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When watching the below clip:

at exactly 11:02, FO says that APU is not required.

What could be the reason for using APU on the ground? What good could this bring? EDIT: Both questions concerning depicted situation only -- i.e. a plane has landed in a quite very hot air.

I am a total noob to flying, but I was always more than sure that APU is used in air only. It is used to produce minimum electrical power to on-board equipment. Which means (as I assume) that there is zero electrical power when starting APU and it must be started when flying airplane generates enough airflow (through APU).

What am I missing? In which scenarios (EDIT: for depicted, landed airplane conditions) on the ground APU "could be required"? And how a ground speed of the after-landing rolling plane (very limited to airborne speed), generating minimum airflow, could start up the APU?

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    $\begingroup$ Starting the engines, air conditioning, AC power if no ground power unit $\endgroup$
    – tedioustortoise
    Commented Feb 13, 2023 at 21:02
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    $\begingroup$ @trejder Your understanding of APU starting is incorrect. An APU is started using electrical power from either a battery, or the aircraft engine generator. You can not start an APU using airflow. $\endgroup$
    – Mike Sowsun
    Commented Feb 13, 2023 at 21:04
  • $\begingroup$ @tedioustortoise The plane in question is rolling through airport after 9+ hours flight. Has both engines fully operational (no need to start them) and providing electrical power, has air conditioning working. $\endgroup$
    – trejder
    Commented Feb 13, 2023 at 21:05
  • $\begingroup$ @MikeSowsun No, it does not. The only on-the-ground reference that I found is starting the engines and providing the AC power (not need after landing) or landing in a very cold conditions (here we have 86F). $\endgroup$
    – trejder
    Commented Feb 13, 2023 at 21:06
  • $\begingroup$ @trejder I was more answering the generic question - in which scenarios is APU on the ground required. $\endgroup$
    – tedioustortoise
    Commented Feb 13, 2023 at 21:11

3 Answers 3

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The APU is primarily used on the ground for:

  1. Engine Starting - the main engines need a bleed air source of pressurised air to get them “turning” before fuel is added and they become self sustaining. Ground starter units can be used, but it is not economical or practical.
  2. Air conditioning - airports may provide air conditioning sources which attach onto the aircraft. But again, most practically the APU provides the pressurised bleed air for air conditioning to the cabin.
  3. AC power. Typically ground power units are provided at airports, but in their absence, or in the interim period between disconnecting all ground support units and engine start, the APU will provide AC electrical power to the aircraft. (The battery with inverters to AC power typically only powers limited resources on larger aircraft).
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    $\begingroup$ Thank you for a very quick and complex answer. I have updated my question to narrow it down to depicted situation only (landed airplane rolling through airport). Can you update your answer to reflect that? If not (meaning that there are no scenarios for landed airplane to use APU) then simply reflect that this is just a procedural wording and APU is never used on the ground for a just landed airplane. Thank you. $\endgroup$
    – trejder
    Commented Feb 13, 2023 at 21:11
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    $\begingroup$ @trejder thanks. I will have a look tomorrow (it’s late here), but no doubt someone else might have got an answer in too in the meantime. :) $\endgroup$
    – tedioustortoise
    Commented Feb 13, 2023 at 21:12
  • $\begingroup$ Waiting for you (or someone else). Will be happy to reward you with +25. Good night (and remember to keep it above the horizon! :) $\endgroup$
    – trejder
    Commented Feb 13, 2023 at 21:13
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It is very common for an arriving airliner to start the APU just before arriving at the gate. This allows the engines to be shut down immediately when the aircraft reaches the gate.

Waiting for ground power to be hooked up can sometimes take a few minutes. Leaving the engines running at the gate is very noisy, burns considerable fuel, creates an unnecessary safety hazard, and delays the offloading of baggage.

Even once ground power is connected, many operators will leave the APU running for heating or air conditioning. (adequate heat or AC may not be readily available)

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  • $\begingroup$ In RJ land, it's common when flying with the APU deferred to leave the right engine running if a ground A/C cart or huffer cart isn't available at the gate. Is that ever done with heavies? $\endgroup$
    – John K
    Commented Feb 14, 2023 at 5:39
  • $\begingroup$ Most airports in Europe prohibit the use of the APU when the aircraft is parked at their gate for noise and environmental reasons. They can only use it in case of a problem with the aircraft or ground equipment. You would never operate deboarding operations with an engine running, the doors are inside the hazard zone. $\endgroup$
    – Benson_YoureFired
    Commented Feb 14, 2023 at 8:44
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TLDR: It replaces "ground support" at less well equipped airports, or unequipped aprons.

The APU starts the main engines

Start with this: Your car has a battery, it starts the engine. Batteries on airplanes aren't big enough to start the main engines.

So the APU is a small engine that the batteries can start. The APU can then start the main engines.

But it also provides hotel power...

Once the APU is running, it also provides hydraulics, electrical controls, lighting and compressed air for engine start and air conditioning/heat for the cabin. It can also be used to power these hotel loads during takeoff, to avoid bleeding energy from engines. Or during flight, as famously used in the Flight 1549 ditching.

Before an airplane can be boarded, the HVAC must be run for quite some time to make the space habitable. You've heard warnings about not leaving babies or dogs in cars - same thing will happen to an airplane cabin left in the sun. That cabin could rise to 60-70C (140-160F) and you cannot board people into that!

Heating is simply: compressed air is pretty warm. Air conditioning uses a machine called a PACK.

If an airplane expects to sit on the ground for an extended period, they will usually shut off the main engines and run the APU to save fuel and engine hours.

...except at better-equipped airports

Shore-side equipment can deliver electrical and/or high-volume compressed air to a jetliner. The latter both starts engines and powers the heat and air conditioning.

The compressed air comes from a machine informally called a "huffer" - if you saw Top Gun: Maverick, you saw them start up a "huffer" to start up a jet fighter.

Simpler versions of this shore electric and air supply is a portable cart that is basically an APU on wheels. Better versions are built right into the apron or gate and are coming off grid power or a central air source. The huffer may be local to the gate, but electric.

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