Engine Instruments Flashcards
Engine tachometer
- AKA “RPM gauge”
- Displays the rotational speed of the engine crankshaft measured in RPM
- Use cable driven from the engine crankcase drive gear that connects to the instrument.
Centrifugal/Mechanical type tachometer
Utilised spinning flyweights to transform the rotational motion of the drive cable into the motion of the indicator needle.
As the speed of the drive cable (RPM) increases, the flyweights are pushed further outward due to centrifugal force.
Outward movement is transferred through a mechanical linage to move the tachometer’s needle across the dial.
Drag cup tachometer.
Utilises rotating permanent magnet connected to the drive cable inside the drag cup in induce a magnetic field which transforms the rotational motion of the drive able into the motion of the indicator pointer.
There is no physical connection between the drive able and the pointer, reducing wear and likelihood of failure.
Electrical tachometer.
Utilises a small engine driven generator which sends electrical power to the cockpit to display RPM.
DC type - used on older aircraft only
AC type - creates a 3-phase electrical signal. Power produced is reliever to a synchronous motor in the tachometer to rotate it at the same speed as the generator.
Does not use a permanent magnet and are completely independent from the aircraft electrical system
Electronic tachometer
Utilises special points (contacts) inside the distributor of a magneto. Works as normal magneto points, however it only send signals to the tachometer.
The rate at which the points open and close is proportional to the engine RPM.
Manifold air pressure gauges
Used to accurately set the engine power in aircraft fitted with a supercharger or Constant Speed Unit (CSU).
Measures the pressure in the intake manifold after the carburettor or fuel control unit.
- Inches of Mercury (“Hg)
- Used aneroid capsule diaphragm to operate
- Any increase or decrease in MAP will expand or collapse the capsule
Boost gauges
- Can be used instead of a MAP gauge.
Measures the amount of extra air pressure in points per square inch PSI.
With the engine shut down, on an ISA day (QNH 1013) and the aircraft at MSL, it will read 0.
Oil pressure gauges
- Measured the pressure of the oil flowing through the engine.
- Indicates PSI
Direct reading type oil gauge
Utilises a bourdon tube to transform oil pressure into the motion of the pointer.
- Tube is designed to deform or flux when pressure is applied. This movement is mechanically transferred to a needle or pointer.
- Only used to light single engine aircraft.
Remote indicating type oil gauge
- Oil pressure will be remotely indicated by relaying the signal electrically.
- More complex than direct reading, but more accurate.
Ratiometer
Uses a diaphragm or bourbon tube which is connected to the oil pressure of the engine.
Typically linked to or is part of a variable resistor (rheostat).
The oil pressure changes the diaphragm’s position, thereby altering the resistance in the circuit.
Ratio meter
Used to send data between the sensor and the oil pressure gauge.
- Includes a transmitter and a receiver linked by wires.
- Oil pressure changes rotate the transmitter’s input shaft and through electromagnetic induction, mirrors this rotation in the receiver’s output shaft and the attached gauge pointer.
Transmitter and receiver consists of:
- Stator - has three fixed coils arranged 120 degrees apart
- Rotor - single coil linked to a single-phase AC power source.
Fuel pressure gauges
Indicate the fuel pressure - calibrated to read in PSI
- Operates similarly to oil pressure gauges (bourbon tube, radiometer, synchro system.
Vacuum gauges
- AKA “suction gauge”
- Measures the suction created by the engine driven suction pump.
- Fitted on aircraft with air-driven gyroscopic instruments
- Bourdon tube used to indicate the suction created.
- Inches of Mercury
If the aircraft has electrically driven gyroscopes, it will not have a suction pump and therefore will not require a suction gauge.
Cylinder Head Temperature Gauge (CHT gauge)
- Indicates the temperature of the cylinder head
- Utilises a thermocouple which is an electrical circuit consisting of two dissimilar metal wires.
Thermocouple produces a small voltage when there is a temperature difference between the hot junction and the cold junction.
Cylinder head temperature prove is normally fitted to the hottest cylinder.
Exhaust gas temperature gauge (EGT)
Indicates the temperature of the gases at the exhaust manifold. Also uses a thermocouple.
- Used to help lean the mixture and to avoid overheating the turbocharger - fitted to higher performance aircraft.
Oil temperature gauges
- Calibrated to read degrees F or C
Mechanical bulb/Vapour pressure thermometer type.
- Probe is filled with a chemical that at normal temperature is part gas, part liquid.
- Connected to a bourdon tube.
- As the temperature increases, the liquid progressively becomes gas and then places pressure on the bourdon tube, causing it to move.
Electrical ratiometer thermometer type
- AKA “resistance thermometer”
- Similar to ratiometer, but uses a temperature probe to change the resistance
Outside air temperature gauges (OAT gauges) - Direct reading type
- Uses bi-metallic strip (two thin strips of metal bonded together
- Reacts to the rise in temperature by bending or curving and reduction in temperature by straightening.
- Brass and iron
Outside air temperature gauges (OAT gauges) - Mechanical bulb type
- Operates of the same principle as the oil temperature gauge using a bourdon tube, hose and probe, filled with part liquid, part gas chemical.
Causes the tube to expand and move the needle pointer.
Fuel quantity gauges - variable resistance
- Most common for light aircraft
- Uses a float inside a fuel tank - alters the resistance of an electrical circuit.
- Indicates the volume of fuel and is easily upset by turbulence
Fuel quantity gauges - capacitance type
- Used in larger aircraft - gives more accurate readings of quantities of large or unusually shaped tankers.
- Measure in mass
- Can store electrical charge when a voltage is applied to two conductive surfaces, or plates.
Fuel flow gauges.
- Essentially a water wheel
- Vane rotate proportionally to the volume of fuel flow
- Rotation is transmitted to the cockpit instrument - synchro system, indicating the rate of fuel usage.
