Early in October, Yeager reached mach 0.94 and had a nasty surprise-he pulled back on the control column and nothing happened. The plane continued to fly as if he hadn't touched the controls. Wisely he shut down the rocket engine; as the plane decelerated, control effectiveness returned to normal. Williams's engineers later determined that a shock wave had formed on the horizontal stabilizer; as the XS-1 increased its speed, the shock wave had moved rearward, "standing" right along the hinge line of the plane's elevator surfaces (which control pitch) at mach 0.94, negating their effectiveness. Fortunately the XS-1 had been designed with an adjustable stabilizer, so the NACA-Air Force team decided to control the craft with the conventional up to where it lost its effectiveness, then use the stabilizer "trimmer" for longitudinal (pitch) control as the XS-1 approached the speed of sound. On October 10,  Yeager again reached an indicated mach 0.94. During the glide earthwards, frost formed on the inside of the canopy, and despite persistent efforts Yeager could not scrape it off. Chase pilots Bob Hoover and Dick Frost, flying Lockheed P-80 Shooting Stars, had to "talk" him down to a blind landing on the lakebed.