CARBURETTOR SYSTEM

 

CARBURETTOR SYSTEM

 

What is it?

The Carburettor System is an aircraft system that controls and supplies spark-ignition type aircraft with a mixture of fuel when it is idling, accelerating, put to a halt, or for power enrichment systems (FAA, 2016).

Figure 1. A Float-type carburetor.

 

Problem

A potential failure that could happen with this system is the formation of ice within the system.

Identification

The identification of icing varies with aircraft type:

• The signs of carburetor icing in a fixed-pitch propeller aircraft is usually a decrease in Revolutions Per Minute (RPM) followed by engine roughness.
• A pilot flying an aircraft with a constant-speed propeller will notice a decrease in manifold pressure instead of an rpm reduction. If an exhaust gas temperature gauge is fitted on an aircraft, it would also very likely display a decrease in temperature.
• In steady level flights, such a decrease in temperatures may be detected before any signs of decreased aircraft performance (EASA, 2013).

 

Causes

To understand the causes, we must first understand the unique design of the system. The Carburettor System has a venturi tube which helps accelerate the flow of the fuel and air mixture. The venturi works by feeding air through a narrow cross-section of the inner area of the Carburettor System. Therefore, this area is narrower and thinner than the rest of the system.

When the air-fuel mixture reaches a temperature below the dew point, condensation would form. The condensation then freezes when the fuel-air mixture reaches below freezing levels. This will cause a build-up of ice in the system. The thin neck of the venturi would quickly be blocked, causing failure in the system as the blockage would change the required fuel to air ratio of the mixture. When the correct mixture is not fed to the engine, it will result in a loss of engine power (EASA, 2013).

It can become particularly dangerous when power is unknowingly reduced during descent (FAA, 2016). This failure is also extremely dangerous as it might not be detected until more power is added. This delay in identification may cause serious incidents as it is too late to carry out corrective actions.

Figure 2. Formation of carburetor ice may reduce or block fuel-air flow to the engine.

 Mitigation Strategy – Carburettor Heat

• Carburetor heat can be used as a preventive measure. It heats the intake air before it reaches the carburetor, keeping the fuel-air mixture above freezing temperatures. 
• It can also be used to melt ice that has already formed but only if the accumulation is not that great. However, prevention is still better than cure, so it is recommended to always use the heating system to maintain above freezing temperatures in the mixture.
• If there is a sudden clog in the Carburettor System due to icing, the Carburettor heat system may also step in and act as a back-up air source.

 

 

References

EASA. (2013). PISTON ENGINE ICING [PDF]. Retrieved from https://www.easa.europa.eu/sites/default/files/dfu/EGAST_GA5-Piston-Engine-Icing-final.pdf

FAA. (2016). Pilot's Handbook of Aeronautical Knowledge Chapter 7 [PDF]. Retrieved from https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/media/09_phak_ch7.pdf