Modern aircraft cockpits are built around the eye reference point. It is used to size the cockpit windows and define the location of all the controls, displays and instruments.
When the pilots align themselves with the eye reference point, they will have adopted the optimum position to operate the aircraft.
An optimized field of view
The cockpit is designed so that when the pilot has aligned themselves to the eye reference point; all of the instruments and displays on the front panel are in their field of view (fig.1).
A pilot needs to have good situational awareness during a flight. Alignment using the eye reference point enables the pilots to have an optimal field of view through the cockpit’s windows to see what is around them outside the aircraft. The eye reference point position ensures the pilot can maintain the best cut-off angle that will provide the longest visual segment (fig.2). This is especially important to get visual references during Low Visibility Operations (LVO).
A consistent viewpoint
Having a consistent viewpoint gives several operational advantages such as easing the handling of the aircraft by providing pilots with a consistent visual reference, repeatable at every flight. This is especially useful during final approach to be familiarized with the final approach path angle and also for the flare phase.
Since the A300 Airbus has provided an eye reference indicator on the centre structure of the windshield in all Airbus aircraft (fig.3). It enables flight crew to adjust their seat position so that their eyes position matches the eye reference point. The indicator is a device that is fitted with 3 balls painted red or white. To achieve a correct seating position, pilots must align the red and white ball meaning that the white ball is hidden when in the correct position.
Using Head Up Display (HUD)
HUD symbols are fully visible when the pilot’s eyes are closest to the eye reference point. An “eye box” is defined as an area around the eye reference point that gives a position tolerance range (fig.4). Hence the pilot correctly sees indications on the HUD when their eyes are positioned inside this virtual box. The HUD eye box area extends further aft than forward to allow HUD readability when seated in a more reclined position for comfort.
An optimized access to controls
A pilot properly seated with their seat harness fastened is able to reach and operate all of the aircraft’s controls through their range of motion or deflection as it is defined by the design certification requirements (fig.5).
Reference/Credit: https://safetyfirst.airbus.com/are-you-properly-seated/