I am a very regular TransAtlantic commuter with various carriers.
I have always wondered why, during long haul flights at cruise altitude approximately 3 hours into the flight, the engines seem to suddenly make an increased noise (of a lower lot deeper tone) for what seems like a minute or so?
Is it the flight crew cleaning the motors in-flight?
Is it an automatic function and what purpose does it serve to the motors?
I would be interested to learn
I am not sure which planes you are referring to. However, I cannot relate your description to any event I know of during the cruise, especially after 3 hours into the flight.
For your information, during the cruise, the plane is on autopilot and the thrust is also on the automation mode.
There is no such thing as cleaning the motors in flight, especially on jet engines. They are indeed very reliable and any 'cleaning' if needed, are done during maintenance or overhaul after thousand of flying hours.
However, there is a possible event that I can think of to cause the change of noise during the cruise. The captain may have reduced the speed prior to entering turbulence and when the turbulence is over, he may have resumed normal speed, causing the auto thrust to apply a slight increase in power to achieve the previous speed.
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