by Oliver Smith, The Telegraph, June 19, 2017
Nervous fliers have a tendency to scrutinise every little bump, whirr and thud they hear on board a plane. Was that a wheel falling off? Did the wing just crack? I could swear I just heard the pilot snoring in the cockpit…
But which sounds are routine and which should have you reaching for the in-flight safety card?
The issue of aircraft noises has been addressed on the excellent website Quora, and the responses – including two from aviation experts – might just help you relax next time you’re in the sky.
Firstly, remember that aeroplanes are generally noisy, creaky beasts, particularly in turbulent conditions.
“The logical assumption is that everything on an aeroplane – the bins, seating sections, loos and galleys – is bolted down,” explains Marc Levy, a mechanical engineer who says he has worked for Boeing. In reality, overhead bins “hang from a sets of tie rods”, lavatories and galleys “are mounted to tracks in the floor”, and so on, meaning “when you give the aeroplane a little jolt, all the stuff on the inside is going to adjust and sometimes bump into things”.
Mike Leary, another former Boeing employee, goes further, providing a checklist of all the normal noises you may or may not hear during your next flight.
That’s the auxiliary power unit at the back of the plane. “That baby hums along while you are boarding the plane to keep every thing up and running while the main engines are off,” says Leary. “In addition, most airports hook the plane up to the ground units to pump air and electricity into the plane to save money.” When the doors close, you may notice the noise changes, and the lights will flicker, as the pilot switches from the airport system to the aircraft system. Next you’ll hear the engine’s being started – first “the whine of the spin-up” and then “the roar”.
What’s that barking sound?
It’s probably not a dog in the hold (although there's a very small chance it might be – more on that here), but a fuel saving device called the PTU, present on all twin-engine Airbus models. “It is making sure the hydraulic pressure is balanced when they only use one engine during push-back and taxiing,” says Leary. “Everyone wants to save fuel so this is their option. It's noisy and a bit unnerving as it cycles on and off, on and off, on and off due to pressure fluctuations. It sounds like a high pitched bark to some: woof-woof-woof-woof...”
Airbus passengers may also hear a “prolonged whine” by the gate prior to departure and then after landing. “It's from an electric hydraulic pump used to open and close the cargo doors,” says Leary.
A whirring noise:
Just before take-off, the pilot flips a switch to make the wings much wider than usual. “Those are flaps which make a whirring sound,” explains Leary. You can see them drop down if you're sitting near the wing.
The bumpy wheels:
Time to go. You’ll hear the engines rev up as you start moving forward.
“You'll be able to tell if a wheel is a little out of round because the bump of the tar strips mingle with the speed of the spinning wheels as they speed up and go blump, blump, blump,” says Leary. When the “slapping” stops, you’re in the air.
More whirring and a “snap”:
“Now you will hear some of those small electric motors as the wheels are pulled up into the plane” then “the snap of the doors closing, first one then the other.”
After that you’ll hear another “whine” as the wing flaps return to their normal position.
Why is the engine changing tone?
The early stages of a flight can be very noisy, explains Patrick Smith, a pilot, in his wonderful book on air travel, Cockpit Confidential. “After breaking ground and raising the landing gear, the pilots follow a profile of target speeds and altitudes. It can be noisy, with multiple power changes, turns, and pitch adjustments. If it feels unusually hectic, chances are the crew is following a noise abatement procedure on behalf of residents below. These can require complicated profiles with low-altitude turns and steeper-than-normal climbs.”
They will gradually quieten down. You may also notice a change in tone as the plane begins its descent, slows down and lowers the nose.
What was that bang?!
As you’re coming into land, the wing flaps may be lowered again, and you’ll “hear the wind like mad now”. Leary adds: “If you are by a window, sometimes see the air is being squished so much, it leaves little vapour trails which follow the airstream over the wing.”
Next there will be a bang. Please don’t be alarmed. “That is the landing gear doors being opened again.” The wheels are then lowered into place until they lock with a click. “The pilot has to see three green lights on his panel to know all the wheels are where they belong and it is safe.”
A final roar – and the dog returns:
After landing, the pilot flips a switch and panels come out of the wings “to force the plane to stay down”. There will then be a loud roar. “Jet engines do reverse, and that’s exactly what you’re hearing,” explains Patrick Smith. It helps the plane slow down. The amount of power used “depends on the runway length, break setting, surface conditions and to an extent which taxiway turnoff the pilots intend to use”.
Expect a bit more whirring as you approach the gate, and then the “dog in the hold” as the PTU balances out again.
Dings and chimes:
Patrick Smith explains: “The chimes you hear are one of two kinds. The first is basically just a phone call. The flight attendant stations and cockpit share an intercom system through which any station is able to call one another.” When a call is made, the phone will ding.
Chimes are also used by pilots as a signalling device for the cabin crew. Airlines have their own rules for how many chimes mean what and when they’re given.
Qantas recently revealed on its blog: “On our Airbus aircraft you’ll hear the ‘dong’ sound shortly after take-off – this sound lets crew know that the landing gear is being retracted.
“Depending on where you are sitting, you can probably hear or feel it moving – if you’re downstairs in the pointy end of one of our Boeing 747’s – you’re basically sitting right on top of the front landing gear.”
A second dong comes when the seatbelt sign is switched off, it added. From that point on, a single chime means a passengers has called for service (look out for the light above their seat), a high-low chime means the cabin crew are calling one another (probably for something mundane like missing snacks), while a priority message from the pilot is signalled using three chimes.
“This could be letting them know there may be turbulence ahead, so they should start putting away the meal carts and be ready in case the fasten seat belt sign comes on,” it said.
So which noises should I be worried about?
Subtle noises are not the ones to worry about.
Complete silence – suggesting all/both the engines have failed – would probably be a concern. But even then, all is not lost – if the engines can’t be restarted, planes can actually glide for a remarkable distance. After Air Transat Flight 236 lost power on August 24, 2001, it managed an emergency landing in the Azores after gliding for 75 miles.
A loud bang and a shudder on the runway might indicate a blown tyre, a potentially serious problem.
A sound like an engine backfiring, while flying relatively close to the ground, could suggest a bird strike.
Lightning strikes – rare and not a danger to the plane – may or may not be noticed by passengers as a dull flash and a thud.
The first indication of a genuine problem might even be the sound of the pilot’s voice on the PA system. Patrick Smith writes: “Passengers will be told about any emergency or malfunction. If you’re informed about a landing gear issue, pressurization problem, engine trouble, or the need for a precautionary landing, do not construe this to be a life-or-death situation. It’s virtually always something minor - though you’ll be kept in the loop anyway. With even an outside chance of an evacuation in mind, you have to be kept in the loop.”
Quora contributors had some other droll suggestions, however.
“The sound of heads hitting the ceiling is bad,” said one. “The very loud sound of 300mph air coming into the cabin is bad, In first class, hearing, 'we are out of champagne' is bad. In any class, hearing 'brace, brace, brace,' is very bad.”