Too Darn Hot
Fire in flight! is one of the scariest things in aviation. An engine fire in a single-engine airplane turns you into a glider, since putting the fire out means shutting off the fuel. But an engine fire in a multi-engine airplane turns you into a brick. One of the axioms of multi-engine flying is that in case of wing fire you must put the airplane on the ground now, no matter where you are. There are tales of multis dropping like a rock on downwind.
I was taught that the reason for this was that the aluminum spar got "soft" when heated. This seemed plausible, so I believed it and passed it on. But that's superstition, which has not place in flight instruction.
So here is a graph from Dennis R. Jenkins's X_15: Extending the Frontiers of Flight, a NASA publication available free here.
The graph shows the strength response to temperature for several materials used in aviation: aluminum, magnesium, titanium, stainless steel, and Inconel X. Inconel X can handle amazing amounts of heat, which is why it was used in the X-15. (The Wikipedia article on Inconel seems pretty good.)
The vertical scale is Tensile Yield Stress, that is, how much stress does it take to make the material yield, "strength" in layman's terms. The horizontal scale is temperature.
The aluminum curve is pretty scary: at low temperatures the strength remains about constant, but at about 200F it starts to decline rapidly, and is halved by about 400F.
That's not very hot. As a cook, I regularly touch stuff nearly that hot (I often stir stuff with my hand rather than a spatula). French fries cook at 375F. Typical ITT temperature for a PT-6 engine is about 1350F, and piston EGTs are in the same ballpark.
Aluminum loses its strength at temperatures that are common in aviation!
So, if you are in an aluminum-sparred airplane and there is heat on the wing land right now. Don't try to reach the airport. Land right now on something that won't hurt anybody else.
A composite airplane like the Diamond DA-42, which I believe has a composite spar, might be different. If this is correct (don't take my advice unless you verify that the spar is composite) then the spar should be able to maintain its strength at much higher temperatures, and you have more options as to landing.
[revised 15 Aug 2011]