What Is An Auxiliary Power Unit (APU)?
An auxiliary power unit, also known as (APU), is a device on a vehicle that provides energy for functions other than propulsion. Auxiliary power units are usually found on large aircraft and naval ships as well as some large land vehicles. In airliners, APU’s are usually located at the very rear of the aircraft, below the tail. The primary purpose of an aircraft APU is to provide power to start the main engines. Turbine engines must be accelerated to a high rotational speed to provide sufficient air compression for self-sustaining operation. Smaller jet engines are usually started by an electric motor, while larger engines are usually started by an air turbine motor. High-speed bleed air from the auxiliary power unit is used to spin up the main engine. Before the engines are turned, the APU is started by a battery or hydraulic accumulator. Once the APU is running, it serves its purpose in providing power (electric, pneumatic, or hydraulic, depending on the design) to start the aircraft’s main engines.
- Cabin air and electric power before the engines are started (saving battery power).
- An emergency source of electric power in the event of engine failure.
- Can start the aircraft engines mid-flight in an emergency.
If the APU fails before engine start, the engines cannot be started without an external “start cart” to provide a source of bleed air. If the APU fails mid-flight, there will be no immediate effect. Without the APU, there are two additional ways to restart an aircraft engine in flight:
- A cross-bleed start, where bleed air from a working engine is used to start a dead engine
- A windmill start, where the aircraft dives and attains enough speed that ram air spins the turbine fast enough to allow an ignition.
Auxiliary Power Units are normally left off during flight but may be turned on for certain long-haul overwater flights as an extra precaution.
The Origin Of The APU
During World War I, the British Coastal class blimps, one of the several types of airship operated by the Royal Navy, carried a 1.75 horsepower (1.30 kW) ABC auxiliary engine. These powered a generator for the craft’s radio transmitter and, in an emergency, could power an auxiliary air blower. One of the first military fixed-wing aircraft to use an APU was the British, World War 1, Supermarine Nighthawk, an anti-Zeppelin Night fighter.
The first German jet engines built during the Second World War used a mechanical APU starting system designed by the German engineer Norbert Riedel. It consisted of a 10 horsepower (7.5 kW) two-stroke flat engine, which for the Junkers Jumo 004 design was hidden in the intake diverter, essentially functioning as a pioneering example of an auxiliary power unit for starting a jet engine. A hole in the extreme nose of the centre body contained a manual pull handle which started the piston engine, which in turn rotated the compressor. Two small petrol tanks were fitted in the annular intake. The engine was considered an extremely short stroke (bore/stroke: 70 mm / 35 mm = 2:1) design so it could fit in the hub of the turbine compressor. For reduction, it had an integrated planetary gear. It was produced in Victoria in Nuremberg and served as a mechanical APU-style starter for all three German jet engine designs to have made it to at least the prototype stage before May 1945: the Junkers Jumo 004, the BMW 003, and the prototypes (19 built) of the more advanced Heinkel HeS 011 engine, which mounted it just above the intake passage in the Heinkel-crafted sheet metal of the engine nacelle nose.
When handling auxiliary power units the test facility and testing are critical to safeguarding the reliability and functionality of the APU. Testing facilities have to be fully correlated and approved by the OEM’s, our facilities are designed to test auxiliary power units at various conditions including a full performance condition with the confidence of producing reliably confirmed data; this will ensure that the APU’s operational characteristics are performing correctly.
By applying high-speed data acquisition systems and performance correction software, APU manufacturers, end users and overhaul agencies will obtain the maximum return on their investment; by optimising the excellent test cell utilisation gained through reliable designed test systems.
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