Hi Landroger...
I don't go along with 'dany4skin' at all...the use of speedbrakes anywhere near that sector of the flight would be very unusual indeed!
My best guess would be that you are near the spot where, as the airplane is slowing on the localizer (electronic signal that is an extension of the runway centerline), probably with 5 flap set. .................................
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Cheers...FD...
Thanks for the detailed answer FD, but I think I'm further out than that. The centerlines for both 27L and 27R extend back to just over the Thames at Battersea and more or less Buckingham Palace. You want acquisition by 10 miles out and 3000' as I understand it? I live about 9 miles
south of that point and aircraft that peeled off the bottom of either Biggin or Epsom stacks (at 5000' is it?) are descending and slowing, but have not yet acquired the ILS.
Tartare
You may be right as I
think aircraft have
some level of flap selected as they pass and a few even seem to extend their undercarriage, but this is less common. My nephew thought it may have been some change in bleed pressures within one or more of the engines, but he admitted he was guessing. I don't think the noise is audible from inside the aeroplane and, unless you live close to the pattern of a large airport - as I do - you may never have heard it.
Its like the slightly desperate sounding
sigh some cattle make - not 'aerodynamic' at all, but its quite load and distinctive.
Roger.