Definition
A metering valve in a vapor-cycle air conditioning system that controls the flow of liquid refrigerant into the evaporator based on the temperature of the refrigerant leaving the evaporator. A temperature-sensing bulb on the evaporator outlet line drives the valve open or closed so that only the right amount of refrigerant enters the evaporator to fully evaporate under current cooling demand.
Plain English
A small automatic valve in an aircraft air conditioning system that adjusts how much cold liquid refrigerant flows into the cooling coil. It senses how warm the line is leaving the coil and opens or closes to keep cooling steady.
Context Anchor
Seen in aircraft air-conditioning system descriptions, especially when discussing refrigerant flow through the evaporator.
Derivation
Thermostatic comes from Greek thermos (heat) and statos (standing or controlled), meaning heat-controlled. Expansion refers to the refrigerant changing from a high-pressure liquid to a low-pressure vapor as it passes through the valve. Together: a heat-sensing valve that controls refrigerant expansion.
Why Pilots Care
Keeps cabin and avionics cooling reliable without risking liquid refrigerant damage to the compressor or icing in the evaporator.
Analogy
It works somewhat like an automatic faucet for the cooling system: it opens or closes just enough to send the right amount of fluid where it is needed.
Intuition Check
Do not read “expansion” as meaning the valve itself expands. The refrigerant expands and cools after passing through the valve.
Example Sentence 1
The mechanic suspected a faulty thermostatic expansion valve after the air conditioning produced only mild cooling on a hot day.
Example Sentence 2
Proper adjustment of the thermostatic expansion valve prevents liquid slugging in the compressor during high-altitude cooling operations.