The mass production of the Spitfire started in 1936 and more than 20,000 planes were produced until 1948. In 1942, the Spitfire was urgently required for operations overseas ane one of the best known versions was the North African prepared Spitfire designated "TROP.", for Tropical. To protect engine from sand, a Vokes air filter was affixed over the carburetor intake. Although the filter was successful, its large mass reduced the aircraft's speed. The problem was solved by 103rd maintenance unit stationed at Aboukir, who designed a smaller more streamlined unit that was equally effective.