While testing an early turn and slip indicator devised by his friend Elmer Sperry in 1918, United States Army Air Corps pilot William Ocker entered a graveyard spiral while flying through clouds without visual references; the turn indicator showed he was in a turn, but his senses told him he was in level flight. Emerging from the clouds, Ocker was able to recover from the dive. In 1926, Ocker was subjected to a Bárány chair equilibrium test by Dr. David A. Myers at Crissy Field; the resulting duplication of the somatogyral illusion he had experienced and a subsequent re-test, which he passed using the turn indicator, led him to develop and champion instrumented flight. Sperry would go on to invent the gyrocompass and attitude indicator, both of which were being tested by 1930. With Lt. Carl Crane, Ocker published the instructional text "Blind Flying in Theory and Practice" in 1932. Influential advocates of instrumented flight training included Albert Hegenberger and Jimmy Doolittle.