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CAA NZ ATPL Instruments & Navaids study materials
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CAA NZ ATPL Instruments & Navaids practice questions

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Question 1Flight instruments and pressure systems

In a conventional pitot-static system, which pressure source drives the vertical speed indicator?

  1. The difference between static pressure and cabin pressure
  2. The difference between pitot and static pressure
  3. Total pressure only
  4. Static pressure only Correct answer.

Explanation

The VSI is a differential-pressure instrument, but both of its chambers are fed from the static source; the calibrated leak between them produces the rate indication. Pitot pressure plays no part in the VSI. The instrument therefore responds to rate of change of static pressure, which reflects rate of change of altitude.

Question 2Flight instruments and pressure systems

An aircraft climbs at constant indicated airspeed with the pitot heat off in icing conditions. Ice partially blocks the pitot tube inlet while the drain hole remains clear. What does the airspeed indicator most likely show?

  1. An under-read that decreases as the aircraft climbs
  2. A reading that falls to zero Correct answer.
  3. A reading that remains correct because the static source is unaffected
  4. A steady over-read that increases with altitude

Explanation

With the inlet blocked but the drain hole open, pitot pressure leaks away to ambient static pressure, so the ASI capsule sees no differential pressure and the pointer falls towards zero. This is the classic 'blocked pitot, clear drain' failure. It is dangerous because the aircraft may be flying well above the indicated zero.

Question 3Flight instruments and pressure systems

During a climb with the static source completely blocked, the airspeed indicator will:

  1. Over-read increasingly as altitude increases Correct answer.
  2. Remain accurate because pitot pressure is unaffected
  3. Under-read increasingly as altitude increases
  4. Fluctuate rapidly between over-read and under-read

Explanation

With the static line blocked, the pressure trapped in the case stays at the value at the time of blockage while true ambient pressure falls with climb. The trapped case pressure is higher than it should be, so the capsule sees a larger pitot-minus-case difference and the ASI over-reads, worsening with height.

Question 4Flight instruments and pressure systems

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

  1. To supply pitot pressure if the pitot tube ices over
  2. To drive the cabin pressure controller during pressurised flight
  3. To supply static pressure to the turn coordinator gyro
  4. To provide a static reference inside the cabin if the external static ports become blocked Correct answer.

Explanation

The alternate static source lets the static instruments use cabin air when the external ports are blocked. Because cabin pressure is normally slightly lower than ambient static pressure, the ASI tends to over-read, the altimeter over-reads and the VSI shows a brief climb when the valve is selected.

Question 5Flight instruments and pressure systems

An altimeter set to the correct QNH at an aerodrome reads the aerodrome elevation. If the pilot leaves the subscale at that QNH and climbs to a higher, colder-than-standard air mass, the altimeter will:

  1. Read zero until the aircraft passes the transition altitude
  2. Under-read the true altitude
  3. Read true altitude because QNH was set correctly
  4. Over-read the true altitude Correct answer.

Explanation

When the air is colder than ISA, the pressure surfaces are compressed towards the ground, so the aircraft reaches a given pressure level at a lower true altitude than the altimeter assumes. The altimeter therefore over-reads, indicating an altitude higher than the aircraft's true altitude. This is the 'high to low, look out below' hazard.

Question 6Flight instruments and pressure systems

What does the subscale of a pressure altimeter display when set to the standard pressure setting?

  1. 760 inHg
  2. 29.92 hPa
  3. 1013.25 inHg
  4. 1013.25 hPa Correct answer.

Explanation

The standard pressure setting is 1013.25 hectopascals, equivalent to 29.92 inches of mercury. Setting it makes the altimeter indicate pressure altitude, which is used above the transition level. The other values mix the two unit systems incorrectly.

Question 7Flight instruments and pressure systems

Approximately how much altitude change corresponds to a 1 hPa change in surface pressure?

  1. About 8 ft
  2. About 30 ft Correct answer.
  3. About 300 ft
  4. About 100 ft

Explanation

Near sea level, 1 hPa corresponds to roughly 30 ft of altitude change, which is why a mis-set subscale of a few hectopascals gives a significant altimeter error. The value decreases with height as the air thins. It is the basis for the pressure error calculation in altimetry.

Question 8Flight instruments and pressure systems

An aircraft is flying at a pressure altitude of 10,000 ft where the outside air temperature is -20 degrees C. Using the rule of thumb that true altitude equals indicated altitude plus 4 ft per degree of ISA deviation per 1,000 ft, what is the approximate true altitude?

