Why do pilots only start up the Auxiliary Power Unit (APU) on take off and landing only on airliners ?

Question

Why do pilots only start up the Auxiliary Power Unit (APU) on take off and landing only on airliners ?

Answer 1

In the event of engine failure - they're too close to the ground to try to go through an APU startup - so they do it in advance.

Answer 2

well your question is not very clear.

first of all, airlines do not start the APU on take-off or landing. they just put it on when they start engines before take-off, or they put it on when ground power is not available. after the landing however, they put it on, because when they shut down the engines, a lot of systems will not work without the APU running.

small private planes do not have APU, that's why they cannot use it.

Answer 3

Most aircraft cannot run the apu while in the air. There would be no point in doing so unless you had a failure of the electrical or hydraulic system. The APU is normally run on the ground to supply electricity, hydraulic power and pneumatics for engine start.

Answer 4

In the private jets I fly, we routinely start it before engine start, but not during takeoff. And start it again, after landing. I have never heard of an APU providing back-up hydraulics, so I have no idea what the others here are talking about. The APU is routinely used as a source of electrical power before engine start and after engine shutdown, and for heating/cooling on the ground. Usually, it can provide back-up electricity in the air, within certain parameters, or back up pressurization, within certain parameters. I have first-hand knowledge of this.

Answer 5

Cost. When the engines are running and the aircraft is in flight, the APU is not needed for any reason. Leaving it running does nothing but burn fuel. The engine driven generators and the bleed air taken from each engine is more than enough to power the aircraft and provide pressurization.

On the ground (or right before landing), the APU is started so that it can take over the electrical and air conditioning loads from the engines once they are shut down (many airliners can single engine taxi on the ground, particularly smaller aircraft like the CRJ).

The reason that some are started right before landing is because some APUs require a unloaded "warmup" period where the APU is not used for either electrical or bleed air (or both) for 1-4 minutes.

Typically an APU is started on the ground at the gate prior to departure (either 30-45 minutes prior if there is no ground power available, or right before pushback if there is). The APU runs until engines are started. Some airlines choose to leave the APU on until after takeoff. This provides "supplemental" air conditioning to the passengers while taxiing. (At idle, the engines don't provide a lot of air to the cabin).

On arrival, the APU is started either after landing during the taxi in or right before landing (typically passing 10,000' or during the approach- but not the landing sequence). It will run until the aircraft is connected to external ground power, or until the crew is ready to shut the airplane down to a secured status (IE everything off and the doors shut/locked for the night).

Answer 6

incase of falilure and it controls the air conditioning

Answer 7

Safety.  In case of some happen, Hydraulics and Electrical are already running.

and that is a SWAG.  :-))

Answer 8

They don't.

We frequently take off without the APU running.  Its only required if performance of 1 engine mandates it.  Pressurizing the airplane takes a lot of power away from the engines, and in bad weather, or on a short runway, it may be required that the pressurization be handled by the APU until 1500' AGL.  However, this is still not a requirement, as pressurization itself can be put off until 3000' AGL, so even if your APU is broken, a legal takeoff can usually still be made.

The main purposes of the APU are to provide electrical power and to provide high-pressure air for engine starting.  The APU is typically used before engine start to provide that air, and once the engines are running, it is shut down.  Sometimes on landing, we have to start it, but only to provide pressurization when we divert engine power to wing de-ice.  Again, it simply provides us with enough safety margin that if we were to lose an engine, we could still outclimb any of the terrain around us on the remaining one engine, even with full de-ice and pressurization running.  If the APU were deferred, we could simply depressurize the airplane at 3000' and be just fine.

Answer 9

the apu is only needed for power in flight and the power supply is provided from the source on the ground at most airports

ie mobile a.p.u,s or hutchins trade name

Answer 10

An aircraft APU is not started on take off and landing.

The APU is started before the main engines are started, to provide electrical power for cockpit instruments, computers, lighting and to provide power to start the main engines.

The APU does not have sufficient power output to supply all the aircraft's main electrical and hydraulic systems.