But in descent, thrust is at idle (normally) - especially if the speedbrakes are out! (you don't put power against drag) - so I dont think it is engine noise.

I fly the A319 and have heard this from inside the aircraft - it is usually upon speedbrakes retraction - so I think it's purely an aerodynamic phenomenon. If the brake are out at hi-speed (>320 knots) the speedbrake retraction takes 30 seconds so it may prolong (or reduce?) the effect.

I'm an Easy pilot based in BRS, and can guarantee its the speedbrakes. Not engines or other random things people suggested. A4 was correct, landing on R27 we are held high due London TMA, so usually before we get to POMAX, we may have to use spoilers to help us get down and slow down. Sorry! :8

at around 2000 - 3000 feet is when you're extending the slats and thats when the engine fan blades accelerate about 7 percent to approach idle.
the engine could also be revving up to maintain slats speed usually close to 180 knots. i avoid this by selecting a lower speed. i wish i could be my own passenger some day and feel the difference.

the sound others are describing when the slats extend is the slat hydraulic servo which you can hear from inside, but impossible from the ground

Just heard the sound again today: an EZY 319 passing at around 3000-6000ft over my place.

Speedbrake-retraction would give an explanation to the WHOOoop, but not to the WHOOoooooOOOOP - unless it's common to retract and re-deploy the brakes in short order.

okay, couple of things here...the sound is from the flaps. You will hear it from different ac, depending on the flap location/size on the wing. The A320 has more of the outboard flaps, which run to almost the full length to the wing tips with a smaller inboard flap, while the B738, has large inboard flaps, and the flaps do not run as far out on the wing.

This has significant effect on the noise generated. (I have not observed any sound from speed brakes, unless its a FOQUA bust!)

In regards to wake vortex, the same principles apply. (while the vortex is generated near the wingtips, the wing tips do not generate the vortex)

AC that have outboard flaps, tend to generate a large core vortex. If you are standing under the flightpath, (best is about 1000m from threshold) this will sound like a deep, low roar...This is the larger volume, slower moving mass of air. For comparison, a 737-400 will make only a large core.

AC that have inboard flaps tend to generate a large core and small a core vortex. The small core is very high velocity, but dont last very long. These sound like tearing a piece of paper on breakup. In comparison, a 738 will make a large and small core vortex.

As misd-again pointed out, you can hear the differences between ac.

When you sit and study this, and observe 10 737-800's in a row, you can tell the difference in flap settings.

A380's are a kick to listen to...

Whilst I'm sure the flaps will generate some noise I still think its speedbrake. I live directly under the approach at appoint where the aircraft are through approximately 4500' at, typically, 220 knots and "clean". It's usual at this point to be told to reduce to 180 knots and continue the descent to 3000'. Jets don't "go down AND slow down" ...... so speedbrake is usually deployed and "wooooooooooo". The flap isn't taken until the aircraft are past my house.

Next time I hear one I'll get my binoculars out :) and see what speed/altitude it's at with "Planefinder".

I can see the flaps for the turbulence creating the noise, but speed brakes get thrown up all the time without the whoop...

I am usually set much closer to the threshold for the vortex measurements.

Good luck...

Engine thrust changes - I hear that over my house frequently which is under a STAR fix with an altitude contraint so aircraft are frequently spooling up as they level off at 5k. Airbus engines more noticeable than most as they spool up in flight at lower altitudes - like someone posted - a Hoover sound alike.

Don't really agree with that.
From my experience you don't often see people needing speed brakes to reduce at this stage unless you are hot & high or with a stonking tail wind.
From 220kts you can already deploy Flap 1 and from 200kts you can deploy Flap 2.. which gives you the drag to slow down to 180 easily.

It may be different with different Airlines or ATC units though.

You said it. Tailwind. For this approach, 9 times out of 10 the decel+descent is also the turn to base leg which also usually has a tailwind. If you don't speedbrake it, you catch the fella in front fairly quickly. The decel with F1 (slats only) is not sufficient to meet ATC's requirement - they do expect us to do as asked promptly not in our own time. Of course if you can see you're going to get a extra couple of miles or there's no critical preceding traffic then you can give it your best shot "sans speedbrakes" - there's always the gear :}

I'm still nailing my colours to the speedbrake mast!

zahnpasta hin, zahnpasta her..

It's not a matter of aircraft type im my opinion.

But, why not do a contest?

All kind of airline jets do a flyby with, let's say 320kts (737 can you do that?), airbreaks on/off:O

Must be fun and this very important thread can be closed thereafter.

I dont think it is the speed brakes..on flight validations, when the Airbus goes to config 1, that whoop takes about 30 seconds..

so that appears to be a slat/flap issue!

beacuse the engines spool up when you select Conf1

Why? When you configure your normally slowing down. Let's say you're at 220 knots selected speed, descending in OP DES (idle thrust) and ATC request 180 knots. "Manage" the speed to start reducing to GN DOT (clean spd approx 205 knots) and take F1 (slats only which takes about 8-10 seconds), target speed now "S" speed (approx 185 knots). You're still descending in OP DES and thrust will only start to spool as you approach level.

Your speed is controlled by pitch (not thrust which will be at idle) in OP DES. In "managed" DES the profile is FIXED and speed is controlled by thrust so if above or on profile even DES should result in idle thrust.

Of course if you held 220 knots and select F1 then yes the engines would spool - but why would you do that? You're then putting power against drag.

I still think it's speedbrakes ;) but I remember being a pax in a BAe 146 many moons ago and the aerodynamic noise on flap retraction after departure was UNBELIEVABLY LOUD! So may be there's a bit of slat stuff as well. (I know the 146 didn't have slats - or did it?)

Could be the Delta P reaching 2.5 in the descent and the outflow valves and doors opening.

Next time in the flight deck after take off, have a listen to the hiss around you; it stops around 4-5000 feet. Have a glance at the Press page - Delta P should be very close to 2.4-2.5. The valves/doors close and the hiss stops.

The 320 does the same thing in reverse on the way down but at a higher altitude.

Might not be loud enough to hear it from the ground though :confused:

I still miss the 146 flap sound every day, after flying it for 4 years. And yes, the 146 doen not have slats. The massive noise you are refering to is when selecting flaps 0 to 18. It certainly raised a few eyebrowse amongs first time 146 pax.

The engines will spool up at some point on the approach, irrespective of THR IDLE annunciated. As MD83FO stated its to do with approach idle - from what I recall its either done from flap lever position not at 0 or upon flaps extension.

I'm still convinced it's engine related for the following reason -

When the noise appears (very abruptly, as mentioned!), after 3-5 seconds it translates into normal engine noise until the aircraft is out of hearing range and fades away naturally.

It 'sounds' like the usual noise of an approaching aircraft (at circa 5,000 - 10,000 feet) has been massively delayed (just silence) due to conditions, wind or otherwise, then it hits you all at once for a few seconds before returning to normal.

A4, the engines on the 319/320 always spool up on selection of flaps. But I do not think that's the cause of the howling noise.

It's either speedbrakes or the initial deployment of the slats. I think it is the slats. When you sit in row 2 or 3 there is a very distinct swoooooooooosh audible when the crew select F1 and I can easily imagine that this is very audible on the ground above and before the engine noise when and idle descent is flown.

But it could also be the speedbrakes since this howling sound on the ground sometimes changes pitch abruptly, which might coincide with selecting half speedbrakes.

Yep - fair call. I forgot about Approach idle :O - but it's only about 5% on N1 I think not enough to generate the noise.