A zoom climb is a climb where the rate of climb is greater than the maximum climb rate using only the thrust of the aircraft's engines. The additional climb rate is attained by reduction of horizontal speed. Before a zoom climb, the aircraft accelerates to a high air speed at an altitude at which it can operate in sustained level flight. The pilot then pulls steeply upward, trading the kinetic energy of forward motion for altitude. The aircraft gains potential energy at the expense of kinetic energy. This is different from a steady climb, where the increase in potential energy comes from mechanical work done by the engines, rather than from the aircraft's kinetic energy. Zoom climbs are somewhat commonly performed by modern fighter aircraft. Typically referred to as an "unrestricted climb", pilots will take off and accelerate to a high speed near the ground and then pull the aircraft vertically or nearly vertical in order to quickly climb up to the aircraft's cruising altitude. In a demonstration of their performance, English Electric Lightning fighter aircraft have sometimes used zoom climbs to climb above a Lockheed U-2, before pouncing on it from above. Zoom climbs have been used to test new aircraft designs and conduct research in different flight regimes. During the proving phase of the McDonnell Douglas F-4 Phantom II, on December 6, 1959, an early version of the aircraft performed a zoom climb to 98,557 feet as part of Operation "Top Flight". The previous record of 94,658 feet was set by a SovietSukhoiT-43-1 prototype. Commander Lawrence E. Flint Jr. accelerated his aircraft to Mach 2.5 at 47,000 feet and climbed to 90,000 feet at a 45-degree angle. He then shut down the engines and glided to the peak altitude. As the aircraft fell through 70,000 feet, Flint restarted the engines and resumed normal flight. An NF-104A Starfighter fitted with an additional rocket engine was also regularly used in zoom-climb research for future spaceflight. On May 7, 1958, the aircraft reached an altitude of 91,249 feet in a zoom climb at Edwards Air Force Base, Calif., setting a new altitude record. The Mach 2 mission took the airplane so high that the standard F-104's jet engine routinely exceeded its temperature limit and had to be shut down. Sometimes the engine simply flamed out for lack of air. Then the pilot steered the aircraft like a returning spaceship to a lower altitude, where he would restart the engine. It was on one such "zoom climb" flight that test pilot Chuck Yeager was nearly killed flying a heavily modified F-104 on December 10, 1963. On 25 July 1973, A. Fedotov reached 35,230 m in a Mikoyan-Gurevich MiG-25 with 1,000 kg payload, and 36,240 m with no load. In the thin air, the engines flamed out and the aircraft coasted in a ballistic trajectory by inertia alone. At the apex the Indicated airspeed had dropped to 75 km/h. Without engine power, the cockpit would depressurize on these missions. Consequently, for protection against the rarified atmosphere, the pilot wore a full-body pressure suit. Properly outfitted with an airtight helmet and suit, the pilot received breathing oxygen without ill effects.
