I85PREP
⌘ K
ATPL 022 Instrumentation study materials
ATPL 022 Instrumentation study resources

ATPL 022 Instrumentation practice questions

One question at a time. Build your ATPL 022 Instrumentation understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Sensors and Measurement Fundamentals

Which physical property of a material is exploited by a resistance temperature device (RTD) used in aircraft air-data systems?

  1. The change of capacitance between two plates with temperature
  2. The change of magnetic permeability with temperature
  3. The predictable increase of electrical resistance with temperature Correct answer.
  4. The generation of a small voltage at a heated junction

Explanation

An RTD is a metallic element (commonly platinum) whose electrical resistance rises in a well-defined, repeatable way as temperature increases. The measuring bridge converts that resistance change into a temperature indication. The other effects listed belong to thermocouples, capacitive sensors and magnetostrictive devices respectively.

Question 2Sensors and Measurement Fundamentals

In a simple unbalanced DC bridge circuit used with a resistive sensor, what does the magnitude of the bridge output voltage represent?

  1. The magnitude of the measured quantity Correct answer.
  2. The direction only of the measured quantity
  3. The rate of change of the measured quantity
  4. The supply frequency of the excitation

Explanation

In a Wheatstone-type bridge the off-balance voltage is proportional to the change in sensor resistance, so its magnitude indicates how far the measured value has moved from the balance (reference) point. The polarity of that voltage indicates the direction of the change, not its size. Excitation frequency is irrelevant for a DC bridge.

Question 3Sensors and Measurement Fundamentals

Why is a servo-driven (null-balance) pressure indicator preferred over a simple direct-reading gauge in many aircraft pitot-static systems?

  1. It measures absolute pressure without a reference chamber
  2. It responds faster because no feedback loop is used
  3. It balances the bridge to zero, so indicator accuracy does not depend on supply voltage Correct answer.
  4. It avoids the need for any pressure-to-displacement transducer

Explanation

In a null-balance instrument the sensing element's signal is opposed by a feedback signal until the bridge is re-zeroed, so the reading depends on the mechanical/electrical balance rather than on the exact excitation voltage. This gives high accuracy and low drift. A direct-reading gauge depends more heavily on stable supply and element linearity.

Question 4Sensors and Measurement Fundamentals

A thermocouple used for exhaust gas temperature measurement produces an output that is proportional to which quantity?

  1. The square of the hot-junction temperature
  2. The resistance of the hot junction
  3. The absolute temperature of the hot junction alone
  4. The temperature difference between its hot and cold junctions Correct answer.

Explanation

A thermocouple develops a small EMF that depends on the temperature difference between the measuring (hot) junction and the reference (cold) junction. This is why cold-junction compensation is needed in the indicator circuit. It is not a resistance device, so resistance of the junction is not the output.

Question 5Sensors and Measurement Fundamentals

What is the primary purpose of cold-junction compensation in a thermocouple indicating circuit?

  1. To amplify the very small thermocouple EMF
  2. To correct the indicated temperature for the temperature of the reference junction Correct answer.
  3. To linearise the thermocouple output over its full range
  4. To cool the hot junction so it does not melt

Explanation

Because the thermocouple EMF depends on the difference between hot and cold junction temperatures, the indicator must add a correction based on the actual reference-junction temperature. Without this, the indicated temperature would drift as the instrument case temperature changes. Compensation is a correction function, not a cooling or amplification function.

Question 6Sensors and Measurement Fundamentals

Which characteristic makes a synchro (or resolver) preferred for transmitting angular position from a gyro or compass to a remote indicator?

  1. It converts the angle directly into a digital word without any converter
  2. It stores the last position mechanically when power is lost
  3. It requires no excitation supply at all
  4. Its output is an electrical signal proportional to shaft angle, allowing accurate remote transmission Correct answer.

Explanation

A synchro transmitter produces a set of stator voltages whose pattern is a function of rotor angle, and a matching receiver or control transformer reproduces or measures that angle remotely. This gives accurate, low-torque position transmission over wiring. It needs AC excitation and does not inherently produce a digital output.

Question 7Sensors and Measurement Fundamentals

An aircraft air-data computer receives a static pressure that is lower than the true local static pressure because of a position error at the static ports. Which indication is affected most directly?

