A thermocouple-type cylinder head temperature (CHT) indicating system generates a voltage based on what principle?
ACS code: AM.III.D
Correct answer: The temperature difference between two junctions of dissimilar metals
Rationale: A thermocouple consists of two wires of dissimilar metals (commonly iron and constantan, or chromel and alumel) joined at a hot junction. When the hot junction is heated relative to the cold (reference) junction, a small voltage proportional to the temperature difference is produced. The handbook explains that CHT thermocouple systems rely on this thermoelectric (Seebeck) effect, not on resistance change (which describes a resistance bulb) or liquid expansion (which describes a vapor/liquid bourdon system).
A typical engine oil pressure gauge of the bourdon-tube type indicates pressure by which mechanical action?
ACS code: AM.III.D
Correct answer: A curved tube tends to straighten as internal pressure increases
Rationale: A bourdon-tube gauge uses a sealed, curved metal tube that tends to straighten when pressurized internally; this motion is transmitted through a gear-and-sector mechanism to the pointer. The handbook describes the bourdon tube as the classic mechanical pressure-sensing element. A diaphragm/resistor describes an electrical transmitter, and a bimetallic strip is a temperature-sensing device, not a pressure element.
On a turbine engine, exhaust gas temperature (EGT) is most commonly sensed using which type of device?
ACS code: AM.III.D
Correct answer: Several thermocouples wired in parallel
Rationale: Turbine EGT systems use multiple thermocouples positioned around the gas path and connected in parallel so the indication represents an average exhaust gas temperature. The handbook explains that averaging several probes reduces error from local hot spots. A bourdon tube senses pressure, and a bimetallic spring is used in simple direct-reading thermometers, not turbine EGT.
A tachometer that uses a small AC generator on the engine driving a synchronous motor in the indicator is best described as which type of system?
ACS code: AM.III.D
Correct answer: An electric tachometer-generator system
Rationale: An electric tachometer uses an engine-driven three-phase AC generator whose output frequency is proportional to engine speed; this drives a synchronous motor and drag-cup assembly in the indicator. The handbook contrasts this with the older mechanical flexible-shaft tachometer that physically rotates the indicator. A thermocouple system measures temperature, not rotational speed.
Turbine-engine tachometers are typically calibrated to indicate engine speed in what units?
ACS code: AM.III.D
Correct answer: Percent of maximum rated rpm
Rationale: Because turbine engines turn at very high rpm and use multiple spools, their tachometers read in percent of maximum rated rpm (with 100% representing the design takeoff speed) so the pilot reads a consistent scale across engine types. The handbook notes this percent-rpm convention. Thrust is inferred from EPR or N1, and exhaust temperature is shown on the EGT gauge, not the tachometer.
What does a manifold absolute pressure (MAP) gauge on a reciprocating engine primarily indicate?
ACS code: AM.III.D
Correct answer: Absolute pressure in the intake manifold
Rationale: The MAP gauge reads absolute pressure (referenced to a sealed vacuum, in inches of mercury) inside the intake manifold, giving the mechanic and pilot a measure of the power being developed and showing supercharger/turbocharger effect when present. The handbook explains it is an absolute, not differential, instrument. It does not measure oil-filter differential pressure or exhaust pressure.
If a thermocouple lead in a CHT system is replaced, why must the resistance of the new leads match the value specified for that system?
ACS code: AM.III.D
Correct answer: The indicator is calibrated for a specific total lead resistance
Rationale: A thermocouple indicator is calibrated to work with a specific total circuit resistance, including the leads, so substituting leads of a different length or gauge changes the resistance and introduces an indication error. The handbook stresses using the correct lead length and resistance when servicing thermocouple systems. The leads do not power the indicator, and lead resistance does not change the actual junction temperature.
In gas turbine engine station notation, what does the term Pt7 mean?
ACS code: AM.III.D
Correct answer: The total pressure at station No. 7.
Rationale: In the standard notation the prefix P denotes pressure and the subscript t denotes the total (stagnation) value, while the figure 7 is the engine station, so Pt7 is the total pressure at station 7. It refers to pressure only, not temperature, and station 7 is not the engine inlet.
