All remaining US Air Force Boeing F-15A-D Eagles have been returned to flight, contingent on completion of individual inspections on each aircraft's fuselage longerons.

The USAF does not expect any of the 149 aircraft to require repair and says there will be no operating restrictions on Eagles that pass these final inspections. But all F-15A-Ds will require repetitive checks for longeron cracks every 400 flying hours.

After a lengthy series of grounding and inspections following the 2 November 2007 crash of a Missouri Air National Guard F-15C, caused by structural failure, longeron cracks have been found in only nine of the US Air Force's 438 A- to D-model Eagles.

The fatigue cracks were found where machining during manufacture had reduced the thickness of the upper fuselage longerons below blueprint specification or where there was surface roughness that could cause stress concentration.

USAF F-15 in flight
                                                                                                  © US Air Force

On 8 January, the USAF returned 259 of the F-15s to flight after they passed inspections for manufacturing deficiencies. The remainder remained grounded because at least one longeron on each aircraft did not meet blueprint specification at one or more points.

The F-15s that remained grounded are undergoing tailored inspections unique to each tail number. The USAF has recommended that aircraft with longerons that do not meet specification, but pass these inspections, be returned to flight, effectively ending the groundings.

Air Combat Command says it has not been decided whether the nine aircraft with longeron cracks will be repaired, although the Warner Robins Air Logistics Center responsible for F-15 depot overhaul is manufacturing a batch of replacement longerons.

Read flightglobal's special report on the F-15 accident investigation.