Definition
A spiral-shaped exhaust collector on a free-turbine turboprop engine that gathers the hot gases leaving the power turbine and routes them outward through one or more exhaust stacks. Because a free-turbine engine separates the gas-producer section from the power turbine, the exhaust must be turned and channeled away from the propeller shaft and reduction gearbox, and the scroll's curved shape performs that redirection.
Plain English
A curled, snail-shell-shaped duct that catches the hot gas coming out of the engine's power turbine and guides it sideways out of the airplane.
Context Anchor
Seen in turboprop engine descriptions, especially when explaining how gas flows through a split-shaft or free turbine engine.
Derivation
Scroll' comes from the Old French escroe, meaning a roll or curl of parchment. The part is called a scroll because its shape spirals outward like a rolled-up scroll, which is exactly how it collects and redirects the exhaust gas.
Why Pilots Care
Efficient exhaust routing lowers back-pressure on the turbine, preserving power output and preventing excessive heat buildup inside the engine compartment.
Grounding Statement
After the turbine has taken energy from the gas, the exhaust scroll gives the remaining hot gas a smooth path out of the engine.
Intuition Check
“Scroll” does not mean a computer screen movement here. It means a curved, scroll-shaped engine passage that directs exhaust gas.
Example Sentence 1
On the PT6, hot gas leaves the power turbine, enters the exhaust scroll, and exits through the side stacks on the cowling.
Example Sentence 2
The mechanic confirmed that the exhaust scroll was securely mounted so turbine gases would exit cleanly without creating drag.