Carburettor Intake Heating
In order to remove ice or to prevent ice formation during flight induction heating must be provided. Intake heating is achieved by hot air being supplied from a jacket around the exhaust system.
Cold air is heated by the hot exhaust then flows into the induction system. An adjustable valve operated by a heat control lever located in the cockpit controls the hot air entering the system. Normally the selection is either fully hot or fully cold with no intermediate position.
When applying carburettor heat the pilot will notice a drop in engine rpm and manifold pressure due to the hot air been less dense which naturally creates a drop in engine power. After applying carburettor heat the air/fuel ratio changes to less air but no change in the amount of fuel. The net effect is a richer mixture.
When hot air is selected it is not filtered, and abrasive material can enter the cylinders increasing the rate of wear of the cylinder barrels. Also hot air reduces density and hence power; this also increases the risk of detonation.
Some installations route the induction system through the oil sump to pre-heat the induction system that helps in vaporisation and also reduces the risk of ice formation. Injection systems require very little or no hot air supply; however they do require alternate air supply to bypass the air filter should it become blocked.
Carburettor Hot Air Check
A carburettor heat check should be carried out immediately before take-off; it should be noted that if this check is carried out some distance from the runway requiring extensive taxiing, then ice can re-form in followed. Generally the procedure is as follows:
- Select the required RPM and select HOT on the hot air lever, noting the RPM decrease.
- If there is no decrease apparent in RPM, then hot air is not entering the induction system. Should rough running occur, then this indicates ice formation in the induction system, therefore continue in the hot selection until rough running ceases. COLD should then be selected noting the increase in RPM. Decreased RPM is due to the reduction of air density of the heated air.
- Take-off should not be attempted with hot air selected since power will be reduced and the likelihood of detonation will be increased.
To monitor cylinder head temp and oil temperatures in the climb, cruise & descent refer to Cylinder Head Temperature Gauge.
Apart from operating the aircraft within its operating limits the pilot has no direct control over the amount of cooling of most small light aircraft. However on most large single engine aircraft and normally on all twin-engine aircraft by the use of cowl flaps the pilot is able to control the airflow through the engine as shown above. When open they allow the maximum amount of air to flow over the engine, and minimum flow when closed, are normally fully open at take-off and climb, and partially or fully closed during the cruise. The cylinder head temperature gauge records the cylinder temperature, and the cowl flaps are adjusted to maintain the correct cylinder head temperature.
Cylinder Head Temperature (CHT)
The cylinder head temperature is a good indication to the pilot when the critical point of detonation is occurring or is about to be approached. Keep in mind that the indication is only reflecting that one cylinder where the measurement is been taken and may appear normal, when another cylinder not been measured may be suffering detonation and over heating.
The pilot can control CHT by varying airspeed or power through changes in manifold pressure or engine rpm, mixture control and by use of the cowl flaps.
During take-off or while in cruise the pilot may set engine power to Take Off Power (TOP) which typically has a time limit of 3 to 5 minutes. During this period the pilot needs to monitor the CHT gauge to ensure temperatures remain within its limits. If take-off power continues to be used the engine will over heat along with the engine oil. Soon the oil will become thin and hot leaving poorly protected components resulting in engine wear. Close attention needs to be made to the oil temperature, pressure and CHT gauges to ensure they remain within their normal operating limits.
Should the engine become hot the pilot can open the Cowl Flaps to circulate cool air around the engine. However though care should be taken not to shock cool the engine which could cause the engine to crack from; for example, a rapid decent or entering a cold air mass.
Carburetors are normally calibrated at sea-level pressure, where the correct fuel-to-air mixture ratio is established with the mixture control set in the FULL RICH position. However, as altitude increases, the density of air entering the carburetor decreases, while the density of the fuel remains the same. This creates a progressively richer mixture, which can result in engine roughness and an appreciable loss of power. The roughness normally is due to spark plug fouling from excessive carbon buildup on the plugs. Carbon buildup occurs because the excessively rich mixture lowers the temperature inside the cylinder, inhibiting complete combustion of the fuel.
This condition may occur during the pretakeoff runup at high-elevation airports and during climbs or cruise flight at high altitudes. To maintain the correct fuel/air mixture, you must lean the mixture using the mixture control. Leaning the mixture decreases fuel flow, which compensates for the decreased air density at high altitude.