In a twin-spool turbine engine with a free power turbine, the Nf indication represents the
ACS code: AM.III.D
Correct answer: free power turbine speed.
Rationale: In an engine with a free power turbine, Nf designates the rotational speed of that free turbine, indicating the speed of the output shaft it drives. It does not indicate compressor speed or thrust, which are tracked by other parameters.
In turbine engine instrumentation, the term Pb refers to burner pressure measured
ACS code: AM.III.D
Correct answer: in the combustion chamber.
Rationale: The term Pb denotes burner pressure, the gas pressure measured within the combustion chamber (burner). It serves as a control and indication parameter representing conditions in the combustor, distinct from pressures sensed at the diffuser case or nozzle guide vanes.
The compensation (cold-junction) device on an EGT system must be re-calibrated
ACS code: AM.III.D
Correct answer: each time a part of the thermocouple system is replaced.
Rationale: The EGT compensation device is matched to the resistance of the specific thermocouple harness and leads, so replacing any part of that circuit changes its electrical characteristics. The compensator must therefore be re-calibrated each time a component of the system is replaced to keep the indicated temperature accurate.
When functionally testing an EGT thermocouple system, the outside air temperature (OAT)
ACS code: AM.III.D
Correct answer: must always be taken into account.
Rationale: A thermocouple measures the temperature difference between its hot and cold (reference) junctions, so the cold-junction/outside air temperature affects the reading. When testing an EGT system, the OAT must always be taken into account to apply the correct cold-junction compensation and verify accuracy.
Engine pressure ratio (EPR) is used primarily to
ACS code: AM.III.D
Correct answer: indicate the thrust the engine is producing.
Rationale: Engine pressure ratio compares turbine discharge total pressure to engine inlet total pressure and is a primary indication of the thrust being produced by the engine. It is not a temperature limiter; EGT is used to monitor temperature limits.
When the resistive element of a resistance-bulb (electrical) thermometer goes open-circuit, the indicator will
ACS code: AM.III.D
Correct answer: read higher than ambient temperature.
Rationale: A resistance-bulb thermometer works on the principle that resistance increases with temperature. An open circuit appears as infinite resistance, which the indicator interprets as a very high temperature, so the gauge reads well above ambient (off the high end).
On an EGT thermocouple system, the hot junction is located
ACS code: AM.III.D
Correct answer: downstream of the combustion chamber, in the turbine or exhaust section.
Rationale: The EGT thermocouple hot junction is positioned in the hot gas stream downstream of the combustion chamber, typically in the turbine or exhaust section, so it samples the temperature of the gases after combustion. Placing it upstream would not give a representative exhaust gas temperature.
In a tachometer system using a synchronous (AC) generator, the indicator pointer is deflected by
ACS code: AM.III.D
Correct answer: a synchronous AC motor driving a drag-cup assembly.
Rationale: In a synchronous (AC) tachometer the generator output drives a small synchronous AC motor in the indicator whose speed follows the generator frequency. This motor, through a drag-cup arrangement, deflects the pointer; a potentiometer or Wheatstone bridge is not used to position it.
On many turboprop and turboshaft engines, torque is displayed by reading
ACS code: AM.III.D
Correct answer: a direct-reading pressure gauge calibrated in torque units.
Rationale: Engine torque on many turboprop and turboshaft engines is sensed as an oil pressure proportional to torque, which is displayed on a direct-reading pressure gauge calibrated in torque units. A tension gauge is unrelated to this hydraulic torque measurement.
In a drag-cup (eddy-current) tachometer indicator, the drag torque on the cup is opposed by
ACS code: AM.III.D
Correct answer: a calibrated hairspring.
Rationale: In a drag-cup tachometer the rotating magnet drags the aluminum cup around, and this torque is opposed by a calibrated hairspring. The balance point between drag torque and spring tension gives a pointer deflection proportional to speed, not one set by counterweights or an adjustment screw.
On a turbine engine, the fuel-flow transmitter may be installed in the fuel supply line
ACS code: AM.III.D
Correct answer: downstream of either the high-pressure or low-pressure pump.
