Definition
A colorless, syrupy organic liquid (chemical formula C2H6O2) used as the principal ingredient in many aircraft anti-icing and de-icing fluids, and as a heat-transfer fluid in some liquid-cooled engine and electronic systems. It lowers the freezing point of water and raises its boiling point, which is why it is mixed with water for these applications.
Plain English
A clear, slightly thick liquid mixed with water to keep the mixture from freezing or boiling too easily. Aircraft use it in de-icing fluids and in some cooling systems.
Context Anchor
Seen in aircraft maintenance discussions of coolant, antifreeze, deicing fluids, anti-icing fluids, leaks, and safe handling of chemicals.
Derivation
From 'ethylene' (a simple two-carbon hydrocarbon) plus 'glycol' (from Greek 'glykys', meaning sweet, because these alcohols have a sweet taste). The name signals the chemical family it belongs to and explains why it must be handled carefully -- it tastes sweet but is toxic if swallowed.
Why Pilots Care
Correct use of ethylene glycol-based fluids is essential for removing or preventing ice on wings and control surfaces and for maintaining reliable engine temperatures during flight.
Analogy
It is similar in purpose to the antifreeze used in many car cooling systems, but aircraft use must follow the specific approved maintenance instructions for that aircraft or fluid.
Grounding Statement
If plain water would freeze in cold weather, an approved ethylene glycol mixture can help keep the liquid usable at a lower temperature.
Intuition Check
Do not assume ethylene glycol is safe because it is used in coolant or deicing fluid. It is poisonous, and only approved fluids and mixtures should be used on an aircraft.
Example Sentence 1
The technician topped off the de-icing cart with an ethylene glycol and water mixture before the morning shift.
Example Sentence 2
Ethylene glycol coolant circulates through the engine to prevent overheating during long flights in warm weather.