The U.S. Marine Corps' troubled V-22 Osprey aircraft has failed to live up to its billing in Iraq and continues to face a multitude of reliability issues and rising costs, a new government report concludes.

As a result of heavier troop equipment, the tilt-rotor aircraft that takes off and lands like a helicopter and flies like an airplane can only carry 20 troops, not 24 as was originally intended.

And the planned installation of a gun in the aircraft's belly to improve its defense could further cut the troop number to 18, the report from the Government Accountability Office finds.

The Osprey's ability to perform well in Afghanistan, where thousands of additional Marines have been assigned in recent weeks and mountains soar above 12,000 feet, is questionable because of the aircraft's problems with its de-icing system and 10,000-foot operating ceiling, the report says.

The 41-page report prepared for Congress and released Tuesday recommends Defense Secretary Robert Gates direct a study to determine whether the Marine Corps should continue the program as designed.

It also suggests the Marine Corps should consider keeping a large fleet of older-generation CH-46 and CH-53 helicopters to ensure all its missions can be fulfilled.

The Government Accountability Office findings fly in the face of consistently glowing Osprey reports from Marine commanders and aviators.

The report also comes as Camp Pendleton and Miramar Marine Corps Air Station are getting ready to place 114 Ospreys on those bases and phase out the CH-46 and CH-53, which have proven to be reliable workhorses since the Vietnam era.

That scheduled 2010 transition to what is supposed to become the primary troop mover could be put on hold.

"I would be surprised if the Defense Department doesn't accept the recommendation to take a new look and perhaps scale back the program," said Phil Coyle, an Osprey critic at the Center for Defense Information in Washington and a former assistant secretary of defense from 1994 to 2001. The center is staffed with former members of the military and analyzes defense issues. "With its high maintenance costs and low readiness rates, the review being called for is appropriate."

The Marine Corps has spent $28.8 billion to date to develop the Osprey, which first flew in 1989. Its development was set back several years by a series of crashes between 1991 and 2000 that claimed the lives of 27 Marines, including 14 from Camp Pendleton and four from Miramar.

Government auditors say the cost of flying the aircraft, which is manufactured by Boeing and Bell, is now $11,000 per hour, more than double the expected price and twice the hourly cost of a CH-46. Research and development costs have ballooned from $4.2 billion to nearly $13 billion, despite the reduction in the number planned for purchase by the Marine Corps and Air Force from 1,000 to 500.

The cost for a single Osprey has skyrocketed 148 percent, from $37.7 million when the program was launched in 1985 to $93.4 million. About 250 Ospreys remain to be built.

"The issue for the Marine Corps is they can buy four conventional helicopters for the price of one Osprey," Coyle said. "I've always felt that the Osprey could be used as a truck, but it's a pretty expensive truck."

A Marine Corps spokesman defended the Osprey while acknowledging the service still faces an array of issues with the aircraft.

"The performance of the Osprey in Iraq has given the ground commanders everything they've asked for, and done so faster, farther and safer than any helicopter could do," said the spokesman, Maj. Eric Dent. "But we know there is still work to be done to improve reliability and maintainability of some components to enhance readiness. We are fully engaged and partnered with industry on this."

While the 12-aircraft Osprey unit in Iraq last year was able to complete most of its missions flying farther and faster than older-generation helicopters, none involved combat. The unavailability of replacement parts rendered many inoperable, and resulted in maintenance crews "cannibalizing" one aircraft to keep another flying, according to the report.

As for serving in Afghanistan, the report notes that the Osprey cannot live up to its promise of being able to fly day or night in all kinds of weather and at varying altitudes.

"Identified challenges could limit the ability to conduct operations … at high altitudes similar to what might be expected in Afghanistan."

No Ospreys are now in Afghanistan. The military is relying on older-model heavy-lift helicopters to ferry troops and equipment.

A spokesman for Marine Corps reservist U.S. Rep. Duncan D. Hunter, R-El Cajon, a member of the House Armed Services Committee who served in Iraq and Afghanistan, said the congressman supports the Osprey while at the same time recognizing it has not yet been tested in combat.

"Congressman Hunter believes the V-22 should be incorporated into the combat mission in Afghanistan and tested in that environment," spokesman Joe Kasper said. "Until then, any criticisms or evaluations of its combat capability are premature."

Call staff writer Mark Walker at 760-740-3529.