Yeager’s pilot report for his 14 October 1947 flight in the X-1. All Mach numbers were presented as “Machi” which meant they were the “indicated” Mach number readings from Yeager’s Mach meter and not necessarily his “true” airspeed at any point in time. Flight instruments of that era were subject to “lag.” Thus, while the Mach meter indicated Mach 0.92, the aircraft could actually be moving at a speed of 0.94 or 0.95 Mach.