Air compression on the outer surfaces caused the cabin to heat up during flight. Every surface, such as windows and panels, was warm to the touch by the end of the flight. Besides engines, the hottest part of the structure of any supersonic aircraft is the nose, due to aerodynamic heating. The engineers used Hiduminium R.R. 58, an aluminium alloy, throughout the aircraft because of its familiarity, cost and ease of construction. The highest temperature that aluminium could sustain over the life of the aircraft was , which limited the top speed to Mach 2.02. Concorde went through two cycles of heating and cooling during a flight, first cooling down as it gained altitude, then heating up after going supersonic. The reverse happened when descending and slowing down. This had to be factored into the metallurgical and fatigue modelling. A test rig was built that repeatedly heated up a full-size section of the wing, and then cooled it, and periodically samples of metal were taken for testing. The Concorde airframe was designed for a life of 45,000 flying hours.