Chapter 38

1. Introduction — Engine Instrument Categories

Engine instruments are divided into two fundamental categories:

2. Types of Display

Both types convey essentially the same information, but the glass cockpit offers greater flexibility and is now the preferred means of displaying both flight and engine instrumentation. A small number of conventional gauges are retained in glass cockpit aircraft for backup in case of electronic display failure.

3. Thrust and Power Measuring Instruments

3.1 Jet Pipe Pressure (P7) Gauge

Measures jet pipe (exhaust) pressure directly. Calibrated in inches of mercury (inHg), pounds per square inch (psi), or percentage of maximum thrust.

3.2 Engine Pressure Ratio (EPR) Gauge

Measures the ratio of jet pipe pressure to engine air intake pressure. This is the most common thrust indicator on turbofan engines.

3.3 Engine Torque (Turboprop/Turboshaft)

Turboprop and turboshaft engines produce torque, not direct jet thrust. Power = Torque × rpm.

Torque is measured between the engine and the reduction gearbox.

Torque indicator units vary by system: psi, inch/foot pounds, newton metres, BHP/SHP, or percentage of maximum. The indicator can show negative torque (windmilling propeller). On FADEC systems, limits can be digitally adjusted.

4. Engine RPM Measurement (Tachometers)

The tachometer (tacho) measures engine rotational speed. For piston engines, it measures crankshaft speed. For gas turbines, it measures compressor speed. There are three basic methods:

4.1 Mechanical (Magnetic) Tachometer

Found on older piston aircraft. Uses a flexible drive shaft connected to a drag-cup indicator:

1. A magnet is driven by the engine via the flex drive
2. The rotating magnet induces eddy currents in an aluminium/copper drag-cup
3. The eddy currents create a torque pulling the drag-cup in the direction of magnet rotation
4. A hairspring opposes the motion; the equilibrium position moves the pointer
5. Temperature compensation devices are incorporated

4.2 Electrical Generator System (Tacho-Generator)

Possibly the oldest system still in use on large aircraft. Uses a small 3-phase tacho-generator driven by the engine. Output feeds an indicator with an asynchronous motor driving a drag-cup and pointer. Displays percentage of maximum engine speed or actual rpm.

4.3 Inductive Probe (Speed Probe) System

Used where tacho-generators cannot be driven from intermediate or low-pressure shafts. A speed probe is positioned on the compressor casing aligned with a phonic wheel (toothed gear) or actual fan blades.

• As the spool rotates, it alters magnetic flux in the probe
• This changes the current in the probe coil
• The frequency of change is directly proportional to spool speed
• The frequency signal drives the indicator

Advantages: fewer moving parts; can provide multiple outputs (speed indication, engine start warning lamp, automatic power control, flight data acquisition).

4.4 Colour Coding on RPM Indicators

4.5 Synchroscope

On multi-engine aircraft, engine speed must be synchronised to reduce structural vibration and noise. The Synchroscope provides a qualitative indication of speed difference between engines:

• One engine is designated master; others are slaves
• Displays whether slave engines are running faster or slower than the master
• Designed to operate from AC generated by the tachometer system

5. Temperature Sensing Equipment

Engine temperature ranges from −56°C to +1200°C. Four types of measuring devices are used:

flowchart LR
    A[Temperature
Measurement] --> B[Expansion Type
e.g. mercury thermometer
bimetallic strip]
    A --> C[Vapour Pressure Type
liquid-to-vapour state change
pressure ∝ temp]
    A --> D[Electrical Type
Resistance bulb RTD
Thermocouple Seebeck Effect]
    A --> E[Radiation Type
Pyrometry
emissivity-based optical]
    B --> F[Low temp
Direct reading]
    C --> F
    D --> G[High temp
Remote sensing]
    E --> G

5.1 Thermocouple (Thermo-electric Type) — Most Important for Turbines

Two dissimilar metals joined at a junction produce a small voltage (thermo-EMF) proportional to the temperature at the junction — the Seebeck Effect.

