Definition
A family of standardized fluid-line fittings, manufactured to Military Standard specifications, that join rigid metal tubing without flaring the end of the tube. The fitting uses a separate sleeve (or ferrule) that bites into the outside of the tube as a coupling nut is tightened against the fitting body, forming a leak-tight seal between the tube, sleeve, and fitting.
Plain English
A type of standard plumbing connector used in aircraft fluid lines. Instead of flaring the end of the metal tube outward to seal it, you slide on a small sleeve, push the tube into the fitting, and tighten a nut. The sleeve grips the tube and seals it.
Context Anchor
Seen in aircraft maintenance when installing or inspecting hydraulic, fuel, oil, or other fluid lines.
Derivation
MS stands for Military Standard, a U.S. government specification system that defines part dimensions and quality requirements so parts from different manufacturers are interchangeable. 'Flareless' means the tube end is left straight rather than being flared (spread outward into a cone shape), which is the older sealing method.
Why Pilots Care
Flareless fittings are common in high-pressure hydraulic systems where flared fittings can crack under vibration. A technician who recognizes them knows the assembly procedure is different — overtightening or reusing the sleeve incorrectly can cause leaks or line failure.
Intuition Check
Flareless does not mean the fitting is weaker or temporary. It means the tube end is not flared; the fitting is designed to seal the straight tube end by clamping it correctly.
Example Sentence 1
The hydraulic line was assembled with MS flareless fittings, so the technician presetted the sleeve onto the tube before installing it on the aircraft.
Example Sentence 2
MS flareless fittings provide a reliable seal in high-pressure systems where flared ends could crack.