Chapter 7
The Machmeter

1. High Speed Flight – Why Mach Number Matters

As an aircraft approaches the local speed of sound, airflow over parts of the fuselage or wings may be accelerated to, or beyond, the speed of sound, forming shock waves. Consequences include:

• Increased drag
• Reduced lift
• Mach tuck — a sudden, severe downward-pitching trim change
• Buffeting
• Reduction in, or loss of, control effectiveness

A limiting Mach number (M_MO) is specified for each aircraft based on flight trials. This must not be exceeded. The Machmeter displays the current Mach number so the pilot can remain below M_MO.

2. Speed of Sound

The speed of sound is not constant — it varies with air temperature only, not pressure or density directly.

graph LR
    A["Temperature Increases"] --> B["LSS Increases"]
    C["Altitude Increases (below tropopause)"] --> D["Temperature Decreases"] --> E["LSS Decreases"]
    F["Above Tropopause (ISA)"] --> G["Temperature Constant −56.5°C"] --> H["LSS Constant ~573 kt"]

3. Machmeter Principle of Operation

The Machmeter uses two capsules:

1. Airspeed Capsule — expands/contracts with dynamic pressure (pitot − static). Senses TAS component.
2. Altitude (Aneroid) Capsule — expands/contracts with changes in static pressure in the instrument case. Compensates for the effect of altitude on the speed of sound.

Their combined mechanical output via the ratio arm and ranging arm drives the pointer to display Mach number directly.

4. Machmeter Construction

• Static pressure enters the instrument case (acts on the altitude capsule).
• Pitot pressure is fed directly into the airspeed capsule.
• Expansion of the airspeed capsule is transmitted via the airspeed link and main shaft to the ratio arm.
• The altitude capsule also governs the position of the ratio arm.
• The spring-loaded ranging arm transmits ratio arm movement to the pointer.

An adjustable Mach limit index can be set via a small knob to the aircraft's M_MO, providing a visual speed warning.

5. Machmeter Errors

5.1 Position Error at High Mach Numbers

At higher Mach numbers, changes in airflow can cause position error to increase in magnitude and even change sign. If position error causes the Machmeter to under-read, this could become dangerous. The normal arrangement in modern jet transport aircraft is to bias instrument and position error so the Machmeter always over-reads — providing a safety margin.

6. Blockages

7. Abbreviations

8. Climb at Constant CAS – Standard Atmosphere

Example: Climbing at 330 kt CAS from sea level to FL360 in ISA:

• TAS increases from 330 kt to 593 kt
• Mach number increases from M 0.5 to M 1.05

This rapid rise in Mach number is why high-performance aircraft are flown at constant CAS (or IAS) in the lower part of the climb, then transition to a constant Mach number for the remainder — to avoid inadvertently exceeding M_MO.

9. Descent at Constant Mach Number – Standard Conditions

During a descent in ISA, temperature increases → LSS increases. To maintain constant Mach number, TAS must also increase (MN = TAS/LSS). As density also increases during descent, CAS increases even more rapidly (Dynamic Pressure = ½ρV²). Example: M 0.8 descent from FL400 to MSL:

• At FL400: TAS = 450 kt, CAS = 242 kt
• At MSL: TAS = 528 kt, CAS = 528 kt (would exceed V_MO)

This is why CAS is used in the descent rather than constant Mach number below a certain altitude.

10. Climb/Descent through Isothermal Layer and Inversion

10.1 Isothermal Layer (temperature constant)

10.2 Inversion (temperature increases with altitude)

11. Climb/Descent Summary

flowchart TD
    A["Climb"] --> B{"What is constant?"}
    B -->|CAS| C["TAS increases
Mach number increases"]
    B -->|TAS| D["CAS decreases
Mach number: depends on temp"]
    B -->|Mach number| E["TAS depends on LSS
CAS always decreases"]

12. Worked Example Problems

13. Mach/Airspeed Indicator

Since commercial aircraft need both IAS and Mach indications, the instruments are combined. Two versions exist:

1. Self-contained instrument — fed directly from pitot and static sources.
2. ADC-fed instrument — receives computed data from the Air Data Computer.

Construction Features

• Airspeed pointer moves clockwise over a fixed scale.
• From M 0.5, Mach number is read from the same pointer against a moving Mach scale that rotates anticlockwise as Mach increases.
• A striped VMO needle may mark the maximum operating airspeed.

Master Reference Tables

Answer Key