This is the  combined effect low pressure & speed of plane 
in sky. Body of aircraft is made up of aluminium due to its 
good tensile strength & good conductor. 
 ( Amritpal Singh Sidhu)

Aluuminium is light weight too.

Up in the sky the temperature is very cold and human body 
cannot sustain such cold, so to maintain a temperature 
sustainable by human body Air conditioning is used inside. 

The plane body is not made of alluminium but aluminium 
alloy i.e duralumin , for high tensile strength, light in 
weight, high toughness to maintain its shape at high speeds 
in the sky , good conductor , easily formed to shape etc.

To maintain a constant temperature and aluminuim's strength 
to weight ratio is more