simply lists the hours an individual aircraft has flown, while EFH measures the fatigue life of an individual aircraft based on an operational assessment.

The second group of 43 aircraft average 21 years old with 5,934 AFH and 4,867 EFH. Cracks were also found in some F-16s flown by other nations.

Lockheed is working with the air force and with other nations who have found cracks.

“Lockheed Martin is actively working with the US Air Force on a permanent repair solution for the canopy sill longeron cracks,” a company spokesman says. “The repair drawing, as it is known, is now available and we will continue to support the Air Force as it works to implement, manufacture and verify the repair installation.”

Murphy said the cracked longerons had not undergone structural improvements during the Falcon Star upgrade program intended to extend the aircraft service life to 8,000h. The affected areas are “original to the aircraft,” she says.

It is still unknown how long the planes will remain grounded, Murphy says. The service is still trying to determine whether the repairs can be done in the field and the scope of testing necessary for return-to-flight will not be known until the repair process is complete, she says.

A temporary fix that would have allowed the affected aircraft to fly sooner, but it was deemed too high-risk and abandoned, she says.