‘What was that noise?’
In previous blogs we have discussed flying in turbulence, high winds and snow, as well as the typical sequence of a pilot’s flight from take-off to landing. Now let’s look at the whistles, bumps, squeaks and clunks that you would hear during the course of a normal, routine flight. Once I’ve explained what they are I hope you will agree that they are normal, and nothing to be concerned about!
Pre-departure & taxi
We start as you board the aircraft. You will hear the airflow from the cabin fans, filtering and circulating the cabin air. Most airports that we visit – Spanish ones in particular – insist that we connect to their ground air conditioning units, a local noise abatement restriction. Whilst quiet for the airport they are not very effective in the cabin, where the temperature can rise uncomfortably at times. Eventually we will have to use our own air conditioning system – which involves starting the auxiliary power unit (APU) in the aircraft tail. If you board at the rear steps you will know all about the roar from the APU, but it does provide a flow of air which we use at this stage for aircraft air conditioning, by switching on one of the air conditioning units or ‘packs’ under the middle of the aircraft. As you board or disembark you will also be aware of the noise from the pack: depending on the exact type of pack some howl like banshees, whilst others emit a muffled roar. Airbus packs tend to be noisier than Boeing ones, although that may change with the arrival of our brand new Boeing 737 Max 8 fleet over the next few years.
Photo by BeckettAlex1 (Instagram)
As you take your seats you might hear noises from under the aircraft floor. This is the loading of the baggage. We no longer operate aircraft that use baggage containers, but on an aircraft that does you will no doubt hear and feel these large metal containers being manoeuvred and then locked into place.
A few minutes before departure there will be a high-pitched whine from a hydraulic pump under the wing, as it drives each hold door to the closed position. (Boeing 737s do not make this noise, as their holds are closed manually.)
It is now almost departure time. The cabin crew will be using their interphone system to pass their checks to the Cabin Manager, and there will be frequent ‘Bing bong’ sounds as they do so. You will hear that same sound several times right through until disembarking, as they pass information back and forth.
We have been cleared to push back, and have turned on our rotating beacons, both approximately in the middle of the aircraft, one above and one below. If the cabin is quiet you might even hear them! When we stop we set the park brake – and on some Airbus aircraft there is an audible clunk from under the floor just behind the wing when we do so.
We would normally start one engine during push-back, and the other during taxi, to save fuel. As we do so the conditioning air is diverted to the engine starter motor, so the cabin is briefly quieter.
Every engine type has its own sound during start, and indeed during take-off. The IAE engines on our A321s start very smoothly and quietly, although the automatic start sequence is quite long. The RR Trents on our A330s were similar. The CFM56 on our A320s start very quickly, and the dull rumble of idle power is soon reached. Air conditioning resumes in the cabin.
With one (or both) engines idling we now configure the aircraft for departure. You will hear a whine as the slats and flaps run out on the wing. Older hands like me also extend the landing lights under the centre of the aircraft at this point, so that light is immediately available if required, and the keen-eared will hear them motor out.
Now we are cleared to taxi. If on two engines, idle power is normally all that is needed, although at some airports like Luton the sloping ground means we might have to apply power. On one engine we will almost certainly need to apply power to move: we pick a suitable power setting and then leave it, until we are able to return to somewhere around idle power. Cabin air noise changes with significant power change, as well as the noise from the engine itself, (more noticeable in the rear left cabin,) and for passenger comfort we avoid changing the power settings continually. (This also has the bonus of reducing engine wear and fuel consumption, as old hands also know.) It all takes practice!
After starting the second engine during taxi out, with the attendant change in cabin air and engine noise, we treat you to an experience which (as far as I know) is uniquely Airbus: the ‘Barking Dog’. The aircraft’s three hydraulic systems are now all up and running, and the power transfer unit (PTU) integrates them. This results in a series of yelping sounds, usually three, coming from under the aircraft floor, as the three systems take their normal load. No two Airbus aircraft make exactly the same sound, and I have never heard it on a Boeing. This is quite normal – if quirky – for Airbus!
We have been cleared for take-off. You will hear us announce: ‘Cabin crew seats for take-off please’, and we then run our final take-off checks. We turn the air conditioning off at this stage, and quiet descends upon the cabin.
On take-off we apply an initial power of about 50% and wait for both engines to stabilise, (above 50% power will build very rapidly, so we want them both producing the same,) then set take-off power. In the forward cabin you will mainly hear engine fan noise; in the rear you will notice the roar from the exhausts. At the same time the pressurisation system automatically reconfigures for flight, which reduces the airflow sound in the cabin. There will be the occasional ‘thump-thump-thump’ as the nose wheel runs over the runway centreline lights; we try to avoid this, but it is not always possible!
