Appleby enlisted into the
Canadian Expeditionary Force for overseas service on 25 September 1914 at
Valcartier, Quebec. He was posted to No. 1 Stationary Hospital as a member of the
Canadian Army Medical Corps, and sailed for England on 3 October 1914. Appleby served in
Near East from 1 August 1915 to 6 November 1916, receiving promotions to
lance corporal on 13 April 1916, and to
sergeant on 22 June. He then returned to England, and was under instruction at the No. 2 Officers Cadet Battalion,
Pembroke College, Cambridge, from 23 December 1916 to 25 April 1917. and was struck off the strength of CEF the same day. Flying as an observer/gunner in
Airco DH.9 bombers, he was credited with six victories over enemy aircraft between 1 July and 15 September 1918, all but one with
Richard Gammon as his pilot. On 1 November 1918 Appleby and Gammon were both awarded the
Distinguished Flying Cross following a particularly hazardous attack by 21 aircraft from No.'s 104 and
99 Squadrons on the
Badische Anilin und Sodafabrik factory at
Mannheim on 7 September, during which they accounted for three enemy aircraft. Their citation read: :Lieutenant (Temporary Captain) Richard John Gammon. :Second Lieutenant Percival Ewart Appleby. ::"Captain Gammon, with Second Lieutenant Appleby as Observer, was the leader of two formations (ten machines in all) on a recent raid. En route the formation was attacked by fifteen hostile aircraft; having driven these off, they reached the objective, which was successfully bombed. While thus engaged the formation was fiercely attacked by fifteen enemy machines, which continued the attack for some distance on the return journey, until they were driven off. Upon nearing our lines the formation was again assailed by seven machines; in the engagement that ensued one of these was destroyed and two driven down by Captain Gammon and his Observer, and, in addition, three others were destroyed by our other machines. The officer who led the whole of the combined formations of this raid speaks in the highest terms of Captain Gammon's leadership and skilful co-operation. Second Lieutenant Appleby was of the greatest assistance to Captain Gammon throughout, keeping him informed of the movements and manoeuvres of the hostile machines. This officer has taken part in numerous raids, displaying on all occasions great keenness and determination." Appleby was transferred to the Home Establishment on 21 October 1918, serving at the No. 1 School of Aeronautics from 18 November 1918, and No. 3 Training Depot Station from 5 May 1919. He was posted to the Repatriation Camp at
Upavon on 18 May,
List of aerial victories ==Post-war career==