WASHINGTON - The Marine Corps announced Sunday the temporary grounding of all 11 of its MV-22 Osprey tilt-rotor aircraft, the hybrid airplane-helicopter that has been under increased scrutiny since one crashed during a training exercise in April, killing all 19 Marines aboard.
The decision to suspend Osprey flight operations was made late Friday by the Naval Aviation Systems Command, which also ordered the Marines to temporarily ground its fleet of CH-53E Super Stallion transport helicopters and AH-1W Cobra attack helicopters, spokesman Lt. David Nevers said.
Nevers said the most significant of the three actions was the grounding of the workhorse CH-53E Super Stallions, because it is likely to take longer to get them back in the air than either the Ospreys or the Cobras.
Nonetheless, the Osprey's problem is likely to draw the most public attention, in part because of recollections of the April crash - the worst Marine helicopter loss in more than a decade - and in part because some members of Congress have criticized the Osprey program as too expensive and technically flawed.
The Osprey takes off and lands like a helicopter but flies like an airplane. It is built by Boeing Co. and Bell Helicopter Textron.
The Osprey fleet was taken out of operation for about two months after the April crash in Arizona. Investigators determined the crash was caused by mistakes made by the pilot and co-pilot, not a mechanical problem.
The Ospreys resumed flying in June.
Last Thursday, an Osprey made a precautionary landing at Camp Lejeune, N.C., and a subsequent maintenance inspection revealed that a coupling on the aircraft's drive shaft had failed. The coupling was repaired and the Osprey returned to its home base at Marine Corps Air Station New River, N.C.
''In the interests of safety, they will check the other aircraft's couplings'' before returning them to flight, Nevers said. He said he could not estimate how long they would be out of operation.
The Marines are counting on the Osprey to eventually replace the corps' Vietnam-era CH-46 helicopters as the primary means of transporting troops into combat from ships offshore. The Marines plan to buy 360 Ospreys, and the Air Force plans to buy 50 for its special operations forces.
The Defense Department has not yet made a final decision to enter full-rate production of the Osprey.
The decision to ground three types of Marine Corps aircraft, albeit for unrelated reasons, puts an unusually large portion of the Marine aviation fleet out of operation. In July, 106 of the Marines' AV-8B Harrier fighter jets were grounded because of an engine bearing problem. Thirty of the Harriers have since returned to flight, but that leaves the majority of the fighters still grounded.
The decision to ground all 165 of the Marines' CH-53E Super Stallions was based on findings from the investigation into the Aug. 10 crash of a Navy MH-53E mine-sweeping helicopter off the coast of Corpus Cristi, Texas, in which four people were killed. Nevers said investigators found a bearing problem, but he was not sure of the exact nature of the problem.
Eight Super Stallions currently are on overseas deployment with Marine Expeditionary Units, Nevers said.
The corps' 198 AH-1W Cobra attack helicopters were grounded after it was discovered this week that some older rotor blades may be susceptible to cracking. Each Cobra will undergo a one-time inspection to identify and replace, if necessary, the suspect blades, Nevers said. Eight are currently deployed abroad.
The Cobras are likely to undergo ''a relatively quick fix,'' Never said, and return to normal operations.