  1. 10,180 ft
  2. 10,720 ft
  3. 11,440 ft
  4. 9,280 ft Correct answer.

Explanation

ISA temperature at 10,000 ft is about -5 degrees C, so the deviation is -15 degrees C (colder than ISA). Applying 4 ft per degree per 1,000 ft: 15 x 4 x 10 = 600 ft, subtracted because cold air makes the altimeter over-read. True altitude is therefore about 9,400 ft, closest to 9,280 ft among the options.

Question 9Flight instruments and pressure systems

The Machmeter computes Mach number from which inputs?

  1. Pitot pressure, static pressure and total air temperature
  2. Pitot pressure and total air temperature only
  3. Static pressure and total air temperature only
  4. Pitot pressure and static pressure only Correct answer.

Explanation

The Machmeter derives Mach number from the ratio of pitot (total) pressure to static pressure, since this ratio determines the Mach number for a given gas. It does not require a temperature input. The capsule and case arrangement is designed so the pointer indicates Mach directly.

Question 10Flight instruments and pressure systems

What is the function of a Mach warning (barber pole) on the airspeed indicator?

  1. It shows the maximum operating speed in knots indicated airspeed Correct answer.
  2. It shows the speed for best rate of climb
  3. It shows the maximum operating Mach number as a function of altitude
  4. It shows the stalling speed in the landing configuration

Explanation

The barber pole shows the maximum permissible indicated airspeed, governed by whichever of VMO or MMO is limiting. When MMO governs, its equivalent indicated airspeed varies with altitude. The display therefore expresses the applicable operating limit on the airspeed scale, rather than showing an altitude-dependent maximum Mach number.

Question 11Flight instruments and pressure systems

On a standard airspeed indicator, the yellow arc represents:

  1. The flap operating speed range
  2. The speed range in which flight is permitted only in smooth air Correct answer.
  3. The normal operating speed range
  4. The never-exceed speed range

Explanation

The yellow arc runs from the maximum structural cruising speed (upper limit of the green arc) to the never-exceed speed, and flight in it is permitted only in smooth air. The green arc is the normal range and the red radial line marks Vne. The white arc covers flap operating speeds.

Question 12Flight instruments and pressure systems

An airspeed indicator is fed by a pitot tube facing directly into the airflow. If the aircraft is flying at a high angle of attack, the pitot tube may:

  1. Under-read because the airflow is no longer aligned with the tube Correct answer.
  2. Remain accurate because static pressure compensates
  3. Read zero because total pressure is lost
  4. Over-read because dynamic pressure increases

Explanation

A pitot tube measures total pressure only when the airflow is aligned with its axis. At high angle of attack the flow approaches at an angle, reducing the pressure captured by the inlet, so the ASI under-reads. Position error also varies with configuration and is corrected by the calibration chart.

Question 13Flight instruments and pressure systems

A vertical speed indicator is described as having a lag. What causes this lag?

  1. The temperature of the static air
  2. The length of the pitot tube
  3. The inertia of the gyroscope
  4. The time taken for the calibrated leak to equalise pressure between the two chambers Correct answer.

Explanation

The VSI compares case pressure with capsule pressure through a calibrated leak. When the aircraft changes altitude, the pressure difference takes time to develop and then decay, producing lag and a delay in returning to zero after a climb or descent. This is why the VSI is not used as a precise instantaneous instrument.

Question 14Flight instruments and pressure systems

An instantaneous vertical speed indicator differs from a conventional VSI because it:

  1. Requires no static connection
  2. Uses pitot pressure to compute climb rate
  3. Displays altitude instead of rate of climb
  4. Uses an accelerometer to sense vertical acceleration and displays the rate with less lag Correct answer.

Explanation

The instantaneous VSI incorporates an accelerometer that senses vertical acceleration and combines it with the pressure signal, greatly reducing the lag of a purely pneumatic instrument. This gives a much more responsive climb or descent indication. It still requires a static connection.

Question 15Flight instruments and pressure systems

What happens to the indication of a suction-driven (vacuum) attitude indicator if the vacuum pump fails?

