When is the APU on?

Question

I know it's on during startup, but I always thought they turned it off after startup. My friend says no, they have it on all the time.

Answer 1

the APU is generally on while the aircraft is on the ground until after the aircraft has pushed back and all engines have started. after startup, the APU is turned off. it isn't on all the time as it's not required to be.

Answer 2

We also turn it on for CAT III approaches as a third source of electrical power.

Answer 3

The APU can be used to full efficiency up until 14000ft and electrical only until 25000ft. The APU uses a centrifugal compressor which is alot small than a radial compressor found in modern jet engines. The centrifugal compressor is not as efficient therefore at higher attitudes the apu cannot provide enough air pressure for bleed and combustion.

Once the apu master switch is turned off it also closes the apu inlet stopping air flowing through the engine which would create drag.

Answer 4

In general, the APU is used whenever a source of electrical or pneumatic air is needed.

This is (obviously) for engine start but also sometimes for pressurization.  Some older planes need it for pressurization on takeoff in order to meet second stage climb gradient requirements.  In that case the small amount of bleed air that is tapped to pressurize the cabin is enough that there is not sufficient power remaining to outclimb all obstacles on a single engine.  In those cases, the APU is used to supply the pressurization up to 3000' or so.

In other cases, it is used to supply the pressurization when extra bleed air must be tapped for wing de-ice (again in order to meet climb gradient requirements in the event of a go-around).

Sometimes it is run as a second or third generator, to supply pneumatics for air-conditioning, or even in some cases to supply power to an additional hydraulic pump to supply backup hydraulic pressure.

It depends on the aircraft but in general, assuming all normal conditions, the APU will be started up immediately prior to engine start and turned off immediately after the last engine is started (unless a cross-bleed start is conducted, the it is possible to shut it down right after the first engine is started).  At stations where ground air conditioning and high-pressure air is available, it may be possible to continue normal operations without ever starting the APU in the first place.

Answer 5

Whenever the engine is off, the APU is on to supply the aircraft of its needs either electrical or air conditioning in ground or in air. though at 25k feet APU can  supply electrical power only.

In ETOPS flight (for twin engine), an aircraft cannot fly with unserviceable APU.  It doesn't necessarily be "ON" but it must be working.

Again as John B says, after engine start it is turned "OFF".

Answer 6

I don't know the newer aircraft but generally the APU is shut down after engine start prior to take off. The 727 even shuts it down when the wheels break ground if the engineer forgets. Many APUs are inflight capable but normally only used during emergencies and at lower altitudes. If they're running them way up at cruise I wouldn't know why. Man, won't be long before I don't even speak the language any more the way technology's headed.

Answer 7

at this point in time hopefully as little as possible due to high price of fuel. but they can be on for giving aircraft electrical power to power the airplane if there is no ground power avail, to run the air conditioning units for the aircraft, and like the first answer in emergencies.

also CAT III landings are when the aircraft lands itself, or the pilot used a Heads up Display(HUD) to land the aircraft. this is mostly because of weather, when the weather is below 600 feet visibility you are required to use at CAT II approach with is basically a autopilot coupled approach. CAT I is just a regualr Instrument Landing System (ILS) approach