Priming, Mixture, Carburettor Fire

Chapter 2.5.3

The Necessary Mixture for the ENGINE START

To start, an engine carburettor needs a fuel mixture able to ignite. Depending on temperatures, different amounts of fuel/air mixture are needed.

The air / fuel mixture must take into account temperatures to start an engine.

The majority of flight training airplanes have two possible ways of injecting fuel to the cylinders:

with the PRIMER:

A mechanical pump which injects fuelinto the cylinders.

with an ACCELERATOR PUMP:

A pump connected to the throttle which injects fuel into the discharge nozzle of the carburettor.

The Necessary Mixture for the ENGINE START

Most aircraft manufacturers describe a standard procedure for engine start in the AFM. This must be followed in any case. In many cases, however, it is necessary to deviate from this standard procedure at high or low temperatures.

Basically, correct injection requires a certain amount of sensitivity and above all a clear understanding of the processes in a carburettor.

Rule of thumb

The Necessary Mixture for the ENGINE START

Even with lower temperatures it is advisable to call a person with experience (flight instructor, flight engineer).

Consider that:

• A hot engine does not require priming.

• there is the possibility of a carburetor fire at low temperatures (too much fuel in the suction chamber).

• especially during low temperatures, a person must be ready with a fire extinguisher.

• during high temperatures the engine “back fires” (too much fuel) if priming is disproportionate.

Carburettor Fire or Engine Fire During the ENGINE START

Carburettor or engine fires occur in the majority of cases due to excess or disproportionate PRIMING. If it happens then we handle it as described in the CHECKLIST for ABNORMAL SITUATIONS / EMERGENCY (part of the AFM).

Since there is always the possibility of a fire, it is therefore important to know the location of fire extinguishers at the airport and in the airplane. Allow a specialist or your instructor to explain the operation of a fire extinguisher.