  1. The pressure altitude indication Correct answer.
  2. The Mach number only
  3. The total air temperature
  4. The indicated airspeed only

Explanation

Static pressure is the primary input for pressure altitude, so a static-source error shifts the altitude indication directly. Airspeed and Mach also depend on static pressure, but altitude is the most direct mapping of static pressure alone. Total air temperature comes from a separate probe.

Question 8Sensors and Measurement Fundamentals

What does a pitot tube measure when connected to a pressure transducer referenced to static pressure?

  1. The total temperature of the airflow
  2. The absolute static pressure at the aircraft altitude
  3. The rate of change of static pressure with altitude
  4. The difference between total pressure and static pressure Correct answer.

Explanation

A pitot tube facing the airflow senses total (stagnation) pressure, and when the transducer's other side is fed static pressure the result is the dynamic pressure, the difference between total and static pressure. This difference is what the airspeed indicator uses. It is not an absolute pressure or a temperature measurement.

Question 9Sensors and Measurement Fundamentals

A capacitive pressure sensor in an air-data system varies its capacitance as pressure changes. Which design feature most improves its linearity and stability?

  1. A very large diaphragm deflection to maximise capacitance change
  2. A resistive strain gauge bonded to the diaphragm
  3. A diaphragm whose deflection is small compared with its diameter, with fixed electrode geometry Correct answer.
  4. An open chamber vented to the measured pressure on both sides

Explanation

Keeping diaphragm deflection small relative to its size makes the capacitance change nearly proportional to pressure and reduces hysteresis. Fixed electrode geometry and rigid construction also improve stability. Large deflection introduces non-linearity, and a resistive gauge is a different sensing principle.

Question 10Sensors and Measurement Fundamentals

What is the function of a flux valve (floating pendulum) in a remote-reading magnetic compass system?

  1. It senses the direction of the Earth's horizontal magnetic field and outputs a corresponding electrical signal Correct answer.
  2. It measures the vertical component of the Earth's magnetic field to give latitude
  3. It provides the gyro-stabilised reference that replaces the magnetic field
  4. It supplies the AC excitation for the compass indicator

Explanation

The flux valve uses a magnetically permeable element that responds to the horizontal component of the Earth's field, producing signals whose phase and amplitude encode magnetic heading. These signals are sent to the indicator or compass system. It does not replace the magnetic reference with a gyro nor measure latitude.

Question 11Sensors and Measurement Fundamentals

In a remote magnetic compass system, why is a gyro unit sometimes used together with the flux valve?

  1. To replace the flux valve permanently in flight
  2. To measure the aircraft's rate of turn for the autopilot
  3. To generate the magnetic field that the flux valve senses
  4. To provide a short-term stable heading reference while the magnetic signal is smoothed or disturbed Correct answer.

Explanation

Magnetic sensors are subject to turning and acceleration errors and need long-term averaging, so a gyro provides a stable short-term heading reference. The two are combined so the gyro is slowly slaved to the magnetic heading. The gyro neither generates a magnetic field nor measures rate of turn.

Question 12Sensors and Measurement Fundamentals

Which statement correctly describes the output of a linear variable differential transformer (LVDT)?

  1. A pulse train whose frequency indicates displacement
  2. A DC voltage proportional to displacement without any demodulation
  3. A change in resistance proportional to displacement
  4. An AC voltage whose amplitude and phase indicate displacement magnitude and direction Correct answer.

Explanation

An LVDT has a primary winding and two secondary windings connected in opposition, so the output is an AC signal that is near zero at the null position. Its amplitude indicates displacement from null and its phase indicates direction. Demodulation is needed to obtain a usable DC signal.

Question 13Sensors and Measurement Fundamentals

A potentiometric position sensor is used to feed an analogue indicator. Which error source is inherent to this type of sensor?

  1. Zero output at the null position
  2. Wear and contact noise at the wiper, causing a noisy or intermittent signal Correct answer.
  3. Phase shift between excitation and output
  4. Saturation of a magnetic core at high displacement

Explanation

A potentiometer relies on a sliding contact, so wiper wear, contamination and contact resistance produce noise and eventual failure. It also has finite resolution between turns of the winding. Null output and magnetic saturation are characteristics of other sensor types.

Question 14Sensors and Measurement Fundamentals

The output of a piezo-electric accelerometer used to sense airframe vibration is best described as which quantity?

