Sperrle's airmen switched to attacking choke-points around Madrid. Operations were complicated by the Sierra de Guadarrama and Sierra de Gredos mountain range. Pilots suffered from circulation problems in non-pressurised cockpits while flying close to the top of the peaks, but only one aircraft and five men were lost to the natural barriers. Influenza and pneumonia outbreaks due to poor living conditions continued to undermine morale. One of the main problems was the inadequate fighter aircraft. The Heinkel He 51 was not up to the job, Sperrle requested Berlin send modern aircraft. In December three Messerschmitt Bf 109 fighters were in Spain, only to return the following February, followed by a single Heinkel He 112 prototype, and Junkers Ju 87 dive-bomber. Richthofen travelled to Berlin to secure more of these types in agreement with chief of staff Albert Kesselring, Ernst Udet, and Milch. Sperrle's command began receiving the Dornier Do 17, He 111, Ju 86 bombers and Bf 109 fighters in January 1937. On 1 April 1937 Sperrle was promoted to "Generalleutnant" (Major General).