Helicopter loss in bin Laden raid highlights risks

WASHINGTON (Reuters) - The loss of a U.S. Black Hawk helicopter during the mission that killed Osama bin Laden reveals the vulnerability of such aircraft, but also reflects important lessons learned from earlier helicopter accidents.

U.S. government officials say the helicopter destroyed during the mission in Pakistan was a newer version of the two Sikorsky UH-60 Black Hawks that were shot down during a raid in Somalia in October 1993 that killed 18 soldiers.

In Pakistan, the helicopter packed with soldiers made a "controlled but hard landing" after encountering higher-than-expected temperatures at bin Laden's compound near Islamabad, Senator Dianne Feinstein told reporters on Tuesday.

The accident was not unusual, military experts said.

Dozens of U.S. helicopters have crashed or been shot down in Afghanistan and Iraq, challenged by sandstorms and high temperatures that reduce the lifting capability of aircraft powered by rotors. In mountainous Afghanistan, there is the added challenge of flying at high altitudes.

"Helicopters are way more reliable than they were in the 1950s, but unfortunately, they're still very sensitive to the environment," said Joseph Trevithick, an analyst with the globalsecurity.org website.

The Pentagon is still investigating the problem in the bin Laden raid, but the helicopter was not damaged by enemy fire, said one defense official, who was not authorized to speak on the record. Officials said it suffered mechanical failure.

U.S. forces quickly destroyed the Black Hawk, which was built by Sikorsky Aircraft, a unit of United Technologies Corp, to avoid any of its sensitive equipment falling into enemy hands, said the defense official.

Unlike fixed wing aircraft, helicopters are more prone to crash if they sustain damage to any one area, Trevithick said.

Helicopter problems forced the U.S. military to abort another high-profile mission in 1980 -- an attempt to rescue U.S. citizens held hostage at the U.S. embassy in Iran.

"They agonized over that. A lot of lessons were learned," including the need for extra helicopters to carry out the mission, and rescue helicopters, said John Pike, founder of the globalsecurity.org website.

Still, helicopters offer huge advantages to the military, allowing the transport of troops and cargo in areas where the few existing roads are often heavily mined with explosives, and providing commanders the ability to get close to targets.

In Sunday's mission, two Black Hawk helicopters were supposed to hover over the bin Laden compound and allow Navy special operations forces to rappel to the ground.

When one of the helicopters ran into problems -- including temperatures that were 17 degrees higher than expected -- and had to land abruptly, two Boeing Co Chinook helicopters were called in to help get the U.S. troops out, said one U.S. government official, speaking on condition of anonymity.

One Chinook would have sufficed, but a second one was sent in case that helicopter also ran into trouble, said Pike.

One retired military helicopter pilot said the Black Hawk likely ran into an issue called "settling with power," when high temperatures, a heavy load and high altitudes force an unplanned landing. "Those conditions just suck the RPM out of the rotor," he said.

Sikorsky's Black Hawks, which typically have a range of 360 miles, are considered reliable and have been real workhorses during the last decade of war, said one congressional aide.

A House Armed Services subcommittee is due to finalize a spending bill for fiscal 2012 that includes $1.3 billion for 71 new UH-60 Black Hawk helicopters.

The Black Hawk, which first began flying in 1978, has a crew of three or four and can carry 11 soldiers equipped for combat. It has a maximum gross weight of 22,000 pounds and can carry up to 9,000 pounds on an external cargo hook. It has a top speed of 187 miles per hour.

(Reporting by Andrea Shalal-Esa with additional reporting by Susan Cornwell and Mark Hosenball; Editing by Tim Dobbyn)