What's that noise...?!

A common question I have raised to me is this...

"When I'm on an aircraft, I often hear strange sounds. Some of these are very loud or don't sound natural - such as grinding or crunching. What are they? Are they normal?"

Firstly...yes, they ARE normal.

Every aeroplane is, by it's design, a machine with literally thousands of working parts. Some are obvious - such as a propellor or wing-flaps, many are subtle and never seen or heard.

Unfortunately for the nervous traveller though...many ARE heard.

So to put your mind at ease, I'll write now a list of common sounds you may hear in the 'plane, and why they are nothing to be scared of.

I'm writing this list based around an airliner, as large jet aircraft simply make the most "strange" sounds! But they may be heard on any plane.

Hissing:

This is merely the airconditioning. Jet aircraft work off a principle that we are not used to on the ground. The higher you fly, the "thinner" the air is, so to speak. Above 10,000 feet, the air is "too thin" to breathe properly, so the cabin of the plane is pressurised - or, simply, pumped with air, so as there is plenty to breathe. The way this is done is quite simple in principle. The engines pressurise air as they "suck" it in. So there is a small valve on each engine that "bleeds" off some of that pressurised air, cools or warms it to a comfortable temperature, then pipes it into the cabin as air conditioning.

It sounds quite different as it is being fed in this way - not through a "normal" car or house air-con style system.

Something else to be aware of is the engine speed. The amount of air being pushed through the air-conditioners is dependent on how fast the engine is working. This affects different planes differently - some more than others. But you might notice the quiet hiss gets more forceful, say, on takeoff.

The only reason for this is now the engine is working faster. So it is compressing more air - and this air is now being fed into the cabin faster. Just like increasing the speed on your car air-conditioner from 1 - 2.

Squeal/Rattle (After Boarding):

A common first fear for people in terms of noise is early in the flight. One of the first things heard after you've taken your seat and made yourself comfortable may be a loud squealing noise, followed by a rattle or, for lack of a better word, a "sealing noise", like someone doing up a nut on a car tyre.

A lot of people worry about this noise. It doesn't sound very natural and in fact, can sound like someone is "forcing" something. But it is in fact a sound that is made before every departure of the plane.

What is happening is down below, in the baggage hold.

When you check your bags in at the airport, they are taken to be loaded in a "baggage hold" - which is a compartment below your cabin, purely for baggage. There are special doors below the main cabin to get to the baggage hold - and the hold itself is also pressurised, so if you have anything in your bag affected by air pressure, it won't get damaged - and also so people can transport pets or live animals in the hold.

To hold pressure, something needs to be sealed, right? Well, same for the baggage hold. The doors are lowered into place after loading the bags - and at this point, they have a mechanical system in them that not only "locks" them shut but also provides a firm seal between the baggage door and outside. As this system locks and tightens into place, it creates a squeal and rattle noise. Sometimes the noise might happen, stop, happen again. This is just the door self-tightening to the right level.

Bell:

Those used to flying will ignore the regular sound of a single bell "ding" through the cabin - or may even find it annoying. But if you've never flown before, it can be unnerving. What does it mean? Should I pay attention? Is there a problem?

The bell can mean one of several things. It IS for getting attention - but not ALWAYS yours! So don't be afraid that you have to listen for it the whole time.

The bell is set off by...

- People pressing a button to get a flight attendant to come to them.
- The flight attendant or pilot are about to make an announcement.
- The "seatbelt" sign is about to be turned on or off (more on that in a minute)
- The pilot needs to talk to the flight attendant. (Don't worry...it's most likely to say what he wants for lunch).

The seatbelt sign is the most important one for you - all it is is a small light above each seat. When the pilot wants you to make sure your seatbelt is done up tightly, he will flick a switch on the flight deck. This small light will then light up with a picture of seat belt buckles and an arrow between them - meaning to do it up. When the light is turned off, you are free to stand up and walk to the toilets or stretch. When turned on, you must stay seated. The bell sounds for this to get your attention and make sure you know - if you don't hear it, the flight attendants will make a call through the cabin PA system too, to remind you and tell you why.

