Conversation so far:
When did Chesley Sullenberger begin his military service?
Sullenberger was appointed to the United States Air Force Academy, entering with the Class of 1973 in June 1969.
Which branch of the military did Chesley Sullenberger join?
Chesley Sullenberger joined the Air Force.

Question: When did he become a pilot?
Self-contained rewrite: When did Chesley Sullenberger become a pilot?