Ice forms on the wings and instrument probes and vanes, but why not on the fuselage? If the temperature of the air is the same, why does the ice form on the wing but not on the fuselage?
It does, but there is much less of it on the fuselage than on the wing.
There are two ways how ice is accumulated on the aircraft's structure:
- When flying through air containing supercooled water droplets, the water will freeze on impact. It does not make much of a difference whether the water impacts the fuselage nose or the leading edge of the wings and tail surfaces: It will form a sheet of clear ice.
- When flying through moist air at a temperature close to freezing, the pressure loss of the air flowing around any obstacle will cool it down, so the moisture condenses and, if temperature allows, will form ice accumulations. Those in turn exacerbate the suction peaks, leading to more ice being collected.
The wing's leading edge shows the most pronounced suction peaks, so they will collect most ice. However, if the icing conditions are right, the fuselage nose will also pick up it's share of ice. Along the length of the fuselage, pressure is similar to ambient, so it will collect ice only when sitting on the ground in freezing rain.
Iced engine nacelle in the NASA Lewis (now Glenn) Research Center icing research tunnel (picture source). Here, water is sprayed into the tunnel and forms supercooled droplets.
In both cases icing has nothing to do with the boundary layer - with supercooled water it is simple ballistics, while the second case of ice accumulation is dominated by the pressure of the flow around the aircraft.
In order to answer you properly I need to first introduce firstly the concept of boundary layer. When an airplane is flying over the air in normal conditions (I mean, usual high Reynolds number), the air flows cretes an small layer around the airplane that is call boundary layer (image source Portland State University)
The previous figure gives you an sketch of how is the boundary layer around an airfoil that is similar to the figure you will see around the airplane. Notice the following behaviours of the air:
- The boundary layer grows downwash.
- The boundary layer is really thin at the beginning.
- The air flow (the inviscid flow...) follows the shape of the limit of the boundary layer, not the shape of the airfoil.
Now, let's say the air contains some water drops. Depending on the size of the water drops they can behave following the air or like bullets following the an straight path. In the reallity the drops have an intermediate behavior, having some inertia and taking some time to adapt to the flow.
Based on that, some drops following the streamlines of slighly over or down the airplane will almost follow the streamlines flying over the boundary layer and not pouring over the surface.
However, some drops will impact directly over the surface of the airplane in what is called "stagnation point", once over that surface they will behave like an humid surface moving in the air... the drops will expand along the surface and eventually leave the surface, in that process due to the air temperature and the surface heat exchange ratios some ice will be formed.
With the previous explanation we can understand why ice is not formed in the rear part of the fuselage, simply because the boundary layer is "protecting" it and the water accumulated in the front is simply not reaching the rear part.
In order to find locations where you can find ice in an airplane, just look for stagnations points (where the flow "impacts" in a simplified way) and the surrondings. Where do we have this areas?
- Leading edges of wings (image source AirDorrin)
- Engine inlets ( image source NASA)
- Instruments (image source AviationWeather)
- Rarely front windows (image source flyjoe180 at Wings over new Zeland)
- Front fuselage (did not find any picture).
Of course, having the airplane on ground removes the "protection" of the boundary layer, leaving the airplane exposed to any ice creation.