Ejection hatches sheared off two B-1B bombers during flights on successive days this week, prompting the Air Force to ground the fleet for the second time in 10 days, Dyess Air Force Base reported Thursday.

In both flights, the same 3-foot-by-4 foot hatch tore off the top of the $280 million aircraft flying out of Dyess on training missions, one Tuesday and one Wednesday, the base said in a statement. No one was injured, but the Strategic Air Command ordered the entire fleet grounded pending an investigation.All 97 B-1Bs had just been returned to service Monday after being grounded March 27 when one of the sophisticated swing wings on another Dyess bomber pivoted too far and punctured a fuel cell.

In a statement issued Wednesday SAC officials said: "We are inspecting all the aircraft and we expect them to be flying late tomorrow afternoon."

The Dyess statement explained that one four-man B-1B crew had been flying near the Texas-New Mexico border Tuesday morning when the cockpit hatch over the navigator tore away at 12,500 feet. Sgt. Stephen Bailey, a base spokesman, said the unarmed jet was climbing when the incident occurred and returned to Dyess safely, where the plane "was taken to a hangar for closer observation."