First of all a note: this post suggestion came courtesy of my brother who wanted to know what some specific airplane noises mean. I realized this might be a great topic for others to ask questions about. So, if there is a particular noise that you hear on airplanes, that you’re curious about, or that worries you, please post in the comments and I’ll address it in a future post. Thanks!
Today I will address “…repeated eeeh-eeeh-eeeh-eeeh electric motor (I think) noise. It sort of reminded me of the sound that electric windshield washers make … Also, you sometimes hear creaks when there is high turbulence. Is that anything to worry about?”
First, the electric motor sound. I assume this is referring to the sounds of the flap and slap motors as they move the flaps and slats to change the shape of the wings. Flaps and slats are panels on the front and back edges of the wings (or, leading and trailing edges, as we say in aviation). Slats are on the front, or leading edge, and slide forward (and often down) and flaps are on the back, or trailing edge and slide back (and, also, often down). They’re used to increase the surface area of the wing and to change the shape in order to give the wings more lift. They’re deployed when the plane is traveling at lower speeds because at lower speeds there’s less air going over and under the wings, so more lift is needed.
Once the plane has reached a speed where it doesn’t need the enhanced lift the flaps and slats retract – the motors that actuate the retraction make a whining sound.
To address the second question: in a word, no. Creaking sounds are perfectly normal on an airplane. An airplane is not a perfectly rigid body, and the wings, especially are made to flex quite a bit. From an engineering perspective, a completely rigid plane would be really heavy and the wings wouldn’t be able to withstand their load if they weren’t able to flex. Think of a tree in the wind and how it bends but remains intact rather than snapping. Now, don’t think of wings snapping if you’re a nervous flyer! They won’t snap.
There are also often creaking sounds in the cabin, especially at takeoff as the plane is gaining speed down the runway, and right after it lifts off. This is just the sound of lightweight materials flexing – the floors, the cabin walls, the places where the lavatories attach to the floors and walls. These are all non-structural parts of the plane, which are made to be as light as possible, and which flex a bit. The loads are carried in the skin of the plane and by the longerons and formers.
About that wing flex – if you’re still nervous, take a look at this video from Boeing. It shows the wing flex test. Look at how far the wing flexes before it breaks. It will never need to flex that far in flight. Planes are way overbuilt.