the substrate the washcoat is typically a metal oxide such as alumina titania silica zirconia or other transition metal oxides or mixtures of these the washcoat and catalyst have good adhesion such that there is no flaking peeling or loss of material in the operating environment of aircraft including high vibrations the adhesion may be ensured by proper formulation of the washcoat as well as treatment of the substrate the washcoat is applied as a slurry of the metal oxide a binder and solvent as discussed in a related u s patent application ser no 101 140 filed sep 18 1998 and which is incorporated herein by reference the exhaust stream purification system may be formed either as a unit separate from the bleed air purification system or within the same structural component as the bleed air purification system the exhaust stream purification system may employ any conventional catalyst known to be effective at removing engine exhaust stream pollutants for example catalysts known as a u201c three way conversion u201d or u201c twc u201d catalysts may be useful in the present invention twc catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing the oxidation of hydrocarbons and carbon monoxide and the reduction of.