  1. A steady DC voltage proportional to constant acceleration
  2. A charge proportional to the applied acceleration, requiring a charge amplifier Correct answer.
  3. A resistance that falls as vibration increases
  4. A capacitance that is independent of acceleration

Explanation

A piezo-electric crystal generates charge when stressed, so its output is proportional to the applied acceleration but cannot sustain a steady DC output. A charge amplifier converts the high-impedance charge into a usable voltage. It is not a resistive or purely capacitive sensing element.

Question 15Sensors and Measurement Fundamentals

Why must the leads from a piezo-electric sensor be kept short or connected to a high-impedance amplifier?

  1. The leads carry the excitation supply and must not drop voltage
  2. The sensor has a very high source impedance, so leakage would otherwise discharge the signal Correct answer.
  3. The sensor output is digital and would suffer timing errors
  4. The sensor output is a large current that would overheat long leads

Explanation

A piezo-electric element behaves as a charge source with very high output impedance, so cable capacitance and insulation leakage can rapidly discharge the small charge. A high-impedance (charge) amplifier preserves the signal. The output is analogue and does not carry excitation.

Question 16Sensors and Measurement Fundamentals

What is the purpose of shielding and twisting signal wires from a low-level sensor to its amplifier?

  1. To provide the excitation supply to the sensor
  2. To cool the sensor during operation
  3. To reduce induced electrical noise from nearby power cables Correct answer.
  4. To increase the voltage output of the sensor

Explanation

Low-level analogue signals are vulnerable to electromagnetic interference from nearby AC power wiring and radio sources. Twisting cancels magnetically induced loops and a grounded shield intercepts electrostatic coupling, improving signal-to-noise ratio. Shielding does not amplify or power the sensor.

Question 17Sensors and Measurement Fundamentals

An ADC with a resolution of 10 bits and a full-scale input range of 10 V has an approximate quantisation step of which value?

  1. Approximately 100 mV
  2. Approximately 10 mV
  3. Approximately 1.0 mV
  4. Approximately 9.8 mV Correct answer.

Explanation

A 10-bit converter divides the full-scale range into 2^10 = 1024 steps, so 10 V / 1024 gives about 9.8 mV per step. This step size sets the smallest resolvable change in the input. The other values correspond to different bit counts or a misapplied division.

Question 18Sensors and Measurement Fundamentals

What is the effect of increasing the number of bits in an analogue-to-digital converter on the digitised signal?

  1. The input range of the converter increases
  2. The need for anti-alias filtering is removed
  3. The conversion rate increases automatically
  4. The quantisation step becomes smaller, so resolution improves Correct answer.

Explanation

Each additional bit doubles the number of discrete levels, halving the quantisation step for a given full-scale range. This improves the ability to resolve small changes and reduces quantisation noise. It does not by itself change conversion speed, input range or the need for anti-alias filtering.

Question 19Sensors and Measurement Fundamentals

Which statement best describes the purpose of an anti-alias filter ahead of an ADC in an air-data acquisition system?

  1. It removes the DC offset from the sensor output
  2. It amplifies the sensor signal to fill the ADC range
  3. It attenuates signal components above half the sampling frequency before sampling Correct answer.
  4. It converts the analogue signal into a pulse-width modulated output

Explanation

Sampling a signal containing frequencies above half the sampling rate folds those components back into the baseband as false low-frequency content. A low-pass anti-alias filter placed before the converter attenuates them. Offset removal and amplification are separate conditioning functions.

Question 20Sensors and Measurement Fundamentals

What does the sensitivity of a measurement system describe?

  1. The change in output for a given change in the measured input Correct answer.
  2. The random fluctuation present in the output
  3. The maximum value the system can measure
  4. The time taken for the output to reach its final value

Explanation

Sensitivity is the ratio of output change to input change, effectively the slope of the calibration curve. A high sensitivity means a small input change produces a large output change. Maximum measurable value, response time and noise are separate characteristics.

Question 21Sensors and Measurement Fundamentals

A pressure sensor's output is specified as 10 mV per hPa but the calibration curve is not a straight line over the full range. Which term describes this departure from a straight-line relationship?

