Definition
An air-filled cavity in the bone of the forehead, located just above the eyebrows, that connects to the nasal passage through a small drainage opening. As one of the paranasal sinuses, it equalizes with outside air pressure during normal breathing, but if its drainage opening is blocked by congestion or inflammation, trapped air cannot expand or compress freely as cabin altitude changes, causing pain during climbs and especially during descents.
Plain English
A small hollow space in the bone above your eyebrows that normally lets air flow in and out through your nose. If a cold or congestion blocks that opening, the trapped air can't adjust to changing cabin pressure, which causes sharp forehead pain when you climb or descend.
Context Anchor
Pilots encounter this term in discussions of sinus pain, colds, allergies, and pressure changes during climbs and descents.
Derivation
From Latin frons (forehead) and sinus (a curve, fold, or hollow). Together: a hollow in the forehead. The Latin origin is literal and matches the location exactly.
Why Pilots Care
A blocked frontal sinus during altitude changes can produce intense pain above the eyes and may distract or incapacitate a pilot.
Analogy
It is like a small air pocket that needs a tiny vent. If the vent is blocked while pressure changes around it, the trapped air can press painfully on the surrounding area.
Grounding Statement
If you descend while congested, the pressure outside your frontal sinus can change faster than the trapped air inside it can adjust.
Intuition Check
Frontal does not mean the front of the airplane here; it means the forehead area of the skull. Sinus does not mean only a sinus infection; here it means the air-filled space itself.
Example Sentence 1
The pilot canceled the flight after waking up with a head cold, knowing that a blocked frontal sinus could cause severe pain on descent.
Example Sentence 2
A pilot with a sinus infection waited until symptoms cleared to avoid frontal sinus trouble in flight.