Airframe Systems

1 January 2017

State three purposes of pneumatic supply system for a typical piston engine powered light aircraft, and give the functions of the piston engine air compressor and receiver, within the system. An aircraft powered by a piston engine usually tends to have a pneumatic air supply system. This allows the function of different systems on the aircraft that are essential to the flight of the aircraft. In a newer system the supply system will provide a vacuum of air for the gyro’s, so that the de icing boots on the leading edge of the wings can inflate, for example, the boots on the jet stream wings inflate.

The vacuum sometimes provides for some of the flight controls. An example of these flight controls would be the auto pilot system. The piston powered aircraft tend to consist of a receiver and a compressor. The compressor collects the air from the intake of the engine, creating a high volume of compressed air and storing it, acting like a storage tank.

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The receiver is there to make sure that the volume of air is consistent and smooth, else parts may move with too much force and damage themselves.

In modern day aircraft, the only pneumatic system used is there to drive the autopilot system and the gyros. P2 – describe how air is supplied to the cabin pressurization system pre-cooler, for a typical passenger aircraft that’s powered by gas turbine engines, include in your answer where the air supply is taken off the engine The cabin pressurization system is designed to keep the pressurization of the inside of the aircraft to a higher level than outside the aircraft, this is done by taking air from the ram stage of the gas turbine.

This air is then supplied to the heat exchanger at minimum pressure. When the aircraft is in flight, the air is bled from the low pressure stage of the compressor, this supplies air to the aircrafts pneumatic systems. But when the aircraft is on the ground at a standstill, there is no pressurization in the gas turbine. The high pressure stage of the compressor is manually operated on the ground when the engine is idling.

A regulating valve is used to regulate the pressure of the bleed air when it has been taken from the pressurization stage of the engine. he air is then passed through a heat exchanger and dehumidified. The hot air is cooled and the heat exchanger is fed by cool air directly from the early compression stage of the engine. P3-with aid of diagrams describe the purpose of the aircraft cabin pressurisation systems, for a typical pressurized airframe. In your answer state the function of the following system features : I. Ram air ii. bleed air iii. the air cycle unit and the humidifier(water extractor) On passenger aircraft, a control system is there to monitor and control the temperatures of the cabin area.

The cabin conditioning and the pressurization system would be -56°c and would be impossible for passengers to survive the lack of pressure and breathe, as the aircraft flies at a very high altitude. The control system is there to control the cabins temperature and maintain the cabins pressure. The control system has 2 related systems, one of them is for the temperature control and the other is for pressurization. Both of these use the engine bleed air. The hot pressurized air passes through the heat exchanger. This cools down the air as it passes through, reducing the air to the desired temperature.

When the hot pressurized air enters the air conditioning pack, it passes through a valve that controls the air pressure before passing through the heat exchanger. The heat exchanger is there to dissipate the heat as the air passes through them, they are positioned inside the ram air ducts, sometime on the underbelly of the aircraft. The ACU’s role is to cool hot bleed air. The system works similar to a refrigerator except there is no condensation or evaporation of a refrigerant involved. The unit consists of a small air expansion turbine and a compressor, connected by shaft.

The system takes the hot air and rapidly decreases its temperature by converting that heat into kinetic energy. The water then condenses as it passes through the turbine and out of the exhaust. Then passes into the water extractor. The extractors job is to remove all the moisture out of the air in the system, ice may form as the the air tends to come out of the turbine at 0c, so a sensor checks the temperature to make sure that ice cannot form. If it becomes too cold, hot air can be directed from the bleed air system through a valve.

M1 – The air supplied to the pneumatic systems of gas turbine powered aircraft, and piston engine powered aircraft, is different. Explain how the pneumatic supply systems differ for the two different aircraft types. Your answers must relate to typical pressurised passenger or cargo aircraft systems. The pneumatic systems on gas turbine powered aircraft are different from those that are on aircraft that are powered by piston engines. Larger aircraft depend more on pneumatic systems due to their increased flying altitude and need a form of environmental control system. For example, air conditioning.

