Supersonic cruise generated fierce aerodynamic heating, raising the skin to temperatures that would alarm passengers on any other jet. At the nose, engineers recorded figures as high as 261°F (127°C). The wing leading edges often reached about 212°F (100–105°C), while most of the fuselage settled between 194 and 203°F (90–95°C). //

At those temperatures, the entire 202-foot Concorde stretched by 7 to 12 inches. That expansion was most visible to the crew at the seam beside the engineer’s station, where the caps went in. //

The ritual of sealing a cap in the fuselage became most famous during the retirement era. On British Airways Concorde G-BOAG’s delivery flight to Seattle in 2003, flight engineer Trevor Norcott slipped his BA cap into the expansion gap while supersonic over Canada. As he later explained, “The Hat was meant as a permanent link between the aircraft and the crews.” Hours later, as the jet cooled on the ramp in Seattle, the seam clamped down, locking the cap inside. To this day, visitors walking past G-BOAG at the Museum of Flight unknowingly pass a hidden time capsule wedged between metal panels.