write:This isn’t a positive clue since the carb heat control can move during impact. Most determinations of carb ice are made inferentially. There is nothing else wrong with the engine

in accordance with the universal gas law
the velocity of the air is increased and its pressure and temperature are both decreased. The venturi effect can lower the air temperature about 5°C. The evaporation of fuel in the fuel-air mixture can lower the tempera-ture up to 40°C. If the temperature is lowered below freezing and the air contains a certain amount of water
it forms instant ice which may block further air flow or impede the operation of the butterfly valve in the carburetor. With no air flow
the engine quits and the plane crashes (or lands prematurely.) Before the investigator shows up
the ice melts and the engine runs just fine. Now what do you do?You might check the carb heat control in the cockpit to see if the pilot thought he had carb ice. This isn’t a positive clue since the carb heat control can move during impact. Most determinations of carb ice are made inferentially. There is nothing else wrong with the engine and atmospheric conditions and probable power settings were conducive to carburetor (or induction system) ice.The term “conducive to carb ice” is not well defined. It depends primarily on the ambient temperature and dew point
but it is also affected by the type of engine and its installation on the aircraft. There is no universal chart that depicts when carb ice will and will not form. The chart in Figure 10-1 was derived from one developed by Transport Canada (Canadian FAA) showing a curve within which conditions might be considered “conducive” to the formation of carburetor ice. Engine manufacturers have more accurate information for each of their engines.B. IGnItIon SyStEm FAILuRE. It is possible to remove a magneto in the field and spin it by hand to determine if it will produce a spark. Before you do this
carefully note the position of the timing marks on the engine. Once you remove the magneto
you won’t be able to tell if the engine timing was correctly set if you didn’t note the marks. Checking the mag in the field
of course
does not tell you much about how well the magneto was working. The magneto and the entire wir-ing harness can be easily checked by a repair station using a magneto tester.C. SPARK PLuG FAILuRE. Keep in mind that the loss of power from the failure of a single plug would be barely noticeable
if at all. It must be something more serious than that. Examination of the plugs can tell a trained engine mechanic a lot about how the engine was running. Be-fore you remove any plugs
remember that you’ve got two per cylinder and it would be nice to know which hole each plug came from. Obtain a plug rack with numbered holes or tag each plug before you remove it.D. CyLInDER FAILuRE. If the engine can still be ro-tated
it is possible to remove one plug from a cylinder and
by closing the plug hole with your thumb
make a rough check for compression as the engine is rotated by turning the prop. If there is some-thing seriously wrong in the cylinder
you will get no com-pression. It is also possible to examine the cylinder and valve faces with a borescope through the spark plug hole.Figure 10-1. Carburetor Icing Probability Chart.