Light cones of the landing light

[legoboyvdlp]

I'm not sure, but at this attitude, should the landing light illuminate the ground?

[gilbertohasnofb]

That looks reasonable to me, @legoboyvdlp! Have you seen it differently anywhere, e.g. in some video shot from the cockpit or from the ground?

[legoboyvdlp]

The best video really are these two: https://youtu.be/zYFy7jGjg4g?t=91 https://youtu.be/eHeglw46mz0?t=70

I was merely making an uninformed guess, but perhaps I am wrong.

This is just a issue report, if someone with actual experience or actual data on the angles that the real light shines could confirm or correct me ;)

[legoboyvdlp]

What didn't seem right to me was that looking roughly from where the ground is illuminated, the landing light is not shining directly onto the ground (judging by the size of the actual light)

as compared to

[gilbertohasnofb]

I think those aircraft in those two videos have a nose landing gear, but I could be wrong. Anyway, given that ALS lights are not true light effects but rather shader magic, what we have might be a limitation that will be difficult to be improved. But perhaps @wlbragg would know better.

[wlbragg]

I think I see what your saying and that is the light is illuminating the runway too far out away from the aircraft, right? In the video it looks like the light doesn't really hit the runway where you can see it until after the flair and touchdown.

We could potential adjust our light to better match that. We should get some more advice from pilots as to how to adjust the lights. This would apply to both the taxi and the landing light.

[okroth]

The maintenance manual says about taxi and landing light adjustment:

The landing and taxi lights are set to specified positions, but you can adjust them as necessary. The procedures that follow give information on the correct landing and taxi light adjustment procedure. The procedures that follow are typical for incandescent and HID lights.

(1) Park the airplane on a flat, level surface with the landing and taxi lights in front of a light-reflecting object. Make sure that the waterline of the airplane is level and that the wings are level. Refer to Chapter 8, Leveling - Maintenance Practices.

(2) Park the airplane so that the distance from the light-reflecting object to the rivet line on the bottom of the front spar is approximately 3 feet.

(3) Set the landing light switch to the LAND position.

(4) Measure the distance from the floor to the center of the beam that shines on the light-reflecting object. The correct distance is 74.41 inches.

(5) Set the landing light switch to the OFF position

(6) Set the taxi light switch to the TAXI position.

(7) Measure the distance from the floor to the center of the beam that shines on the light-reflecting object. The correct distance is 73.29 inches.

(8) Set the taxi light switch to the OFF position.

(9) To adjust the beam to the correct position, add or remove washers between the spacers and the plate.

[okroth]

However, these instructions leave me scratching my head. First: to level the plane, the main gear needs to be raised, as the plane sits tail down on the floor. Second: the x-distance is approx. 3 feet. the y-distance is 73.29 or 74.41 inches, three orders of magnitude more precise (0.01" ~ +/-0,13mm vs. half a foot ~ +/- 150mm). Older service manuals give quite precise instructions about the adjustments, but don't specify where the beams will land. In any case, the landing light will be higher than the taxi light. I'll have to check but it could be that the landing light is just straight forward to warn other traffic

[okroth]

May this be helpful.... http://blog.aveoengineering.com/2018/02/19/landing-and-taxi-lights-in-c172-comparison/

[wlbragg]

Interesting, I would imagine they get away with it because the "bright spot" is at some point in time (right when they are turned on) the equivalent of the measurement they portray. But the "which would you rather have" is still unequivocal.

[dany93]

As far as the spacial distribution (independently from the temporal degradation): It might be because the luminous intensity (candela or watt / steradian) measured only in a given direction is enough to be able to write this. The direction range is very narrow for the GE version, large for AVEO HERCULES version. An integration, or a distribution curve, of the intensity over a large angle would make the difference.

[okroth]

Correct beam angle of 19° is the angle where at least 50% of the peak intensity is reached; however, that could be a peak 1°, and a flat 18° just above 50%. Doesn't look that way in the photos. And, to measure the candelas, you sit at the point the light goes to. The LED lights seem to have a wide code which means a lot of light is produced (high lumen) and scattered light illuminates the scene additionally, which the narrow cone of the incandescent lights can't. So of course these pictures look better than the measurements. Actually, I wanted to show the photos with the narrow cones, as these might give a hint where to point the lights to. Although I am IMPRESSED of the light show :-)

[wlbragg]

Here is the current image of our landing light when sitting on the runway. Position would be the only adjustment I know of that could be made and I think this is reasonably close to reality. How the position when airborne is affected I have no idea. So if this image is close to correct, I am not sure there is anything that we can do to change the airborne behavior.

We also have the exterior ALS landing light, but again it looks to be reasonably close to the images? I am not sure about what all can be adjusted other than position with this as well.

[legoboyvdlp]

I'm going to investigate this issue - if it needs correcting, I will, if not I'll close this :)

[okroth]

Found in the service manuals that the landing light uses GE 4596 or 4591 lamps, and the taxi light is a GE 4587 or 4626 lamp. They have different light distributions; Landing light 4591 = 100W 12°H, 6°V 4596 = 250W 11°H, 12°V Taxi light: 4587 = 250W 40°H, 12°V 4626 = 150W 40°H, 9°V The GE spec says that the angles define the 10% intensity border, i.e. where light intensity dropped to 10%