During a descent from high altitude, the opposite is true. The mixture must be enriched, or it may become too lean. An overly lean mixture causes detonation, which may result in rough engine operation, overheating, and a loss of power. The best way to maintain the proper mixture is to monitor the engine temperature and enrichen the mixture as needed.
Some aircraft are fitted with an Exhaust Gas Temperature gauge, which gives an indication of combustion temperature. The aircraft's-operating manual provides instructions on how to use the EGT gauge to obtain the desired mixture (i.e.-best power or best economy). Best economy mixture is usually not recommended with power settings over 75% MCP. This is because high power settings with lean mixtures result in high combustion temperatures and high risk of detonation.
As altitude increases air density decreases and the mass of air entering the combustion chamber with each stroke is less. Therefore, as the aircraft climbs, the pilot must lean the mixture to maintain the correct proportions and on descent the mixture must be enriched again.
The ratio of the mass of air to the mass of fuel entering the combustion chamber is known as the mixture. An engine will operate over a wide range of mixtures; however the most efficient mixture which provides the best burning of the fuel is approximately 15:1 (ie 15 parts of air to 1 part fuel by mass). This is known as chemically correct mix (CCM). CCM produces the highest combustion temperature therefore peak EGT, however the mixture which produces the best power is richer than peak EGT. The mixture for best economy is between peak EGT and best power.
Aeroplane Mixture Adjustment
- Step: Identify the mixture control. The control is generally either a lever or a push rod. Traditionally, the handle of the control is colored red or orange. However, it should also be labeled with words.
- Step: Learn the settings. As a rule of thumb, pushing the lever forward, or in, makes the mixture rich. Pulling it backward, or out, makes the fuel mixture lean. Rich means that a lot of air will be mixed with the fuel. Lean means that less air will be mixed with the fuel. When the engine is not running, the mixture control is normally set to "lean." This reduces the likelihood that the engine will start.
- Step: Set the mixture to "rich" prior to starting the engine. This is the normal procedure if the airport is at a low altitude above sea level. If the airport is at a higher altitude, over 3,000 feet, then it may be advisable to lean the mixture slightly before starting.
- Step: Lean the mixture gradually during flight, as you climb to higher altitudes. Generally, pilots begin leaning the mixture at approximately 4,000 feet msl. The higher you climb the more it is necessary to lean the mixture.
- Step: Locate the appropriate mixture setting for your given altitude. Do this by very gradually adjusting the mixture control while watching the tachometer. While you do this the throttle should not be adjusted, and the propeller control should be set for max RPM. As you pull the mixture back, you will see the RPM begin to rise a little. Continue slowly pulling the control back until the RPM begins to drop. Then begin slowly pushing the mixture control forward toward the rich setting once more. Locate the point at which the RPM is the highest. This is the optimum mixture setting.
- Step: Use the mixture control to shut down the engine after parking at your destination. This is normal practice in most aircraft of this type. Simply pull the mixture control all the way back to the fully lean position and the engine will shut down by itself. Then turn off the key and master switches.
Oil Pressure Gauge
This is the most important gauge for satisfactory engine operation. Should the oil pressure fail then bearing failure will occur very quickly. A green arc on the face of the gauge shows the normal pressure range, a yellow arc for the caution range and a red line for maximum oil pressure. Oil pressure should register on the gauge within 30 seconds of the engine starting or slightly longer on a cold day. Should the oil pressure not register within this time then the engine must be shut down.
A high oil pressure is normal on a cold engine at start up due to the oil temperature been cold. Cold oil has a high viscosity, meaning it is thick and slow to move around the engine. After a few minutes the oil will warm becoming thinner and oil pressure should come down to its normal operating range.
Low oil pressure is usually the result of low oil quantity. A lower level than normal of oil will lead to hotter oil temperatures and poor lubrication causing engine wear. The hot oil is too thin and falls away from engine components leaving them exposed to friction forces.
Oil Temperature Gauge
This gauge is normally fitted after the oil cooler and has a green arc for the normal temperature range and a red line for maximum temperature. Temperature gauges on light aircraft are really pressure gauges, since liquid is sealed in the line to the gauge; as the temperature increases then expansion of the liquid occurs, which causes the pressure to rise, which in turn results in the pointer moving over a temperature scale.
For seasonal influence on the choice of appropriate grades of oil viscosity refer to Oil Grades.
For symptoms of fuel vaporisation and the methods of rectification refer to Vapour Lock and Booster Pump.