  1. Threshold
  2. Resolution
  3. Non-linearity Correct answer.
  4. Hysteresis

Explanation

Non-linearity is the maximum deviation of the actual transfer characteristic from the best straight line over the specified range. Hysteresis is a different effect, where the output differs depending on whether the input is increasing or decreasing. Threshold and resolution describe small-signal behaviour.

Question 22Sensors and Measurement Fundamentals

A sensor shows a different output when a pressure is increasing compared with the same pressure when decreasing. Which characteristic is being demonstrated?

  1. Zero drift
  2. Hysteresis Correct answer.
  3. Linearity
  4. Sensitivity

Explanation

Hysteresis occurs when the output for a given input depends on the direction of the previous change, producing a loop in the transfer characteristic. It arises from mechanical or magnetic lag in the sensing element. Linearity, sensitivity and zero drift describe different aspects of performance.

Question 23Sensors and Measurement Fundamentals

Why is a ratiometric measurement scheme often used with a resistive or potentiometric sensor?

  1. It eliminates the sensor's non-linearity completely
  2. It removes the need for any excitation supply
  3. The ADC reference is derived from the same supply as the sensor, so supply variations cancel Correct answer.
  4. It converts the sensor output directly into a frequency

Explanation

If the sensor excitation and the ADC reference come from the same source, a change in supply voltage affects both proportionally and cancels in the digital result. This improves accuracy without a precision voltage reference. It does not remove the need for excitation nor correct non-linearity.

Question 24Sensors and Measurement Fundamentals

What is the main advantage of a digital air-data system over individual analogue instruments for the flight deck?

  1. It allows the aircraft to fly without any pressure measurements
  2. It eliminates all position and instrument errors
  3. Sensor data can be processed, corrected and shared digitally among multiple users Correct answer.
  4. It removes the need for any pitot or static probes

Explanation

A digital air-data computer can apply corrections, combine inputs and distribute computed parameters to displays, autopilot and other systems over a data bus. Physical probes are still required, and residual errors such as position error must still be corrected.

Question 25Pressure Instruments and Air Data

In a conventional altimeter, the static pressure from the aircraft's static port is fed to which side of the sealed aneroid capsule assembly?

  1. The case surrounding the capsule, while the capsule remains sealed at a fixed reference pressure Correct answer.
  2. Both the capsule interior and the case, so the capsule never deflects
  3. The inside of the evacuated capsule, while the case receives static pressure
  4. A pitot pressure line, so the capsule senses total pressure

Explanation

A conventional altimeter has a sealed, partially evacuated capsule stack inside a case that is connected to the static source. As static pressure falls with altitude, the capsule expands and its motion is geared to the pointers. The capsule interior is never connected to the static line; only the case is.

Question 26Pressure Instruments and Air Data

An aircraft on the ground at an aerodrome with QNH 1013 hPa shows an altimeter reading of 40 ft. What does this indicate?

  1. That the instrument has a leak in the capsule
  2. That the static system is blocked
  3. A normal servicing tolerance with no significance
  4. That the altimeter subscale is set to 1013 hPa Correct answer.

Explanation

With QNH set, the altimeter should read aerodrome elevation. A reading of about 40 ft indicates the subscale is set to the standard pressure 1013 hPa, so the indicated altitude reflects the pressure difference between the aerodrome and the standard datum.

Question 27Pressure Instruments and Air Data

What does the vertical speed indicator actually measure?

  1. Difference between pitot and static pressure
  2. Rate of change of static pressure Correct answer.
  3. Absolute static pressure
  4. Rate of change of total pressure

Explanation

The VSI compares the instantaneous static pressure with a delayed version of the same pressure held in a calibrated leak or chokelike element. The resulting differential is proportional to the rate at which static pressure changes, which is displayed as a climb or descent rate.

Question 28Pressure Instruments and Air Data

During a steady climb at a constant indicated airspeed, the airspeed indicator tends to under-read unless corrected. What is the main cause?

  1. The capsule becomes stiffer at low static pressure Correct answer.
  2. The dynamic pressure falls as true airspeed rises with altitude
  3. The pitot line is heated and the air expands
  4. The static pressure is higher than the pitot pressure

Explanation

The ASI capsule deflects in response to dynamic pressure, but its stiffness relative to that pressure changes as static pressure decreases. At high altitude the same true dynamic pressure produces a smaller capsule deflection, so the instrument under-reads and a position or compressibility correction is needed.