5.2 Temperature Probe Locations and Terminology

Probe position depends on the material's ability to withstand the temperature encountered. TIT/TET locations have the highest temperatures; EGT probes are downstream where temperatures are lower and metal endurance is sufficient.

5.3 Air Temperature — SAT, RAT, TAT

6. Pressure Gauges

Pressure = force per unit area. Common units: psi, inHg, bar (1 bar = 14.5 psi), pascal (1 bar = 100,000 Pa). Pressure gauges measure gauge pressure (difference between absolute and atmospheric pressure).

Elastic Pressure Sensing Elements

Remote Indicating Pressure Systems

Where it would be impractical to run fluid pipelines to the cockpit (e.g., outer engine of a B747), remote indicating systems are used:

• A transmitter at the pressure source
• An indicator (receiver) on the cockpit panel
• Can be AC or DC operated
• Advantage: hazardous fluids (e.g., engine oil, hydraulic fluid) need not be piped into the cockpit
• Saves weight by reducing pipeline length

7. Engine Vibration Monitoring

Vibration Monitoring Equipment (VME) is fitted to almost all commercial jet aircraft. Gas turbines have very low inherent vibration; any increase is indicative of damage that may lead to failure.

Sensor Types

VME Operation

Both sensor types are suspended within a fixed coil carrying 115 volts at 400 Hz. The sensor moves in sympathy with engine vibration. The signal is:

1. Filtered: only frequencies indicative of damage pass through (normal engine frequencies are erased)
2. Amplified by the amplifier
3. Rectified and sent to the vibration indicator

Vibration is measured and displayed in Relative Amplitude (Rel Ampl). If vibration exceeds a predetermined threshold, a warning light illuminates on the instrument.

8. Fuel Quantity and Flow Measurement

8.1 Volume vs. Mass Measurement

8.2 Float System (DC Volume)

A float rests on the fuel surface. Its movement repositions a wiper on a variable resistor, altering current to a cockpit indicator. Simple but has two significant errors:

1. Fuel tanks are rarely symmetrical — float level is not a true measure of quantity
2. Errors occur during manoeuvres as attitude changes affect float position

The gauge is set to be accurate only at the low and empty positions.

8.3 Capacitance Fuel Gauge System

The standard system on commercial aircraft. Measures mass using the relationship between fuel level (dielectric permittivity) and capacitance.

Typical Dielectric Values

Reference Unit and Temperature Compensation

• A Reference Unit is always submerged in the unusable fuel to improve accuracy when permittivity changes from normal
• A Compensating Capacitor corrects for temperature-induced changes in fuel density and Er, allowing the system to indicate mass rather than volume
• The system senses changes in Specific Gravity (SG) to indicate mass

Fail-Safe and Test Features

• On failure: indicator slowly drives to zero (fail-safe)
• A test circuit simulates tank emptying; on release, pointer returns to original position
• A Fuel Totalizer can display the sum of all tank gauges

8.4 Fuel Flowmeter

Measures instantaneous fuel consumption rate. Units: volume flow (gallons/hr or litres/hr) or mass flow (lb/hr or kg/hr).

Modern flowmeter uses an electrical sensor with a helical vane impeller containing an embedded magnet. Fuel flow rotates the impeller → pick-off coil generates a sinusoidal signal at frequency proportional to volume flow rate. Temperature correction converts this to mass flow.

The flowmeter is located in the high-pressure fuel line to the burners. An Integrated Flowmeter displays both fuel flow rate and total fuel consumed (rate integrated over time).

9. Remote Signal Transmission & Flight Hour Meter

Remote indicating systems are used to indicate the position of valves, flaps, or levers on engines to the cockpit. Each system comprises:

• A Transmitter at the source position to be measured
• A Receiver (indicator) on the appropriate cockpit panel
• Can be AC or DC operated

Flight Hour Meter

Records engine and systems usage time. Activated automatically via the weight-on-wheels switch or (more commonly) an airspeed switch.

Master Reference Tables

Numerical Values — Quick Reference

Answer Key