Definition
Air or other gas held inside a body cavity that cannot freely escape or equalize as outside pressure changes. As the aircraft climbs, the surrounding pressure drops and the trapped gas expands; as it descends, the gas contracts. Common sites include the middle ear, sinuses, teeth (around fillings or infections), and the gastrointestinal tract.
Plain English
Gas stuck inside a part of the body that cannot easily get out when pressure changes. As you go up, it expands. As you come down, it shrinks. Either way, it can hurt.
Context Anchor
Seen in aircraft welding, metal repair, and inspection discussions.
Derivation
Trapped comes from the idea of being caught and unable to get out. The phrase simply describes gas that is sealed inside a space and cannot move with pressure changes the way outside air does.
Why Pilots Care
Unresolved trapped gas can cause barotrauma, severe pain, or incapacitation that compromises flight safety.
Analogy
It is like small air bubbles frozen inside an ice cube. Once the ice hardens, the bubbles are stuck inside instead of rising out.
Grounding Statement
Picture a sealed plastic water bottle on the ground. Take it up to altitude and it puffs out; bring it down and it crumples. The same thing happens to gas pockets inside the body.
Intuition Check
Do not assume this means fuel vapor or cabin air trapped somewhere in the airplane. In metal and welding use, it means gas sealed inside solid metal or a weld.
Example Sentence 1
The instructor briefed the student on trapped gas, explaining why a head cold is a good reason to cancel a flight.
Example Sentence 2
Trapped gas in the intestines can become painful at altitude if not addressed before flight.