Photo by cleared4to (Instagram)
We are airborne, and we select landing gear up. On some larger aircraft with more complicated landing gear, this has been unkindly described as sounding like all the toilets in the world being flushed at once, but on the A320/321 (and the Boeing 737) it’s a much simpler affair. The wheel doors temporarily open, which results in aerodynamic noise as well as the sound of the hydraulic system driving the heavy gear up into the retracted and locked position, and then they close: the aerodynamic noise reduces. With the wheels up the cabin emergency exit lights extinguish, accompanied by a single chime.
We maintain take-off power usually up to 1000ft, and then reduce to climb power. The first air conditioning pick is switched back on, so cabin air noise returns. Flap retraction starts, and you will hear (and feel) the hydraulic system driving the flaps in. The second pack is switched on, and the air noise increases a little further. There is a rumble as the landing lights retract. Shortly afterwards we normally announce ‘Cabin crew, you are released’, to allow them to leave their seats and start their cabin service.
If we have to level off at intermediate altitudes then engine power will reduce, increasing again as we climb further, and cabin air noise will change in the same way. But if Air Traffic Control are being kind to us we will have a continuous climb, in which case the engine and air conditioning noise will remain constant until we reach our cruising altitude. As aircraft weight reduces during the cruise we may well climb further, but apart from that the noise in the cabin should remain fairly constant. You will be looking forward to your cabin service – and I will be looking forward to a cup of tea!
Photo by Louise Copland
In an ideal world we would reduce power to idle at top of descent, and leave it there all the way down to about five miles from touchdown. Unfortunately it rarely happens like that nowadays, and we will usually have to level off at various points in the descent. Engine and cabin air noise will vary as we do so. Air Traffic Control will also require specific speeds and rates of descent on the way down, so in addition to changes in engine power we may need to use speed brakes on the wing. These can produce a rumble, and a slight buffeting, particularly in the rear of the aircraft. We avoid speed brakes if at all possible (I was always taught: ‘Every time you use the speed brakes a fairy dies, and we don’t want that do we children?’) but with today’s congested skies it is rarely possible (and fairies die). As we start our approach we are frequently required to avoid flying level (for noise reasons), so continually change our rate of descent – with the resultant change in engine and cabin air noise. As we extend the flaps there will be the same aerodynamic rumble, and the whine of the hydraulic system.
With the engines at lower power on the approach, you will be more aware of other noises. If there is turbulence you will hear the cabin fittings creaking and rattling, and you may see the overhead bins moving slightly. This is normal, and designed to happen!
Photo by Laura Thurgood
At about five miles from landing we select the gear down. Again the wheel doors open, with resultant aerodynamic and hydraulic noise, until the gear is locked down and the emergency exit lights illuminate with a single chime. (On the Boeing 737 the landing gear is not hydraulically powered down, but rather drops through a combination of gravity and clever aerodynamics on the gear doors. There are no noisy main wheel doors either.) The hydraulics whine as we select landing flap. Once down, you will feel a slight buffet.
If you are seated in the front of the cabin you will probably hear sounds from the flight deck. When the autopilot is switched off there is a distinctive triple sound which Airbus calls ‘cavalry charge’. A horn sounds as we approach our desired altitude. A synthetic voice calls out specific heights above the runway (the Boeing 737 does not have this last system).
On landing we do not select full reverse thrust if conditions (and runway length) permit, which of course makes for a quieter landing roll. If we do use full reverse thrust we cancel it as we slow towards taxi speed. For safety reasons, flaps and speed brakes normally remain extended until we vacate the runway, and then we ‘clean up’ – all are retracted, with accompanying hydraulic noises. We start the APU, and then normally shut down the right engine, both of which causing slight flickering of the lights as the power source switches.
Keeping the aircraft moving requires some anticipation of power setting, and again the aim is to set a particular power and leave it there.
As we halt on the stand we set the parking brake (that audible ‘clunk’ again possibly), and shut down the remaining engine(s). There will be some clicks and flickers as the APU generator takes over from the engine. Shortly afterwards the air conditioning pack will be switched off, ground power will be plugged in, the APU will be selected off and relative peace and quiet will descend. As you leave the aircraft by the forward steps you might hear a ticking sound from the freewheeling engine fans: the blades when not loaded are slightly loose in their mounts, and they chatter contentedly.
As you travel home you can reflect that your journey might have included a cavalry charge, howling banshees, a barking dog and the sad demise of fairies. Who would have expected such things on a normal, routine flight?
Featured image by Captain Paul Fox (@flying_foxoir on Instagram)