Pitot-Static Systems

Introduction:

Pitot Tube:

  • Pitot Tube
    Pitot Tube
  • Invented by Henri Pitot in 1732
  • Placed in front of airfoil to prevent disturbed air from entering
  • Used only for the Airspeed Indicator (ASI)
  • Pitot Heat:

    • Pitot Tubes are often electrically heated which can prevent, and remove ice accumulation
    • Note that these devices should only be utilized during ground operations when necessary
      • Leaving them on unnecessarily can cause heat to the point of damage/malfunction

Static Port:

  • Static Port
    Static Port
  • Samples ambient still air atmospheric pressure
  • Normally flush mounted on the side of the aircraft where air is undisturbed
  • Senses movement of the aircraft through air both horizontally and vertically
  • Placed in one more more places where the air is not disturbed
  • Some ports are heated
  • Dual ports remove errors due to slips and skids
  • Responsible for Airspeed Indicator, Altimeter and Vertical Speed Indicators
  • The POH/AFM contains any corrections that must be applied to the airspeed for the various configurations of flaps and landing gear
  • Instrument Flying Handbook. Figure 3-1, A Typical Electrically Heated Pitot-Static Tube
    Instrument Flying Handbook, A Typical Electrically Heated Pitot-Static Head
  • Instrument Flying Handbook. Figure 3-1, A Typical Electrically Heated Pitot-Static Tube
    Instrument Flying Handbook, A Typical Electrically Heated Pitot-Static Head
  • Static Port
    Static Port

Pitot-Static System Failure:

  • Blockages in the system can cause a variety of errors
  • To prevent these errors you must complete a thorough pre-flight
  • Blockages can occur from FOD, striking an object, insects, and icing
  • Consult the Pilot's Operating Handbook/Airplane Flight Manual (POH/AFM) to determine the amount of error
  • Pitot Tube Blockage (static open):

    • At the altitude where the Pitot tube becomes blocked, the airspeed indicator remains at the existing airspeed and doesn't reflect actual changes in speed
      • At altitudes above where the Pitot tube became blocked, the ASI displays a higher-than-actual airspeed increasing steadily as altitude increases
      • At lower altitudes, the ASI displays a lower-than-actual airspeed decreasing steadily as altitude decreases
  • Pitot Tube Drain Hole Blockage:

    • Would cause ASI to malfunction
  • Pitot tube and drain hole become blocked:

    • Would cause ASI to malfunction
  • Static Port Blockage:

    • Would cause ASI, altimeter, and VSI to malfunction
    • If in a real emergency, malfunctions can be corrected with alternate air or breaking the glass on a Pitot static instrument (VSI)
    • If the static ports become blocked, the ASI would still function but could produce inaccurate indications
    • At the altitude where the blockage occurs, airspeed indications would be normal
      • At altitudes above which the static ports became blocked, the ASI displays a lower-than-actual airspeed continually decreasing as altitude is increased
      • At lower altitudes, the ASI displays a higher-than-actual airspeed increasing steadily as altitude decreases
    • The trapped pressure in the static system causes the altimeter to remain at the altitude where the blockage occurred
    • The VSI remains at zero
    • On some aircraft, an alternate static air source valve is used for emergencies
    • If the alternate source is vented inside the airplane, where static pressure is usually lower than outside static pressure, selection of the alternate source may result in the following erroneous instrument indications:
      1. Altimeter reads higher than normal
      2. Indicated airspeed (IAS) reads greater than normal
      3. VSI momentarily shows a climb

Common Training Aircraft Pitot-Static System Characteristics:

  • Piper Arrow:

    • Composed of a heated Pitot tube on the lower left wing
    • Two static ports are located on each side of the fuselage
    • Alternate static air (below pilot control yoke) provides static pressure from inside the cabin
  • Cessna 172:

    • Composed of a heated Pitot tube on the lower surface of the left wing
    • An external static port is located on the lower left side of the forward fuselage
    • Pitot Tube consists of a heating element, a 5-amp switch/breaker, and associated wiring
    • Alternate static (below throttle) provides pressure from inside the cabin

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