UPDATED. 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.
Airplane cabin sounds are from different sources
The many strange sounds you may hear once you board your airplane cabin is a normal series of harmless and different sounds you are going to hear throughout your entire flight experience.
You will hear several additional sounds when you are inflight. Here we only explain the different airplane cabin sounds you may hear when you are boarding your plane.
What is that continuous humming airplane cabin sound?
The “humming” sound is a result of the Auxiliary Power Unit (APU) running.
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
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.
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.
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.
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.
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.
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?
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.