Hengl joined the German army as an ensign in 1914, aged 16, serving initially in Reserve Infantry Battalion Nr. 21 near Ypres in 1914. The following year saw him transferred to the Eastern Front to serve in Russia. In October 1915, he was transferred south to the Serbian sector. He transferred back to France in 1916, to serve near Verdun; on 23 March he was promoted into the officer's ranks as a Leutnant. He then returned to duties in Russia. After requesting a transfer to aviation duty, he started aerial observer's training on 23 February 1918. Upon graduation, he was posted to the Kingdom of Bavaria's FA(A) 295. His usual pilot in the two-seater reconnaissance aircraft was Johann Baur. The duo were credited with six confirmed aerial victories together, beginning with a double victory over SPADs on 17 July 1918 over Courton Wood. The aircrew of Hengl and Baur were shot down behind British lines during the Third Battle of the Aisne; however, they were rescued from captivity by troopers from Württemberg. The pair would score another four victories in October 1918, with Hengl scoring a seventh while crewing for another pilot.