Still, the Quiet One had its problems. The CIA had to contend with issues the Army did not.

For one, the CIA needed helicopters that could fly long distances without refueling. And the pilots would have to navigate—in an era before GPS—in total darkness.

So the agency’s first order of business was to find a new, more efficient motor for its new helicopter. The Allison Division of General Motors had intentionally limited the output of its engines in existing OH-6s to 250 horsepower for safety reasons.

“Talk about bureaucracy!” the unnamed author of the CIA journal article, a member of the development team, wrote. “The G.M. people were afraid that we would change one ‘hair’ on this [engine] without years of testing, and they cited ‘liability’ after ‘liability.’”

In the end, Allison lent one of its engineers to the CIA so it could do the work in house and away from G.M.’s lawyers. The spooks hand-selected a small number of rebuilt engines from an unnamed company in Burbank, California.

The technicians picked the six best performing engines out of a batch of 100, the CIA article noted. These motors could reportedly produce almost twice the horsepower as the factory models.

With the new motor, the 1,100 pound aircraft could lift more than 2,000 pounds of people, gas and specialized equipment. To safely land in an emergency, Hughes suggested the CIA limit the total weight to 2,400 pounds.

The helicopter’s added power also meant the aircraft could carry the fuel needed to get deep into North Vietnam. The technicians installed a special fuel tank—like a pair of saddle bags attached together through the main cabin—to hold the extra gas.

To try and keep noise to an absolute minimum, the agency tweaked the new engines so that parts wouldn’t rattle or scrape against each other. The technicians also designed a further improved muffler.

“We … found an engineer who was working at Lockheed who was working on quieting problems of the Rolls Royce RB-211 engine,” the Studies in Intelligence piece noted.

Rolls Royce built the engines for the Lockheed TriStar airliner. RB-211 engines are still used today on commercial planes.

“We tried to obtain his services, but Lockheed said he could not be spared from their programs,” the author added. “He wound up doing this [work] in his garage at night, and, within a week had produced a design that was almost perfect.”

The heavily redacted article did not state how quiet the final configuration was in the end.

But with a new, muffled engine, the agency’s engineers just had to figure out how the crews would pilot their small choppers into North Vietnam. The technicians decided on a combination of infrared cameras and night vision gear.

Flying 100 feet or less above the ground, the pilots would have to fly their route by looking through a small television screen. The co-pilot would wear night vision goggles to keep an eye out for other obstacles.

It would be dangerous, hard flying. But the helicopter had an inertial navigation system. This is a computer that takes data from a combination of sensors — such as gyroscopes — and figures out where the aircraft is, what direction it’s pointed and how fast it’s going.

“At the time, the best unit available had an error rate of about one nautical [mile per hour],” the CIA article stated. So manufacturer Singer-Kearfott “put some packages together that used ‘selected’ gyros that yielded us an error rate of less than one-fourth nautical [mile per hour].”

Today, the Kearfott Corporation still makes navigation and guidance computers for aircraft, ships and ground vehicles.

Army aviators “loaned” to the CIA found the modified aircraft handled well. The pilots had no trouble flying the choppers at night using the infrared camera.

Two years after ARPA started its Quiet Helicopter Program, the CIA quietly registered four Hughes helicopters through its Air America front company. The small fleet included two modified Model 500Ps.

“Most official Air America papers … speak of only two Air America Hughes 500s,” historian Joe Leeker wrote in his deeply researched Aircraft of Air America database. “But those two aircraft only served as cover for two others—the ‘Quiet Ones.’”

With approval from the White House, the CIA ferried the top-secret helicopters to a base known by the code “PS-44” in neighboring Laos. “With[in] a month, however, pilot error claimed one of the two modified choppers as it landed hard in front of the hangar,” Leeker noted.

At the time, Taiwanese crews—who Washington also employed in other secret programs—were training to fly the choppers to Vinh. After the accident, the CIA sent Taipei’s airmen home and replaced them with Air America pilots.

On the night of Dec. 6–7, 1972, the agency’s aviators flew the remaining Quiet One to Vinh and back without incident. With the Model 500Ps in standard Army colors, the pilots planned to claim they were lost if North Vietnamese forces discovered or shot down the helicopters.

After the operation, both Air America discreetly loaded the Model 500Ps onto C-130 transport planes and flew them out of the country. The CIA then sent the two regular Model 500s to Laos as a way to cover up the entire project.

The mission was reportedly a success, but we don’t know exactly what information the CIA’s analysts actually got for their comrades’ troubles. In 1973, the bulk of America’s troops left South Vietnam after Pres. Richard Nixon finalized the peace deal with Hanoi.

In late April 1975—40 years ago now—North Vietnamese troops captured Saigon, the capital of South Vietnam, effectively ending the country’s long-running civil war.

The agency turned over at least one of the Quiet Ones to the Army. We don’t know how long the ground combat branch ended up working with the special chopper. But we do know that the Pentagon and the CIA continued developing quiet and stealthy helicopters after the mission to Vinh.

The Defense Advanced Research Projects Agency still leads various—and sometimes outlandish—military projects. In 2011, the public managed to get a brief glimpse of the next generation of these stealthy choppers.

During the mission that led to the death of Osama Bin Laden in Pakistan, a super-secret chopper crashed at the Al Qaeda leader’s compound.

This aircraft—nicknamed a “Stealth Hawk” as it appeared to be a modified UH-60 Black Hawk—probably isn’t the only one of its kind in Washington’s arsenal.