The wind-chill factors one commonly sees on earth are for turbulent flow, usually over exposed human skin, which loses heat by evaporation and convection.
Laminar flow over smooth dry metal, which is what aircraft designers aim for, is much less efficient at transferring heat. At 29,500 feet, air density is about one-third of its value at sea level: it is as if the plane were flying in a vacuum flask.
At speeds above 300 miles per hour, there is significant frictional heating of the outer surface of any aircraft. Parts of Concorde got 390°F warmer in flight, and the skin of a returning spacecraft of course gets red hot.
The human metabolic power density in watts per cubic yard inside an aircraft packed with passengers is hundreds of times higher than in even the smallest of houses, and the surface-to-volume ratio of a smooth cylinder is much less than that of an irregularly shaped house.
The air inside a pressurized cabin is climate controlled and circulated.
Several megawatts of surplus heat is available from the engines to drive the air conditioning system, so there’s no problem maintaining a comfortable cabin air temperature in flight. The trick is to line the cabin with just enough plastic so you can’t touch any cold bits of metal, and to fill the cavity between the skins with fairly ordinary foam or fiber insulation, with properties very similar to the walls of a house.
So it’s pleasant in the cabin, but it’s right to assume that parts of the aircraft do chill. The tail cone and rear baggage compartment do indeed get very cold on a long flight.
It’s worth noting that a plane on the ground with the engines switched off is no warmer than an unheated trailer.
The temperature outside a plane at high altitude is very low but the aircraft’s skin can become very hot. These extreme temperatures require thermal insulation but, apart from a couple of areas such as those above and below the passenger compartment, the main insulation consideration is acoustic.
To keep the inside of the plane protected from engine and wind noise, the insulation is thicker than that required for thermal considerations.
Fiberglass batting, the material used, has the very small fiber diameter necessary to provide the best acoustic insulation. The insulation usually varies between 5 inches in the roof to 3 inches in the sides and 1 inch in the floor, with obvious variations between different types of plane.
The type of fiberglass used in planes is especially light, but similar products are widely available to the public and commonly used in buildings for thermal insulation.
