1966 Collision of XB-70, F-104 Drew Attention to Wing Vortex Issue
In 1961, President John F. Kennedy signed an initiative endorsing the development of an American supersonic transport – SST for short – capable of flying three times the speed of sound.
As part of that initiative, NASA's Flight Research Center (today the Dryden Flight Research Center) at Edwards Air Force Base, Calif., began flight experiments with a Navy A-5A Vigilante in 1963, exploring landing approaches of such a high-speed aircraft in a crowded air traffic environment. Pilots flew the aircraft on approaches to Edwards, but also into Los Angeles International Airport for more realistic simulations of integrating an SST into landing patterns in a high-volume commercial airport.
The next phase of the effort would involve North American Aviation's XB-70.
The Valkyrie, as it was named, was enormous for its time: its cockpit was three stories in the air, its delta wings stretched 105 feet and it was 185 feet long. Six General Electric YJ93 jet engines could propel the plane at speeds up to Mach 3, three times the speed of sound. Weighing a half-million pounds due to its stainless steel rather than aluminum construction, the Valkyrie was designed as an intercontinental bomber and it featured an advanced aerodynamic design, including canards and drooping wing tips. The program was cancelled before the aircraft went into production, however, leaving two prototypes that became research aircraft.
The Air Force planned to turn over the second XB-70 to the Flight Research Center in June 1966 for a NASA SST research program. But before it did so a photo shoot was planned. Like many other Air Force aircraft, the XB-70 was powered by General Electric jet engines, and the manufacturer wanted a group formation photo of those planes.
On June 8, 1966, an F-4B Phantom, a YF-5A Freedom Fighter, a Lockheed F-104N Starfighter, and a T-38A Talon formed up on the huge, white XB-70 over California's high desert near Edwards. The photo chase aircraft that day was a Learjet owned by singer Frank Sinatra. Flight Research Center chief pilot Joe Walker, who had flown the X-15 rocket plane, the Lunar Landing Research Vehicle and many other unique research aircraft, was flying one of the center's F-104s just off the Valkyrie's right wing.
Without warning, Walker's F-104 was suddenly drawn in toward the bomber. His aircraft clipped the right wing tip, rolled up and over, struck the XB-70's right vertical fin, sheered off most of the left vertical fin, and exploded into a ball of fire as it glanced off the left wing. Walker died instantly.
"Mid-air! Mid-air! Mid-air!" yelled one of the chase pilots over his radio.
For 16 seconds the XB-70 continued to fly straight and level. Then the experimental bomber began a slow roll into an inverted spiral; portions of a wing broke away and fuel began streaming from the stricken aircraft.
"Eject! Eject! Eject!" yelled another chase pilot, as the airplane began falling away toward the hills north of Barstow, Calif. North American Aviation pilot Al White managed to eject, although he was severely injured in the process. Air Force Maj. Carl Cross could not get out, however, and died in the crash.
The crash investigation pointed to the wake vortex of the XB-70's wingtips as the reason for the F-104's sudden roll over and into the bomber. Not much was understood about wake vortices at the time, although they are now recognized as very powerful and potentially deadly mini-tornados trailing from an aircraft's wingtips. Regardless of the cause, in less than two minutes, the Air Force and NASA had lost two aircraft and, much worse, two talented test pilots.
Although not as capable in performance as the aircraft that crashed, the remaining XB-70 was pressed into service. It flew for another two years conducting sonic boom research in connection with the SST program until it was retired when that program was cancelled.
In early 1969, NASA research pilot Fitzhugh Fulton and Air Force Lt. Col. Emil "Ted" Sturmthal ferried the surviving XB-70 to Wright-Patterson Air Force Base, Ohio, where it remains on display today at the National Museum of the U.S. Air Force.
By Christian Gelzer
NASA Dryden Flight Research Center