Outer engine compartment Stainless steel shroud 350° F Fiberglass Aluminum foil Fiberglass Silver foil Exhaust duct 900° F Exhaust gas 1,000° F Cooling air 120° F Figure 6-62.  Typical engine insulation blanket. blanket contains fiberglass as the low conductance material  and aluminum foil as the radiation shield. The blanket is  suitably covered so that it does not become oil soaked.  Insulation blankets have been used rather extensively on  many installations in which long exhaust is needed. Some  auxiliary power units (APU) mounted in the tail cone of  transport aircraft have air that surrounds the exhaust tail pipe  that provides cooling and protects the surrounding structure.  6-41