SANTA CLARITA — An X-31 experimental jet spun out of control and crashed in the desert north of Edwards Air Force Base on a test flight earlier this year because ice had built up on an unheated air speed measuring device, an investigatory committee has concluded.
The pilot, who parachuted, survived with serious injuries.
An ad hoc committee of U.S. and German military officials, in findings released Tuesday, said that the ice caused the device to give incorrect data to the computers that help fly the plane.
The advanced experimental jet, one of only two in existence, was being used in a $230-million program managed by the U.S. government's Advanced Research Projects Agency for the United States and Germany.
The aircraft was in the final phase of the 43-minute flight on Jan. 19 when ice formed in or around a small tube located in the nose of the plane known as a Kiel probe. The tube was not heated because "flying in the desert, [researchers] thought it was unnecessary," said J.D. Hunley, a spokesman for NASA's Dryden Flight Research Center at Edwards.
The ice formation led to a false transmission of air pressure data to the cockpit, the flight control computers and to the mission control centers at Dryden. Wrongly concluding that the plane was flying more slowly than it was, the X-31's computer reconfigured the controls for a slower 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," according to a NASA statement.
The pilot, Karl-Heinz Lang of Germany, ejected safely from 20,000 feet. But he suffered serious injuries, according to the committee's Aug. 18 report.
The X-31 is equipped with paddles that can change the direction of the jet exhaust, enabling the pilot to turn more tightly and point at targets more quickly than conventional fighter craft.
"The X-31 . . . was designed to fly in these difficult positions to test out the aerodynamic theory and push forward the envelope of flight," Hunley said.
Two X-31s, which together had more than 500 flights, were built by Rockwell Aerospace and Deutsche Aerospace as part of a joint project by NASA, the U.S. Navy, the U.S. Air Force, the German Federal Ministry of Defense, Rockwell Aerospace and Daimler-Benz.
Hunley was unable to provide the exact cost of the downed jet. But Lt. Col. Donna Parker, a Pentagon spokeswoman, put the U.S. contribution to the program at $190 million, and the German contribution at $40 million in 1993, the latest year for which figures were available.
The board recommended changes in X-31 equipment and training, but the crash came near the end of the X-31 test program. It did not affect the outcome, said NASA spokesman Don Haley. "The program was better than a 100% success," Haley said.
Soon after the crash, changes were made in the remaining X-31 and it flew several more flights at Edwards before a final appearance in the Paris Air Show last June, Haley said.
"There are no plans at this time to return it to flight status," Haley said.