Structural Steel starts to lose strength at around 700* F, and is below half strength around 1000* F. So melting isn't required for materials to fail. Materials used inside jet engines are designed to operate within their strength tolerances at their operating temperatures, which is not flame temps. One of the tricks to maintaining material strength in a jet engine is boundary layer air flow over the turbine blades and other parts subjected to high stress.

A raging building fire can easily reach 1800* F. Jet fuel wouldn't increase the temperature much, if at all. Unless it's contributing to a chimney effect - pulling in air with enough velocity to make a fire hotter. A blast furnace can melt steel, and I think they get to around 3000* F.

Anyway, just some basic info on materials. Not a commentary on anything.