Why are stationary airplanes noisy?


Hello, I’m Korosuke, and I’m glad to see that the airport is quite crowded and lively in the middle of GW.

I heard that the bullet train is also quite crowded, so it is quite difficult to move around as a customer,..,

Now, for the first time in a while, I would like to explain something about aviation.

Why are stationary airplanes so noisy?

Have you ever tried to approach an airplane parked on the tarmac?

You can hear the loud noise from the observation deck, but why is it so loud even though the engines are off?

In this article, I will try to explain the reason for this, the system related to airplanes.

The reason for the loud sound is another small engine

To answer the question, the loud noise is caused by another small engine on the plane, the APU( Auxiliary Power Unit).

APUs are called “auxiliary power units” in Japanese, and like engines, they are powered by fuel.

The APU is located around the tail, which is called the butt of the airplane.

尾翼付近のAPU Exhaust(wikipediaより)

The size of the main unit varies from aircraft to aircraft, but it is about this size.


The role of the APU is explained next.

Three main roles of APU

APU has three main roles.

1. power supply and cabin temperature control during engine shutdown

Airplanes basically depend on the electricity and heat supply from the engines to provide cabin temperature, service, etc. during flight. But keeping the engines running on the tarmac is dangerous and not good for the environment. This is where the APU comes in. Although small in size, it is a solid and sufficient power source for passenger aircraft, and the hot air created to run the APU can be used to regulate the cabin temperature.

2. assist in engine start

Starting the engine requires outside air, hot air and a power source to start the engine running. The APU plays that role.

3. emergency power supply

If the engines stop in flight, the APU is basically turned on immediately to ensure that the plane does not run out of electricity and bleed (hot air). This APU standard is especially important when flying away from land, such as over the Pacific Ocean, because of the ETOPS standard.

Items used in place of APU

As explained above, APUs use fuel to operate, which emits exhaust gases that are bad for the environment, and they are also very loud, so many airports (Haneda, Kansai, Narita, Fukuoka, etc.) have restricted their use as a noise reduction measure.

Here are some typical substitutions that are made at that time.

1.GPU(Ground Power Unit)

The power supply will be like an outlet embedded in the basement of the tarmac.

2.ASU(Air Start Unit)

Similar to a GPU, to assist with temperature control and engine start provided on the ground!

3. power supply vehicle

In some tarmac areas, there are no GPUs or ASUs, in which case the power and air supply is mounted on a vehicle such as a power truck.

For the full article click here.

Summary of why stopped airplanes are noisy

How was it?

Quite a few people know about the APU, so if you are aware of the sound when you fly, you will realize that this is the APU sound.

In fact, it is quite noisy when heard up close, so pilots often wear ear muffs when they go for external inspections.