Chainat joined the army in October of 1913. After serving with an artillery regiment, he transferred to the French Air Service in April of 1914. He received a Pilot's Brevet on June 8, 1915 and was promoted to corporal later the same month. Chainat was injured in crashes twice and wounded in combat on June 19, and September 7, 1916.

Médaille Militaire
"Pilot of great skill and bravery. On 26 March 1916, he attacked and shot down a plane which fell in flames in front of our lines." Médaille Militaire citation, April 5, 1916

Chevalier de la Légion d'Honneur
"First class pursuit pilot. Has shot down six enemy planes between March 26, and July 12, 1916. He was wounded on June 16. Already cited four times in orders and received the Médaille Militaire." Chevalier de la Légion d'Honneur citation,August 3, 1916