This is a Significant Operational milestone.
This was a free-hover flight. The pilot was David Howe.
Source: "The X-14 - 24 years of V/STOL Flight Testing", Society of Experimental Test Pilots, Technical Review, Vol. 16, No. 2, 1981, p. 3-21.
Source: "X-14 Flies: Transition Test to Follow", Aviation Week, July 8, 1957