# What the APU Does

> The APU, or auxiliary power unit, is a small engine carried on many aircraft.

- Published: 2026-03-19T09:00:00.000Z
- Updated: 2026-08-15T20:04:19Z
- Category: Aircraft & Systems
- Canonical page: [https://flymap.app/learn/aircraft-systems/what-the-apu-does](https://flymap.app/learn/aircraft-systems/what-the-apu-does)

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The **APU**, or auxiliary power unit, is a small engine carried on many aircraft.

It is not used to fly the plane through the sky, but it does support the aircraft in important ways.

## 🔌 1. The APU Provides Power on the Ground

When the main engines are off, the APU can supply electrical power to the aircraft.

That helps run:

- cabin lighting
- onboard systems
- cockpit equipment

This is especially useful at the gate.

## 🌬 2. It Can Also Supply Air

The APU can provide compressed air used for:

- cabin systems
- starting the main engines

That makes it a very useful support system before departure.

## ✈️ 3. It Helps the Aircraft Be More Independent

Without the APU, an aircraft may need more ground support equipment for power and air.

With it, the aircraft can handle more by itself.

This gives airlines and crews more flexibility.

## ✨ What It Means

The APU is like a support engine for the aircraft.

It helps with:

- electrical power
- air supply
- engine start support

Even though passengers rarely think about it, it is a very practical part of operations.

## 💡 Simple Way to Think About It

The APU is like:
> a helper engine in the background...  
> keeping the aircraft ready before the main engines take over.

## 🟢 Quick Fact

On many aircraft, the APU is located in the tail section.

The APU does not get much attention from the cabin -  
but it plays a major role in making aircraft operations smoother and more self-sufficient.

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