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 United States 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 underscores the extent to which American 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, 2001, terrorist attacks, the 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 E. Panetta, the C.I.A. 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 that 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 L. Aboulafia, an aviation analyst at the Teal Group in Fairfax, Va.
Mr. Aboulafia said the changes appeared to include the addition of special coatings to the skin to absorb radar beams and the replacement of sharp edges on the helicopter with curved ones. The gentler curves could scatter the reflections of other radar beams in too many directions for an air-defense system to put together a coherent picture of the plane, he said.
Bill Sweetman, the 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 that most helicopters make.
A cover on the rotor that looks like a dishpan or a hubcap in the news photographs may have also helped reduce so-called radar signature of the craft, the analysts said.
Lawmakers who were 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 of the Seal members later tried to destroy the craft, presumably to hide the secret stealth components, before boarding larger backup helicopters that carried them to Afghanistan.
Mr. Aboulafia and Mr. Sweetman both said it was harder to quiet a helicopter than a winged plane, given all the whirling blades.
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.
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.
Mr. Aboulafia said the latest modifications seemed similar to plans for the stealthy Comanche helicopter, which were canceled in 2004 after billions of dollars in cost overruns. Those plans also 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,” Mr. Aboulafia said, “unless you were also looking at the Pakistan dimension.”
Some analysts wondered whether the C.I.A. might have also used a stealthy 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.
The Pentagon announced in late 2009 that it was testing a bat-winged stealth drone, the RQ-170 Sentinel, in Afghanistan, and it was quickly dubbed the Beast of Kandahar. Military officials have not mentioned it publicly since then, and they would not say this week whether it had been involved in the hunt for Bin Laden
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