posted on Dec, 29 2011 @ 08:06 PM
Yes, I know the exact noise you are describing and some of you may note that particular types like the 777 seem to do it whilst others dont appear to.
It is not as one poster here suggested, the engine Thrust Reverser's(T/R's) engaging. That is forbidden, highly dangerous and in fact virtually
impossible due to multiple safeties designed to stop it from ever happening. If it did the end result would be something akin to the Lauda Air 767
accident that killed everyone onboard. These days it should be impossible for the T/R to cycle until you have weight on wheels and your throttle
setting and air speed have reached there thresholds. The CF-6 for example (as used on the 767) has mechanical and electro-mechanical locks and brakes
designed to compliment each other. In fact after the Lauda Air tragedy all CF-6 engines were mandated to be fitted with the second system to prevent
such an accident from happening in the future.
Although I am in the business I must confess that I dont know the exact cause of the noise but I can make an educated guess. It is almost certainly
the auto throttle system cycling to maintain a selected airspeed and altitude. The noise may be caused by the engine's Engine Electronic Control unit
(EEC) changing the scheduling of say the Variable Stator Vanes (VSV's) in the compressor and therefore how it sounds. It is definitely not the
doppler effect however.
As I said I dont know the exact cause but that is my educated guess. I have asked one or two other colleagues for an explanation but they were not
sure either. It is something I do note that we dont hear around the immediate vicinity of the airport, or during takeoff, or engine runs we conduct on
the ground whether idle, low or high power runs, so that is why I believe it may be the auto throttle maintaining a selected airspeed in flight. I
have as one other poster eluded noticed it some 5-10 mins before they would touchdown at a height of 2-5,000ft. Anyone who can answer that here I
would also appreciate it as well.