Definition
The transparent optical element of a Head-Up Display (HUD) positioned in the pilot's forward line of sight, onto which flight symbology and information are projected so the pilot can read them while still looking outside the aircraft.
Plain English
The clear screen in front of the pilot's eyes that shows HUD information without blocking the view out the window.
Context Anchor
Seen in discussions of head-up display parts and how the pilot views HUD information while maintaining an outside view.
Derivation
From 'eye' (the part of the body that sees) and 'piece' (a single component). The term comes from optical instruments like telescopes and microscopes, where the eyepiece is the lens nearest the user's eye. In a HUD, it serves the same role: the optical surface the pilot looks through and reads from.
Why Pilots Care
Correct eye-piece alignment keeps HUD symbols registered with the horizon and runway, preventing misinterpretation of attitude or flight-path data.
Analogy
It is similar to the viewing end of binoculars: your eye has to be in the right place to see the intended image clearly. In a head-up display, the difference is that the pilot is still looking forward through the aircraft’s view, not away from it.
Intuition Check
Do not think of the eye piece as a separate instrument the pilot stares into instead of looking outside. In this context, it is the viewing part that lets HUD information appear in the pilot’s forward view.
Example Sentence 1
As the aircraft descended through the clouds, the pilot read airspeed and altitude directly from the HUD eye piece without shifting focus inside the cockpit.
Example Sentence 2
During the instrument scan the instructor reminded the student to keep the eye piece centered so the HUD remained usable.