There he was promoted on 27 January 1896 to lieutenant, and served in the following years as a battalion adjutant and graduated from the Institute of War 1908.

Following a temporary command of the air force, where he was trained as a pilot, Haehnelt was transferred on 1 October 1913 to the staff of the 4th Aviator Battalion to Strasbourg.

On 31 December 1920, Haehnelt's position was eliminated as a result of the Treaty of Versailles and the consequent prohibition of air forces in Germany from active military service.

Haehnelt was taken as an E officer and colonel in the Air Force, adopted on 1 July 1938 in active service and acted as head of the War Department in Military Science of Aviation.

By 1 March 1942, Haehnelt was made a General of the air force but retired from active service roughly a month later.

After the war Haehnelt was arrested, though no crimes were specified, in May 1945 by the Soviet occupation forces and interned in the Sachsenhausen camp.