Engine howl

[goudie]

I live in the vicinity of Luton and invariably Easyjet et al are overhead. Sometimes I hear a change in the engine note which produces a brief howling noise. I'm aware of the 146 howl but never see them, so assume the noise is from a 737. Any ideas as to cause/origin?

[SNS3Guppy]

Hard to know exactly what it is you're hearing. Much of the whine comes from the front of the engine, whereas the whoosh comes from the back. You may be hearing the doppler effect as the engine passes overhead, or you may be hearing the transition from one sound effect to another just because of the passage of the airplane. You may be hearing other sounds, too, like pumps or actuators. Some airplanes increase engine speed as a function of flap setting; you may be hearing that. Or you could just be hearing a change in engine sound as the flaps are lowered and the engine noise and exhaust gasses are directed at a different angle.

[Rainboe]

I live on the south coast of England and often hear this effect from jets descending into Gatwick as they pass overhead at about 10,000'. In particular, it is a change of note over about 5 seconds plus, but it cannot be doppler, it sounds like a low note becoming a higher note, which is opposite to doppler. Been in aviation for 40 years and cannot understand it,

[treadigraph]

A320 family sometimes make an odd howl occasionally: sounds rather like blowing across the mouth of a milk bottle and otherwise pretty much as Rainboe describes. I hear it most often as the aircraft has come off the hold at Biggin, heads west towards Ockham then turns back for a 27 approach into Heathrow.

I was on an EasyJet A319 landing at Gatwick several years ago, sitting just aft the wings and thought I could hear it faintly as the flaps began to extend, so they might be the culprit.

PS, would you have flaps coming down that early, off the hold at Biggin?

The landing at Gatwick (from Edinburgh) was more or less straight in, we flew west of Guildford, the flaps were starting to extend before we passed Dunsfold for a landing on 08 as I recall.

[Groundloop]

I'm aware of the 146 howl but never see them

The 146 howl is not engine related, it's an airflow problem at a particular stage of flap during extension or retraction. BAe spent a long time trying to solve the problem but eventually gave up.

[BigJoeRice]

It could also be the engine compressor/surge bleed valves opening/closing as power is reduced or modulated as the aircraft is trimmed for landing. The 14th stage bleed on the PW2037 powered 757 was so loud it used to scare the passengers when it dumped overboard.

[goudie]

Thanks for replies guys. Power reduction/bleed valve operation seems the most likely cause.

[Localiser]

Speedbrake makes a noticeable howl too, possibly that. Especially if left high before dropping into LTN.

[davidash]

The "howl" is definitely caused by the speedbrake on the A319's. I fly on the 319 often and notice the noise change when the speedbrakes are extended on the descent.

[Double Hydco]

My money is on the A319 speed brakes being the source of the noise you can hear.

On approach to STN from the west, we are often kept a little high to cross over the top of LTN arrivals from the east and STN departures. If that's the case, then northeast of Luton out come the speed brakes to regain our approach profile.

The faster we're going, the louder they seem to be!

DH

[goudie]

Well reading these recent posts the A319 speedbrake is more likely the answer. The sound is, in fact, usually coming from a point N.E. of Luton and sometimes it's louder than others. So thankyou guys for another explanation.