Rationale: The fuel-flow transmitter is fitted in the engine fuel supply line so it measures the fuel actually being delivered. Depending on the installation it may be located downstream of either the low-pressure (boost) pump or the high-pressure (main) pump, so its position varies between engine types.
A Bourdon-tube instrument may be used to indicate
ACS code: AM.III.D
Correct answer: pressure and temperature.
Rationale: A Bourdon tube responds to the pressure inside it, so it can indicate pressure directly and, in a vapor-pressure thermometer where temperature produces a corresponding pressure, it can also indicate temperature. It is not suited to measuring position or quantity.
Oil-temperature thermocouples are usually constructed of
ACS code: AM.III.D
Correct answer: iron and constantan.
Rationale: Oil-temperature ranges are relatively low, so iron-constantan thermocouples are used because that pairing is accurate over the lower temperature band. Chromel-alumel is reserved for the much higher temperatures of exhaust-gas (EGT) measurement.
The RPM indication of a synchronous AC-motor tachometer is governed by the generator's
ACS code: AM.III.D
Correct answer: frequency.
Rationale: A synchronous tachometer generator produces an AC output whose frequency is directly proportional to its rotational speed. The indicator's synchronous motor runs in step with that frequency, so the RPM reading is governed by the generator frequency, not by its voltage or current.
Instruments that measure relatively high fluid pressures, such as oil-pressure gauges, are usually of what type?
ACS code: AM.III.D
Correct answer: Bourdon tube.
Rationale: A Bourdon tube tends to straighten under internal pressure and is robust and accurate for measuring relatively high pressures such as engine oil pressure. Diaphragm and bellows elements are reserved for low or differential pressures where they give greater sensitivity.
Instruments that provide readings of low or negative pressure, such as manifold pressure gauges, are usually of what type?
ACS code: AM.III.D
Correct answer: Diaphragm or bellows.
Rationale: Diaphragm and bellows elements deflect appreciably under small pressure changes, making them ideal for low or negative (below-atmospheric) pressures such as manifold pressure. A Bourdon tube is comparatively insensitive and is better suited to higher pressures.
In what units are gas-turbine engine tachometers calibrated?
ACS code: AM.III.D
Correct answer: Percent of rated engine RPM.
Rationale: Gas-turbine tachometers are calibrated as a percentage of maximum rated RPM, with 100 percent representing the engine's maximum design speed. This lets the same scale be used across different engines and makes limit comparison straightforward, rather than reading actual RPM figures.
In a turbine engine, where is the turbine-discharge-pressure indicator sensor located?
ACS code: AM.III.D
Correct answer: Immediately aft of the last turbine stage.
Rationale: Turbine discharge pressure is sensed immediately aft of the last turbine stage, where the gas has given up its energy to the turbine. This location provides the turbine outlet total pressure used in computing engine pressure ratio.
The exhaust gas temperature (EGT) indicator on a gas-turbine engine provides a relative indication of the
ACS code: AM.III.D
Correct answer: turbine inlet temperature.
Rationale: True turbine inlet temperature is too high to measure reliably with a probe, so EGT is taken farther downstream and used as a relative indication of turbine inlet temperature. Monitoring EGT therefore protects the turbine against overtemperature.
Engine pressure ratio (EPR) is determined by
ACS code: AM.III.D
Correct answer: dividing turbine outlet total pressure by engine inlet total pressure.
Rationale: Engine pressure ratio is defined as turbine (exhaust) outlet total pressure divided by engine inlet total pressure. This ratio of outlet to inlet pressure reflects the pressure rise developed through the engine and is used to indicate thrust.
A red triangle, dot, or diamond mark on an engine instrument face or glass indicates
ACS code: AM.III.D
Correct answer: the maximum limit for high transients such as starting.
Rationale: A red triangle, dot, or diamond on an engine instrument marks a maximum limit permitted only for short transient conditions, such as the brief temperature peak during starting. It differs from a continuous red radial line, which marks the maximum for all normal operations.
What is the primary purpose of the tachometer on an axial-flow compressor turbine engine?
ACS code: AM.III.D
Correct answer: To monitor engine RPM during starting and to indicate overspeed conditions.
