a Spitfire V up to 30,000 feet and above is very much better. At 38,000 feet it is capable of a true speed of 368mph and is still able to manoeuvre well for fighting. Although the Mk IX's airframe did not have the aerodynamic and strength improvements, or the modified control surfaces of the Mk VII and VIII, the Mk IX still proved to be an effective counter to the Focke-Wulf Fw 190. In September 1942, the "High Altitude Flight" was set up at RAF Northolt to counter the threat of the high altitude Junkers Ju 86 R bombers. Two Spitfire Mk IXs converted from Vcs were stripped of armour, the .303 Brownings and other equipment and repainted in a lightweight PRU blue finish. One of them intercepted a Ju 86R at over . Production of the Mk IX finished at Supermarine in June 1943, with production continuing exclusively at the Castle Bromwich factory. Several major and large numbers of minor improvements were progressively introduced to Mk IXs, some of which were used on other Spitfire variants. The Merlin 61 was phased out early in 1943 in favour of the Merlin 63 and 63A. The new engine had increased power resulting from engine improvements and engine limitations of +18 lbf/(sq in) and 3,000 rpm (5 minute combat). During the second half of 1943, production of the Merlin 63 powered F Mk IX was discontinued in favour of the Merlin 66 powered LF Mk IX. Early production Spitfire Mk IXs suffered from vapour locks in the fuel lines resulting from fuel evaporating if the aircraft was parked in direct sunlight. As a result of this the gun-camera was moved from the port wingroot to the starboard wingroot and a fuel cooler, fed by a small round air-intake was fitted in its place. This fuel cooler was also fitted to early PR Mk XIs. Production of the LF Mk IXs, frequently referred to as the Spitfire Mk IXb, initially ran in parallel with the Merlin 63 powered Marks. This version first became operational in March 1943 with the Biggin Hill Wing, comprised at the time of 611 and 341 (Free French) Squadrons. This type was by far the most produced of the Spitfire Mk IX variants, with over 4,000 built. The maximum power of the Merlin 66 was at 5,750 ft (1,283 kW at 1,752 m) and the maximum speed of the