A question on jet engine noise?

[KeithE]

Something that has been intriguing me that im sure someone on here will know

I live just NW of Hastings under the flight path on the approach to Gatwick from Europe. As the planes go over there is a distinct change in engine noise. It sounds almost as if they are changing down a gear 🤭.

What is going on?

[Tony1963]

Might be that the throttle levers have been pulled right back, just gliding for most of the remaining flight.

[KeithE]

That’s feasible, they have certainly started their descent

[bearman68]

Could it be doppler effect if it is happening right over you?

[KeithE]

That’s feasible, they have certainly started their descent

[James 33]

KeithE said:

Something that has been intriguing me that im sure someone on here will know

I live just NW of Hastings under the flight path on the approach to Gatwick from Europe. As the planes go over there is a distinct change in engine noise. It sounds almost as if they are changing down a gear ??.

What is going on?

You must live close to me then as im in Hollington and have also heard this.

[KeithE]

bearman68 said:

Could it be doppler effect if it is happening right over you?

Doesn’t sound right for that so I don’t think so

[KeithE]

James 33 said:

You must live close to me then as im in Hollington and have also heard this.

I’m in Ninfield, west of Battle

[Eric Mc]

Aircraft adjust their throttle settings during descent and approach. That causes the engine note to change.

[mcdjl]

Tony1963 said:

Might be that the throttle levers have been pulled right back, just gliding for most of the remaining flight.

It might be the opposite of that: they often throttle up to over come the extra drag of landing gear / flaps being deployed and causing more drag.

[bobbo89]

I live on the edge of the pennines on the approach to Manchester and you really do get some strange noises from descending planes. When it's wet and cloudy they can sometimes sound like they're in a nose dive crash! It's also sometimes very hard to pinpoint where exactly they are as if the sounds are bouncing off hills and reverberating all over the place.

I get what you mean about the changing gear thing, if it were a car it'd it'd be the equivalent of dropping it a gear and just letting the rev's run down without any throttle.

[djc206]

KeithE said:

That’s feasible, they have certainly started their descent

They’ve not just started their descent they’re 2/3 to 3/4 of the way down, they’ll have started their descent before hitting the French coast if coming from the south.

It could be you’re hearing them slow down possibly including the use of speed brakes as the Speed Limit Point is about 1.5 miles off the coast of Bexhill and they (sometimes) enter the TIMBA hold which is just south of Heathfield.

[djc206]

Here’s a link to the arrival chart from the south with lat and longs for the waypoints

http://www.ead.eurocontrol.int/eadbasic/pamslight-...

[KeithE]

bobbo89 said:

I get what you mean about the changing gear thing, if it were a car it'd it'd be the equivalent of dropping it a gear and just letting the rev's run down without any throttle.

Exactly that

[KeithE]

mcdjl said:

Tony1963 said:

Might be that the throttle levers have been pulled right back, just gliding for most of the remaining flight.

It might be the opposite of that: they often throttle up to over come the extra drag of landing gear / flaps being deployed and causing more drag.

It won’t be an adjustment for the gear, way to far out for that, flaps is a possibility but I’ve never noticed a flap adjustment when I’ve been flying in. I’ll have a look next time I up there

Edited by KeithE on Tuesday 26th June 07:16

[Mabbs9]

I'm an Airbus driver and I'm 99% sure you're hearing the noise of the speedbrake being deployed. You get the opposite sound when they're retracted. Almost like a Hoover being switched on or turned off? (The mini Airbus that is. I've never heard it from the widebodies btw).

[KeithE]

Mabbs, that sounds exactly right, thank you. I’ve lived here for 13 years and often wondered but just got around to asking. I knew someone on here would know.

This also makes sense as not all do it, so presumably only those that need to slow for some reason.

Would I be able to see the speed brake being deployed from the cabin? Intuitively I think I should (and I think I know what it looks like) but I’d only be guessing!

[Mabbs9]

Yes you would see them. A set of spoilers on the upper surfaces of the wings. 5 on each. Not all used for speed brake. Some deploy in turns for roll control and all deploy on the landing rollout.

Oddly they're not very effective compared to other types of been on. But they do make the most noise!

[Mabbs9]

I'll add that the 320 family are very slippery. So good for fuel burn but not very easy to slow down. This makes speed brake use pretty normal on an approach, where as other types often have sufficient drag to slow down satisfactorily without their use.

Atb

[uncinqsix]

It's not a step-down approach is it? The change in noise could be when the aircraft levels off at one of the steps.

I would assume Gatwick has a constant descent rate approach though...