Question 29Pressure Instruments and Air Data

An aircraft flies at high altitude and high Mach number, and the ASI over-reads. Which error explains this?

  1. Density error
  2. Instrument friction
  3. Position error
  4. Compressibility error Correct answer.

Explanation

At high Mach numbers the airflow ahead of the pitot tube is compressed, so the measured pressure rise exceeds the value that the instrument's calibration assumes for incompressible flow. This compressibility effect causes the ASI to over-read, and it is corrected by a Mach-dependent calibration or by an air data computer.

Question 30Pressure Instruments and Air Data

What is the purpose of the altimeter's barometric subscale (pressure setting window)?

  1. To damp capsule oscillations during turbulence
  2. To display the aircraft's rate of climb
  3. To correct for instrument temperature error
  4. To allow the reference pressure to be set so the altimeter indicates altitude relative to a chosen datum Correct answer.

Explanation

The subscale adjusts the linkage so the pointer reads altitude above the pressure datum selected by the pilot, such as QNH, QFE or standard pressure. Changing the setting shifts the whole indication without altering the capsule's response to static pressure.

Question 31Pressure Instruments and Air Data

An aircraft descends with the altimeter set to QNH. As it passes through the transition level, the pilot changes the setting to standard pressure 1013 hPa. If the local QNH is 1003 hPa, how does the indicated altitude change at the moment of reset?

  1. It increases by about 30 ft
  2. It decreases by about 300 ft
  3. It remains unchanged
  4. It increases by about 300 ft Correct answer.

Explanation

The pressure difference is 10 hPa, and near the surface 1 hPa corresponds to roughly 30 ft, so the change is about 300 ft. Setting a higher reference pressure (1013 versus 1003) makes the altimeter indicate a higher altitude for the same static pressure.

Question 32Pressure Instruments and Air Data

Why does a VSI show a momentary lag at the start of a climb before settling to a steady rate?

  1. The subscale must be reset for the new altitude
  2. The capsule has to overcome static friction and the calibrated leak takes time to establish the differential Correct answer.
  3. The instrument is connected to pitot pressure
  4. The static port is blocked during climb

Explanation

The VSI responds to the difference between instantaneous and delayed static pressure, and the delay element plus mechanism friction means the pointer takes a few seconds to reach the true rate. This lag is why instantaneous vertical speed indicators use accelerometers or air data computations to give a quicker response.

Question 33Pressure Instruments and Air Data

What is indicated airspeed (IAS) defined as?

  1. The speed shown after position and instrument corrections
  2. The speed corresponding to the Mach number
  3. The reading of the ASI before any correction is applied Correct answer.
  4. True airspeed corrected for wind

Explanation

IAS is the raw reading of the airspeed indicator, uncorrected for position error, instrument error or compressibility. Applying the appropriate corrections gives calibrated airspeed, and further correction for density gives true airspeed.

Question 34Pressure Instruments and Air Data

Which relationship correctly links calibrated airspeed to equivalent airspeed?

  1. EAS equals CAS at all altitudes
  2. EAS equals CAS multiplied by the square root of the density ratio Correct answer.
  3. EAS equals CAS corrected for compressibility
  4. EAS equals CAS divided by the square root of the density ratio

Explanation

Equivalent airspeed is the speed at sea level in standard conditions that would produce the same dynamic pressure as the actual flight condition. Because dynamic pressure depends on density, EAS equals CAS multiplied by the square root of the density ratio, and the two are equal only at sea level in the standard atmosphere.

Question 35Pressure Instruments and Air Data

What is the function of the pitot-static system's drain valve?

  1. To calibrate the ASI at the maintenance base
  2. To provide a backup static source if the port ices over
  3. To allow accumulated moisture to be removed from the lines Correct answer.
  4. To vent static pressure to the cabin during unpressurised flight

Explanation

Moisture can condense in the pitot and static lines, and a drain valve lets this water be removed before it freezes or blocks the lines. Blocked or water-filled lines cause erroneous indications in all three pressure instruments.

Question 36Pressure Instruments and Air Data

The static port becomes blocked during level flight. What happens to the ASI, altimeter and VSI?

