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Flight Deck Cutoff Angle - Forward Visibility

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Flight Deck Cutoff Angle - Forward Visibility

Old 18th Mar 2023, 10:26
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Flight Deck Cutoff Angle - Forward Visibility

Hello all.

Looking for some generic numbers to describe the loss in forward visibility if a pilot seated X cm too low in cockpit when approaching minima on ILS.

ie.. if a pilot is seated 5cm too low, what reduction in forward visibility would they experience? (Ie from 800m for CAT I at 300 feet).
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Old 18th Mar 2023, 11:22
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The FCOM/FCTM has the diagrams for the normal approach scenario. Just do the trigonometry for yourself.
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Old 18th Mar 2023, 17:03
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Capt Fathom's suggestion is good for a specific aircraft type (assuming one has the documents).

For a GENERIC ESTIMATION, this GENERIC document on the problem provides some dimensionless diagrams that may help figure a GENERIC geometry. Just choose and apply reasonable dimensions to the diagrams (eye distance from panel or other obstruction horizontally; pitch angle of aircraft on 3 glideslope (usually about 2-3 nose-up); actual height and slope of glareshield; and so on.

Of note, many modern aircraft (as you likely know) have eye reference indicators above the glareshield, to ensure correct seating (height above and distance from, the cockpit panel) for a correct eye position. And recent aircraft, originally designed in the glass-cockpit era, now have less-tall instrument panels to give pilots better downward vision (no doubt accounted for in the eye reference indicator's construction).

Also of note, it mentions just how a pilot could end up 5cm low even if their eye position was correct at the beginning of the flight - slouching from muscle fatigue after a long flight.

https://skybrary.aero/sites/default/...shelf/4247.pdf

Last edited by pattern_is_full; 18th Mar 2023 at 17:15.
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Old 18th Mar 2023, 19:42
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Interesting link but it seems a bit Airbus specific. I find Boeings tend to put you so far forward if you line up the little balls, or the rudder adjuster and the yoke markings, it becomes difficult to use the yoke fully and it feels rather uncomfortable. YMMV.
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Old 18th Mar 2023, 20:21
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Cat 1 is typically 550m at 200ft so why are you asking about 800m?
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Old 18th Mar 2023, 20:54
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You could do the math on this (and I'm almost bored enough to do it just for fun) but the visibility lost from seating position is off the near end, while atmospheric conditions (the "visibility" on an approach around minimums, etc) is at the far end. So I don't see the related applicability. If you can't see the runway because of the fog, sitting higher won't help.
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Old 18th Mar 2023, 21:20
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But the near end of the visual field includes the nearest approach lights - highly relevant at DH
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Old 18th Mar 2023, 21:37
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Originally Posted by m-dot View Post
Hello all.

Looking for some generic numbers to describe the loss in forward visibility if a pilot seated X cm too low in cockpit when approaching minima on ILS.

ie.. if a pilot is seated 5cm too low, what reduction in forward visibility would they experience? (Ie from 800m for CAT I at 300 feet).
Originally Posted by Capt Fathom View Post
The FCOM/FCTM has the diagrams for the normal approach scenario. Just do the trigonometry for yourself.
Originally Posted by pattern_is_full View Post
Capt Fathom's suggestion is good for a specific aircraft type (assuming one has the documents).

For a GENERIC ESTIMATION, this GENERIC document on the problem provides some dimensionless diagrams that may help figure a GENERIC geometry. Just choose and apply reasonable dimensions to the diagrams (eye distance from panel or other obstruction horizontally; pitch angle of aircraft on 3 glideslope (usually about 2-3 nose-up); actual height and slope of glareshield; and so on.

Of note, many modern aircraft (as you likely know) have eye reference indicators above the glareshield, to ensure correct seating (height above and distance from, the cockpit panel) for a correct eye position. And recent aircraft, originally designed in the glass-cockpit era, now have less-tall instrument panels to give pilots better downward vision (no doubt accounted for in the eye reference indicator's construction).

Also of note, it mentions just how a pilot could end up 5cm low even if their eye position was correct at the beginning of the flight - slouching from muscle fatigue after a long flight.

https://skybrary.aero/sites/default/...shelf/4247.pdf

For off nominal seating, go measure your eye to coaming distance and apply a 1:60 for the quick solution, using the basic data from the FCOM, or do the trig. The eye height directly affects the visual cutoff but also the segment of observed surface, and that has issues with perception of rates. pitch attitude (speed/configuration/wind component) alter the cutoff as well. approximately each 5 kts changes the attitude by 1 degree... Every transport has a reference eye height, it is a design requirement and found in the AC 25-773-1 4(b) to meet the requirements of the Part. 25.773(d)
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