Definition
A small-diameter tube used in aircraft systems to carry a sensing fluid or refrigerant between a remote bulb or reservoir and an instrument or control device. The narrow bore allows the tube to transmit pressure changes accurately while protecting the sensing fluid from outside influence, and is commonly found connecting the temperature-sensing bulb of a remote-reading thermometer to its gauge, or metering refrigerant flow in vapor-cycle cooling systems.
Plain English
A very thin tube that links a sensor out in the aircraft to a gauge or control in the cockpit, or that meters fluid flow in a cooling system.
Context Anchor
Seen in aircraft maintenance when working with instruments, temperature-sensing systems, and some fluid-control systems.
Derivation
From the Latin capillaris, meaning 'of or relating to hair.' A capillary tube is named for its hair-thin internal bore, which is what allows it to do its job — the tiny passage carries fluid or pressure signals over distance without significant lag or loss.
Why Pilots Care
Blockage or damage can cause incorrect fuel quantity or temperature readings, affecting flight planning and safety.
Intuition Check
Do not read capillary here as a blood vessel. In aircraft maintenance, it means a very small tube used in a system or instrument.
Example Sentence 1
The mechanic replaced the capillary tube running from the oil temperature bulb to the cockpit gauge after finding it pinched against the firewall.
Example Sentence 2
Capillary tubes allow the sensing fluid in certain temperature gauges to expand and contract with heat changes.