What does a tachometer measure in aircraft?

A tachometer measures the angular speed (RPM or percentage) of an engine's rotating shaft.

What are the main types of tachometers used in aviation?

The main types are mechanical (rotating magnet), electrical (tachogenerator), and electronic (speed probe with phonic wheel).

Why are mechanical tachometers unsuitable for multi-engine or remote engine installations?

Mechanical tachometers are limited by cable length and are only practical when the engine is close to the cockpit.

How does an electronic tachometer work?

It uses an inductive probe and a toothed wheel to generate pulses, which are processed to indicate RPM digitally.

Can engine torque be displayed as a percentage?

Yes, engine torque is often displayed as a percentage, especially in turbine-powered aircraft.

Tachometer and Engine Instrumentation

Medium4 min readInstrumentation

Moderately Examined

Why this matters

Accurate engine speed monitoring is vital for safe flight, engine health, and performance management. Misreading or misunderstanding tachometer indications can lead to engine damage or loss of control, especially during critical phases like takeoff or emergency situations.

Tachometers and engine instrumentation are essential for monitoring engine speed and performance in aircraft. Different types of tachometers—mechanical, electrical, and electronic—are used depending on engine type and aircraft design. Understanding how each system works is crucial for interpreting engine data and ensuring safe operation.

Quick Check

Which type of tachometer is most commonly found in small piston-engine aircraft with the engine close to the cockpit?

AI Tutor

Go beyond the textbook.

Ask Avi AI about Tachometer and Engine Instrumentation

In depth

Explanation

Types of Tachometers in Aircraft

Mechanical Tachometer: Commonly found in light piston-engine aircraft, this type uses a flexible cable connected directly to the engine. The cable spins a magnet inside the indicator, inducing eddy currents in a metal cup. The resulting force moves a pointer to display RPM. Mechanical tachometers are simple, reliable, and require no external power, but are limited by cable length—typically only suitable for engines close to the cockpit.
Electrical Tachometer (Tachogenerator): Used when engines are further from the cockpit, this system employs a small generator mounted on the engine. As the shaft turns, it produces an electrical signal (DC, single-phase AC, or three-phase AC) proportional to engine speed. The cockpit gauge reads this signal. These systems do not need external electrical power, but DC types can suffer from voltage drops and radio interference due to brush wear.
Electronic Tachometer: Essential for turbine engines and modern aircraft, electronic tachometers use a speed probe and phonic wheel. An inductive sensor detects the passage of ferrous teeth on a rotating wheel, generating an AC signal. This is processed into digital pulses, which are counted to determine RPM. These systems require aircraft electrical power and are highly reliable for remote or high-speed engines.

Display and Units

Analogue Displays: Traditional dials with needles and colour-coded arcs are common in older or non-glass cockpit aircraft.
Digital Displays: Modern aircraft may use digital readouts or integrate engine data into electronic flight instrument systems (EFIS).
Units: Piston engines display RPM (revolutions per minute), while turbine engines typically show speed as a percentage of maximum rated RPM. Engine torque can also be displayed as a percentage.

System Comparison

Mechanical systems are heavier and limited by installation constraints but are independent of electrical power.
Electrical and electronic systems are lighter and allow for flexible installation, but may depend on aircraft power for operation.

Engine Instrument Errors

Mechanical and electrical systems can be affected by cable length, voltage drop, or electromagnetic interference.
Electronic systems are robust against signal loss but will not indicate RPM if electrical power fails.

The essentials

Key Points

Tachometers measure engine rotational speed (RPM or %RPM).

Mechanical tachometers use a spinning cable and magnet; no external power needed.

Electrical tachometers use tachogenerators, producing voltage proportional to RPM.

Electronic tachometers use inductive probes and phonic wheels, requiring electrical power.

Piston engines display RPM; turbines typically use percentage of max RPM.

Analogue and digital displays are both used, depending on aircraft type.

Engine torque can be displayed as a percentage on some instruments.

Watch out

Exam Traps & Typical Mistakes

Confusing tachometer with hour meters or vibration indicators.

Assuming all tachometers need electrical power—mechanical and some electrical types do not.

Overlooking the limitations of mechanical tachometers (cable length, single-engine use).

Mixing up the error sources: voltage drop affects electrical, not electronic tachometers.

Believing all turbine engines use mechanical tachometers—most use electronic systems.

Test yourself

Example Exam Questions

Question 2Medium

What is a key disadvantage of electronic tachometers compared to mechanical or electrical types?

Question 3Easy

In turbine-engine aircraft, engine speed is typically displayed as:

Still not fully confident?

Deepen your knowledge with an AI tutor built specifically for EASA ATPL students.

Built from thousands of ATPL knowledge references, real exam references and official learning objectives.

Open Avi AI Tutor

Keep going

Related Concepts

Still have questions?

Ask questions in plain English and get exam-focused explanations from an AI tutor built specifically for EASA ATPL students.

Open Avi AI