#aviation #classnotes

The Pitot Static System

Private Pilot License (PPL) Notes

The pitot-static system drives the airspeed indicator, altimeter, and vertical speed indicator. It is crucial for pilots, especially new student pilots, to understand how this system works.

Airspeed Indicator and Types of Airspeed

  1. The airspeed indicator measures the difference between pitot (impact) pressure and static pressure.
  2. Types of airspeed:
    1. Indicated Airspeed (IAS): Read directly from the airspeed indicator without corrections.
    2. Calibrated Airspeed (CAS): IAS corrected for instrument and position errors.
      1. Errors may be significant at certain flap settings and airspeeds.
      2. At normal cruise speeds, IAS and CAS are usually similar.
      3. Use the Pilot's Operating Handbook (POH) for calibration tables or graphs.
    3. True Airspeed (TAS): CAS corrected for air density (altitude and temperature) variations.
      1. TAS increases over CAS by approximately 2% per 1,000 feet of altitude.
      2. The airspeed indicator shows TAS accurately only at sea level on a standard day.

Airspeed Indicator Markings

  1. The airspeed indicator has color-coded arcs specific to the aircraft:
    1. Green Arc: Normal operating range.
      1. Lower limit: Stall speed power-off, flaps up, at maximum weight.
      2. Upper limit: Maximum structural cruising speed.
    2. Yellow Arc: Caution range; fly only in smooth air.
    3. Red Line: Never-exceed speed.
    4. White Arc: Flap operating range.
      1. Upper limit: Maximum speed with flaps extended.
      2. Lower limit: Stall speed power-off with flaps extended.
  2. Maneuvering Speed (Va):
    1. Not marked on the airspeed indicator.
    2. Varies with aircraft weight.
    3. Consult the POH for maneuvering speeds at different weights.

Altimeter

  1. An aneroid barometer measuring atmospheric pressure to indicate altitude in feet.
  2. Adjustment for current atmospheric pressure using the altimeter setting:
    1. Altimeter setting is the sea level pressure used as a common reference.
    2. Adjusting the setting changes indicated altitude (approximately 1 inch Hg = 1,000 ft).
    3. Lowering the altimeter setting decreases indicated altitude, and vice versa.

Types of Altitude

  1. Indicated Altitude: Read from the altimeter set to the current setting; approximate height above mean sea level (MSL).
  2. Pressure Altitude: Altitude when the altimeter is set to 29.92 in Hg; used for performance calculations.
  3. Density Altitude: Pressure altitude corrected for non-standard temperature.
    1. Important for calculating aircraft performance (takeoff, climb, landing).
    2. Higher-than-standard temperatures increase density altitude.
  4. Absolute Altitude: Actual height above ground level (AGL); varies with terrain.
  5. True Altitude: Actual vertical distance above mean sea level; charted elevations are in true altitude.

Vertical Speed Indicator (VSI)

  1. Measures rate of change of pressure to indicate climb or descent rate in feet per minute.
  2. Has a lag of approximately 6 to 9 seconds in indications.

Alternate Static Source

  1. Used if the external static source becomes blocked.
  2. Allows the static system to draw air from inside the cabin.
    1. May affect instrument indications.
    2. Consult your instructor and POH for specifics on your aircraft.

Understanding the pitot-static system is essential for dealing with any erroneous indications due to system or instrument malfunctions. Always ensure the pitot tube, static source, and alternate static source are checked as per the checklist. The system is reliable and should not give you trouble during your flying career.