Photography of U.S. military technology left behind in the May 1 raid against Osama bin Laden shows that previously unseen stealth-like enhancements to rotorcraft played a critical role in the mission to take down the Al Qaeda leader.

The Aviation Week analysis, made shortly after the Abbottabad, Pakistan, operation, proves that the potential to capture or kill the 9/11 mastermind was important enough to U.S. leaders to risk the lives of at least two dozen of the most highly trained U.S. special operators. In addition, the raid would have been a major international embarrassment for Washington if something went wrong and might have exposed cutting-edge military and intelligence technology.

In the end, only the exposure of technology might have occurred—and even then, the otherwise highly successful operation still has left tantalizingly few, albeit significant, new clues to military advancements.

According to widely published photographs from Abbottabad, at least one previously undisclosed, low-observable helicopter apparently was part of the U.S. task force that killed and recovered bin Laden's body. It appears to be a significantly modified version of a Sikorsky H-60 Black Hawk, although whether an MH-60K, L or M version is still unknown. The U.S. Army's 160th Special Operations Aviation Regiment (Airborne), the Night Stalkers, uses all three types of MH-60s.

Pentagon leadership remains tight-lipped about operational details. But an intelligence source says two Black Hawks and two Boeing HH-47 Chinooks were used. The Black Hawks participated in the assault, and the Chinooks were used to refuel the Black Hawks and for backup. Indeed, when the one Black Hawk was downed, a Chinook came in to help complete exfiltration of the Navy Seals.

What is as intriguing as the stealth adaptations is how well they apparently worked. “The attack on bin Laden did not occur in some remote area outside Pakistani control, but in a compound in a city of some 100,000 and less than 100 mi. from a major Pakistani population center like Islamabad, and one occupied by a brigade from the Pakistani army's second division and the location of the Army's military academy,” notes analyst Anthony Cordesman of the Center for Strategic & International Studies.

According to the chairman of the House Armed Services Committee, who was briefed last week by the military on the issue, it was not a mechanical failure or a problem by the pilot that downed the Black Hawk—it was a miscalculation of temperature in and outside the compound. The Black Hawk ran into lift trouble due to a 15F difference inside the courtyard, says Rep. Buck McKeon (R-Calif.). “They couldn't hold the hover.”

The Seals appear to have destroyed almost all of the airframe that landed inside the compound, but part of the helicopter's tail landed outside the wall and escaped demolition during the roughly 40-min. ground operation.

The public photos show that the destroyed Black Hawk's tail features stealth-configured shapes on the boom and the tail-rotor hub fairings, swept stabilizers and a “dishpan” cover over a five- or six-blade tail rotor. It has a silver-loaded infrared (IR) suppression finish similar to that seen on the Bell-Boeing V-22 Osprey.

Stealth enhancements for rotorcraft are not new and were applied extensively to the Boeing/Sikorsky RAH-66 Comanche, canceled in 2004. Compared with fixed-wing stealth, more emphasis is usually placed on noise and IR signatures.

Noise can be reduced and made less conspicuous by adding blades to the main and tail rotors. It can also be reduced by aerodynamic modifications and flight control changes that make it possible to reduce rotor rpm, particularly in forward flight below maximum speed. Under any such effort, a reduction in IR would be critical; the Comanche had an elaborate system of exhaust ducts and fresh-air ejectors in its tail boom.

Classic radar cross-section (RCS) reduction measures include flattened and canted body sides, making landing gear and other features retractable, and adding fairings over the rotor hubs. But it is believed that a helicopter cannot yet be made as radar-stealthy as a fixed-wing airplane, as helicopters generally operate at low altitude and against ground clutter. Still, reducing RCS makes jamming more effective, whether from the helicopter itself or from a standoff jammer.

According to Pentagon budget data from fiscal 2010, there have been plans for an MH-60M version. Following standard practice, this would have been the most recent Black Hawk variant for the Army to be upgraded for U.S. Special Operations Command (Socom) standards.

A Socom official says the M, or “Mike,” upgrade kit costs $18 million per unit. It includes specialized avionics, aerial refueling capability, engine upgrade and aircraft survivability equipment, as expected. Such radar and noise-suppression capabilities also could be favored by the U.S. Air Force, which is known to be eyeing more Mikes for combat search-and-recue missions.

With Jen DiMascio and Michael Bruno in Washington.