# High Speed Flight: Definitions

> Mach number, speed of sound, and flight regimes.

- **Subject:** 081 — Principles of Flight
- **Section:** 081-02 — High Speed Aerodynamics
- **Topic code:** 081-02-01
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/principles-of-flight/high-speed-aerodynamics/speeds/

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## Speed of Sound ($a$)
The speed at which small pressure disturbances travel through a medium.
*   Depends ONLY on **Temperature** (in air).
*   Formula: $a = 38.94 \sqrt{T}$
    *   $T$: Absolute temperature in Kelvin.
    *   As altitude increases (in troposphere), Temperature decreases -> Speed of Sound decreases.

## Mach Number ($M$)
The ratio of True Airspeed (TAS) to the Local Speed of Sound ($a$).
$$ M = \frac{TAS}{a} $$
*   **Constant Mach Climb**: As altitude increases, $a$ decreases. To maintain constant $M$ ($TAS/a$), TAS must decrease.
*   **Constant Mach Descent**: As altitude decreases, $a$ increases. To maintain constant $M$, TAS must increase.

## Flight Regimes
1.  **Subsonic**: Airflow over the entire aircraft is subsonic ($M < 1.0$). Typically $M < 0.75$.
2.  **Transonic**: Some airflow is subsonic, some is supersonic. Typically $M 0.75 - M 1.20$.
    *   This is where shockwaves begin to form.
3.  **Supersonic**: Airflow over the entire aircraft is supersonic ($M > 1.20$).
4.  **Hypersonic**: $M > 5.0$.

## Critical Mach Number ($M_{crit}$)
The free stream Mach number at which the airflow over **some point** on the aircraft first reaches the speed of sound ($M = 1.0$).
*   Flying faster than $M_{crit}$ creates shockwaves.

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