More than half of the U.S. Air Force’s F-16D fighter have been grounded due to cracks found during inspections.

Eighty-two of 157 F-16Ds, primarily used for training, were removed from flight status after cracks were found in canopy sill longerons found between the front and rear pilot seats, the Air Force announced Aug. 19. Longerons run the length of the aircraft and transfer loads throughout the structure and skin of the platform. Program overseers issued a time compliance technical order for these aircraft after initial cracks were found in the structures during post-mission inspections, according to Air Combat Command officials.

Lt. Col. Steve Grotjohn, deputy chief of the Weapon System Division at the program office, attributes the cracks to "fatigue from sustained operations." F-16Ds average 24 years old with roughly 5,500 hr. of flight time.

Pentagon officials found Boeing to blame for faulty longerons in its twin-engine F-15; in 2007 one of the fighters broke apart in flight due to defective manufacturing. Of 441 F-15s, 182 were found to have structural components below design specification.

Service officials have not yet determined the effect of the recent grounding on graduating F-16 pilots.

The cracks are likely to fuel debate about what the future mix of Air Force aircraft should be. F-16s have been used to support routine missions – such as monitoring the skies in North America and over Afghanistan — though some industry officials say less-expensive aircraft are suitable for such a mission. Textron AirLand has made such an argument the centerpiece of its campaign to sell the twin-engine Scorpion aircraft to the Air Force; company officials boast a low operating cost of $3,000 per hour.

Meanwhile, Lockheed Martin announced the completion of a critical design review for the first active electronically scanned array (AESA) radar on the aircraft; the company had dubbed the system the F-16V.

Though Taiwan is the launch customer for the F-16V as part of an upgrade program for its 144 F-16A/Bs, Lockheed Martin and radar manufacturer Northrop Grumman are hoping to get U.S. Air Force buy-in eventually.

The Air Force, however, has opted to forgo an avionics upgrade to its F-16s due to tight budgets and a priority placed on the single-engine, stealthy F-35, also made by Lockheed.