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From this question:

What are these Lights on Center Holm between Windows on the A320?

I learn that the pilots' eye position is important. Enough to justify an extra piece of equipment.

But why? Can't a pilot tell if they see enough, above/below/on each side, by just looking?

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The correct seating position is important to ensure

  1. A consistent viewpoint, which results in the final approach and flare always looking the same to the pilot. This is important when hand-flying the aircraft to ensure consistent landings.

    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.

  2. Best forward visibility, which maximizes the area in front of the aircraft you can see (Visual Segment):

    Visual Segment

    Having the seat further backwards or lower would reduce the Visual Segment.

  3. Good View of the Instruments, which allows seeing the PFD and ND without having to move your head down. This is how it looks with the correct seating position:

    PFD and ND with correct seating

    And this is how it looks if you are seated too high or too far forwards:

    PFD and ND with incorrect seating

All sources: Airbus - Are You Properly Seated?


You also asked

Can't a pilot tell if they see enough, above/below/on each side, by just looking?

Yes, not all aircraft have the three balls for finding the optimum seat position. You can do it by looking over the glareshield and ensuring good visibility of the PFD/ND. On Boeing aircraft, this is how it is supposed to be done since they don't have a dedicated eye reference indicator:

Pilot Seat Adjustment

Adjust the seat position with the appropriate controls to obtain the optimum eye reference position. Use the handhold above the forward window to assist. The following sight references are used:

  • Sight along the upper surface of the glareshield with a small amount of the airplane nose structure visible (A)
  • Sight over the control column (in the neutral position) until the bottom edge of the outboard display unit is visible (B).

B737 Eye Reference Position

(Boeing 737 NG FCOMv2 - 1.40.44 Airplane General - System Description)

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    $\begingroup$ It's even more critical in aircraft with a HUD. Proper eye position ensures the attitude reference and flight path vector are consistent with the actual aircraft path. And if it's also equipped with enhanced or synthetic vision, it ensures the supplemental vision information overlays the 'real world' image acurately. $\endgroup$ – Gerry Aug 16 at 17:55
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It's just a shortcut aid based on the OEM's ergonomics engineering. The sweet spot is considered to be with your eyes high enough to see down to the base of the windshield without obstructing anything on the instrument panel.

This puts your preferred sight line roughly on the plane of the coaming/glareshield extending toward you from the base of the windshield (or slightly above or below), when the seat is positioned to the typical for/aft position for a comfortable hold on the control wheel (roughly a forearm's length from your torso).

When you get in there are 3 steps:

  1. Set the seat for and aft to suit your forearm length to the control wheel or side stick.
  2. Set the seat height to align the balls, or whatever your personal preference is.
  3. Set the rudder pedal adjustment (called the stature adjustment) to your leg length, making sure you can apply full rudder plus push the brake on that side fully on with your toe while your leg is stretched out.

Not everybody likes the recommended location set by the balls. When I was flying CRJs, which had the balls on the center post, I would ignore them most of the time and just set the height to put my sight line where I could just see the top surface of the glareshield, which, as it happens, more or less aligns the balls.

The eye height most pilots will set with or without the balls tends to be the same, and the seat setting you find when you get in tends to reflect the torso length of the previous pilot. I have a long torso and short legs, kind of orangutan-ish, so the next person after me usually would find the seat set pretty close to the floor.

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To answer the second part of your question, the short answer is ‘No’.

We use perspective to judge taxiing manoeuvers (turns) and to achieve the correct initial pitch attitude on take-off and on landing.

We need certain visual cues to determine correct positioning over the threshold of the runway. When perspective is used, a reference point is needed by which to gauge your position. That reference point must remain the same the entire time.

An example:
On take-off in poor visibility, we need to see 4 runway lights to confirm whether or not we can depart. An incorrectly adjusted seat could mean a delayed take-off or an illegal one.

Another example:
On landing, at 50ft we need to see the threshold disappear under the nose. This helps to ensure that we are neither too high nor too low (both of which can lead to disaster). Adjusting seat height alters that visual cue with potential serious consequences.

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  • $\begingroup$ Welcome to Aviation. This appears to be very good info, if you would edit it to break it up a bit, it will be much more readable and make it a very good answer. $\endgroup$ – FreeMan Aug 16 at 12:15
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Airplane designers analyze and test the heck out of the airplane-pilot system. That very hard problem becomes less hard if they remove the flexibility of where can the pilot eyes be.

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