Dear Friends and followers, today's question is another absolute classic question
Joe, could you answer, What is the APU?
The APU's a very important topic, so let's get started!
I'm pretty sure many of you have seen and heard the APU before
So lets imagine that you're in a bus driving towards your plane, which is parked in an outside position
and as you get off the bus and on to the mobile stairs towards the plane you can hear this sound.
(APU in action)
That my dear friends is the sound of a running APU.
APU stands for Auxiliary Power Unit.
On most commercial jets, the APU is placed at the far most end of the fuselage
And its main purpose on ground is to make the plane self-sustainable
providing it with electrical power, and bleed air pressure for the air conditioning system
and especially for engine start.
Now, most APU's are a single spool turbine which power an electrical generator
And air compressor. The APU has its own electrical control box,
which monitors the start sequence, the turbine speed, the exhaust temperature,
the oil and fuel supply and the bleed air outlet
and in case of a failure it controls an automatic shutdown to prevent any further damage.
It even has its own fire protection system
,which automatically activates internal fire extinguisher, reducing the risk of fire.
Okay! Let's look at how you would start the APU,
following the cockpit preparation checklist you will connect the batteries first,
then you would test the fire protection system, if its the first flight of the day, following the APU start sequence
Testing the fire protection.
By pressing the APU master push button, the air inlet flap below the APU will open.
Then, pressing the APU start button is like turning the ignition key in your car
The batteries power the APU's electrical starting motor
And, the electronic control box regulates the start sequence.
Once the APU as it is at the correct speed, the electrical generator is interconnected by a relay,
and powers the airplane's electrical system
Now, most of the elctrical system works same as it would be running oil supply by the main engines
As mentioned before, the APU also provides bleed air for the air conditioning system
Once you press the bleed air push button, the bleed valve turns into the open position
pressured air to the cooling packs
and the packs consist of an air cycle machine and the heatings change up
but more about that the air conditioning packs in another detailed video
Other than that, one other primary purpose of the APU is to start the main engines
Again, using bleed air which is guided to an air starter,
consisting of a little radial airflow turbine or propeller, you might say
which sits on a planetary reduction drive or a centrifugal clutch,
Which then is connected to the turbine gearbox and which is then connected
which is then connected to the high pressure turbine. But to make a long story very short,
the APU and its bleed air system start the main engines.
I'm gonna have to make another detailed video just about the engine start sequence on a jet turbine.
So obviously the APU uses jet fuel.
'Cause it would'nt make any sense having an extra fuel system just for the APU.
So on the Airbus A320, for example, the left wing tank
provides fuel for the APU. And, the APU can be very thirsty.
Up to a 160 litres per hour depending on its load and external factors.
And as far as I know, the APU on a Boeing 747 nearly uses upto
400 liters per hour, so you do not want to have that running for hours!
So this is a very basic introduction of the APU and its main purpose.
I will make more detailed videos about APU and engine start sequences in the future
I hope you enjoyed this short little video on the APU. So please hit the subscribe button, tell your friends..
about my channel and
spread the word!! All the best ,farewell. Your Captain Joe.
The next time you sit on a passenger jet see what happens whilst the main engines get started.
Because the engine start sequence uses up most of the APU bleed air , the Air conditioning system is temporarily
cut off, for the time the engines spool up.
So you won't be feeling any cool air coming from the panels above, see for yourself on your next time in the plane