Total Materia: Al-Cu-Mg-Ag Alloys
A technical article describing the kinds of alloys found in jet engines. These alloys are sloughed off by the engines during flight and land on areas beneath the flight path.
MIT: How to get sprayed metal coatings to stick
This is a rather technical article. However it’s well worth reading as it provides an explanation as to why the toxic metals (Thorium, Beryllium, Ytterbium, Gadolinium, etc.) used to coat aircraft engines sloughs off, thus falling on areas under the flight paths.