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EASA ATPL 081 Principles of Flight study materials
EASA ATPL 081 Principles of Flight study resources

EASA ATPL 081 Principles of Flight: High-Speed Flight: Compressibility, Mach Number, Shock Waves, Critical Mach, Buffet and Mach Trim

EASA ATPL 081 Principles of Flight · Work through your exam, one topic at a time.

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High-Speed Flight: Compressibility, Mach Number, Shock Waves, Critical Mach, Buffet and Mach Trim

This domain covers what changes when the air can no longer be treated as incompressible: local Mach number, shock waves, wave drag and the buffet boundary. Items test Mach calculations, critical Mach, and the handling effects at high Mach.

Mach and the speed of sound

  • M = TAS/LSS. The speed of sound depends only on temperature: a = 38.94√T in knots, or 20.05√T in m/s, with T in kelvin.
  • ISA sea level gives 661.5 kt; at the tropopause (−56.5 °C, 216.65 K) it is about 573 kt.
  • IAS and Mach are the measured quantities; TAS and LSS are derived from them.

Critical Mach and shock waves

  • Mcrit is the free-stream Mach at which the local flow somewhere first reaches M = 1, usually at the upper-surface suction peak.
  • Beyond Mcrit a shock wave forms, bringing wave drag, buffet and possible shock-induced separation (shock stall).
  • Sweepback raises Mcrit approximately in proportion to cos of the sweep angle; thinner sections also raise it.
  • Tuck-under: the centre of pressure moves aft at high Mach, giving a nose-down pitching moment; the Mach trim system counteracts it.

High-altitude limits

  • The coffin corner is the altitude where the low-speed buffet (stall) and high-speed buffet (Mach) boundaries converge.
  • Mmo and Vmo are separate limits; either may be reached first.

Worked example

At FL350 the SAT is −54 °C, or 219 K. Then a = 38.94 × √219 = 576 kt. At M 0.80 the TAS is 0.80 × 576 = 461 kt. If the temperature rises to −45 °C, 228 K, the speed of sound becomes 588 kt, so Mmo is reached at a higher TAS but at the same Mach number - the Mach limit is unchanged, the TAS at which it occurs is not.

Traps

  • Mach number depends on temperature, not on pressure or density directly.
  • Mcrit falls as CL rises, so a heavy, slow or high-altitude cruise reaches Mcrit sooner.
  • Buffet is not always a stall; high-speed buffet comes from the shock wave.
  • Overspeed in IAS and overspeed in Mach are different events with different limits.
  • Shock-induced separation can cause control reversal or pitch-up, not just drag.