when you are using reduced power, such as during a descent.  You may not notice it during the descent until you try to add  power. Indications of carburetor icing are a decrease in engine  rpm or manifold pressure, the carburetor air temperature  gauge indicating a temperature outside the safe operating  range, and engine roughness. A reciprocating engine with a  governor may mask the formation of carburetor ice since it  will maintain a constant manifold pressure and rpm.  Since changes in rpm or manifold pressure can occur  for a number of reasons, closely check the carburetor  air temperature gauge when in possible carburetor icing  conditions. Carburetor air temperature gauges are marked  with a yellow caution arc or green operating arcs. In most  cases, it is best to keep the needle out of the yellow arc or  in the green arc. This is accomplished by using a carburetor  heat system, which eliminates the ice by routing air across  Some fuel systems contain a small hand-operated pump  called a primer. A primer allows fuel to be pumped directly  into the intake port of the cylinders prior to engine start. The  primer is useful in cold weather when fuel in the carburetor  is difficult to vaporize. A fuel quantity gauge located on the pilot’s instrument panel  shows the amount of fuel measured by a sensing unit inside  the tank. Most fuel gauges will indicate in gallons or pounds  and must be accurate only when empty. It is worth noting that in accordance with Title 14 of the Code  of Federal Regulations (14 CFR) section 27.1337(b)(1), fuel  quantity indicators “must be calibrated to read ‘zero’ during  level flight when the quantity of fuel remaining in the tank  is equal to the unusable fuel supply.” Therefore, it is of the  utmost importance that the pilot or operator determine an  accurate means of verifying partial or full fuel loads. It is  always a good habit, if possible, to visually verify the fuel on  board prior to flight and determine if adequate fuel is present  for the duration of the flight. Additionally, 14 CFR section 27.1305(l)(1) requires newer  helicopters to have warning systems “provide a warning  to the flight crew when approximately 10 minutes of  usable fuel remains in the tank.” Caution should be used  to eliminate unnecessary or erratic maneuvering that could  cause interruption of fuel flow to the engine. Although these  systems must be calibrated, never assume the entire amount  is available. Many pilots have not reached their destinations  due to poor fuel planning or faulty fuel indications. Regardless of the device, the reciprocating engine and the  turbine engine both use the ignition and combustion of the  fuel/air mix to provide the source of their power. Engine  fuel control systems utilize several components to meter  the proper amount of fuel necessary to produce the required  amount of power. The fuel control system, in concert with  the air induction components, combines the proper amount of  fuel and air to be ignited in the combustion chamber. Refer  to the Pilot’s Handbook of Aeronautical Knowledge for a  detailed explanation and illustration. The effect of fuel vaporization and/or a decrease of air  pressure in the venturi causes a rapid decrease in air  temperature in the carburetor. If the air is moist, the water  vapor in the air may condense causing ice to form in the  carburetor. If ice is allowed to form inside the carburetor,  engine failure is a very real possibility and the ability to  restart the engine is greatly reduced. Carburetor icing can  occur during any phase of flight but is particularly dangerous  Engine Fuel Control System Carburetor Ice 4-14 Figure 4-23.  When the carburetor heat is turned ON, normal air flow  is blocked, and heated air from an alternate source flows through  the filter to the carburetor.  Manifold pipe is connected to exhaust manifold Filter Carburetor heat collector To carburetor Carburetor Heat Off Carburetor Heat On