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Airplane Cabin Sounds that you hear during Passenger Boarding explained

Have you ever wondered what the different airplane cabin sounds are that you sometimes hear when you board your airplane?

I know it puzzles a lot of passengers because I get asked about it quite often! But, most passengers don’t give much thought to the sounds, as they simply look forward to flying.

Updated and republished on July 20, 2020.

Continental Airlines TV commercial 1974. Continental Airlines.

Airplane cabin sounds come from different sources

Airplane cabin sounds when boarding

The many strange sounds you may hear once you board or deplane your airplane cabin is a normal series of harmless and different sounds you are going to hear throughout your entire flight experience.

Airplane cabin sounds inflight

You will hear several additional sounds when you are inflight. In this article, we only explain the different airplane cabin sounds you may hear when you are boarding and deplaning your plane.

What is that continuous humming airplane cabin sound?

The “humming” sound is a result of the Auxiliary Power Unit (APU) running.

APU inlet and exhaust on an A380, airplane boarding cabin sounds
The visible parts of an Auxiliary Power Unit (APU) of an Airbus 380. The cylinder-shaped tube sticking out of the tail is the exhaust-part of the APU. Photo: Airbus.

What is an APU and what does it do?

An APU is a separate small jet engine located at the tail of large airplanes.

It is used to provide electrical power to the airplane when the engines are not running on the ground. The APU is also hooked up to the pneumatic system to power the air conditioning system.

You can compare it to a fuel-operated electrical generator you can buy at Home Depot.

access panel of an airplane apu, airplane boarding cabin sounds
This APU engine is shown with the access panel open. Photo: CaptainJetson.com.

Air that is powered from the APU can also be used to start the airplane’s engines.

Furthermore, it can act as a back-up generator in the air, as a safety measure should the airplane’s engine-generators malfunction.

A jet has one separate generator for each engine.

If you didn’t have the APU as a back-up in the air, then only the battery would be able to provide electrical power should the airplane’s main generators malfunction.

As a matter of fact, on the old Boeing 727 aircraft, the APU was even designed to provide a small additional amount of inflight thrust!* It did that by generating a small amount of thrust from the exhaust of the APU unit itself.

*If you are interested in a basic explanation of how an airplane flies and what thrust is you can find out more by reading our article on the subject here.

What is that low-pitch, low volume continuous humming sound?

Airport Terminal External Airplane Power sound

We prefer external electricity to come from the airport terminal powering the airplane on the ground.

These power units are hooked up to the jetway, also called the jetbridge.

Jetway terminal electrical power, airplane boarding cabin sounds
An airplane stationary Ground Power Unit (GPU). Photo: CaptainJetson.com.

This is because the APU is running by jet fuel from the airplane’s fuel tanks, which in turn is expensive.

The electrical hookup produces a sound similar to that of plugging in your cordless electrical garden equipment into a power charger.

External Electrical Power Cart sound

The next best thing to external power hookup from the terminal is to have the airplane get its ground electricity from an external electrical power cart (“a portable electrical generator on wheels”), also called a GPU, or a Ground Power Unit.

The GPU, when supplying the airplane with electricity will also create that low-pitch continuous sound, just like the units located at jetways.

In addition, this cart produces outside noise like any other portable generator would do.

Airplane external power cart
An airplane portable Ground Power Unit (GPU). Photo: CaptainJetson.com.

The APU will be started to supply the airplane with electricity if neither the jetway-connected GPU or the portable GPU is available.

What is that whining sound?

Airplane’s Air Cycle Machine

Another sound you may hear is a hollow-type whining sound. The Air Cycle Machine is responsible for producing that sound.

The Air Cycle Machine is a part of the air conditioning system.

Airplane Air Cycle Machine
An airplane’s Air Cycle Machine is part of the air conditioning system keeping your temperature comfortable in the cabin. Photo: Captainjetson.com.

The Air Cycle Machine (ACM) is located in the belly of the airplane. Its purpose is to produce and process tempered air-conditioned air for the cabin, for the comfort of passengers and the crew.

On some jets, if you sit directly above the Air Cycle machine the sound may be quite noticeable.

What is that hissing sound?

No, that hissing sound is not from snakes on your airplane.

Air Diffusers

Usage of the air conditioning system on the ground and in the air sometimes results in a “hissing-sounding” and even fog blowing out of the cabin air diffusers in the cabin.

Fog can occur in the cabin when the temperature differences and moisture mix get to the point where fog occurs. This is only temporary and completely harmless.

Have you experienced the cabin fogging up on any of your flights?

Fog in airplane cabin
Fog in the airplane cabin. Photo: Passenger-submitted to CaptainJetson.com.

Did these explanations help you understand the cabin sounds you hear when boarding your airplane? Comments or questions? You can contact us here.

Featured Image: Unsplash.

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