  1. All three instruments freeze at their current readings
  2. ASI reads as an altimeter, altimeter freezes, VSI reads zero Correct answer.
  3. ASI under-reads, altimeter over-reads, VSI oscillates
  4. ASI behaves like an altimeter, altimeter freezes at the trapped pressure, VSI remains at zero

Explanation

With the static port blocked, the static pressure trapped in the case stays constant while the pitot pressure varies with speed. The ASI then responds to changes in pitot pressure as if they were altitude changes, the altimeter remains frozen, and the VSI shows zero because the case pressure no longer changes.

Question 37Pressure Instruments and Air Data

What is the purpose of an alternate static source in a light aircraft?

  1. To supply pitot pressure if the pitot tube ices
  2. To increase the sensitivity of the VSI
  3. To provide a static reference from inside the cabin if the external port blocks Correct answer.
  4. To allow the altimeter to be set to QFE

Explanation

If the external static port becomes blocked, a selector can connect the instruments to a static source inside the cabin. The cabin pressure differs from the true static pressure, so the indications are inaccurate and must be corrected, but the instruments remain usable.

Question 38Pressure Instruments and Air Data

A pitot tube becomes completely blocked by ice while the aircraft climbs at constant indicated airspeed. What does the ASI indicate?

  1. A steadily decreasing airspeed
  2. A steadily increasing airspeed Correct answer.
  3. Zero airspeed
  4. A speed that remains constant

Explanation

With the pitot tube blocked, the pressure trapped in the capsule stays constant, but the static pressure in the case falls as the aircraft climbs. The growing difference between the trapped pitot pressure and the falling static pressure makes the ASI over-read, appearing as an increasing airspeed.

Question 39Pressure Instruments and Air Data

What does the Machmeter display?

  1. True airspeed divided by the local speed of sound Correct answer.
  2. The ratio of true airspeed to the speed of sound at the tropopause
  3. The ratio of indicated airspeed to the speed of sound at sea level
  4. The ratio of true airspeed to the speed of sound at sea level

Explanation

Mach number is the true airspeed divided by the local speed of sound, which depends on the local outside air temperature. A Machmeter computes this ratio from pitot and static pressures and an air data input for temperature, so the same TAS gives a higher Mach number in colder air.

Question 40Pressure Instruments and Air Data

An aircraft flies at constant true airspeed while the outside air temperature falls. How does the Mach number change?

  1. It increases because the local speed of sound decreases Correct answer.
  2. It increases because the local speed of sound increases
  3. It remains constant because true airspeed is constant
  4. It decreases because the local speed of sound increases

Explanation

The speed of sound depends on temperature and falls as the air becomes colder. Since Mach number is true airspeed divided by the local speed of sound, a constant TAS with a lower speed of sound gives a higher Mach number.

Question 41Pressure Instruments and Air Data

What is the purpose of the maximum allowable airspeed (barber's pole) pointer on an ASI?

  1. To indicate the maximum speed for the current flap configuration
  2. To indicate the best rate of climb speed
  3. To indicate the maximum speed for the current altitude and Mach number Correct answer.
  4. To indicate the speed at which the aircraft stalls

Explanation

The barber's pole moves with altitude and Mach number to show the maximum speed that can be flown without exceeding structural or Mach limits. It is driven by an air data computer or an aneroid mechanism and shifts as the aircraft climbs.

Question 42Pressure Instruments and Air Data

What is the function of the air data computer in a modern aircraft?

  1. To control the cabin pressure schedule
  2. To display engine parameters on the flight deck
  3. To compute the aircraft's heading from magnetic sensors
  4. To compute altitude, airspeed, Mach number and temperature from pitot, static and temperature inputs Correct answer.

Explanation

The air data computer processes pitot pressure, static pressure and total air temperature to derive altitude, airspeed, Mach number, vertical speed and other parameters. These outputs feed the displays, autopilot and flight management system.

Question 43Pressure Instruments and Air Data

An altimeter is set to QFE at an aerodrome with elevation 1500 ft and QNH 1013 hPa. What does the altimeter read when the aircraft is on the ground?

  1. Zero feet Correct answer.
  2. The pressure altitude of the aerodrome
  3. The aerodrome elevation corrected for temperature
  4. 1500 feet

Explanation

QFE is the pressure at the aerodrome reference point, so with the subscale set to QFE the altimeter reads zero on the ground. It then indicates height above the aerodrome rather than altitude above mean sea level.

