from FlightGlobal:

USAF F-15s back on ground with new crack fears
By Stephen Trimble

The US Air Force has detected a possibly "catastrophic" design weakness in the Boeing F-15A-D. Air Combat Command on 28 November grounded all F-15s excluding the F-15E Strike Eagle.

Preliminary findings from an investigation of an accident on 2 November - in which an F-15C disintegrated in mid-air - revealed cracks in the F-15's upper longerons near the canopy seal.

Moreover, recent inspections found cracks in the same area on two other F-15s. According to the USAF, simulations by Boeing indicate such cracks could cause "catastrophic failure".
more here: http://www.flightglobal.com/articles...ack-fears.html