Definition
A method of attaching turbine blades to a turbine disk in which the base of each blade is shaped with a series of tapered serrations resembling the branches of a fir tree. The matching slot in the rim of the disk has the same tapered serrated profile, so the blade slides into the slot from the side and is held in place by the interlocking shape against the enormous centrifugal load produced when the disk spins.
Plain English
A way of locking turbine blades into the spinning disk by cutting the bottom of the blade into a shape like a small fir tree, then sliding it into a matching slot. The interlocking shape stops the blade from flying outward when the engine is running.
Context Anchor
Seen in aircraft turbine-engine construction and maintenance, especially when discussing how compressor or turbine blades are held in the engine disk.
Derivation
Named for the visual resemblance of the serrated blade root to the branched profile of a fir tree. The shape is functional: each pair of serrations adds another mechanical interlock, spreading the centrifugal load along the entire root rather than concentrating it at one point.
Why Pilots Care
This attachment keeps turbine blades securely in place at high speeds and temperatures, directly affecting engine reliability and preventing blade separation.
Analogy
Think of a key sliding into a lock with several matching steps. The steps keep the part from pulling straight out when force is applied.
Intuition Check
Do not read “fir-tree” as a material or a wooden part. Here it describes the shape of a metal blade root and its matching slot.
Example Sentence 1
The mechanic carefully inspected each fir-tree attachment for cracks before reinstalling the turbine blades.
Example Sentence 2
The fir-tree attachment design lets the blades expand as the turbine heats up without coming loose.