Thermal and structural limits were also present. Air is heated considerably when compressed by the passage of an aircraft at supersonic speeds. The airframe absorbs heat from the surrounding air, the inlet shock cone at the front of the aircraft becoming the hottest part. The shock cone was composed of fibreglass, necessary because the shock cone also served as a radar radome; a metal shock cone would have interfered with the AI 23's radar emissions. The shock cone was eventually weakened due to the fatigue caused by the thermal cycles involved in regularly performing high-speed flights. At and , the heating conditions on the shock cone were similar to those at sea level and indicated airspeed, but if the speed was increased to at , the shock cone was exposed to higher temperatures than those at Mach 1.7. The shock cone was strengthened on the later Lightning F.2A, F.3, F.6, and F.53 models, thus allowing routine operations at up to Mach 2.0.