Rationale: On an axial-flow turbine engine the tachometer's main role is to monitor RPM during starting (to confirm correct acceleration and avoid hot starts) and to give warning of overspeed. Thrust is set primarily by EPR, so RPM is not the prime thrust-setting instrument.
Engine pressure ratio (EPR) is the total-pressure ratio between the
ACS code: AM.III.D
Correct answer: front of the compressor and the rear of the turbine.
Rationale: Engine pressure ratio is the total-pressure ratio between the engine inlet (front of the compressor) and the turbine discharge (rear of the turbine). Measuring across the whole gas path in this way gives the indication of thrust developed.
On an aircraft turbine engine operating at a constant power setting, the application of engine anti-icing will result in
ACS code: AM.III.D
Correct answer: a noticeable shift in EPR.
Rationale: Selecting engine anti-icing diverts compressor bleed air, altering the engine's pressure conditions, so at a constant power setting there is a noticeable shift in the EPR reading. The effect is a genuine change in EPR, not a false or erroneous indication.
The indicator of a tachometer system is responsive to a change in
ACS code: AM.III.D
Correct answer: frequency.
Rationale: The tachometer generator's output frequency is proportional to engine speed, and the indicator responds to that frequency to display RPM. It is not driven by changes in voltage or current, which can vary independently of speed.
Both motor-driven (impeller-and-turbine) and motorless-type fuel-flow transmitters provide a measure of
ACS code: AM.III.D
Correct answer: fuel mass flow.
Rationale: Both the motor-driven impeller-and-turbine type and the motorless (autosynchronous) type transmitters are designed to measure fuel mass flow, because thrust and fuel consumption depend on the mass of fuel burned, not its volume. Mass flow remains accurate despite temperature-driven changes in fuel density.
In addition to fuel quantity, how many of the following does a computerized fuel system with a totalizer indicator provide: fuel flow rate, fuel used since reset, fuel time remaining at current power setting, and fuel temperature?
ACS code: AM.III.D
Correct answer: Three (flow rate, fuel used, and time remaining).
Rationale: Besides fuel quantity, a computerized fuel system with a totalizer provides three additional readouts: instantaneous fuel flow rate, total fuel used since reset, and fuel time remaining at the current power setting. Fuel temperature is not part of this totalizer indication.
In a motor-driven impeller-and-turbine fuel-flow indicating system, the indicator rotor and needle are driven by
ACS code: AM.III.D
Correct answer: an electrical signal from the transmitter.
Rationale: In a motor-driven impeller-and-turbine fuel-flow system the transmitter sends an electrical signal that drives the indicator's rotor and needle, typically through a synchro arrangement. The pointer is positioned electrically, not by a direct mechanical coupling or gear train from the transmitter.
A motor-driven impeller-and-turbine type fuel flow transmitter sends its data to the indicator
ACS code: AM.III.D
Correct answer: electrically, using aircraft electrical system power.
Rationale: The motor-driven impeller-and-turbine type fuel flowmeter uses a powered AC motor and a synchro system, so it transmits the flow data electrically using aircraft electrical system power. It does not rely on a purely mechanical link or on fuel pressure to drive the indication.
In an electric tachometer system, which unit sends speed information to the indicator?
ACS code: AM.III.D
Correct answer: The three-phase AC generator.
Rationale: An electric tachometer system uses a small three-phase AC generator driven by the engine; its three-phase output sets the speed of the synchronous motor in the indicator. The generator, not the indicator motor, is the unit that sends the speed information.
An engine oil temperature gauge is normally calibrated to indicate the temperature of the oil
ACS code: AM.III.D
Correct answer: as it enters the engine.
Rationale: The oil temperature gauge senses the oil at the engine inlet, after the cooler and just before it enters the bearings, because this is the temperature most relevant to lubrication and protection. Reading it entering the cooler or in the tank would not reflect the condition of the oil being delivered to the engine.
Thermocouple leads used in an exhaust gas temperature system
ACS code: AM.III.D
Correct answer: are designed for a specific installation and must not be altered in length or material.
Rationale: Thermocouple leads are manufactured to a fixed total resistance matched to the indicator, so their length and material cannot be altered without upsetting the calibration. They are designed for a specific installation; arbitrary lead swapping or splices that change resistance are not permitted.