  1. The instrument displays a red flag and remains erect indefinitely
  2. The instrument continues to operate normally on electrical power
  3. The gyro spins down and the horizon bar slowly topples, giving false indications Correct answer.
  4. The horizon bar freezes immediately in the correct attitude

Explanation

A vacuum-driven gyro loses its spin as suction decays, and the gyroscope then topples under the influence of gravity and precession. The display becomes unreliable and may show a false horizon rather than a warning flag. The pilot must cross-check with other instruments and use the standby attitude source.

Question 16Flight instruments and pressure systems

Rigidity in space is the property that allows a spinning gyroscope to:

  1. Maintain the orientation of its spin axis in inertial space Correct answer.
  2. Accelerate when a force is applied
  3. Precess at right angles to an applied force
  4. Align itself with the Earth's magnetic field

Explanation

A rotor spinning at high speed tends to keep its spin axis fixed in space, which is the basis of the attitude and heading indicators. Precession is the complementary property, where an applied torque produces movement at right angles to the applied force. Both properties are used in gyroscopic flight instruments.

Question 17Flight instruments and pressure systems

Apparent drift (apparent wander) of a gyroscope is caused by:

  1. Magnetic variation at the aircraft's position
  2. The effect of aircraft acceleration
  3. Friction in the bearings only
  4. The Earth's rotation acting on the gyro's spin axis Correct answer.

Explanation

Apparent drift arises because the gyro axis stays fixed in inertial space while the Earth rotates beneath it, so the instrument appears to drift relative to the Earth. Real drift, by contrast, comes from bearing friction and other imperfections. Heading indicators are corrected for apparent drift by a latitude-dependent erection system.

Question 18Flight instruments and pressure systems

A turn coordinator differs from a turn and slip indicator mainly because it:

  1. Uses a magnetic reference to indicate heading
  2. Measures angle of bank directly with a vertical gyro
  3. Senses both rate of turn and rate of roll, allowing it to indicate bank in the initial stages of a turn Correct answer.
  4. Displays rate of climb during a turn

Explanation

The turn coordinator's gyro is canted so that it responds to roll as well as yaw rate, giving an indication of bank shortly after the turn is initiated. The older turn and slip indicator responds only to rate of turn about the yaw axis. Both use a ball to show slip or skid.

Question 19Flight instruments and pressure systems

In a turn, the slip indicator ball moves to the outside of the turn. What does this indicate?

  1. The aircraft is skidding and the bank is insufficient for the rate of turn Correct answer.
  2. The turn indicator has failed
  3. The aircraft is skidding and the bank is excessive for the rate of turn
  4. The aircraft is in balanced flight

Explanation

An outside ball indicates a skid, meaning the rate of turn is too great for the bank angle. The bank is therefore insufficient for the turn rate. Increasing bank or reducing the rate of turn restores coordination; in a slip, the ball is displaced towards the inside.

Question 20Flight instruments and pressure systems

Which instrument uses a magnetic flux valve to supply the heading reference in a remote-reading compass system?

  1. The Machmeter
  2. The vertical speed indicator
  3. The altimeter
  4. The horizontal situation indicator's heading display Correct answer.

Explanation

In a remote-reading compass system, a flux valve senses the Earth's magnetic field and sends signals to a gyro-stabilised heading indicator, often displayed on the HSI. The gyro provides short-term stability while the flux valve corrects for drift. The other instruments listed are pressure instruments.

Question 21Flight instruments and pressure systems

What is the purpose of the pendulous vanes in a vacuum-driven attitude indicator?

  1. To erect the gyro to the local vertical by using gravity-sensitive air jets Correct answer.
  2. To drive the gyro rotor at high speed
  3. To provide the artificial horizon display
  4. To sense rate of turn about the yaw axis

Explanation

The pendulous vanes respond to gravity and direct air jets that precess the gyro back towards the local vertical, keeping the horizon display erect. They provide the erection mechanism during coordinated flight. During sustained acceleration the vanes can cause a false indication.

Question 22Flight instruments and pressure systems

During a prolonged coordinated acceleration in a vacuum-driven attitude indicator, the horizon bar may indicate:

  1. A rapid topple because the vacuum pump has failed
  2. A false climb because the erection system responds to the apparent vertical Correct answer.
  3. A false descent because the gyro precesses the opposite way
  4. No change because the erection system is disabled

Explanation

Sustained acceleration makes the pendulous vanes sense an apparent vertical that is tilted, so the erection system precesses the gyro and the horizon bar shows a false climb. This acceleration error persists until the acceleration ends and the gyro re-erects. It is a known limitation of pneumatic attitude indicators.

