NASA Dryden News Releases

Ice In or On Static System Cause of X-31 Crash

November 7, 1995

Release: 95-33

A Mishap Investigation Board studying the cause of the X-31 experimental aircraft accident on Jan. 19, 1995, has concluded that an accumulation of ice in or on the unheated pitot-static system on the aircraft provided false airspeed information to the flight control computers, causing the aircraft to go out of control and crash.

The aircraft was one of the two X-31s operated by an International Test Organization (ITO) located at NASA's Dryden Flight Research Center, Edwards, Calif. It was being flown back to Edwards Air Force Base following a research flight when it departed from controlled flight at an altitude of 20,000 feet and crashed near the northern boundary of Edwards. The pilot, Karl-Heinz Lang, Federal Republic of Germany, ejected safely.

The pitot-static pressure system, using a small tube called a Kiel probe at the nose of the aircraft, provided air speed data to instruments in the cockpit, the aircraft's flight control computers, and to the mission control center monitors at Dryden.

Near the final portion of the approximately 43-minute flight, ice formed in or around the pitot tube. This led to a false reading of total air pressure data and caused the aircraft's flight control system to automatically misconfigure for a lower speed. The aircraft suddenly began oscillating in all axes, pitched up to over 90 degrees angle of attack and became uncontrollable, prompting the pilot to eject.

The Board recommended that training be conducted on the system safety analysis process, that procedures be implemented to assure all test team members receive configuration change notices, and that improvements be made in the remaining X-31 to prevent similar single-point failures from causing catastrophic consequences.

The X-31 was being flown to study the use of thrust vectoring as a way of enhancing the maneuverability of future fighter aircraft. The project was managed by the Advanced Research Projects Agency (ARPA), and included participation by NASA, the U.S. Navy, U. S. Air Force, Rockwell Aerospace, the Federal Republic of Germany and Daimler-Benz (formerly Deutsche Aerospace).

Initial flight phase of the highly successful program, which began in October 1990, was based at Palmdale, Calif., where the aircraft were assembled by Rockwell Aerospace. The ITO was located at Dryden in February 1992.

The mission flown by Lang on Jan. 19 was the 524th of the X-31 program. The flight set a record for the most flights of any experimental aircraft flown at Dryden. After the accident, the remaining X-31 was brought back to flight status and in June 1995 appeared at the Paris Air Show. Reviewers of the air show commented that "the X-31 maneuvers steal the show" and "the Rockwell/DASA X-31's daily flight display brought all other activity at the show to a stop."


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