Definition
Speed regimes are the recognised ranges of airspeed in which an aircraft operates, each with distinct aerodynamic behaviour relative to the speed of sound. The four regimes are subsonic (below about Mach 0.75), transonic (about Mach 0.75 to 1.20), supersonic (about Mach 1.20 to 5.00), and hypersonic (above Mach 5.00). The boundaries are set by how airflow over the aircraft behaves as it approaches and exceeds the local speed of sound, not by any single fixed airspeed.
Plain English
Different speed bands an aircraft can fly in, each one behaving differently because of how air flows around the aircraft as it gets closer to or past the speed of sound.
Context Anchor
Seen in thrust and performance discussions, especially when comparing takeoff, climb, and cruise behavior.
Derivation
Regime' comes from the Latin 'regimen', meaning a system or set of rules. A speed regime is therefore a speed range with its own set of aerodynamic 'rules' — what works in one regime may not work in another.
Why Pilots Care
Pilots must recognize when entering different speed regimes to adjust power and pitch appropriately and avoid hazards like the back side of the power curve.
Grounding Statement
The same airplane can produce and use thrust differently at slow speed than it does at higher speed.
Intuition Check
Do not read regime as a political system here. In this context, a speed regime means a range of speeds where the airplane or airflow behaves in a similar way.
Example Sentence 1
Light piston aircraft operate entirely within the subsonic speed regime.
Example Sentence 2
In high-speed regimes, induced drag decreases while parasite drag increases significantly.