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Flight Instruments

Objective​

Review the pitot-static and gyroscopic instruments, their operation and errors, and emphasize how they are relied upon for IFR flight.

Timing​

45 minutes

Format​

Overview​

Elements​

Pitot-Static Instruments​

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  • Pitot: Pitot tube, often on the wing of GA aircraft
  • Static port: Usually flush with the fuselage on the side of an aircraft
  • Which instruments are pitot static instruments?

Altimeter (Sensitive altimeter)​

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  • Static ports
    • Fed by one or more static ports on the side of the aircraft
    • Some static ports may be heated
  • A traditional altimeter contains an aneroid wafer with a vacuum inside
    • As ambient pressure changes, the wafer expands and contracts
    • This expansion causes the needle to move up/down
  • Types of altitude
    • Indicated altitude: Read from the altimeter
    • True altitude (MSL): How many feet the aircraft is above true mean sea level
    • Pressure altitude: Read from the altimeter when set to 29.92"
      • Also, the height in the standard atmosphere where that ambient pressure would be found
    • Density altitude
      • The height in the standard atmosphere where an equivalent pressure is found after adjusting for non-standard temperature and pressure
    • Absolute altitude (AGL): Actual height above the terrain
  • Altimetry errors
    • Cold weather errors: Cold air is more dense, therefore at the same indicated altitude the aircraft would be flying lower than is indicated
      • Think of the cylinder of air below the aircraft condensing in cold weather
    • Warm weather errors: Warm air is less dense, therefore at the same indicated altitude the aircraft would be flying higher than is indicated
      • Think of the cylinder of air below the aircraft expanding in hot weather
    • Altimeters also need to be set to a nearby altimeter setting to account for changes in ambient pressure
      • Flying from high to low pressure: Altimeter will indicate higher than true altitude
      • Flying from low to high pressure: Altimeter will indicate lower than true altitude
      • Altimeter should read within 75 feet of the airport elevation (AIM 7-2-3)

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Vertical speed indicator​

  • Also fed by the static port
  • Uses a "calibrated leak" to determine how fast the altitude in changing

Airspeed indicator​

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Airspeed = (RAM air pressure - static pressure)

  • Airspeed errors
    • Pitot tube is mounted at a specific angle of attack
    • As the AoA increases, fewer molecules enter the pitot tube, thus a lower airspeed is indicated
    • Calibrated airspeed corrects for this
  • Types of airspeed
    • Indicated airspeed: Read from the altimeter
    • Calibrated airspeed: Airspeed corrected for installation and position errors
    • True airspeed: The speed at which the aircraft is moving through the airmass
      • Changes in temperature and ambient pressure result in different air densities
      • This means the aircraft could be moving faster or slower through an airmass at the same indicated airspeed
      • True airspeed corrects for this
  • Pitot-static errors and malfunctions
    • Blocked pitot tube (drain tube free): Indicated airspeed slowly drops to zero
    • Blocked pitot tube (drain tube blocked): Indicated airspeed drastic increases as the aircraft climbs, and decreases as it descends
    • Blocked static port: Altimeter will read altitude where it became blocked
  • Alternate static source: Used for situation where the static port becomes blocked
    • Pressure inside the cabin is lower than the pressure on the outside of the fuselage
      • Altimeter reads slightly higher than actual
      • Airspeed reads slightly faster than actual

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Gyroscope Instruments​

A spinning object exhibits the following qualities:

  1. Rigidity in space
  2. Precession

Gyroscopic instruments are often powered by:

  • An electric motor
  • A vacuum pump which vacates air from the instrument case, causing the gyroscopic wheel to turn

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Attitude Indicator​

  • Sometimes called an "artificial horizon"
  • Contains a vertical spinning wheel, spun at high speeds
  • The disk is attached to a gimbal that allows the aircraft to pitch and bank around the gyro
  • Gyroscopic AI's can take up to 5 minutes to stabilize
  • Some AI's have a slight acceleration error
    • Slight nose-up indication when accelerating
    • Slight nose-down indication when decelerating

Gyro-driven Heading Indicator​

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  • These are not compasses and are not north-seeking
  • They must be periodically set to the magnetic compass
    • The Earth constantly rotates at 15° per hour
    • Thus, the heading error will accumulate at least at 15° per hour
    • Standard practice is to compare the compass and HI every 15 minutes

Turn Coordinator​

  • Operates in much the same way as the heading indicator with a spinning gyro
  • Used to make standard-rate turns
  • Often are powered by a DC motor, for redundancy if the main vacuum pump were to fail

Slip/Skid Indicator​

  • Also called a coordination ball or inclinometer

Electronic Flight Instruments​

  • Electronic Flight Information System (EFIS): Digital cockpit information display
    • Primary flight display (PFD)
    • Multi-function display (MFD)
  • AHRS: Attitude and Heading Reference System (AHRS)
  • Air data computer:
    • Integrates static, pitot, and air temperature information
    • Compute airspeed, altitude, vertical speed, and true airspeed to be output on a PFD

References​