Carburettor icing

Carb ice can choke off the engine's air even on a warm day. As fuel vaporises and air speeds up through the narrow throat, the incoming air cools by up to about 21 C, so moisture freezes onto the throttle plate and starves the engine of air (TP 1102).

When it happens:

• You need moist (humid) air and outside air roughly -13 C to +38 C, so it happens on warm days too (TP 1102).
• It is most likely at LOW power (descent, approach), where the nearly closed throttle cools the air the most, though it can still strike at cruise or high power (TC AIM AIR).

What you notice depends on the propeller:

• Fixed-pitch: the RPM drops slowly, then the engine runs rough.
• Constant-speed: the governor holds the RPM, so a manifold-pressure drop shows up first.

What to do: turn on FULL carb heat (not partial). At first the RPM drops further and the engine runs rougher as the melted water passes through, then it recovers as the ice clears. Carb heat pulls warm, UNFILTERED air from around the exhaust, which enriches the mixture and costs a little power. On a long descent, turn it on early to stop ice forming.

Traps:

• Do not lean the mixture or just add throttle: neither one clears the ice, full carb heat does.
• The first RPM drop after you turn on the heat is normal (the ice is melting), not a reason to turn it off.
• Carburettor engines use carb heat; fuel-injected engines open an alternate air source instead (TP 1102).

Practice