CAF Apologist Michelle Arnold has the answer to your question:
Question: Why is St. Therese of Lisieux (1873–1897) a patron of aviators?
Answer: In the early years of the twentieth century, St. Therese’s spiritual testament Story of a Soul became a sensation in Europe and, later, around the world, bringing Therese a legion of devotees. Students of Therese’s life would eventually call this period the “storm of glory,” because of all of the favors from heaven it was reported that Therese obtained for those petitioning her intercession. During this time, World War I broke out and many soldiers on both sides reported seeing visions of a young nun—Therese had not yet been canonized—comforting wounded men of various nationalities along the battle lines. In response to this, many soldiers began carrying pictures of Therese with them into battle. She became a special favorite of French pilots, and this is where her intercession for aviators began.