Hot answers tagged

75

Whenever you see titanium being used on aircraft, look for heat sources. The metallic alternative aluminum is usually lighter and much cheaper. In most commercial aircraft the APU (Auxiliary Power Unit) is in the tail, behind the rear pressure bulkhead and the empennage. (modified from aerospaceweb.org) The APU is a small turbine and its exhaust gases are ...


70

Weight. Piston APUs for trucks are designed for frugal and quiet operation. This one produces 5.2 kW electrical power and weighs 375 lbs. The APU for the A320 and B737 is a noisy screaming unit that produces 90 kW electrical power, 445 shaft kW. It weighs 375 lbs as well. The main difference is in the weight of the engine itself. The turbine engine of the ...


43

The sound you're hearing is the APU (Auxiliary Power Unit) running. Turbine (jet) engines turn at a much higher RPM than reciprocating engines. This means that an electric starter motor would have to be bigger and heavier in order to produce enough torque to get the engine moving fast enough. The bigger the engine, the bigger the starter (and the batteries ...


38

This answer applies to the AVRO RJ (BAe 146), because this is the aircraft I've flown. Other aircraft may or may not have similar features/issues/restrictions. Reasons for APU use in mid-air For takeoff, the APU bleed-air can be used to supply the cabin air pressure system to reduce load on engines. This leads to a significant reduction in runway length ...


33

APUs are like small jet engines, small enough to be started electrically. So you would use the power of the battery or an electric ground power cart to spin up the APU by its electric starter motor, and then introduce fuel into it once its spinning fast enough, just like a jet engine is started. Depending on the design, the electric motor will then either ...


31

It is the Auxiliary Power Unit (APU) inlet door. Image from Boeing Aero Magazine


31

Dead space... Not much room in the tail to put other stuff, and it fits. Plus it has a nice empty space just outside for the exhaust pipe to exit without the hot gasses hitting anything. Put the APU in the nose and you need a long pipe somewhere to get rid of the exhaust. More weight, more space, etc. Put it in the wing and you're looking at uneven weight ...


28

The full NTSB report can be found here and is the official source of information on the incident. According to that report Sullunberger did immediately start the APU which did deliver power and they determined they did not need to manually deploy the RAT. However the onboard systems deployed it automatically likely as a result of the initial power loss and ...


28

No. It doesn't have an APU — it has two of them. They are located close to the main landing gears. From An-225 Mriya is the world’s largest aircraft (English version): Auxiliary power plant consisting of two TA-12 turbofans installed in the left and right chassis fairings provides independent power to all systems and starts the engines. I think "...


24

This varies due to operational need and airline standard operating procedures. On my aircraft (an Embraer E-Jet) we turn on the APU roughly ten minutes prior to pushback and shut it down shortly after starting the engines. If we plan on taxiing on one engine we will shut it down after the first engine start and perform a cross-bleed start on the second ...


23

There are a few reasons that the APU is located in the tail in most modern aircraft. As previously mentioned, the space in the tail is otherwise mostly unused. Aside from hydraulics and such for the control surfaces, there isn't much use for the space. Most aircraft don't have any other spaces like that. Alternate locations like the wing root are generally ...


23

Other than the APU, there are multiple ways to provide electrical power to an aircraft: Battery: The battery is typically the first thing you would turn on and it usually provides DC power to emergency systems only (at least on an airliner, smaller aircraft are fully powered by the battery). Running only on battery power will however deplete the battery ...


22

Yes, the solution to start APU was important. The ditching procedure directs the use of maximum available slats and flaps for the final approach and touchdown (source, chapter 10.3). This is not possible without APU, as Airbus A320-214 cannot move flaps if only powered by RAT (only blue hydraulic line, same source, chapter 9.3). The running APU adds the ...


21

You may be expecting the rotor to spin up in lock-sync with the engine. That's not quite what happens on a turbine helicopter. Turbine helicopter engines (turboshaft engines) have a turbojet engine inside them that makes thrust. This engine has a shaft of its own - so that its compressor blades are connected to its own turbine blades. The turbojet's ...


19

APU may be used in flight as second source of power or bleed air if one of the engine systems is inoperative. If it is so used, it might leave a contrail the same way engines will, though as DeltaLima notes, it's exhaust is hotter and the air behind the fuselage is more turbulent, so the range of conditions where it will leave a contrail is narrower than for ...


