The F-16s airframe has been subject to more heavy loads than was predicted in 1979. This resulted in several unpredicted hair cracks in some of the airframe's bulkheads. Before an aircraft can be offered for MLU modification, the current state of the airframe is examined in the extensive Aircraft Structural Integrity Program (PACER SLIP). In this modification program, all bulkheads of the aircraft will be examined and repaired (using the so called Cold Working method) if necessary. After PACER SLIP, the aircraft will be able to last at least another 5,000 flying hours and can complete its life expectancy of 30 years.
Note that hair cracks are not uncommon in aircraft design. To predict the acceptable number and type of cracks, the aircraft manufacturer used information that reflected the exact capabilities of the aircraft. The F-16 was designed as a light and highly maneuverable aircraft that could withstand 9G and last for a minimum of 8,000 flight hours. All data is recorded in a load spectrum that specifies the use of the aircraft (i.e. type of missions), aircraft load and predicted number of landings.