Engine "Backfire" - Leaning the Mixture
If you have ever experienced a sudden power loss due to carburetor icing this technique will improve your chances of landing safely with power.
Lycoming and Continental engines in conventional carburetor equipped aircraft experience carburetor icing. The carburetor can cool below the freezing point of water quickly. The use of carburetor heat and idling the engine can keep the temperature of the carburetor system above 32 degrees F. On take off, when power is applied, the amount of fuel vaporized is increased and that vaporization absorbs heat from the carburetor and lowers its temperature.
Humidity plays a important part whether the moisture in the air can freeze the water onto the surface of the venturi of the carburetor. Greater the humidity the greater the chance for the moisture in the air to form ice.
This is directly related to the dewpoint of the air at the time of your flight. The dewpoint is that temperature where the water in the air begins to form water droplets. (like fog and clouds, for example) If the dew point is 68 degrees F on a nice clear summer day where the ground temperature is 90 degrees F you would not think of icing on a day like that.
THe adiabatic lapse rate for air is 4.5 degrees F per 1000 feet of altitude. This means if the aircraft reaches an altitude of approximately 5000 feet the temperature is 68 degrees F. The plane can begin to form ice in the carburetor at this altitude.
I spent some time in a normally aspirated Cessna 182. Several times a year it would develop ice in the carb! I would try the normal procedures for melting the ice but an effective technique is to reduce power and lean out the mixture. The engine will back-fire and blow out the ice formed in the carburetor. Its effective.
In flight when the throttle is reduced too rapidly the fuel/air mixture goes with it, but the fuel valve does not react as fast as the air reduction. Therefore the fuel left behind explodes.
For it to be heard it must be done in one of the cylinders exhaust stroke. There is no spark at the exhaust stroke so the little fuel or un-burnt carbon is heated to its flash point from the hot cylinders. Its ignited by the severe heat when the fuel is reduced (the fuel keeps the CHT cool, reducing it makes less fuel so less mass to be superheated in a very short time). Overal the fuel did not finish its complete burn before the exhaust and comes out as a "pop of flames".
On the ground this "Back fire" will also happen if the fuel/air is reduced to rapidly and then the un-burnt fuel is left to reach its flash point.
During the run-up the leaner the mixture the more backfire you will have. This is also because the fuel is too lean for the air mixture making it burn at very high temperatures. This increases the CHT, EGT, OIL Temp to where the fuel can also get to its lash point and explode uncontrollably.
The freezing that occurs in fuel-injected engines is when ice forms on the intake screen of the air source for the fuel before it enters the engine.
