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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
      c. Air intake
        1) Air induction system ice
          a) Impact ice is formed by moisture-laden air at temperatures below freezing, striking and freezing on elements of the induction system
          b) Ice may build up on such components as the air scoops, heat or alternate air valves, air intake filters or screens, and protrusions in the carburetor
          c) Be particularly alert for such icing when flying in snow, sleet, rain, or clouds, especially when ice forms on the windshield or leading edge of the wings and temperatures are near 25°F when super cooled moisture in the air is still in a semi liquid state
          d) Affects both fuel injected an carbureted engines
          e) Usually preferable to use carburetor heat or alternate air as an ice prevention means, rather than as a deicer, because fast forming ice which is not immediately recognized may significantly lower the amount of heat available from the carburetor heating system
          f) Activation of carburetor heat (Cessna TR-182) opens valve allowing hot (but unfiltered) air within engine cowling to bypass air filter and flow directly to turbocharger, carburetor and engine, restoring power.
            i) If engine roughness is due to air intake blockage with ice, the engine may smooth out immediately when carb heat is applied, and immediately return to roughness if heat is turned off
            ii) With carb heat on the fuel-air mixture is enriched (hotter, less dense air)
          C TR-182 Induction System
        2) Carburetor ice
          a) Can occur at temperatures between -7°C (20°F) and +21°C (70°F) when there is visible moisture or high humidity
          b) Vaporization of fuel plus expansion of air flowing through carburetor causes sudden cooling to freezing temperatures in sometimes less than one second
          c) Causes drop in RPM and/or manifold pressure (MP) with, usually, rough engine operation
          d) Cessna TR-182
            i) Monitor carburetor air temperature guage: beware and pull carb heat knob out to ON for temperatures below or in yellow arc range, -15°C to +5°C
            ii) Suspect if unexplained or exaggerated drop in MP
            iii) To clear ice
            • Advance throttle to 31 in. Hg. or full ON (<31 in. Hg.)
            • Pull carburetor heat knob full out - ON
            • Observe return to smooth engine operation
            • When engine smooth, push carburetor heat off
            • Readjust throttle
            • Monitor carburetor temperature guage
            iv) If continuous carburetor heat is necessary
            • Use minimum carburetor heat needed to prevent ice formation (monitor carburetor temperature guage)
            • Lean mixture for smoothest engine operation
            v) High altitude
            • Application of carburetor heat may drop MP by up to 10 in. Hg.
            • During carburetor heat application, advance throttle to lesser of full or desired power

      References:

Greg Gordon MD, CFII
Updated: