Aircraft Instrument Systems practice questions

Answer: D) The normal operating range

• AIncorrect. Green shows normal continuous operation.
• BIncorrect. Caution ranges are yellow arcs.
• CIncorrect. Limits are marked by red radial lines.
• DCorrect. Green arcs mark normal operation.

Answer: D) About 4.5–5.5 inHg

Vacuum-driven attitude and heading indicators typically need about 4.5–5.5 inHg of suction, kept there by the suction relief valve. Always check the manufacturer's limits for the specific aircraft.

• AIncorrect. This much suction would not move enough air past the rotor vanes to spin the attitude and heading gyros up to operating speed.
• BIncorrect. Standard sea-level pressure is about 29.92 inHg, so 25–30 inHg of suction would be close to a total vacuum, far more than gyro instruments need or are designed for.
• CIncorrect. This is roughly half an atmosphere of suction, far above normal gyro requirements. It may come from confusing the 14.7 psi of standard atmospheric pressure with inches of mercury.
• DCorrect. Most systems hold about 4.5–5.5 inHg for the attitude and heading gyros, set by the suction relief valve. The exact limits come from the aircraft and instrument manufacturer.

Answer: A) The pressure altimeter

The pressure altimeter uses only static pressure. The airspeed indicator and Machmeter need both pitot and static pressure, and the manifold pressure gauge measures engine induction pressure.

• ACorrect. The pressure altimeter senses only outside static pressure through its case connection and shows it as altitude. It has no connection to pitot (ram) pressure.
• BIncorrect. The airspeed indicator works on the difference between pitot (ram) pressure and static pressure, so it needs both lines.
• CIncorrect. A Machmeter uses both pitot and static pressure. It combines an airspeed mechanism with an altitude mechanism to show speed as a fraction of the speed of sound.
• DIncorrect. A manifold pressure gauge shows absolute pressure in the engine's induction manifold. It is an engine instrument and is not connected to the airframe static system.

Answer: B) An electrical resistance bulb

• AIncorrect. Bourdon tubes are mainly pressure sensors.
• BCorrect. Resistance changes with temperature to drive the gauge.
• CIncorrect. Aneroids sense pressure.
• DIncorrect. Gyros sense motion.

Answer: B) Attitude and heading reference system (AHRS) using solid-state inertial sensors

In glass cockpits, an AHRS with solid-state sensors replaces mechanical attitude and heading gyros and supplies that data to the PFD. The ADC provides air data, and a magnetometer provides only a heading reference.

• AIncorrect. The air data computer uses pitot and static pressure to provide airspeed, altitude, and vertical speed. It does not sense pitch, roll, or heading.
• BCorrect. An AHRS uses solid-state sensors to supply attitude and heading data to PFDs, autopilots, and other avionics, replacing mechanical spinning gyros.
• CIncorrect. A magnetometer gives only a magnetic heading reference, often as an input to the AHRS. It cannot provide pitch and roll attitude by itself.
• DIncorrect. A turn coordinator shows rate of turn and roll rate. It is not a source of attitude and heading data for an electronic display.

Answer: B) So a gyro instrument stays available if the vacuum pump or system fails

Powering the turn coordinator electrically while the attitude and heading indicators run on vacuum gives a second power source. If the vacuum system fails, the pilot still has a working gyro instrument.

• AIncorrect. A turn coordinator shows rate of roll and rate of turn, plus slip or skid on its inclinometer. It does not show pitch attitude, whatever powers it.
• BCorrect. With a different power source, a vacuum failure does not take out every gyro instrument. The pilot still has turn-rate information.
• CIncorrect. The canted gimbal is a mechanical design feature. It does not depend on the power source, and vacuum-driven turn instruments exist.
• DIncorrect. The inclinometer is a ball in a fluid-filled curved tube. Gravity and inertia move it, so it needs no power.

Answer: A) The flux gate (flux valve), mounted remotely, often in a wingtip

A remote-indicating compass uses a remotely mounted flux gate (flux valve) to sense Earth's magnetic field, and the heading gyro is slaved to that signal.

• ACorrect. The flux gate/flux valve detects Earth's magnetic field at a location away from cockpit magnetic interference, and the heading gyro is continuously corrected (slaved) to agree with it.
• BIncorrect. The gyro gives a stable heading reference but cannot sense magnetic north. It is slaved to the flux gate's signal.
• CIncorrect. Compensating magnets are adjusted during a compass swing to reduce deviation. They do not sense Earth's field.
• DIncorrect. The slaving amplifier processes the error signal it receives from the flux gate and drives the correction. It is not the magnetic sensor.

Answer: D) A Bourdon tube

• AIncorrect. Pitot tubes sense ram air pressure.
• BIncorrect. Aneroids are used for low pressures like altitude.
• CIncorrect. Thermocouples sense temperature.
• DCorrect. A curved tube straightens as pressure rises, moving the pointer.

Answer: D) Absolute pressure in the intake manifold

• AIncorrect. Manifold pressure is on the intake side.
• BIncorrect. That is the oil pressure gauge.
• CIncorrect. That is the fuel pressure gauge.
• DCorrect. It reads in inches of mercury.

Answer: A) If the glass with its markings has rotated

• ACorrect. A misaligned mark means the markings may be wrong.
• BIncorrect. That is a red line.
• CIncorrect. That is a green arc.
• DIncorrect. That is a yellow arc.