Supermarine Spitfire Mk VI

In February 1941 the Air Ministry asked, at a meeting held at the RAE Farnborough, for a Spitfire whose cabin could hold a differential of one pound per square inch at 40,000 ft. The fear was that the Luftwaffe would mass-produce high-flying bombers such as the Junkers Ju 86. The Mk VI that resulted doubled the requirement, holding two pounds per square inch — enough to make 37,000 ft feel like 28,000 ft to the pilot, who still had to wear an oxygen mask.

Sealing a fighter was not simple. Both cockpit bulkheads were fully enclosed and every control and electrical run passed through rubber grommets. The side door was replaced by plain skin, and the hood no longer slid: the pilot climbed in, four toggles locked it down and an inflatable rubber tube sealed it, with jettisoning the only way out in an emergency. A Marshall compressor on the starboard side of the engine, fed by a long intake beneath the exhaust stubs, supplied the air. Power came from a Merlin 47 turning a four-blade Rotol propeller chosen for its thrust in thin air, and the rounded wingtips gave way to extended, pointed ones that took the span to 12.2 m.

The German high-altitude offensive never happened, and only two squadrons, Nos. 124 and 616, were fully equipped with the type; others used a handful as a stopgap. In practice Mk VIs spent most of their time at medium altitudes where ordinary Spitfires outperformed them, and it turned out that modified Mk Vs did almost as well high up. Down low the sealed cockpit became stiflingly hot and pilots often flew with the hood removed altogether, unconvinced they could jettison it when it mattered. At 573 km/h at 21,800 ft and a ceiling of 11,900 m the aircraft did what it promised; the trouble was that nobody needed it any more.

In 1943 five Mk VIs in Egypt became the first pressurised photo-reconnaissance Spitfires. No. 680 Squadron and the Aboukir maintenance unit stripped the armament and installed vertical F8 cameras in the rear fuselage; tropicalised with Vokes filters and painted in desert camouflage, they flew a few sorties over Crete and Greece in April and May 1943 before being withdrawn in August. Of the 100 Mk VIs built by Supermarine, that was the last useful task.