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speed flight and/or high-altitude flight, the measurement of speed is expressed in terms of a “Mach number”—the ratio of the true airspeed of the aircraft to the speed of sound in the same atmospheric conditions. An aircraft traveling at the speed of sound is traveling at Mach 1.0. Aircraft speed regimes are defined approximately as follows: 

Subsonic—Mach numbers below 0.75 

Transonic—Mach numbers from 0.75 to 1.20 

Supersonic—Mach numbers from 1.20 to 5.00 

Hypersonic—Mach numbers above 5.00 

While flights in the transonic and supersonic ranges are common occurrences for military aircraft, civilian jet aircraft normally operate in a cruise speed range of Mach 0.7 to Mach 0.90. 

The speed of an aircraft in which airflow over any part of the aircraft or structure under consideration first reaches (but does not exceed) Mach 1.0 is termed “critical Mach number” or “Mach Crit.” Thus, critical Mach number is the boundary between subsonic and transonic flight and is largely dependent on the wing and airfoil design. Critical Mach number is an important point in transonic flight. When shock waves form on the aircraft, airflow separation followed by buffet and aircraft control difficulties can occur. Shock waves, buffet, and airflow separation take place above critical Mach number. A jet aircraft typically is most efficient when cruising at or near its critical Mach number. At speeds 5–10 percent above the critical Mach number, compressibility effects begin. Drag begins to rise sharply. Associated with the “drag rise” are buffet, trim, and stability changes and a decrease in control surface effectiveness. This is the point of “drag divergence.” [Figure 5-65] 

V$_{MO}$/M$_{MO}$ is defined as the maximum operating limit speed. V$_{MO}$ is expressed in knots calibrated airspeed (KCAS), while 

M$_{MO}$ is expressed in Mach number. The V$_{MO}$ limit is usually associated with operations at lower altitudes and deals with structural loads and flutter. The M$_{MO}$ limit is associated with operations at higher altitudes and is usually more concerned with compressibility effects and flutter. At lower altitudes, structural loads and flutter are of concern; at higher altitudes, compressibility effects and flutter are of concern. 

Adherence to these speeds prevents structural problems due to dynamic pressure or flutter, degradation in aircraft control response due to compressibility effects (e.g., Mach Tuck, aileron reversal, or buzz), and separated airflow due to shock waves resulting in loss of lift or vibration and buffet. Any of these phenomena could prevent the pilot from being able to adequately control the aircraft. 

For example, an early civilian jet aircraft had a V$_{MO}$ limit of 306 KCAS up to approximately FL 310 (on a standard day). At this altitude (FL 310), an M$_{MO}$ of 0.82 was approximately equal to 306 KCAS. Above this altitude, an M$_{MO }$ of 0.82 always equaled a KCAS less than 306 KCAS and, thus, became the operating limit as you could not reach the V$_{MO }$ limit without first reaching the M$_{MO}$ limit. For example, at FL 380, an M$_{MO}$ of 0.82 is equal to 261 KCAS. 

## Mach Number Versus Airspeed 

It is important to understand how airspeed varies with Mach number. As an example, consider how the stall speed of a jet transport aircraft varies with an increase in altitude. The increase in altitude results in a corresponding drop in air density and outside temperature. Suppose this jet transport is in the clean configuration (gear and flaps up) and weighs 550,000 pounds. The aircraft might stall at approximately 152 KCAS at sea level. This is equal to (on a standard day) a true velocity of 152 KTAS and a Mach number of 0.23. At FL 380, the aircraft will still stall at approximately 152 KCAS, but the true velocity is about 287 KTAS with a Mach number of 0.50. 

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