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CFI Instrument Practical Test Standards, FAA-S-8081-9B, June 2001

II. Technical Subject Areas

A. Aircraft Flight Instruments and Navigation Equipment

    3. Anti-ice/deicing and weather detection equipment and their operating characteristics
      e. Pitot-static system
        1) Pitot tube is particularly vulnerable to icing
          a) Even light icing can block the entry hole (pressure inlet) where ram air enters the pitot-static system
            i) Only instrument affected by this is ASI
            ii) If pitot tube has a water drain hole that is also blocked by ice (but aircraft static ports are open), then pressure is trapped in the pitot tube and the ASI may "act like an altimeter" with IAS increasing during a climb and decreasing during a descent
            iii) If water drain hole is present and open normally, then blocked pitot tube pressure may escape and IAS will decrease to zero
            iv) See diagram
          b) Most airplanes are equipped with a pitot heating system to keep ice away
            i) Usually consists of coiled wire heating element wrapped around the air entry tube
            ii) Apply pitot heat before entering an area of suspected icing
            iii) Beware of potentially significant current drain of pitot heater
          c) Cessna TR-182
            i) Pitot heat system consists of heating element in pitot tube, rocker PITOT HEAT switch, 10-amp push-to-reset circuit breaker and wiring
            ii) Preflight check: Master ON, PITOT HEAT ON, feel pitot tube for heat
        2) Static ports may become blocked by structural ice
          a) If static ports blocked by ice
            i) ASI becomes inaccurate in that IAS would be lower-than-actual at altitudes above that where ports became blocked and IAS would be higher-than-actual below that altitude
            ii) ALT would remain at altitude where blockage occured
            iii) VSI would remain at zero
          b) Many aircraft have static port heating systems
          c) Some aircraft have an alternate static source vented inside the airplane cabin
            i) Static pressure in cabin is usually lower than outside so
            ii) Use of alternate static source may result in erroneous instrument indications
            • ALT reads higher than normal
            • IAS reads greater than normal
            • VSI momentarily shows a climb
            • Ignore this initial false "climb up fast" indication
          d) In absence of an alternate static source in an unpressurized aircraft, pilot could break the glass on the VSI to provide a source of static pressure to the ASI and ALT
            i) This could cause additional instrument errors
            • ASI and ALT will respond more sluggishly than normal (VSI capillary tube effect)
            • VSI will indicate in reverse
            ii) So, it has been suggested instead (Kershner) to either
            • Break the VSI by punching through to leave a good-sized hole in the VSI diaphragm (ASI and ALT would not then lag), or
            • Carefully break the glass only of the ASI (then ASI, ALT, VSI should indicate essentially as accurately as expected using a cabin alternate static pressure)
          e) Cessna TR-182
            i) Static pressure alternate source valve installed adjacent to parking brake
            ii) Pull ALT STATIC valve out ON if external static source blockage suspected
            iii) See POH Section 5 for IAS corrections (no more than 3 knots if windows closed)

    References:

Greg Gordon MD, CFII
Updated: