Engine Instruments and the Clock - ATPL 022 — Instrumentation

The module closes with the instruments that watch the engine itself: how fast it turns, how hard it pushes or twists, whether its rotors run smoothly, and, on a multi-engine propeller aircraft, whether the engines agree with each other. Each is a short measurement chain of its own, and the exam asks for each chain by name: the sensor, the principle, the unit, and what the display looks like.

Tachometers: three ways to measure rotation

The tachometer reports the engine's rotational speed, and three generations of the instrument coexist.

Mechanical (rotating magnet) — a flexible drive shaft from the engine spins a magnet inside a light metal drag cup. The rotating field drags the cup around by the eddy currents it induces (small circulating currents stirred up in the metal, whose own magnetic field chases the magnet), a hairspring resists, and the cup settles at an angle proportional to speed, carrying the pointer with it. Entirely self-contained, the light-aircraft rpm gauge.
Electrical (three-phase tacho-generator) — an engine-driven generator produces three-phase alternating current whose frequency is set by the engine's speed. The three wires drive a small synchronous motor in the indicator, which turns a drag-cup mechanism of its own. The signal is made by the engine, so the system powers itself and keeps indicating with the aircraft's electrical system dead: the classic fit on older turboprops and larger piston twins.
Electronic (speed probe and phonic wheel) — a toothed phonic wheel turns with the shaft, and each passing tooth induces a pulse in a fixed magnetic speed probe. Electronics count the pulse rate and compute the speed: no drive shaft, negligible drag on the engine, the modern standard. The counting electronics require electrical power: with the supply lost, this type stops indicating, which is exactly the dependence the syllabus wants named.

The units split by engine type. Piston engines indicate in rpm (revolutions per minute), the number the propeller and its limits are written in. Turbine engines indicate in percentage of a nominal speed: the spools turn at tens of thousands of rpm, numbers with no operational feel, so the design speed is called 100% and every limit and setting becomes a small, comparable figure. Each spool gets its own indication, named from the fan inward:

N1 for the fan and low-pressure spool.
N2 (and N3 where fitted) for the spools behind it.

Representing thrust: N1 and EPR

A jet engine has no thrust gauge; thrust is not measured in flight but represented by a parameter that tracks it. Manufacturers use one of two:

N1 — the fan speed: the fan produces most of a modern turbofan's thrust, so its speed is an honest stand-in, sensed by the ordinary tachometer chain above.
EPR (engine pressure ratio) — from the first lesson's pressure family: turbine outlet pressure divided by compressor inlet pressure, sensed by pressure probes at the two stations, a direct measure of how much the engine has raised the pressure of the air passing through it.

The EPR chain's weakness is its inlet probe. If the compressor inlet probe ices over or is blocked, the ratio is computed from a wrong reference and the indicated EPR is no longer the engine's true pressure ratio: set thrust by that figure and the engine delivers something other than what was commanded, the classic case being an over-reading EPR that seduces the crew into setting too little actual thrust. The certified answer is reversion to N1 mode: when EPR is missing or suspect, the crew or the engine electronics set thrust by N1 instead, the tables and displays carrying both parameters for exactly this case. Displays for both are dials or vertical tapes on the engine page, with the command value marked by a bug and the limit by a red line.

Torque: the turboprop's power gauge

Torque is a turning moment: force times the arm it acts on, the twisting effort a shaft transmits. For a propeller engine it is the honest power figure, because the power a rotating shaft delivers is its torque times how fast it turns:

Read the relationship both ways: at fixed rpm, power and torque move together, so a torque gauge on a constant-speed turboprop is effectively a power gauge; and at fixed power, torque rises as rpm falls, which is why a reduction gearbox that slows the propeller multiplies the torque its shaft must carry. Units: the newton metre (N·m) in SI, inch-pounds or foot-pounds imperial, and in the cockpit torque is commonly displayed simply as a percentage of a reference value, like the turbine speeds above.

Two torquemeter families measure it:

Mechanical (oil-pressure) torquemeter — uses the reduction gearbox itself: helical gear teeth push axially in proportion to the torque they transmit, that thrust loads a piston, and the oil pressure needed to hold the piston in place is proportional to torque, read on a pressure gauge.
Electronic torquemeter — measures the twist of a shaft section directly: toothed phonic wheels at each end of the section feed speed probes, and the phase shift between their pulse trains grows with the twist, which the electronics convert to torque.

Comparing the two: the mechanical system needs its oil circuit, piston and plumbing, making it the heavier and more complex installation; the electronic system is lighter, with no hydraulic parts, and suits electronic displays directly. Displays: a dial or tape in per cent, or the mechanical system's native pressure units (psi) on older turboprops.

The synchroscope

On a multi-engine propeller aircraft, engines running at slightly different speeds produce an audible beat and lasting vibration. The synchroscope exists to remove it: its purpose is to show whether the other engine runs faster or slower than the master, so the pilot can match them exactly. The principle is a small motor fed by both engines' tacho-generator signals: fed identical frequencies it stands still, and any difference turns it at the difference frequency. The display is a rotating pointer, often drawn as a little propeller: rotation one way means the slave engine is slow, the other way fast, and a stationary pointer means the engines are synchronised. Direction gives the correction; a glance replaces listening to the beat.

Vibration monitoring

A jet engine's rotors are balanced to fine tolerances, and a change in vibration is often the earliest warning of trouble: ice or dirt on the fan, a damaged blade, bearing wear, or imbalance after a birdstrike. That early warning is the purpose of the vibration-monitoring system. Its sensors are accelerometers mounted on the engine casings, of two kinds:

piezoelectric crystal — which produces a small electric charge whenever it is squeezed, so vibration becomes an oscillating signal directly.
magnet type — a suspended magnet and coil in relative motion, vibration inducing a voltage in the coil.

Both feed electronics that extract the vibration level of each rotor.

Vibration has no specific unit on the flight deck: the indication is a dimensionless level on an arbitrary scale, judged against specified numeric threshold values rather than against a physical unit. The display is a small dial or tape per engine, typically on the engine page's lower half, with an advisory band or threshold mark; crossing the threshold prompts monitoring or maintenance action, and a sudden change after an event (a suspected birdstrike, a heavy icing encounter) is treated as evidence even inside the band.

The on-board clock

The least glamorous instrument on the panel is a system in its own right: the on-board clock provides the time reference for several aircraft systems. ACARS (the aircraft communications addressing and reporting system, the data link that carries messages between the aircraft and the ground) stamps every message with it; the maintenance systems stamp engine exceedances and fault records with it, so an event can be matched to a phase of flight afterwards; and flight-deck times (estimates, elapsed time) are counted from it. It runs in UTC (coordinated universal time), and setting it wrongly quietly corrupts every record downstream.

What the exam asks

The tachometer families and the power-torque-rpm relationship carry the calculations; the rest is precise recall. The overview, following the learning objectives:

List the tachometer types, their operating principles and uses; explain rpm vs percentage; state which types need electrical power

Describe N1 and EPR, the EPR operating principle, the consequences of incorrect or missing EPR and N1 reversion, and their displays

Define torque, relate power, torque and rpm, list the units and displays, and compare the two torquemeter types

State the synchroscope's purpose, explain its principle, and give examples of display

State the purpose of vibration monitoring, describe its sensors, and explain its unit-less thresholds and displays

Explain the on-board clock as the time reference for systems including ACARS and maintenance