The assault team that killed Osama bin Laden sneaked up on his compound in radar-evading helicopters that had never been discussed publicly by the U.S. government, aviation analysts said Thursday.

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The assault team that killed Osama bin Laden sneaked up on his compound in radar-evading helicopters that had never been discussed publicly by the U.S. government, aviation analysts said Thursday.

The commandos blew up one of the helicopters after it was damaged in a hard landing, but news photographs of the surviving tail section reveal modifications to muffle noise and reduce the chances of detection by radar.

The stealth features, similar to those used on advanced fighter jets and bombers, help explain how two of the helicopters sped undetected through Pakistani air defenses before reaching the bin Laden compound in Abbottabad. The use of the specially equipped helicopters also illustrates the extent to which U.S. officials wanted to get to bin Laden without tipping off Pakistani leaders.

Analysts said the raid was a rare case in which stealth aircraft, devised for conventional warfare during the Cold War, became critical to fighting terrorism.

Military and intelligence agencies have refused to comment about the use of stealth aircraft in this raid. But since the Sept. 11 terrorist attacks, special forces have spared no expense in developing technology to hunt terrorists, and aviation experts said the debris from the damaged helicopter provided further evidence of that.

Leon Panetta, the CIA director, has said that two Black Hawk helicopters carried about 25 Navy SEAL members to the compound, where they killed bin Laden and three other people in an operation that lasted nearly 40 minutes.

But several analysts and executives in the helicopter industry said the rear section that was left behind looked nothing like the tail of a regular Black Hawk, a popular midsize helicopter made by Sikorsky. Rather, they said, it appeared that the Black Hawks had been modified to incorporate some of the features of a proposed stealth helicopter the Pentagon canceled in 2004.

“They would have learned an awful lot from that, and a lot of it would have been relevant to a program like this,” said Richard Aboulafia, an aviation analyst at the Teal Group in Fairfax, Va.

Similar to previous plan

Aboulafia said the modifications seemed similar to plans for the stealth Comanche helicopter, canceled in 2004 after billions of dollars in cost overruns.

Plans for the Comanche, a joint Boeing and United Technologies project, arose during the Cold War. But with the lack of anti-aircraft threats in Iraq and Afghanistan, Army officials decided that full-scale production of stealth copters was not worth the cost.

“Stealth never made sense in an Afghan context,” Aboulafia said, “unless you were also looking at the Pakistan dimension.”

Bill Sweetman, editor of a military trade publication owned by Aviation Week, reported that the damaged helicopter appeared to have five or six blades in its tail rotor, instead of the four in a standard Black Hawk. That could have allowed operators to slow the rotor speed and reduce the familiar chop-chop sound most helicopters make.

A cover on the rotor that looks like a dishpan or a hubcap in the photographs may have also helped reduce the radar signature of the craft, the analysts said.

Lawmakers briefed on the mission said the damaged helicopter had not malfunctioned, as initially described by senior administration officials.

Instead, they said, it got caught in an air vortex caused by higher-than-expected temperatures and the high compound walls, which blocked the downwash of the rotor blades.

As a result, the helicopter lost its lift power while hovering over the yard and had to make a hard landing, clipping one of the walls with its tail. Some SEAL members later tried to destroy the craft, presumably to hide the stealth components.

It was not clear whether the special forces had used the stealth helicopters in any earlier raids in Afghanistan, Iraq or Pakistan’s tribal areas.

Extension of technology

Indications that stealth features were added to the helicopters suggest an extension of a technology that was created to protect American fighter jets and bombers from sophisticated air defenses in countries like Russia and China.

The top stealth fighter, the F-22, has never been flown in combat. The long-range B-2 bombers have been used sparingly, including a recent bombing run that destroyed an airfield in Libya.

Some analysts wondered whether the CIA might have also used a stealth drone to gather intelligence before the raid on bin Laden’s compound and possibly to monitor the attack.

In addition to satellite photographs, the special forces rely on Predator and Reaper drones in Iraq and Afghanistan to provide video showing how many people are living in insurgent compounds and their patterns of activity.

But the Predators and Reapers would be easy for almost any air-defense system to track.