15

The APU is usually located at the rear of the aircraft, in the tail, so the intake is generally somewhere on the side of this location. Aircraft place the inlet in different places though, and some even put the APU somewhere different. For some examples: 737 intake, the small NACA inlet at the bottom E145 (the place they don't want you to put de-icing ...


15

On multiple PPRUNE threads they incorrectly say: The APU bleed is much cooler than the engine bleed system, and would be ineffective for WAI. But the reason is: The APU bleed temperature is unregulated and thus could damage the wing slats if used for anti-ice. Highlight shows regulation of engine bleed temperature.


15

Usually an APU provides electricity and bleed-air, just like the aircraft engines do. An all-electric APU only provides electricity. This can be done in the Boeing 787 because of the bleedless architecture where systems that would normally require hydraulic or pneumatic pressure are fully electrified.


15

Onboard batteries for DC and a ground power supply for AC. The ground supply can come either from an airport vehicle or from the stand itself. Since the standard AC in aviation is 115V and 400Hz, the usual ground power supply needs to be converted to a higher frequency. This used to be done with a rotating converter, i.e. a motor (at 110V and 60Hz or ...


13

An APU lacks the critical component that produces thrust, a propelling nozzle. A propelling nozzle converts a gas turbine or gas generator into a jet engine. and Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. There are several different types of gas turbine engines, but all turbine ...


13

The engines provide electrical and hydraulic power to the systems that control the flight path of the aircraft: elevator, aileron etc. If all power is lost, a Ram Air Turbine (RAT) is deployed which spins up and powers the systems from the airspeed. However, this creates extra drag. The RAT deploys automatically upon loss of AC bus 1 and AC bus 2. This ...


13

It comes from the left side of the fuel manifold, which means it is most often fed from the #1 main tank, typically by turning on either or both boost pumps in that tank. However, it can be fed from the center tank (using the left center tank boost pump) or from the #2 main tank using a boost pump there and opening the crossfeed valve. If no boost pumps are ...


12

Three words: They’re much heavier. Gas turbines have a very high power to weight ratio As opposed to reciprocating diesel engines. So for the needs of an airplane which requires large amounts of electrical and hydraulic power in order to operate, a gas turbine makes a lot of sense for use as an APU.


11

Yes, you can operate without an APU. If the APU is inoperative you will need to write it up and coordinate with maintenance and dispatch to operate under the stipulations of the MEL. This may restrict certain kind of operations but if we assume a route completely over land this has no effect other than needed ground services. The requirements for no-APU ...


11

Auxiliary Power Units (APUs) are small gas turbine engines used primarily during aircraft ground operation to provide electric power, air conditioning etc and to start the main engines. In flight, the APUs can be used for the following: The APUs can be used to provide backup electric power during in-flight operation, in case the main supply fails (or is ...


10

The link you provided is for a Canadian PBY-5A which were called "Canso" in Canada. I flew these aircraft in water bombing operations during the early 1980's. In your link it describes the 1955 installation of a 28v Homelite APU. It was installed on the left (port) side of the aircraft in the "living quarters" compartment behind the landing gear. The ...


9

As you have already imagined, it's the APU fuel line shroud drain, as visible here: (Source) Detail: The fin gives an aerodynamic profile to the drain. The reason this drain exists seems to be: Natural fuel leaks from the auxiliary power unit (APU) fueling line were supposed to drain harmlessly out the bottom of the structure as designed, ...


9

Yes it is. Without the APU the airconditioning and electronics also won't work when the engines are shut down, so a ground power unit is connected to provide the electricity and enough compressed air to run the air-conditioning. Starting the engines will need a air start unit to start one of the engines using compressed air provided by hoses. After that ...


9

The A400M is designed to operate from unpaved surfaces. With the RAT on top, this avoids FOD issues, and provides ground clearance. For the APU, it's space and proximity. You've got the wing box above the fuselage, plenty of space to add things there. Also easier maintenance access. Proximity as in less wiring and tubing to run inside the airframe, saves ...


9

You need to look at the overall picture. Yes, a diesel is cheaper, but it will weigh more and be less reliable. Also, the operating time of an APU is rather a small fraction of total airframe operating time, so it will mostly be a dead weight. To arrive at a number representative of overall cost, add to the acquisition cost all the maintenance and all the ...


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