Machmeter

Everything about the Machmeter, including:

• what it is,
• how Mach is measured and calculated,
• how a mechanical Machmeter works, and
• how its linkage system operates — all in one clear explanation 👇

✈️ Machmeter — Principle, Measurement, and Mechanism

🧭 1️⃣ What Mach Means

The Mach number (M) is the ratio of the aircraft’s True Airspeed (TAS) to the local speed of sound:

$$ M = \frac{\text{TAS}}{\text{a}} $$

Where:

• TAS = True Airspeed
• a = speed of sound (depends on air temperature)

At sea level, the speed of sound ≈ 340 m/s (661 kt), but it decreases with altitude because air temperature drops.

⚙️ 2️⃣ How Mach Is Measured in Aircraft

Modern aircraft calculate Mach using pressures from the pitot-static system:

• Total (pitot) pressure ( p_t )
• Static pressure ( p_s )

The Machmeter (or Air Data Computer) determines Mach from their ratio:

$$ M = \sqrt{\frac{5}{2}\left[\left(\frac{p_t}{p_s}\right)^{2/7} - 1\right]} $$

This formula comes from compressible airflow laws (using γ = 1.4 for air).

✅ So — the Mach number depends only on total pressure and static pressure.

📟 3️⃣ What a Machmeter Does

The Machmeter shows the aircraft’s Mach number, not its airspeed. It’s crucial at high altitudes because the speed of sound changes with temperature — so IAS or TAS alone don’t show how close the aircraft is to its Mach limits (MMO).

⚙️ 4️⃣ Mechanical Machmeter — How It Works

Before digital systems, the Mach number was indicated mechanically using only pitot and static pressures.

🔹 Main Components

The instrument compares total and static pressures to display Mach number directly.

🧩 5️⃣ Principle of the Linkage

Inside the Machmeter, a mechanical linkage (levers, gears, and a cam) combines the two pressure effects.

🔸 Step-by-step:

1. Pitot capsule expands as total pressure increases → pushes the pointer toward higher Mach.
2. Static capsule also expands as altitude increases (pressure decreases) → adjusts the pitot effect.
3. The linkage blends these two movements so that the pointer shows the correct Mach ratio for any altitude.

👉 The linkage is shaped so that the movement matches the non-linear relationship between pressure ratio and Mach number.

🪀 Conceptual Diagram

Pitot (total pressure) capsule  → expands with speed
        │
        ▼
   Lever/Cam system ←── Static (altitude) capsule
        │
        ▼
    Mach pointer (dial)

• More pitot pressure → higher Mach
• Less static pressure (higher altitude) → higher Mach for the same speed

🧠 6️⃣ Why It’s Important

• At high altitude, IAS decreases but Mach increases for the same TAS.
• Aircraft must stay below a maximum Mach limit (MMO) to avoid compressibility effects and shock waves.
• The Machmeter gives an accurate, altitude-compensated indication of this limit.

✅ 7️⃣ Summary

✈️ In short:

The Machmeter measures how fast the aircraft is flying relative to the speed of sound. In a mechanical Machmeter, pitot and static pressures move two capsules connected by a linkage system that combines their effects — showing Mach number directly, automatically compensating for altitude and temperature.