In civil airliners such as the B747 the air ducting from the APU to the
main distribution manifold, and also the main ducting in the wing leading
edges, all have overheat detection wires running parallel to the duct. My
experience of the 747 is that the "duct leak" messages (on the 747-400)
are fairly common, but normally are a genuine air leak, normally in the
"centre duct" section, often in the air conditioning area.

Assisted engine starts are allowed in flight, depends on alltitude mainly
whether the starter is utilised or windmill start is used.

Regards