The air conditioning system provides a more suitable living atmosphere pressure inside the cabin, by supplying oxygenated air to breathe. The bleed air helps to pressurise the airframe of the aircraft at a high altitude. Because the piston engine powered aircraft don’t tend to fly at a high altitude, there is no need for a large pneumatic supply system to power the climate control. This is how the two types differ from each other. On gas turbine powered aircraft, there is an environmental control system that pressurises the airframe and supplies the cabin with a decent air temperature and oxygen level.

The systems that run on large aircraft consist of pressurisation of the fuel tanks, de icing of the wings and engine, pressurisation of the fuel tanks, the hydraulic oil. The pressurisation from the APU is also capable of starting the turbine engines of the aircraft. A continuous flow of air is constantly bled from different stages of the compressor and fed into power certain systems, the stages may change due to the air density at clime and the rpm of the engine. The hot air coming from the exhaust stage of the engine can exceed temperatures of 250°c, so the air needs to be cooled before it can pass through certain parts of the aircraft.

The gas turbine pneumatic system consists of 2-3 bleed ports, the air is normally taken from the low pressure stage, the intermediate pressure stage and the high pressure stage of the engines compressor. The system also has shutoff valves so that the air can be stopped from the compressor. A non return flow valve stops high pressure air getting into the low pressure bleed port. The bleed air is then taken through a heat exchanger , this is cooled using the ram air taken from the front section of the engine. Because of high pressure there is also a flow regulator that controls the flow of air through the system.

The air passes through another check valve to prevent the air from being fed back into the system. The air then passes through a pressure regulating valve and then into the consumption systems. Piston engine powered aircraft use a different air supply because there is less need for pneumatic air to run into the different systems. The air is bled from each engines compressor, then it is moisture separator, then it is fed through a non return valve.

The air isn’t at a high pressure like from the gas turning engine. his means that there is less a need to have things such as pressure regulating valves, ran air cooling and various bleed ports in the systems. The air is fed through filters, and then fed into the receivers. There are usually three storage bottles with a twin piston engine, the brake bottle, the emergency bottle and the primary bottle. The air gets taken from the primary bottle, then through a filter. The filtered air then goes into consumption systems such as passenger doors. M2 – Explain how on a pressurized airframe the environmental control system is monitored, and controlled, from the crew compartment.

In your answer you must include the control of both cabin pressure and temperature and also the following: i. Sensors. ii. Duct stats (temperature probes). iii. Motorised valves the environmental control system is all controlled from the cockpit. The crew has a pneumatic panel that enables them turn the bleed air on or off, and select the pneumatic air to be fed into the left or right wing anti ice systems. This is because the system requires pneumatic air to function. The panel also has an alert panel, so that the crew can see if the air is being dual bled, whether the ram air doors are closed, or if anything in the system is overheating.

The environmental control system at cruise altitude should have a pressure of about 9 psi, and the temperature should be between 15-25°C. The system needs to be constantly monitored and controlled from the cockpit. The crew can set the air-conditioning panel to auto or manual. The pneumatic system that is used to pressurise the airframe is always controlled automatically using sensors and motorised valves, allowing the correct volume of air inside the cabin, preventing depressurisation. i. Sensors- sensors are used in the ECS to establish the flowe rate of the air that runs through the system.

This is so that the correct flow and pressure of air gets into the consumer systems. There are also sensors that detect excessive temperatures in the system. This reacts to the temperature and uses the ram air to enable cooling or shuts down the system to regain a moderate temperature. ii. Duct stats (temperature probes)- the duct temperature probes measure the statistics of the ducts, they send the temperature data from the duct that supplies air to the cabin and the cockpit air distribution system.

There are usually three probes that are in the system. There is normally one that is in cabin conditioned air duct upstream of the left hand pressure penetration. The next probe is in the cockpit contisioned air duct on the right hand side pressure penetration. The last probe is upstream the moisture separator iii. Motorised valves- the environmental control system and other pneumatically run systems use different types of motorised valves controlling the flow of air.

In the ECS there is usually three temperature control valves. These valves are butterfly valves , they regulate the flow of hot air to be mixed with cold air from the ECU. The other type of motorised valve are used to discharge air out of the rear cabin. These valves are used to keep the correct cabin pressure . the outflow valves are operated during normal operation using a DC motor. in an emergency operation, the valves run on AC motors as the valves will need to open and close quickly.

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