Definition
A ring-shaped exhaust collector that surrounds the rear of a free-turbine turboprop engine and channels the spent gases outward to one or more exhaust outlets on the sides of the engine, rather than straight out the back. Because the propeller shaft on a free-turbine engine often runs through or near the centerline of the engine, the exhaust must be routed around it through this peripheral (outer) housing.
Plain English
A donut-shaped exhaust housing wrapped around the back of the engine that gathers the hot gases and lets them escape out the sides, since the middle of the engine is taken up by the propeller shaft.
Context Anchor
Seen in turboprop engine diagrams, especially when describing the gas path in a split-shaft or free turbine engine.
Derivation
Peripheral comes from the Greek peripherein, meaning 'to carry around.' Scroll refers to a curved or rolled shape, like a rolled sheet of paper. Together it describes a curved housing that wraps around the outside of the engine to carry exhaust away.
Why Pilots Care
Efficient collection and expulsion of exhaust reduces back-pressure on the turbine, preserving available power and preventing overheating or performance loss.
Analogy
Think of it like a curved drain channel around the edge of a sink: it gathers flow from one area and guides it toward an outlet.
Intuition Check
Peripheral does not mean unimportant here; it means located around the outside edge. Scroll does not mean a piece of paper; it means a curved, spiral-like passage for exhaust gas.
Example Sentence 1
On the PT6, hot gases leave the power turbine and pass through the peripheral exhaust scroll before exiting through the two stacks on the sides of the cowling.
Example Sentence 2
During preflight the mechanic checked the peripheral exhaust scroll for signs of erosion from high exhaust temperatures.