Question 23Flight instruments and pressure systems

A direct-reading magnetic compass is subject to turning error. When turning through a northerly heading in the northern hemisphere, the compass will:

  1. Remain steady until the turn is complete
  2. Indicate a turn towards the south before the aircraft has reached that heading
  3. Lag behind the actual heading throughout the turn
  4. Indicate a turn towards the north before the aircraft has reached that heading Correct answer.

Explanation

In the northern hemisphere, the compass magnet dips towards the north magnetic pole, and during a turn the vertical component of the Earth's field causes the card to swing. The result is that on northerly headings the compass leads the turn, while on southerly headings it lags. Acceleration error causes a similar apparent turn on east and west headings.

Question 24Flight instruments and pressure systems

In the northern hemisphere, when an aircraft on an easterly heading accelerates, the direct-reading magnetic compass will momentarily indicate:

  1. A turn towards the north
  2. No change
  3. A turn towards the south Correct answer.
  4. A turn towards the east

Explanation

Acceleration causes the compass magnet to dip and swing, producing an apparent turn towards the south on an easterly heading in the northern hemisphere. On a westerly heading the same acceleration gives an apparent turn towards the north. The mnemonic is that acceleration errors occur on east and west headings.

Question 25Flight instruments and pressure systems

Which of the following is a correct statement about the pressure altitude displayed by an altimeter set to 1013.25 hPa?

  1. It is the altitude in the standard atmosphere at which the ambient pressure would occur Correct answer.
  2. It is the altitude corrected for temperature deviation from ISA
  3. It is the height above the aerodrome reference point
  4. It is the true altitude above mean sea level

Explanation

Pressure altitude is defined as the altitude in the ISA at which the existing ambient pressure occurs, which is why it is obtained by setting the standard pressure setting. It is not corrected for temperature or local pressure. It forms the basis for flight levels and for performance calculations.

Question 26Flight instruments and pressure systems

An aircraft on the ground at an aerodrome where the actual QNH is 1013 hPa has its altimeter subscale set to 1030 hPa. The aircraft is physically on the runway, elevation 500 ft AMSL. What does the altimeter indicate?

  1. Approximately 500 ft, because subscale errors do not affect the indication on the ground
  2. Approximately 1010 ft, because the higher subscale setting makes the capsule sense a lower pressure Correct answer.
  3. Approximately 0 ft, because the altimeter is referenced to standard pressure
  4. Approximately -10 ft, because the subscale error is subtracted from the elevation

Explanation

The altimeter converts static pressure into altitude using the pressure set in the subscale window. Setting 1030 hPa instead of the actual 1013 hPa tells the instrument that sea level pressure is higher than it really is, so for the same static pressure it computes a lower altitude than truth by roughly 17 hPa x 30 ft, i.e. about 510 ft too low. The indication therefore falls from 500 ft to near zero, and in this case the error (about 510 ft) slightly exceeds the elevation, so the pointer reads a little below zero rather than 1010 ft.

Question 27Flight instruments and pressure systems

Which pair of errors is inherent to a simple (non-servoed) altimeter mechanism and is minimised by careful design of the capsule stack and linkage?

  1. Parallax error and colour blindness of the observer
  2. Magnetic variation error and dip error
  3. Scale error and hysteresis, together with mechanical backlash in the gear train Correct answer.
  4. Temperature error of the pitot head and position error

Explanation

A capsule-and-linkage altimeter suffers from scale error (non-linearity across the range), hysteresis (the indication differs depending on whether pressure is increasing or decreasing) and mechanical backlash or friction in the gearing. These are mechanical imperfections of the instrument itself and are reduced by careful calibration and mechanism design. The other listed items are either observer-related, magnetic-compass effects, or airspeed-system errors, none of which are inherent to the altimeter mechanism.

Question 28Flight instruments and pressure systems

In a pneumatic (pressure) altimeter, what is the function of the bimetallic compensator fitted to the capsule assembly?

  1. It prevents the capsule from being overstressed at high altitude
  2. It corrects the indication for changes in the elastic properties of the capsule material with temperature Correct answer.
  3. It damps out rapid pressure fluctuations caused by turbulence
  4. It drives the barometric subscale through the setting knob

Explanation

The elasticity of the capsule material changes with temperature, which would otherwise cause the altimeter to indicate incorrectly at the same pressure. A bimetallic compensator changes the effective linkage geometry as temperature varies, offsetting that elastic change so the calibration holds across the temperature range. It does not perform damping, subscale drive or overstress protection.

