The Central Intelligence Agency and Overhead Reconnaissance; the U-2 and Oxcart Programs, 1954-1974

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Secret NOFORN
Chapter 2

and could not survive the stresses of loops and barrel rolls. Moreover, the original U-2s were placarded, which meant that they could not be flown at sea level faster than 190 knots in smooth air or 150 knots in rough air. At operational altitude, where the air was much less dense, they could not exceed Mach 0.8 (394 knots). Speeds in excess of these limits could cause the wings or tail section to fall off.

Airspeed was a very critical factor for the U-2. At maximum altitude only 6 knots separated the speeds at which low-speed stall and high-speed buffet occurred. Pilots called this narrow range of acceptable airspeeds at maximum altitude the "coffin corner" because at this point the U-2 was always on the brink of falling out of the sky. If the aircraft slowed beyond the low-speed stall limit, it would lose lift and begin to fall, causing stresses that would tear the wings and tail off. A little too much speed would lead to buffeting, which would also cause the loss of the wings or tail. Flying conditions such as these required a U-2 pilot's full attention when he was not using the autopilot. Airspeed was such a critical factor that Kelly Johnson added a vernier adjustment to the throttle to allow the pilot to make minute alterations to the fuel supply. $ ^{54} $

Among the unique devices developed for the U-2 was a small sextant for making celestial "fixes" during the long overflights. Because cloud cover often prevented U-2 pilots from locating navigational points on the earth through the periscope, the sextant turned out to be the pilots' principal navigational instrument during the first three years of deployment. When clouds were not a factor, however, the periscope proved highly accurate for navigation. During the final tests before the aircraft became operational, U-2 pilots found they could navigate by dead reckoning with an error of less than 1 nautical mile over a 1,000-nm course. $ ^{55} $

## FINAL TESTS OF THE U-2

Flight-testing of the U-2 continued throughout the fall and winter of 1955-56 in order to test all the various systems. By mid-January 1956, SAC officials were so impressed that they also wanted to purchase a fleet of these planes. On 30 January, DCI Dulles agreed to