Flying exposes passengers to an array of sounds and sensations that can only be experienced onboard an airliner. Seasoned travelers will know the cause of many of these noises but might not understand the reasons behind them. Let’s talk about some of the more noticeable sounds you’ll hear on your next flight and why you will notice them when you do.

The Auxiliary Power Unit and Conditioned Air

You are likely familiar with the sound of air circulating through the cabin. This sound could be described as a plane’s “white noise” and is usually audible as soon as you pass through the boarding door. This sound might sometimes be weaker than expected. If this is the case, your plane is probably connected to “PCA.” Pre-conditioned air provides less powerful air conditioning when compared to air sourced from the APU. The PCA hose is the big yellow tube connected to the underside of the aircraft and is conditioned by units attached to the jet bridge or an air cart that sits on the ramp. If you turn your overheard air conditioning nozzle to little avail, the PCA unit might be in use. PCA is highly valuable on cold mornings. Ground crews heat the aircraft using the PCA before pilots arrive to ensure that passenger boarding is not delayed due to an unduly chilly cabin.

The PCA hoses connected to the underside of a Boeing 777.

Photo: Heffalum I Shutterstock

Sometimes you might not hear any “air sound” when you board or right after you pull up to the gate. This means that the pilots have not turned on the APU, or auxiliary power unit. The APU provides power to the aircraft when the engines aren’t on or when ground power is not being used. The APU is audible to passengers, particularly in the rear of the plane. You can hear it powering up because it is a small turbine engine and sounds accordingly. The APU provides the best source of conditioned air on most planes because of how powerful it is. If the engines are off, the PCU is not connected, and the APU is off, there will be no air circulating in the cabin and the plane will be quieter than most passengers expect. Pilots will normally have the APU running whenever the engines are off to ensure passenger comfort. If you notice the APU is not operating, it might be because it is temporarily inoperative.

Hydraulics Systems

Everyone has boarded your flight and the boarding door has closed. The flight attendants are making their mandatory announcements. In the background, you might hear a high-pitched squealing. If you are on an Airbus aircraft, you might describe what you hear as a back-and-forth scrubbing sound or even barking dogs in the cargo hold. While there may indeed be dogs traveling as cargo (in air-conditioned, pressurized spaces), the sound you hear is the hydraulic system(s) being powered. Most commercial aircraft have three hydraulic systems that operate around 3000 PSI. The sounds you hear are the hydraulic accumulators charging the reservoirs to the required pressure to operate brakes, steering, reverse thrust, and control surface movements. Pilots power-up hydraulics only when the jet bridge has been removed and ground personal are in position to ensure that inadvertent movement of control surfaces doesn’t cause damage or injury.

Flaps and Slats

Your plane has pushed back from the gate and the engines have started. Before you taxi, you notice a hydraulic, medium-pitched sound that lasts between 10-20 seconds. Passengers seated at the window would notice the slats (on the leading edge of the wing) and flaps (on the trailing edge) moving while this noise is audible. Most airlines have their pilots set the flap position for takeoff before taxi unless deicing is required. The sounds of flap and slat movements vary by aircraft and are usually hydraulically operated. These sounds are similar to what you will hear when the flaps and slats are retracted as the aircraft climbs through its acceleration altitude. Likewise, the sounds of flaps and slats will again be heard as the plane approaches the destination and begins configuring for landing. However, it’s easiest to hear the flaps and slats move on the ground since the engines are at an idle power setting and the plane is not in flight.

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American Airlines Boeing 777-200ER with flaps and slats extended for landing.

Taking The Runway

Your plane is now taxiing to the departure runway and you hear a double chime. The pilots have likely been given clearance to line up and wait on the departure runway or have been cleared for takeoff. The protocols for taxi and takeoff are different at every airline, so this might not be true in every case. However, most airlines have their pilots tacitly notify the flight attendants of imminent departure via a double chime. This practice might appear minor, but it is a safety-critical routine meant to ensure flight attendants and passengers are seated before departure. Most pilots will be able to tell you about a time when they were lining up on the runway and received a frantic call from the flight attendants notifying them that a passenger was up or in the lavatory. When this happens, pilots let the tower know they will need a momentary delay and might even have to taxi off the runway if the airport is busy.

10,000 Feet

You have been airborne for a few minutes and you hear another chime or a ding-dong noise, depending on the plane you’re on. This chime is the pilot’s indication to the flight attendants that they are climbing through 10,000 feet. The flight attendants can get up from their jump seats if the air is smooth, and in-flight announcements and promotions will commence. Ten thousand feet is also the point that “larger electronic devices may be used,” or whatever the airline-specific verbiage happens to be.

I thought 10,000 feet was an arbitrary number until I became a pilot. Unless otherwise permitted, pilots cannot exceed 250 knots indicated airspeed below 10,000. Once at or above 10,000 feet, pilots are allowed to continue climbing at an airspeed of their choosing. Crucially for flight attendants’ safety, pilots reduce the pitch attitude of the plane to accelerate and maintain a higher airspeed. The deck angle will decrease from roughly 10 degrees nose-up to 5-6 degrees. This pitch reduction makes walking up and down the aisles much easier and safer.

 A flight attendant serves food and drinks to passengers on board of Singapore Airlines Airbus A380.

Photo: Sorbis I Shutterstock

Another significant consideration in tandem with the 10,000-foot ding is the implication of a “sterile flight deck.” Pilots’ conversation below 10,000 feet must be pertinent to the safe operation of the flight, and extraneous chatter is not allowed. Operating below 10,000 feet generally induces a higher workload on pilots, thus the reason for the sterile rule. Pilots’ notification to the cabin crew that they have passed through 10,000 feet indicates that they can call the flight deck with questions unrelated to safety or emergencies.

The noises you hear while flying will change depending on the aircraft you’re on. Likewise, the cadence of the protocol-related chimes will differ based on airline-specific operating procedures. Hopefully, this discussion has given some insight into the noises you hear at different phases of your flight and explanations for them.


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