The most obvious manifestation of this was a gap that opened up on the flight deck between the flight engineer's console and the bulkhead. On some aircraft that conducted a retiring supersonic flight, the flight engineers placed their caps in this expanded gap, wedging the cap when the airframe shrank again.[114] To keep the cabin cool, Concorde used the fuel as a heat sink for the heat from the air conditioning.[115] The same method also cooled the hydraulics. During supersonic flight the surfaces forward from the cockpit became heated, and a visor was used to deflect much of this heat from directly reaching the cockpit.[116]