Question 44Pressure Instruments and Air Data

Which statement correctly describes the relationship between true altitude, indicated altitude and temperature?

  1. Temperature has no effect on the relationship between true and indicated altitude
  2. In air colder than standard, the true altitude is higher than the indicated altitude
  3. In air colder than standard, the true altitude is lower than the indicated altitude Correct answer.
  4. In air warmer than standard, the true altitude is lower than the indicated altitude

Explanation

The altimeter is calibrated for the standard atmosphere, and cold air is denser, so the pressure levels are closer together than the instrument assumes. When flying from a region of standard temperature into colder air, the aircraft is lower than indicated, which is why cold-temperature altitude corrections are applied on approach.

Question 45Pressure Instruments and Air Data

What is the purpose of the static pressure port's flush mounting on the fuselage?

  1. To reduce position error by sensing pressure close to the free-stream static value Correct answer.
  2. To drain water from the system
  3. To provide a pitot pressure reference
  4. To heat the static line

Explanation

The static port is positioned and shaped so that the local pressure at the port is as close as possible to the true free-stream static pressure. Poor location or disturbed airflow produces position error, which affects all three pressure instruments.

Question 46Pressure Instruments and Air Data

An aircraft with a pressurised cabin has its static source selected to the alternate (cabin) source while cruising at high altitude. How do the indications change?

  1. The altimeter over-reads and the ASI under-reads relative to the true values
  2. The indications are unaffected because cabin pressure equals static pressure
  3. The VSI is the only instrument affected
  4. The altimeter under-reads and the ASI over-reads relative to the true values Correct answer.

Explanation

Cabin pressure at altitude is higher than the external static pressure, so the altimeter senses a pressure corresponding to a lower altitude and under-reads. The higher static pressure in the ASI case reduces the pressure difference across the capsule, so the ASI over-reads for the same pitot pressure.

Question 47Pressure Instruments and Air Data

An aircraft climbs at a constant indicated airspeed while the Mach number increases. What happens to the difference between indicated and true airspeed?

  1. True airspeed increases less than indicated airspeed
  2. True airspeed increases more than indicated airspeed Correct answer.
  3. True airspeed and indicated airspeed remain equal
  4. True airspeed decreases while indicated airspeed stays constant

Explanation

As altitude increases, the air density falls, so a given indicated airspeed corresponds to a higher true airspeed. The ratio of true to indicated airspeed grows with altitude, meaning true airspeed increases faster than indicated airspeed during a constant-IAS climb.

Question 48Pressure Instruments and Air Data

What causes the altimeter to indicate a value different from the true altitude when flying through a region where the temperature is much colder than standard?

  1. The capsule becomes stiffer and under-reads the pressure change
  2. The static port position error increases in cold air
  3. The pressure levels are compressed and the aircraft is lower than the standard-atmosphere calibration assumes Correct answer.
  4. The subscale expands with the cold temperature

Explanation

The altimeter converts static pressure to altitude using the standard atmosphere, in which temperature falls at a fixed lapse rate. In colder air the pressure surfaces are closer together, so the aircraft at a given pressure altitude is physically lower than the calibration implies.

Question 49Pressure Instruments and Air Data

Which instrument uses both pitot and static pressure?

  1. The vertical speed indicator
  2. The airspeed indicator Correct answer.
  3. The altimeter
  4. The Machmeter only

Explanation

The airspeed indicator compares pitot (total) pressure with static pressure to obtain dynamic pressure. The altimeter and VSI use static pressure only, while the Machmeter also uses both pitot and static pressure but is a separate instrument from the ASI.

Question 50Pressure Instruments and Air Data

Which pressure is applied internally to the capsule stack of a simple altimeter so that it can display altitude?

  1. Total pressure from the pitot tube
  2. Static pressure from the static port Correct answer.
  3. Dynamic pressure from the pitot-static difference
  4. Cabin pressure from the pressurisation system

Explanation

A simple altimeter is an aneroid barometer whose sealed capsule stack is referenced to a fixed internal pressure, while the case around it receives static pressure from the static port. As the aircraft climbs, static pressure falls and the capsule expands, driving the pointer through the mechanical linkage. Only static pressure changes with altitude, so the instrument responds to that source alone.