Definition
An older standard of flared tube fittings used in aircraft fluid lines, in which the end of a metal tube is flared outward at a 35-degree angle and clamped between a male fitting and a sleeve and nut to form a leak-tight seal. The AC standard was later superseded by the AN (Army-Navy) standard, which uses a 37-degree flare and is dimensionally different, so AC and AN fittings are not interchangeable.
Plain English
An old type of metal tube connection used on aircraft, where the end of the tube is opened out into a cone shape so it can be squeezed tight against a matching part to stop fuel, oil, or hydraulic fluid from leaking. AC fittings were the first version of this design and were later replaced by a slightly different version called AN.
Context Anchor
Seen in aircraft maintenance when inspecting or replacing metal fluid lines, especially in older aircraft or older maintenance references.
Derivation
AC stands for Air Corps, the pre-1947 branch of the U.S. Army that originally specified these fittings. After the Army Air Corps became the Army Air Forces and later the U.S. Air Force, the standard was reissued jointly with the Navy as AN (Army-Navy), with a slightly different flare angle. Knowing the origin explains why two similar-looking fitting families exist and why they cannot be mixed.
Why Pilots Care
These fittings maintain leak-free connections in pressurized fluid systems; improper installation can lead to fluid loss, fire risk, or loss of control authority.
Analogy
Picture the end of a drinking straw widened into a small cone, then pressed tightly against a matching cone-shaped opening. The seal comes from the two cone shapes being pulled firmly together.
Intuition Check
AC does not mean alternating current here. In this term, AC identifies the older Air Corps fitting standard.
Example Sentence 1
The mechanic noted that the vintage trainer still had original AC flared tube fittings on its fuel lines and ordered correct-standard replacements rather than substituting AN parts.
Example Sentence 2
Before the preflight inspection the mechanic checked all AC flared tube fittings in the engine compartment for signs of seepage or loose nuts.