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What Are Those Noises Inside the Aircraft?

By: Norman Thomson - Updated: 19 Aug 2012 | comments*Discuss
Aircraft Noise Noises Passengers Runway

There are many noises that can be heard inside an aircraft and, although these noises are quite normal, they can be distracting to passengers. However, once people know why the noises occur, they usually forget all about them.

On the Ground

After boarding and while getting comfortable in their seats, the first noise that most passengers are aware of is a low background humming sound. This is the auxiliary power units that keep the aircraft supplied with electrical power during boarding of passengers and stowage of luggage. These units provide external power to the aircraft, as it would be quite costly for the power to be generated via the aircraft engines at this stage.

After boarding, the aircraft is secured for take-off. The main engines are started and spooled up, which often produces a sound inside the aircraft. Engines have to be checked before take-off and sometimes the pilots will increase the speed of the engines while sitting on the ground, so that checks can be made. After the engines have been started and running properly, the aircraft pushes back from its stand, often followed by a short wait until the pilots get clearance to taxi to the runway.

Pressurised Air

During taxi to the runway, air compressors start to alter the cabin pressure, resulting in air flow that can be heard by passengers. Humans need oxygen to provide energy for muscles, including the brain. At altitudes above 10000ft the air pressure is very low. At such altitudes, it becomes difficult for the lungs to get fully saturated with oxygen, because the pressure isn’t high enough to force the air into the small sacks, or alveoli, in the lungs. So aircraft have to be pressurised to levels that are safe and healthy for passengers and crew.

As the aircraft increases in altitude, the air pressure inside the cabin is constantly adjusted. However, it should be remembered that the air pressure inside the cabin at 40000ft is not actually the same as one would encounter at ground level; the cabin is not pressurised to normal atmospheric pressure. In fact, the actual air pressure when flying at such altitudes is likely to be around the equivalent of standing on a mountain that is 6000ft above sea level. So even with pressurised cabins, the air is still lower pressure than one would normally experience at sea-level, which explains the reason why alcohol consumption should be kept to a minimum when flying. Low pressure can cause dehydration and so too can overindulging in alcohol!

On the way back down, the air pressure inside the cabin is adjusted again so that it returns to normal on landing. This explains the ‘popping’ of passengers ears when the aircraft starts to descend. Equalizing the cabin pressure often causes ear pain, due to the ear drums being forced inwards. Holding your nose and blowing is a good way to help push the ear drums outwards, thus allowing them to function properly.

How Noisy are the Engines?

During normal flight, the most noticeable noise inside the cabin comes from the constant drum of the engines. Modern aircraft have excellent noise reducing components, resulting in cabin noise levels that are well below those that can cause harm to health.

Flaps are Noisy

During the last stage of landing, passengers often become aware of mechanical sound, sometimes likened to the sound made by a mechanical digger when it scoops up a pile of dirt from an excavation. This noise comes from the flaps being lowered.

Flaps are short panels at the back of the wings that are essential for controlling the speed of the aircraft. During preparations for landing, pilots will reduce the air speed of the aircraft to a speed that is safe for touch-down. Flaps are used to help change the aerodynamic shape of the wings, enabling them to provide enough lift to keep the aircraft in the air but to allow slower speeds for landing. As the aircraft gets nearer to the airfield, more flaps are set, and thus more noise is usually heard inside the cabin.

Gear Down

Of course, landing gear is required for landing! It would be uneconomical for an aircraft to fly the entire journey with their wheels in the down position – this would increase drag resulting in more fuel being burned. So, wheels are stored inside the aircraft and are lowered for landing. The sound produced when the wheels are lowered can be quite loud, especially if you are sitting near to the centre of the cabin. Many passengers hear a ‘clunk’ as the wheels lock into position, but this is quite normal.

Do Engines Get Faster During Landing

Passengers often say that engines speed and get louder during the final approach to landing. Well actually there is some truth to this. Drag on the aircraft increases after the wheels have been lowered and pilots sometimes have to increase the engine speed to counter the increased drag. Weather plays a part too. A head wind during final approach may be experienced and, if this is the case, engines are increased a little to counter the effects of the wind.

Reverse Thrust

On landing, jet engines are usually put into reverse thrust, to help assist with braking. Again, this is quite normal. Relying only on the wheel brakes to slow the aircraft would result in excessive wear on the brake discs, so braking is assisted by engines going into reverse. That sudden roar of the engines can frighten people if they are not expecting it. However, the sound only lasts for a few seconds before the engines are returned to a much lower speed for final taxi to the terminal.

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