AP4ATCO - Piston Engine
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- FAA Pilot’s Handbook of Aeronautical Knowledge - Chapter 06: Aircraft Systems]
- The following SKYbrary Articles:
An aircraft piston engine, also commonly referred to as a reciprocating engine or "recip", is an internal combustion engine that uses one or more reciprocating pistons to convert pressure into a rotational motion. The aircraft piston engine operates on the same principles as the engines found in most automobiles. However, modifications, such as dual ignition systems, to improve redundancy and safety, and air cooling to reduce weight, have been incorporated into engines designed for aviation use. Turbochargers and, less commonly, superchargers can be added to piston engines to improve performance. Aircraft piston engines are most commonly fueled with AVGAS but diesel fueled engines are becoming more common, especially in light aircraft.
Engine design has varied tremendously in the century that has passed since the first powered flight. Most engines installed in current generation aircraft are of the horizontally opposed configuration. Horizontally opposed engines are often referred to as boxer or flat engines. They have two banks of cylinders staggered on opposite sides of a central crankcase. The design is simple, reliable and easy to maintain. However, there are other engine types still being flown in production, experimental and vintage aircraft.
Often referred to as “Carburettor Icing”, Induction Icing is the build-up of ice in the fuel induction system and can affect all types of piston engined aeroplanes, helicopters and gyroplanes.
There are 3 types of fuel induction system icing that may affect piston engines:
- Impact Ice. This is formed by the impact of moist air at temperatures between -10°C and 0°C on air scoops, throttle plates, heat valves, etc. It usually forms when visible moisture such as rain, snow, sleet, or clouds are present. Most rapid accumulation can be anticipated at -4°C. This type of icing can affect fuel injection systems as well as carburettor systems and is also the main type of icing hazard for turbocharged engines.
- Fuel Ice. This forms at, and downstream from, the point that fuel is introduced into the carburettor when the moisture content of the air freezes as a result of the cooling caused by fuel vaporisation. It generally occurs between +4°C to +27°C, but may occur at even higher temperatures. It can occur whenever the relative humidity is more than 50%.
- Throttle Ice. This is the most common, earliest to show and most serious carburettor icing. It is formed at or near a partly closed throttle valve (sometimes called the “butterfly valve”). The water vapour in the induction air condenses and freezes due to the venturi effect cooling the air as it passes the throttle valve. Since the temperature drop is usually around 3°C, the optimum temperature for forming throttle ice is between 0°C to +3°C although a combination of fuel and throttle ice could occur at higher ambient temperatures.
Although “Carburettor Icing” is most likely to occur when the temperature and humidity are in the ranges indicated above, it can also occur under conditions not depicted.
“Carburettor Icing” is much more likely at reduced power, so pilots select carburettor heat before power is reduced for the descent, especially if they are intending to lift off again e.g. a practice forced landing or helicopter autorotation.
The first signs of carburettor icing are normally:
- Fixed Pitch Propeller - a reduction in available power measured by RPM
- Constant Speed Propeller - a reduction in manifold pressure
If left untreated:
- Rough running engine
- Further loss of performance
- ENGINE STOPS due to fuel starvation
Prevention is better than the Cure.
For extended periods of flight with reduced power settings, a pilot will routinely increase the power and apply FULL carburettor heat for around 15 seconds to ensure that the engine stays sufficiently warm to melt any ice.
If flying in conditions that appear conducive to the development of carburettor ice, carburettor heat will be applied at regular intervals as both a preventative measure against the formation of ice and a method to test whether or not carburettor ice is developing.
The use of carburettor heat will decrease engine performance by up to 15% so pilots should beware of flying around with it continuously selected; the aircraft will use more fuel than planned for.
Q1: Most engines installed in current generation aircraft are of the following type:
- Horizontally Opposed
Q2: Throttle Ice is most likely to occur when the temperature is in the range
- +0°C to +3°C,
- +4°C to +27°C, and whenever the relative humidity is more than 50%.
- -10°C to +0°C, and whenever the relative humidity is 100%.