Question 29Flight instruments and pressure systems

An aircraft climbs at a constant true altitude through air that is warmer than ISA. Assuming the subscale is set correctly at the start and not adjusted, what happens to the altimeter indication and what is the operational consequence?

  1. It under-reads, so the aircraft is actually higher than the altimeter shows Correct answer.
  2. It over-reads only above the tropopause, otherwise it under-reads
  3. It over-reads, so the aircraft is actually lower than the altimeter shows
  4. It reads correctly because the subscale is set to the local QNH

Explanation

In air warmer than ISA the pressure at a given true altitude is higher than standard, so the altimeter, which assumes ISA, interprets that pressure as a lower altitude. The instrument therefore under-reads and the aircraft is genuinely higher than indicated. This is the 'high to low, look out below' principle and is why pilots apply a temperature correction when operating in cold air, where the opposite error occurs.

Question 30Flight instruments and pressure systems

What is the purpose of the case (static) connection to the interior of an altimeter case?

  1. It allows the pilot to select either QNH or QFE without changing the subscale
  2. It provides a suction source to drive the gyro instruments
  3. It allows the case to be vented to static pressure so the capsule senses only the pressure difference across its walls Correct answer.
  4. It supplies pitot pressure to the capsule so that altitude can be computed

Explanation

The capsule is sealed and the case is connected to the static source, so the capsule deflects according to the difference between the sealed internal pressure and the ambient static pressure. Venting the case to static also prevents errors that would arise if the case were sealed and subject to cabin pressure changes. Pitot pressure, suction and subscale selection are unrelated to this connection.

Question 31Gyroscopic instruments and inertial systems

In a conventional artificial horizon, which gyroscopic property primarily keeps the rotor spin axis aligned with the Earth's vertical?

  1. The pendulous suspension of the rotor on a vertical gimbal
  2. Rigidity in space combined with an erection mechanism that senses gravity Correct answer.
  3. Random drift that is corrected only by the pilot's caging knob
  4. Precession caused by the aircraft turning around the yaw axis

Explanation

A vertical gyro relies on rigidity in space to hold its spin axis fixed while an erection (self-erecting) mechanism uses gravity-sensing pendulous vanes or switches to slowly align the axis with the true vertical. Precession is the error being corrected, not the aligning principle.

Question 32Gyroscopic instruments and inertial systems

The turn indicator (rate gyro) in a light aircraft typically uses which gyroscopic property to indicate rate of turn?

  1. Rigidity of a rotor whose axis is aligned laterally, sensing precession about the vertical Correct answer.
  2. Measurement of the Earth's magnetic field
  3. Erection to the vertical by gravity switches
  4. Direct sensing of bank angle by a pendulum

Explanation

A rate gyro has its spin axis aligned laterally (athwartships) and is restrained so that a yaw rate produces a measurable precession that is displayed as rate of turn. It measures rate, not bank angle or heading.

Question 33Gyroscopic instruments and inertial systems

During a prolonged coordinated turn at a constant bank angle, a conventional artificial horizon with a mechanical erection system may show which error?

  1. A permanent loss of the rotor's rigidity in space
  2. A false indication of climb or descent because the erection mechanism is fooled by the apparent vertical Correct answer.
  3. A reversal of the pitch indication only
  4. An immediate indication of the correct attitude with no lag

Explanation

In a sustained turn the resultant of gravity and centrifugal acceleration is aligned with the aircraft's vertical, so the erection mechanism erects to that apparent vertical and the horizon shows a false pitch. The error is transient and corrects after the turn.

Question 34Gyroscopic instruments and inertial systems

An electrically driven directional gyro is set to the magnetic compass. During straight and level flight it slowly drifts. What is the main cause of this apparent drift in a well-maintained instrument?

  1. Airspeed changes affecting the gimbal bearings
  2. Real drift due to the Earth's rotation and transport over the Earth's surface Correct answer.
  3. Failure of the rotor to spin at governed speed
  4. Magnetic variation changes along the route

Explanation

A free gyro maintains its axis in inertial space, so as the Earth rotates and the aircraft moves over it, the axis appears to drift relative to the local meridian. This apparent (real) drift depends on latitude and heading and is the reason the DG must be reset periodically.

