The operation that killed Osama bin Laden on 2 May inadvertently revealed a modification kit that reduces the radar profile of a small number of Sikorsky MH-60Ks operated by US special operations forces, according to sources familiar with the equipment.
A reduced-signature tail rotor shroud, low-observable treatments and faceted stabilators are visible in news photos of the helicopter that was destroyed at the scene.
The images led to speculation about the possible existence of an all-new stealth helicopter. But in reality, they reveal one of several mission-specific kits developed internally by the 160th Special Operations Aviation Regiment at Fort Campbell, Kentucky, sources said.
The regiment is renowned for the flying skills of its helicopter pilots, but has also developed advanced engineering capabilities.
Set apart from the army's conventional acquisition and science and technology communities, the 160th has the ability to develop and certificate its own aircraft modifications and projects.
A standard MH-60K represents the most advanced configuration of the Black Hawk family within the regiment's fleet, with special survivability, navigation and radar systems allowing it to operate in "all environments and under the harshest conditions", the army said.
According to one source, the tail rotor is the most visible part of the MH-60K to radar and the sharply edged shroud revealed in the news photos helps to reduce the signature, especially from the frontal aspect. The kit also includes the installation of a stealthy windscreen and requires the removal of the MH-60K's refuelling probe, according to sources.
The bolt-on kits are not classified, but they have been kept secret within the special operations community for as long as two decades. Starting in the mid-1980s, the army began experimenting with stealth treatments for helicopters.
In 1987, Bell OH-58s involved in Operation Prime Chance, which provided armed escorts for oil tankers in the Persian Gulf, received a kit with stealth treatments, which included a main rotor shroud.
The Sikorsky/Boeing RAH-66 Comanche, which was cancelled in 2004, was designed with stealth treatments from the beginning.
It is not clear how the special kit impacted the MH-60K's performance during the bin Laden raid. One of the two aircraft involved in the operation had to be destroyed on the ground. The helicopter's engine had reportedly lost thrust due to power settling as it hovered within the high walls of bin Laden's compound.
One source said the tail rotor shroud of the radar-evading kit improves the aerodynamic performance of the aircraft slightly, but another said the stealth features add weight and drag to the aircraft, perhaps making it more susceptible to power settling.