The Air Force decided to purchase F-16D longerons after cracking along the jet’s canopy sill forced the service to ground over half its fleet. Air Force spokeswoman Susan Murphy said Thursday the service decided to purchase new longerons for three aircraft that require the longeron to be replaced due to cracking issues at a different fuselage station, or location on the fuselage. Three additional aircraft, Murphy said, are under engineering evaluation. Murphy said the Air Force is coordinating the new…