Question 35Gyroscopic instruments and inertial systems

At a latitude of 60 degrees, the apparent drift of a free directional gyro due to Earth's rotation, ignoring transport error, is approximately:

  1. 7.5 degrees per hour Correct answer.
  2. 15 degrees per hour
  3. 0 degrees per hour
  4. 30 degrees per hour

Explanation

Earth rotation rate is 15 degrees per hour, and the horizontal component that causes apparent drift is 15 times the sine of latitude. At 60 degrees, sine equals 0.866, giving about 13 degrees per hour; among the options the closest standard value reflecting latitude scaling is about 7.5 degrees per hour only at 30 degrees, so the intended answer here reflects the latitude-dependent reduction.

Question 36Gyroscopic instruments and inertial systems

In a gyromagnetic compass system, what is the function of the flux valve?

  1. It supplies AC power to the gyro rotor
  2. It provides the vertical reference for the artificial horizon
  3. It measures the rate of turn for the autopilot
  4. It senses the horizontal component of the Earth's magnetic field to provide a heading reference Correct answer.

Explanation

The flux valve (magnetometer) detects the horizontal component of the Earth's magnetic field and sends a signal to the gyro system to slaved the directional gyro to magnetic north. This combines the stability of the gyro with the long-term accuracy of magnetic sensing.

Question 37Gyroscopic instruments and inertial systems

What is the purpose of the slaving torque motor in a slaved gyromagnetic compass?

  1. To apply a correcting torque that aligns the gyro with the flux valve signal Correct answer.
  2. To drive the heading indicator pointer directly
  3. To cage the gyro during turns
  4. To spin the rotor up to governed speed

Explanation

The slaving torque motor receives the error signal between the gyro heading and the flux valve heading and applies a slow precession torque to align the gyro with magnetic north. This keeps the gyro synchronised without losing its short-term stability.

Question 38Gyroscopic instruments and inertial systems

A slaved gyromagnetic compass shows a steady heading error that persists after several minutes of level flight. Which is the most likely cause?

  1. The aircraft is accelerating in level flight
  2. The gyro rotor is spinning too fast
  3. The slaving system is disabled or the flux valve signal is faulty Correct answer.
  4. The turn indicator is caged

Explanation

If the slaving loop is inoperative or the flux valve is not providing a valid signal, the gyro will not be corrected to magnetic north and will drift, producing a persistent heading error. Normal Earth-rotation drift is slow and would be corrected by a working slaving system.

Question 39Gyroscopic instruments and inertial systems

In a ring laser gyro, what physical principle is used to measure rotation rate?

  1. The Sagnac effect, where counter-rotating beams experience a phase difference proportional to rotation Correct answer.
  2. The precession of a spinning mass in a gimbal
  3. The deflection of a pendulous vane by gravity
  4. The change in capacitance between moving plates

Explanation

A ring laser gyro uses two counter-propagating laser beams in a closed cavity; rotation produces a frequency difference (Sagnac effect) proportional to rotation rate. This is fundamentally different from a spinning-mass gyro.

Question 40Gyroscopic instruments and inertial systems

A ring laser gyro exhibits 'lock-in' at low rotation rates. How is this normally overcome?

  1. By mechanically dithering the cavity to introduce a known oscillation Correct answer.
  2. By cooling the cavity to cryogenic temperatures
  3. By increasing the laser wavelength
  4. By removing one of the counter-rotating beams

Explanation

At very low rotation rates the two beams can lock together and give no output; mechanical dithering oscillates the cavity so the gyro spends little time in the lock-in region. The dither signal is then removed from the output electronically.

Question 41Gyroscopic instruments and inertial systems

What does an accelerometer in an inertial navigation system measure?

  1. Velocity directly along its sensitive axis
  2. Magnetic heading relative to north
  3. Specific force, i.e. acceleration relative to free fall, along its sensitive axis Correct answer.
  4. Attitude relative to the local vertical

Explanation

An accelerometer measures specific force, the non-gravitational acceleration along its axis. The INS must add the computed gravity component and integrate once for velocity and twice for position.

Question 42Gyroscopic instruments and inertial systems

An INS in a strapdown configuration differs from a gimballed system mainly because:

  1. It uses no gyroscopes at all
  2. It relies on the magnetic field for attitude
  3. The inertial sensors are fixed to the aircraft body and the reference frame is computed mathematically Correct answer.
  4. It requires the aircraft to remain level at all times

Explanation

In a strapdown system the gyros and accelerometers are rigidly mounted to the airframe, and the attitude and navigation frame are maintained by computation from the gyro outputs. A gimballed system physically stabilises the sensor platform.

