AET Aircraft Instrument Systems — Questions and Answers
Question 1: Which set of flight instruments receives input directly from the pitot-static system?
- Attitude indicator, heading indicator, and turn coordinator
- Altimeter, airspeed indicator, and vertical speed indicator (VSI) (Correct answer)
- Magnetic compass, altimeter, and attitude indicator
- GPS, altimeter, and airspeed indicator
Correct answer: Altimeter, airspeed indicator, and vertical speed indicator (VSI)
The pitot-static system supplies air pressure data to three instruments: the altimeter (static pressure), the airspeed indicator (pitot minus static pressure), and the vertical speed indicator (rate of static pressure change). Gyroscopic instruments use vacuum or electrical power, not pitot-static pressure.
Question 2: What type of pressure does the pitot tube sense?
- Static atmospheric pressure only
- Dynamic (impact) pressure only
- Total ram air pressure (static + dynamic) (Correct answer)
- Differential pressure between the upper and lower wing surfaces
Correct answer: Total ram air pressure (static + dynamic)
The pitot tube is forward-facing and captures total (ram) air pressure — the sum of static atmospheric pressure and dynamic pressure created by the aircraft's forward motion. The airspeed indicator then subtracts static pressure (from the static port) to derive dynamic pressure, from which airspeed is calculated.
Question 3: If the static port becomes completely blocked during a climb, what indication will the altimeter display?
- It will over-read, showing higher altitude than actual
- It will under-read, showing lower altitude than actual
- It will freeze at the altitude where the blockage occurred (Correct answer)
- It will fluctuate rapidly and become unreliable
Correct answer: It will freeze at the altitude where the blockage occurred
A blocked static port traps the air pressure inside the static system at the value present when the blockage occurred. Since no new pressure reference can enter the system, the altimeter, VSI, and airspeed indicator are all frozen at their last readings. The altimeter will not change as the aircraft climbs or descends.
Question 4: What fundamental gyroscopic property allows an attitude indicator (artificial horizon) to maintain a reference to the earth's horizon?
- Precession — the gyro tilts 90° from an applied force
- Rigidity in space — a spinning gyro resists any change to its orientation (Correct answer)
- Pendulosity — the gyro is weighted to seek the gravitational vertical
- Resonance — the gyro vibrates at the earth's natural frequency
Correct answer: Rigidity in space — a spinning gyro resists any change to its orientation
Gyroscopic rigidity in space (also called gyroscopic inertia) means that a rapidly spinning mass tends to maintain its plane of rotation in a fixed orientation relative to inertial space. The attitude indicator exploits this to keep the gyro aligned with the true horizon as the aircraft maneuvers around it.
Question 5: What is the primary function of an Air Data Computer (ADC) in a modern glass-cockpit avionics suite?
- To manage autopilot mode selection and engage/disengage logic
- To convert raw pitot-static pressure inputs into digital altitude, airspeed, and air temperature outputs for other avionics systems (Correct answer)
- To monitor and distribute aircraft electrical power to avionics buses
- To compute and display fuel quantity and fuel flow data
Correct answer: To convert raw pitot-static pressure inputs into digital altitude, airspeed, and air temperature outputs for other avionics systems
The ADC receives analog pressure inputs from the pitot and static systems, applies temperature compensation and calibration, and outputs calibrated digital data (pressure altitude, indicated and true airspeed, Mach number, outside air temperature) via ARINC 429 buses to the flight displays, autopilot, and other avionics.
Question 6: What is the purpose of the electrically-heated element inside the pitot tube?
- To calibrate the pressure sensor over a range of temperatures
- To improve the aerodynamic flow of air into the tube at high angles of attack
- To prevent ice from forming and blocking the pitot tube opening, which would cause false airspeed readings (Correct answer)
- To warm the cockpit instruments in cold-weather operations
Correct answer: To prevent ice from forming and blocking the pitot tube opening, which would cause false airspeed readings
Ice can form rapidly inside the pitot tube at altitude or in icing conditions, completely blocking the pressure inlet and causing the airspeed indicator to show erroneous or frozen readings. The pitot heater prevents ice accumulation by maintaining the tube above freezing temperature.
AET Aircraft Instrument Systems
Which set of flight instruments receives input directly from the pitot-static system?