Icing freezing airspeed indicator

[willisxdc]

@TrueCaliber

The other half of that system is the “Static Port”. Speed is measure as the difference between the static air pressure and the ram air pressure. While there is a way to deice the Pitot Tube, there is no provision for deicing the Static Port. This is not normally a problem as the Static Port is normally mounted on the side of the fuselage and is not normally affected by ice. However, in cases of freezing rain the Static Port can become obstructed.

The Static Port provides ambient air pressure to the Airspeed Indicator, Altimeter and Vertical Speed Indicator. There is an Alternate Static Source available, usually as a small plunger type control that will allow for switching to a static source inside the cockpit on unpressurized aircraft. If not equipped, in an emergency, breaking the glass on any of the static port instruments will restore a static reading, albeit slightly lower than the primary port.

Quiz time.

What happens if the Static Port gets obstructed while cruising, but the Pitot Heat prevents or removes any ice obstructing the Pitot Tube?

Answer - Don't cheat. Post your answer first, then read this.

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

• Pitot0Tube 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 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