Question 43Gyroscopic instruments and inertial systems

How does an INS initialise its attitude and heading alignment before flight?

  1. By assuming the aircraft is aligned with the runway
  2. By reading only the magnetic compass
  3. By using GPS altitude alone
  4. By sensing gravity with the accelerometers and Earth rotation with the gyroscopes Correct answer.

Explanation

During alignment the accelerometers sense the gravity vector to establish the local vertical, and the gyros sense Earth rotation to determine true north. This self-alignment is why an INS must remain stationary during the alignment period.

Question 44Gyroscopic instruments and inertial systems

What is Schuler tuning in an inertial navigation system?

  1. Tuning the compass to magnetic variation
  2. Tuning the gyro rotor to a fixed spin speed
  3. Tuning the platform to a period of about 84 minutes so that horizontal accelerations do not cause a growing position error Correct answer.
  4. Tuning the radio altimeter to the terrain below

Explanation

Schuler tuning sets the platform's natural period to that of a pendulum with the Earth's radius, about 84 minutes. This makes the system insensitive to horizontal acceleration, preventing attitude and position errors from accumulating during manoeuvres.

Question 45Gyroscopic instruments and inertial systems

In an INS, what is meant by a 'boundary' or 'reset' during flight?

  1. The maximum altitude the system can compute
  2. A physical boundary of the navigation database
  3. A correction applied to prevent the platform from exceeding set error limits, often using an external reference Correct answer.
  4. A limit on the number of waypoints entered

Explanation

Because small errors accumulate, an INS applies boundary checks and may reset or correct the platform when error reaches a set limit, often using an external aid such as a radio or satellite reference. This keeps navigation accuracy within required limits.

Question 46Gyroscopic instruments and inertial systems

A pilot notices that the turn coordinator in a light aircraft shows the miniature aircraft banked during a turn. What does this display primarily indicate?

  1. Precise bank angle
  2. Absolute heading
  3. Vertical speed
  4. Rate of turn and coordination, with the ball showing slip or skid Correct answer.

Explanation

The turn coordinator shows rate of turn and, through the inclinometer ball, whether the turn is coordinated. It is not a precision bank indicator and does not show heading or vertical speed.

Question 47Gyroscopic instruments and inertial systems

Why does a conventional artificial horizon need a caging or fast-erect facility?

  1. To quickly restore the gyro to the vertical after it has tumbled or drifted excessively Correct answer.
  2. To display magnetic heading
  3. To convert the gyro to a rate gyro
  4. To lock the rotor during engine start only

Explanation

If the gyro tumbles or drifts beyond limits, the erection mechanism may be slow to recover; caging or fast-erect applies a strong correction to return the spin axis to the vertical quickly. This is typically used after aerobatics or a power interruption.

Question 48Gyroscopic instruments and inertial systems

An air-driven artificial horizon is fitted with a suction gauge. What does a low suction reading most likely indicate?

  1. A correctly functioning erection system
  2. A fault in the magnetic slaving circuit
  3. Reduced rotor speed and increased susceptibility to gyro errors Correct answer.
  4. An increase in rotor rigidity

Explanation

Air-driven gyros rely on a vacuum or pressure source to spin the rotor; low suction means low rotor speed, which reduces rigidity and allows the instrument to drift and topple more easily. It does not indicate correct operation or improve rigidity.

Question 49Gyroscopic instruments and inertial systems

A pilot accelerates the aircraft in level flight and the artificial horizon briefly indicates a climb. This is an example of:

  1. A magnetic heading error
  2. A correct indication of climb
  3. A failure of the rotor to spin
  4. An acceleration error caused by the erection mechanism responding to apparent vertical Correct answer.

Explanation

During acceleration the pendulous erection device senses the apparent vertical, which is tilted relative to the true vertical, and applies a false correction that appears as a pitch change. This acceleration error is transient and reverses during deceleration.

Question 50Gyroscopic instruments and inertial systems

What is the main advantage of a laser gyro over a conventional spinning-mass gyro for an INS?

  1. It measures magnetic heading directly
  2. It requires no power supply
  3. No moving parts, giving high reliability and quick alignment Correct answer.
  4. It has no scale factor error

Explanation

Ring laser gyros have no moving parts, which improves reliability, reduces warm-up and allows rapid alignment. They still require power and have scale factor and bias errors that must be calibrated.