As an airplane approaches its coffin corner, the margin between stall speed and critical Mach number becomes smaller and smaller. Small changes could put one wing or the other above or below the limits. For instance, a turn causes the inner wing to have a lower airspeed, and the outer wing, a higher airspeed. The aircraft could exceed both limits at once. Or, turbulence could cause the airspeed to change suddenly, to beyond the limits. Some aircraft, such as the Lockheed U-2, routinely operate in the "coffin corner". In the case of the U-2, the aircraft was equipped with an autopilot, though it was unreliable. The U-2's speed margin, at high altitude, between 1-G stall warning buffet and Mach buffet can be as small as 5 knots.