Roaring:

There are two stages when "roaring" may be at a point where it is very hard to hear over - or even making you nervous to if it is too much. These are during takeoff and just after landing. I'll explain why.

Simply, this noise is from the engines doing one of two things. The first and simplest is that during takeoff is the one stage of flight where the pilots will run the jet engines (each plane will have 2, 3 or 4) at their absolute highest speed.

To fly - a plane must get air flowing across it's wings. To do that, we have to move forward into the air. The faster, the better. So to get moving into the air quickly enough is the job of the engines - and is why they are run hard on takeoff.

The reason it is so much noisier than a car is this...

Think of your car at home. It might weigh one tonne and seat 4 - 5 people, right? Well, the engine is a fair size just for that.

Now take a jet. It can seat 100 - 550 people, plus baggage, plus fuel, plus the weight of the plane itself. That could be the same number of people as 110 cars alone. So naturally we need more engine power.

Jet engines do this well - they produce a lot of power for a reasonably small size...the only downside is they are as noisy as hell at full power.

Don't stress. The noise inside the cabin is cut down a lot. It won't make you deaf - although it may be irritating.

After takeoff, the plane is comfortably in the air and going fast enough. It doesn't need to keep accelerating - so the pilots slow the engines down to a quieter speed to cruise.

On landing approach, the engines are very quiet as we slow down. But once you touch down, you may often hear a loud roaring again, like on takeoff.

This noise now is what they call "reverse thrust". Put simply, the engine is running - but instead of pushing air out the rear and pushing the plane forward, this air is now pushed forward to act like a brake. The engine is sped up again to push more air forwards - and you hear the noise.

Why do we use reverse thrust? Simple again - it saves wear and tear on the brakes and tyres, saving the airline money - and ultimately, helping keep ticket prices cheaper.

Thud:

You'll hear this twice during the flight.

It sounds quite strange, as it is from BELOW the plane. People often tell me it sounds like something is broken - or has been hit. It's not!

In fact, to hear the thud means all is working well.

I can tell you this. You will hear it shortly after takeoff and then again a short while before landing. Sometimes it might sound like a whirring noise first too...or maybe two smaller thuds.

All it is, is simply the landing gear, or wheels, being folded up and stored away in the belly of the plane. On landing, the doors around them open again (the whirring noise) and the wheels are lowered back out (the thud).

Why fold them away? Put it this way. Next time you are in the car, hold your hand out the window. See how it drags the air and resists against it? Well, the same thing happens in the plane. Everything outside is dragging a bit against the wind - and the engines have to work to overcome that. More engine work means more fuel is burnt - so it's a wise idea to reduce this as much as possible. It's why the front of planes are smoothed out - not just a flat box. It's why we fold up the wheels - so they are out of the wind while flying and get put back down on landing.

You'd be amazed how much extra fuel a jet actually uses by keeping the wheels down! Most international flights would need up to three or four more fuelling stops (if not more) if they flew with them down.

Sudden quiet:

If you have been on a long flight, you'll have become accustomed to the drone of the engines as the flight continues. So why then, all of a sudden, do they go suddenly quiet...?

Don't be afraid! It doesn't mean something has gone wrong.

In fact, it's most likely meaning the flight is nearly over. Pilots calculate what is called a "top of descent" point. The point at which they need to start descending the plane back down to be at ground level by the time they reach the airport. Just as I said it takes a lot of engine power to get moving and gain height - by reverse, they simply slow the engines to an idle speed (the engine is still moving but slowly...just like idling your car) and gently glide the plane down the descent. The plane will pick up speed as it descends - just like a car on a hill - so there is no need for the pilots to run the engines and waste extra fuel.

As you get closer to ground and they have to level off, you might notice the engine speed pick up again. Or they may get a quiet day at the airport and be able to cruise straight in - in which case it may well stay quiet.

Hope this sums up a lot of the common unnerving noises heard on planes!

Any more you want to ask about? Leave a comment!