Definition
A description of how flareless tube fittings make a seal in aircraft fluid lines. Instead of flaring (belling out) the end of the tube, a metal sleeve is slipped over the tube. When the fitting nut is tightened, the sleeve is squeezed (compressed) onto the tube, biting into it slightly and forming a leak-tight seal between the tube and the fitting body.
Plain English
Some tube connections don't flare the tube end. Instead, a small metal collar slides over the tube and gets squeezed tight when the nut is screwed down. That squeeze grips the tube and seals the joint.
Context Anchor
Seen when installing, inspecting, or maintaining aircraft fuel, oil, brake, or hydraulic lines that use rigid tubing.
Derivation
Compression here means 'pressing or squeezing together' (from Latin comprimere, to press together). A sleeve is a tube-shaped piece that fits over something, like a sleeve covers an arm. So a compression sleeve is a small collar that gets squeezed onto the tube to hold and seal it.
Why Pilots Care
A properly installed compression sleeve prevents fluid leaks that could lead to system failure or fire.
Analogy
It is like sliding a small ring over a straw and then tightening a cap so the ring pinches the straw firmly in place. The ring is not just covering the straw; it is being squeezed to hold it.
Intuition Check
Do not read “sleeve” here as only a protective cover. In this fitting, the sleeve is squeezed so it helps grip and seal the tube. Do not read “compression” as engine compression. Here it means mechanical squeezing of the sleeve around the tube.
Example Sentence 1
The hydraulic line uses a flareless fitting, so the seal depends on the compression sleeve gripping the tube when the nut is torqued.
Example Sentence 2
During preflight inspection, check that all fittings use a compression sleeve around the end of the tube to confirm no loose connections exist.