[thedoc (OP)]

Jan Potgieter asked the Naked Scientists:
   Hello Dr Smith,

I phoned in to 702 Talk Radio a few weeks ago with a question regarding jet airliners. I don't think I explained myself very well and in return you didn't answer the question to my satisfaction.

I live on the initial approach path towards OR Tambo airport in Johannesburg. An aircraft would be approaching straight towards me before making its penultimate left turn towards the runway. The aircraft is still fairly high, perhaps 10 000 ft or more and perhaps 10 to 20 km away from me. I assume, therefore, that it is moving with the wind at this stage as it is in the opposite direction of what it would be during landing.

The aircraft, at this point, have made no sound whatsoever so I have not yet noticed it. Very suddenly a sound appears, as audible as an approaching airliner would normally be. The sound starts off very high in pitch, dropping gradually to a much lower pitch within about one second. It then continues to be audible, in this lower pitch, for the rest of its approach until it has passed me. This final, lower pitched sound is the same as an approaching aircraft would normally sound like and there is nothing at all strange about it.

I would love to record this phenomenon but it happens, well, seemingly randomly and not at all that often. I notice it perhaps once in two months. I have searched the net for answers and although I have come close (something referred to as airplane froops), it is not quite what I was looking for.

Please help me if you have the time. The curiosity is killing me.

All of the best to you.

Jan Potgieter

What do you think?

[alancalverd]

The instrument approaches to Tambo begin at 8000 ft or more. From cruise altitude (say 30,000 ft) down to this level the planes are usually flying "clean", in a cruise-descent attitude with low power (engines idling) and no flaps extended - in effect a long glide at 250 - 300 knots.  In this condition they make very little noise as they are flying at maximum aerodynamic efficiency.

Entering the approach at about 8 miles you need to slow down to not more than 160 knots. This is done by increasing the angle of attack, and deploying spoilers and flaps, in the order and at speeds determined by the manufacturer's handbook. Each of these manouvers dissipates the kinetic energy of the plane as turbulence. The first movement of the flaps creates a slot between the flap and the wing which produces a high frequency whistle: the frequency reduces as the gap increases and the speed decreases. At some point in the approach trajectory you will increase engine power to maintain a steady approach speed and angle, and from then on until touchdown the noises are fairly constant.   

If you search for "instrument approach plates Tambo" you can download the plans and profiles, and work out exactly where you'd expect to hear the noises for any given wind direction.