Mach number, the Machmeter and the air data computer
Navigation · Chapter 24 · DGCA CPL and ATPL
Why fast jets fly by Mach, how Mach links TAS and temperature, how the Machmeter reads it, what CAS, TAS and Mach do in a climb, and the air data computer.
Remember
- Mach = TAS ÷ LSS; LSS = 38.95 √K (temperature only): 661 kt at +15 °C, 573 kt at −56.5 °C.
- Machmeter: (pitot − static) ÷ static; errors instrument, position, manoeuvre-induced.
- Blocked: SUC SOD POC PUD (as the ASI).
- Climb at constant CAS: TAS, Mach up. Constant Mach: CAS down. Constant TAS: CAS down. Descent reverses.
- ADC: pitot, static, TAT in; altitude, CAS, Mach, TAS, V/S, SAT out. Not an emergency altimeter.
What happens to the air over a wing near the speed of sound?
Why jets need a Mach limit
- Faster over the top • Air speeds up over the curved top of the wing • Blue slow, red fast
- Critical Mach • At the critical mach number the air over the top of the wing first reaches Mach 1, though the aircraft itself is slower
- A shock wave • Beyond it a supersonic patch ends in a shock wave • Drag rises, lift falls, the flow separates: buffet
- MMO high, VMO low • So jets have a Mach limit, mmo, that bites high up • And a CAS limit, VMO, that bites low down. Descending at the limit, you meet MMO first
What is Mach number, and why does it change as you climb?
Mach = TAS ÷ LSS
- Speed of sound • The lss (local speed of sound) depends on temperature only • +15 °C: about 661 kt · −56.5 °C: about 573 kt
- Mach • Mach = TAS ÷ LSS: your speed as a fraction of the speed of sound
- Climbing, LSS falls • Colder up high → slower sound → the same TAS is a bigger Mach • Above the tropopause (−56.5 °C) LSS stays 573 kt
Solve it: TAS, Mach and temperature
Solve it: Mach sums
This lesson is part of the Navigation plan. See plans
Same Mach and same TAS at two levels: what happened to the temperature?
Same Mach, same TAS
This lesson is part of the Navigation plan. See plans
How can one instrument read TAS ÷ LSS?
Inside the Machmeter
This lesson is part of the Navigation plan. See plans
Which errors does a Machmeter have?
Errors and the combined dial
This lesson is part of the Navigation plan. See plans
Static or pitot blocked: what does the Machmeter do?
Blocked lines
This lesson is part of the Navigation plan. See plans
Climb at a constant CAS, Mach or TAS: what do the others do?
Hold one speed, watch the others
This lesson is part of the Navigation plan. See plans
Height up, speed across: which line leans which way?
The three graphs
This lesson is part of the Navigation plan. See plans
Same flight level, the air warms up: what changes?
Same level, warmer air
This lesson is part of the Navigation plan. See plans
One box for every air-data user: what goes in, what comes out?
The air data computer
This lesson is part of the Navigation plan. See plans
Test yourself on chapter 24
Test yourself
This lesson is part of the Navigation plan. See plans