The combustion byproducts are toxic and some uncombusted propellants are left in the pipes and can leak out. Has to be washed.
The emergency APU on F-16 fighters uses hydrazine and if it was used and the jet landed on a civilian airport (e.g. engine failure) then you need to have it safely washed before anyone can approach the plane. https://youtu.be/Puia_yQxir8?t=178
The emergency APU on F-16 fighters uses hydrazine and if it was used and the jet landed on a civilian airport (e.g. engine failure) then you need to have it safely washed before anyone can approach the plane. https://youtu.be/Puia_yQxir8?t=178