Kammala wannan jumla: Hengl ya shiga rundunar sojojin Jamus a matsayin dan jarida a shekara ta 1914, yana da shekaru 16, yana aiki da farko a cikin Battalion Reserve Infantry No. 21 kusa da Ypres a shekara ta 1914. A shekara ta gaba aka tura shi zuwa Gabas ta Gabas don ya yi hidima a Rasha. A watan Oktoba 1915, an tura shi kudu zuwa yankin Serbia. An sake shi zuwa Faransa a 1916, don yin aiki a kusa da Verdun; a ranar 23 ga Maris an daukaka shi zuwa matsayin jami'in a matsayin Leutnant. Daga nan sai ya koma aikinsa a Rasha. Bayan ya nemi a canza shi zuwa aikin jirgin sama, ya fara horar da mai lura da iska a ranar 23 ga Fabrairun 1918. Bayan kammala karatunsa, an tura shi zuwa FA ((A) 295 na Masarautar Bavaria. Jirginsa na yau da kullun a cikin jirgin sama mai kula da kujeru biyu shine Johann Baur. An ba da lambar yabo ga duo tare da nasarar da aka tabbatar da nasara guda shida tare, wanda ya fara da nasara biyu a kan SPADs a ranar 17 ga Yuli 1918 a kan Courton Wood. An harbe jirgin saman Hengl da Baur a bayan layin Birtaniya a lokacin yakin basasa.
Yaƙin Aisne na Uku; duk da haka, sojojin da suka fito daga Württemberg sun cece su daga bauta. Ma'aurata za su ci nasara hudu a watan Oktoba na 1918, tare da Hengl ya ci nasara a karo na bakwai yayin da yake neman wani matukin jirgi.