12.12.20(a) . atmospheric and throttle setting conditions conducive to the formation of carburettor ice; 

If outside air temperature is between -10deg C and +20deg C carburettor ice is possible. Particularly with high relative humidity and low throttle openings.

12.12.20. Describe the:

12.12.18. Explain the formation of refrigeration, throttle and impact ice in a carburettor andintake system. 

Refrigeration Ice
– when fuel is introduced into the carburettor airstream the temperature of the resulting fuel – air mixture is lowered
– vapourisation of the fuel requires heat energy which is absorbed from the surrounding air
– forms downstream of the main metering jet in the throat of the carburettor when temp of fuel – air mixture drops below 0 degrees C
– if atmospheric conditions are moist water will condense from the incoming air and freeze onto surfaces of inlet manifold walls

Throttle Ice
– as fuel-air mixture accelerates through the venturi and especially past throttle valve there is a decrease in static pressure accompanied by a drop in temperature
– causes ice to form on and in the vicinity of the throttle butterfly
– carburettor ice is more likely at lower throttle settings

Impact Ice
– occurs when water droplets which have been cooled to below freezing point impact on the forward facing surfaces of the aircraft
– immediately turn to ice
– can also form on the inlet air scoop and ducting to the carburettor and affect operation if the engine
– occurs whe ambient temperature is near to or below oC
– and if the aircraft is flying through visible moisture with water droplets at or below zero oC

12.12.16. Explain the causes and likely effects of detonation and pre-ignition and the measures which can be taken to avoid them. 

Detonation
Causes
– primary cause is that the temperature of the fuel-air charge is excessive
– unnecessary use of carburettor heat at high ambient temperatures and power settings
– time-expired fuel
– wrong grade of fuel
Effects
– Loss of power
– damage to engine
Avoidance
– use correct fuel.
– operate in accordance with manufacturers recommendations.
Pre Ignition
Causes
– hot spots in combustion chamber
– overheated spark plugs
– high power with too lean mixture.
Effects
– Rough running
– Sudden rise in cylinder head temp
– engine damage
Avoidance
Use correct fuel and operate in accordance with manufacturers directions.

– over-lean mixture at high power settings
– overheated engine

Effects
– a knocking noise
– a high cylinder head temperature (CHT) if gauge fitted
– often nil
Avoidance
– enrich the mixture
– throttle back to reduce cylinder pressure
– increase airspeed to reduce cylinder temperature

Pre-Ignition
Causes
– lead deposits in the cylinder
– high power when the mixture is too lean
– overheated spark plugs
–
Effects
– rough running
– back-firing
– sudden rise in cylinder head temperature
– engine damage eg burnt piston, broken cylinder head, scuffed cylinder wall,
– damage to valves and spark plugs

Prevention
Use correct fuel and observe operating limits of the engine

12.12.14. Describe the abnormal combustion conditions of detonation and pre-ignition, and distinguish between them. 

Detonation
– explosive combustion
– flame fronts within the cylinder suddenly accelerate to as much as 25 times normal speed
– detonation normally occurs in all cylinders
Pre-Ignition
– Is a condition where ignition of the fuel/air occurs before the normal time.

Pre-ignition
– early ignition ahead of the spark
– fuel = air charge ignites before it receives a spark from the plug
– can be caused by a hot spot in the cylinder such as lead deposit
– occurs in one cylinder only
– is related to the condition of a particular cylinder

12.12.12(b) . over-lean mixture settings. 

over lean mixture settings:
may, at high power settings lead to high cylinder head temperatures and possibly detonation.

12.12.12(a) . over-rich mixture settings; 

over rich mixture can cause:
loss of power
rough running
fouling of spark plugs
bore washing
increased fuel consumption

12.12.12. Explain the consequences of operating with:

12.12.10. Explain the correct operational use of a manual mixture control and idle cut-off.

For take-off and landing mixture is normally kept full-rich unless operating at high elevations.
To lean the mixture the lever is moved slowly towards the lean position. With a fixed pitch propeller the RPM will increase as the correct mixture is obtained, and then decrease if leaned further. With constant speed propeller exhaust gas temp. will increase then start to decrease. Normally a setting to the rich side of the peak is selected.
For detailed information on leaning refer to individual aircraft flight manuals.

12.12.8. Explain the function of a manual mixture control and idle cut-off.

A manual mixture control allows the pilot to adjust the fuel/ air ratio in order to compensate for changes in the operating environment. (eg increased altitude)
The idle cut-off system allows the motor to be stopped by cutting off the fuel at the carburetor. This systems ensures that there is on combustable mixture left in the cylinders, but the carburetor is left full of fuel for the next start.