What is the Aircraft Air Start Unit?

When an aircraft is readying itself for departure and needs to initiate engine operations, a ground support equipment device known as an aircraft air start unit (ASU) is extremely useful. Utilized by countless types of aircraft across the globe, aircraft air starter units ensure that the engine is capable of cranking until a self-sustaining speed is reached. As an equipment piece that is used during the boarding process until the aircraft is ready for pushback and departure, having an understanding of the aircraft air start unit’s functionality may be beneficial for any aerospace personnel involved with aircraft starting procedures.

Upon reaching the gate of an airport terminal after a flight operation, the aircraft will park itself and turn off all engines so that passengers can prepare for disembarkment. Prior to the departure, a tow tractor will deliver an air start unit to the aircraft in preparation, and procedures will begin once the okay is given by the pilot. At that point, the discharge nose of the air start unit is attached to the aircraft high pressure receptacle before the system is turned on. As the air start unit is known to be one of the loudest ground support equipment devices during operations, it is crucial that all surrounding personnel have proper ear protection during the process.

With the power provided by the air start unit, internal engine components such as the engine fan, compressor, and turbine blades are all driven or rotated. As speed increases, the pilot within the cockpit will then activate the fuel injection system so that the combustion chamber can begin the ignition of fuel and air mixtures. Once a self-sustaining speed and operation is reached, the air start unit may then be switched off and disconnected from the aircraft. In general, the air start unit is only used to initiate the processes of one engine, the other typically being activated while taxiing towards the runway for departure.

In order for the air start unit to supply the power necessary to drive engine components, there are a number of processes and parts that they rely on. As a type of engine driven air compressor, the unit takes in air from the atmosphere and then filters out any potential contaminants that may present damage to the engine. After passing through filters, the air is then directed towards the jet engine to kickstart processes. Generally, the engine will come in the form of a diesel engine paired with an air compressor, utilizing a gearbox for speed management. There are also various control valves and regulation valves to maintain air pressure and control.

With the use of a control panel, gauges and meters may be referred to in order to monitor discharge air pressure, temperature, engine speed, oil temperature, and other various conditions. The control panel is also where operating modes may be selected from as well. While most aircraft vary in their air flow rate, air pressure at 42 PSI is most common for engine startup procedures. While the air start unit is the most commonly used device for startup procedures, one may also use equipment such as an auxiliary power unit (APU) as well.

Despite the air start unit mostly being reserved for initiating the operations of engines, they may also be used to provide onboard air conditioning while the aircraft is on the ground. Through the use of the control panel, air pressure and air flow settings can be changed to suit air conditioning needs, and air will pass through heat exchanger devices, compressors, and an expansion turbine to create air that is suitable for treating a cabin. In general, air for the air conditioning system may be provided by the air start unit, APU, or the main jet engines.

Internet of Components is a leading distributor of aircraft air start unit components and quality aviation parts, and we can help you secure competitive pricing on heat exchanger products, engine parts, and other various items. Take the time to explore our vast offerings as you see fit, and our team of industry experts are always readily available to assist customers through the purchasing process as necessary. When you are ready to initiate the purchasing process, give our team members a call or email at your earliest convenience and they would be more than happy to assist you however they can.


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