The Meaning of Mechanical Sounds You Hear on an Airplane

In addition to the beeps and bells you hear on an airplane, you may also hear various mechanical sounds before and during your flight, as well as after you land. If you don’t have aviation experience or knowledge (or are traveling with someone who does), you may be curious or worried about what causes these noises and what they mean. Here are some examples.

High-pitched squealing, preening, or barking dogs.

If you’re traveling on an Airbus and hear what sounds like scrubbing back and forth or dogs barking in the cargo hold, you’re probably hearing power being supplied to the hydraulic system(s) (usually there are three). “Up,” says Jack Hurstam , an airline pilot and former flight instructor. On other types of aircraft, the sound may sound more like a high-pitched screech, he adds.

“The sounds you hear are the hydraulic accumulators charging the reservoirs to the pressure needed to operate the brakes, steering, reverse thrust and steering movement,” explains Herstam. You will most likely hear these hydraulic noises after the air bridge is removed and the airport employees on the ground are safe.

Humming of varying intensity

The dull hum in the background is the sound of airplane engines. If you’ve heard of this sound, you’ve probably also noticed changes in noise levels throughout the flight: it gets quieter when less engine power is needed (such as when descending), and it gets louder when the plane needs more power (such as when descending). when turning). ).

And if the roar of the engines makes you sleepy, you’re not alone. The sound is an example of brown noise , which some people seek out to put them to sleep or, in other cases, to help them concentrate . Here’s how Dan Berlau, professor of pharmaceutical sciences at Regis University , explains it :

Like white noise, brown noise contains all the frequencies that the human ear can hear. The difference is that brown noise enhances low frequencies and softens high frequencies.

White noise in the background

The air circulating in the aircraft cabin adds another layer to the background sound. Most of the time, you’ll hear this high-frequency white noise as soon as you step on board, says Hirstam, adding that its intensity can vary before, during and after the flight.

Most of this is the sound of the air conditioning, thanks to the auxiliary power unit (APU). The APU provides power to the aircraft when the engines are not turned on and is not connected to the ground network, explains Hirstam. The sound is typically loudest at the rear of the plane.

If the sound is weak before takeoff, the plane is likely connected to pre-conditioned air (PCA), which comes from the large yellow tube that connects to the bottom of the plane, Hirstam notes. At this point, there won’t be a strong stream of air coming out of the nozzle above your seat, but it will get stronger once the APU turns on.

Finally, if you don’t hear that distinctive air sound at all when boarding the plane or it’s approaching the gate after landing, Hirstam says, it means the APU is off and the PCU isn’t connected. This will make the plane unusually quiet (and likely stuffy) since there is no air circulating in the cabin.

Medium-pitched hydraulic sound.

Everyone is on board: the plane’s doors are closed, the engines are started, and the plane departs from the gate. Before taxiing, you hear a medium-pitched hydraulic noise for 10–20 seconds.

If you sit by a window overlooking the wings, you may notice that the slats (at the front of the wing) and flaps (at the rear) – the cause of this noise – are moving when you hear it, Hirstam. speaks. You’ll also likely hear this sound as the plane climbs and the flaps and slats retract, and then again as it approaches its destination.

Knocking sounds

The knocking noise you hear after landing and before the plane leaves the gate is most likely the sound of luggage being loaded into the cargo hold. If you’ve ever observed this process, you know that luggage is often thrown around (not necessarily gently), which can contribute to the knocking noise. When you hear a particularly loud bang followed by a thump, it means the cargo door is closing. Flights also usually end with an explosion : this time the landing gear doors open and the wheels open.