to attack England, the V-1s were employed in attacks against strategic points in Belgium, primarily the port of Antwerp. Launches against Britain were met by a variety of countermeasures, including barrage balloons and aircraft such as the Hawker Tempest and newly introduced jet Gloster Meteor. These measures were so successful that by August 1944 about 80% of V-1s were being destroyed (Although the Meteors were fast enough to catch the V-1s, they suffered from frequent cannon failures, and accounted for only 13.) In all, about 1,000 V-1s were destroyed by aircraft. The intended operational altitude was originally set at . However, repeated failures of a barometric fuel-pressure regulator led to it being changed in May 1944, halving the operational height, thereby bringing V-1s into range of the 40mm Bofors light anti-aircraft guns commonly used by Allied AA units. The trial versions of the V-1 were air-launched. Most operational V-1s were launched from static sites on land, but from July 1944 to January 1945, the launched approximately 1,176 from modified Heinkel He 111 H-22s of the 's Kampfgeschwader 3 (3rd Bomber Wing, the so-called "Blitz Wing") flying over the North Sea. Apart from the obvious motive of permitting the bombardment campaign to continue after static ground sites on the French coast were lost, air launching gave the the opportunity to outflank the increasingly effective ground and air defences put up by the British against the missile. To minimise the associated risks (primarily radar detection), the aircrews developed a tactic called "lo-hi-lo": the He 111s would, upon leaving their airbases and crossing the coast, descend to an exceptionally low altitude. When the launch point was neared, the bombers would swiftly ascend, fire their V-1s, and then rapidly descend again to the previous "wave-top" level for the return flight. Research after the war estimated a 40% failure rate of air-launched V-1s, and the He 111s used in this role were vulnerable to night-fighter attack, as the launch lit up the area around the aircraft for several seconds. The combat potential of air-launched V-1s dwindled during 1944 at about the same rate as that of the ground-launched missiles, as the British gradually took the measure of the weapon and developed increasingly effective defence tactics. Experimental, piloted, and long-range variants Piloted variant Late in the war, several air-launched piloted V-1s, known as , were built, but these were never used in combat. Hanna Reitsch made