Summary of Aircraft Engine Instrument Systems

## Introduction Aircraft engine instruments monitor engine health and performance to ensure safe flight. This guide explains common engine instruments used on reciprocating and turbine engines, how they work, where they read values, and how to interpret their readings. > **Definition:** Engine instruments are cockpit instruments that display engine operating parameters such as RPM, manifold pressure, oil pressure, oil temperature, fuel flow, exhaust gas temperature, and torque. ## Tachometers (Engine Speed Indicators) ### Reciprocating engine tachometers - Calibrated in **hundreds of RPM**. For example, a displayed value of 22 means $2200$ RPM. - On geared reciprocating engines, the tachometer indicates the **crankshaft** speed, not the propeller shaft. > **Definition:** Tachometer — an instrument that measures and indicates the rotational speed of an engine component, typically displayed in RPM or percent of rated RPM. ### Turbine engine tachometers - Calibrated in **percent of the engine's rated takeoff RPM** (often shown as %N1, %N2). For multi-spool engines, separate tachometers show different spool speeds (e.g., N1 and N2). - On a two-spool gas turbine: the **N1** tachometer indicates low-pressure compressor RPM; the **N2** tachometer indicates high-pressure compressor RPM. Fun fact: Pilots monitor percent RPM on turbines because turbine speed limits and performance are expressed relative to rated takeoff speed, simplifying thrust and power management. ## Manifold Pressure Gauge (Reciprocating Engines) - Reads pressure from the **intake manifold** of the engine. - Used with constant-speed propellers to set power via manifold pressure and propeller RPM. > **Definition:** Manifold pressure — the absolute pressure in the engine intake manifold, usually measured in inches of mercury (inHg). ## Oil Pressure and Oil Temperature Indicators ### Oil pressure gauges - Most have a **restrictor** in the line between the engine and the instrument. - Purpose: the restrictor **damps pressure pulsations**, preventing the needle from oscillating. - Common sensing mechanism on reciprocating engines: a **Bourdon tube**. > **Definition:** Bourdon tube — a curved tube that straightens slightly as pressure increases; its movement drives the instrument needle. ### Oil temperature (ratiometer-type) indicators - If the indicator needle **pegs to the high side** as soon as the master switch is turned on, it indicates an **open in the bulb circuit**, causing the instrument to see infinite resistance. - Note: For ratiometer systems, **higher resistance in the bulb circuit yields a higher temperature indication**. ## Fuel Flow Measurement (Fuel-Injected Reciprocating Engines) - Measured by the **pressure drop across the injector nozzles**. - This differential corresponds to flow rate through the injectors. ## Exhaust Gas Temperature (EGT) and Cylinder Head Temperature (CHT) ### Turbine engine EGT - Measured by a set of **thermocouples arranged in an averaging circuit**. - Typically installed in the **tail pipe** of the engine to sample exhaust gas. > **Definition:** Thermocouple — a sensor made by joining two dissimilar metals that produces a voltage proportional to temperature difference. ### Thermocouple materials for turbine EGT - Common alloys: **Chromel** and **Alumel**. ### CHT (thermocouple-type) when engine is off - Should read **ambient (outside) air temperature** when the engine is not operating. Did you know that thermocouple circuits used for EGT average multiple sensor inputs to give a representative exhaust temperature, protecting engine components from localized hot spots? ## Torque Measurement (Turboprop Engines) - A **torquemeter** measures torque by sensing pressure produced in a torque sensor through engine oil. - That pressure is proportional to the **strain in the torsional shaft** that drives the reduction gears of the turboprop engine. > **Definition:** Torquemeter — an instrument that indicates the twist