Henri Farman
Waller was commissioned as a temporary second lieutenant to serve in the Army Service Corps on 7 September 1914, alongside his brother William Hastings de Warrenne Waller (who also became a pilot, eventually retiring from the RAF with the rank of wing commander in 1935.) He was promoted to lieutenant on 6 November 1914, and to captain on 5 September 1915. Waller transferred to the Royal Flying Corps to train as a pilot, being granted Royal Aero Club Aviators' Certificate No. 4015 after soloing a Henri Farman biplane at the Military School, Joyce Green, on 3 December 1916. He was appointed a flying officer and transferred to the General List on 10 February 1917.