Where is the HUD optical center relative to the aircraft waterline?

• A. Waterline
• B. Four degrees below waterline.
• C. Four degrees above waterline.
• D. Ten degrees below waterline.

Answer: (B)

The important idea is how the HUD is aligned with the aircraft’s reference line so the pilot sees the flight cues where intended in the head-up view. The HUD optical center is set slightly below the waterline, a small downward tilt of the projection axis. This offset accounts for the pilot’s seating position and typical cockpit attitude, so in level flight the horizon and other symbols appear at the correct place in the pilot’s line of sight. If the optical center were at the waterline or above it, or offset too far, the horizon and cues would sit too high or too low in the HUD, making them harder to read and potentially misleading during scanning. So the correct placement is below the waterline by a small amount, ensuring the HUD presents imagery where the pilot expects it during normal flight.