Lega volunteered for aviation in late 1916. He qualified as a pilot on 1 September 1916, and was granted his license on 1 November 1916. By 30 January 1917, he had finished pilot's training, as he was then posted to reconnaissance duty with "21a Squadriglia". He was further rated in Savoia-Pomilio SP.2s on 14 February 1917; he was also promoted to tenente. His duties as a reconnaissance pilot between May and November 1917 earned Lega a Silver Medal for Military Valor. He remained with this squadron until it was withdrawn from service in the wake of the Battle of Caporetto. He was selected for fighter pilot training, which he began on 16 November 1917 at Malpensa. When that was complete, he went for gunnery training; on 27 December 1917, he passed his gunnery test with a "mediocre" rating. Lega then returned to action as a member of "76a Squadriglia". He would fly 46 combat sorties with them.