During the 1930's Trenkle was a flying instructor with the Luftwaffe and in 1937 he became Chief Test Pilot of the Messerschmitt-Zwigweks where he test flew the first ME-108 to be produced and went on,during WW11,to test fly the ME-109 and ME-262. In 1945 he claimed to have shot down a P-38 Lightning. It was during this test flying service that in 1942, Trenkle was granted the title of Flugkapitan at the age of 30, the youngest age at which this honour was ever granted.