Why do many helicopters use a dual-needle (split) tachometer system?
ACS code: AM.III.D
Correct answer: One needle indicates engine RPM and the other indicates main rotor RPM.
Rationale: A helicopter dual (split) tachometer compares engine RPM and main rotor RPM on one instrument. This allows the crew to confirm clutch and freewheel engagement and to detect autorotation, where rotor and engine speeds diverge.
If the thermocouple leads were inadvertently crossed during installation, the EGT gauge pointer would
ACS code: AM.III.D
Correct answer: move off scale on the low (zero) side of the meter.
Rationale: A thermocouple produces a polarity-sensitive EMF, so reversing (crossing) the leads sends current the wrong way through the meter. The pointer is therefore driven off scale on the low (zero) side rather than reading normally or high.
A common type of electrically operated oil temperature gauge operates using
ACS code: AM.III.D
Correct answer: a Wheatstone bridge or ratiometer circuit.
Rationale: Electrical oil temperature gauges use a temperature-sensitive resistance bulb whose resistance change is read by a Wheatstone bridge or a ratiometer circuit. These resistance-based methods, not a self-generating thermocouple or a vapor-pressure switch, are typical for oil temperature indication.
Engine pressure ratio (EPR) is used on a turbine engine to indicate
ACS code: AM.III.D
Correct answer: the thrust produced by the engine.
Rationale: Engine pressure ratio is the ratio of turbine discharge total pressure to compressor inlet total pressure, and this ratio rises directly with the thrust the engine is developing. It therefore serves as the cockpit thrust indication on many turbojet and turbofan engines, rather than a direct reading of shaft power.
Turbine outlet temperature (TOT) on a turbine engine is measured
ACS code: AM.III.D
Correct answer: downstream of the turbine.
Rationale: Turbine outlet temperature is sensed by thermocouples placed in the gas stream immediately aft of the turbine, where the gas leaves after giving up energy. Measuring downstream gives a usable, cooler temperature that still reflects turbine operating conditions, whereas combustion-section gas is far too hot for the probes to survive.
A thermocouple-type temperature indicator is basically a
ACS code: AM.III.D
Correct answer: millivoltmeter calibrated in degrees of temperature.
Rationale: A thermocouple generates a small self-induced EMF in the millivolt range proportional to the junction temperature difference, so the indicator is essentially a millivoltmeter calibrated in degrees. It responds to voltage, which is why a milliammeter or milliohmmeter would not represent the sensing principle correctly.
On a high-bypass turbofan engine, thrust is normally indicated by
ACS code: AM.III.D
Correct answer: N1 RPM or engine pressure ratio (EPR).
Rationale: On a high-bypass turbofan most thrust comes from the fan, so fan speed (N1) is a good thrust indicator, and engine pressure ratio (EPR) is the alternative used on engines designed for EPR indication. N3 speed and a P1/P4 ratio are not the standard thrust-setting parameters for this engine type.
On a triple-spool turbine engine, how are the individual spool speeds (such as N1 and N2) commonly sensed?
ACS code: AM.III.D
Correct answer: By pulse-type (phonic) speed probes.
Rationale: On a triple-spool engine the individual spools are not all accessible to a geared tachometer, so rotational speeds are sensed by pulse (phonic) speed probes that count the passing of a toothed wheel or blade tips and output a frequency proportional to RPM. This non-contact method suits the multiple concentric shafts.
Modern oil-pressure servo transmitters typically sense
ACS code: AM.III.D
Correct answer: differential pressure.
Rationale: Modern oil-pressure servo transmitters measure the difference between supply oil pressure and a reference (often air or return) pressure, giving a differential pressure. Using a differential sensing element cancels out ambient effects and gives a true reading of working oil pressure rather than an absolute value referenced to a vacuum.
Exhaust gas temperature (EGT) thermocouples are usually made of
ACS code: AM.III.D
Correct answer: chromel and alumel.
Rationale: Exhaust-gas-temperature thermocouples are commonly chromel (nickel-chromium) and alumel (nickel-aluminum) wires, chosen for their stable, near-linear output over the high temperature range seen in the exhaust. Platinum-based pairs are used elsewhere but are not the standard EGT combination.