What is "Visual segment = 90m"?
 

Hi, there..

I've been studying many things this forum.

Today, I joined as a member, and this is my first question.

What is "Visual segment = 90m"?

I'm studying Take off minima in low visibility procedures.

What does it mean?

Is anyone explain me in detail?

Sure.

To takeoff in LVO both the reported and actual observed visibility must be above certain minimum values.

The REPORTED vis is usually that which is detected by transimometers (Damn!! Can't ever spell that right!!) and is typically 125m in all three segments (150 for D cat aircraft).

90m Visual segment is what the pilot must actually observe in front of the aircraft before comencing the takeoff roll.

In practical terms, as runway centreline lights are spaced at 15m intervals for CatIII operations, need to be able to see 6 lights in front of you before you can commence the takeoff.

Hope that helps.

Wouldn't seeing (exactly) 6 lights mean that you have a visual segment of just 75 meters? I'd say you need to see 7 lights, which translates into 6 spaces of 15 meters = 90 meters...!

If you can see six lights 'over the nose' then you are most likely fifteen meters behind the first light, so that gives you 90meters.

And since the Runway edge lights ("at each side") mostly are spaced 60 meters apart, he was quite right to do so.

And I'm still curious where the 90m comes from. True, there is one light under the nose so 6 lights = 90m; BUT, it also says that, for most of us mortals, you need 125m or takeoff minima. So, if you can count 6 lights, and assume one under the nose, to achieve 125m there need to be 3 lights out of sight, i.e 9 lights. All very confusing. I have read such a document which states minima 125m = 6 lights = 90m visual segment, but there was no explanation to square up this baffling arithmetic.

Rat 5,

The rest of what you can see is made up of what you can't see :p

When you look out of the window you're looking down at an angle: sitting at a height, looking down over the nose and unable to see directly beneath you. The slant range means the first thing you see is a number of metres ahead of you.

Simply add the 90m to the bit you would be able to see if you could look straight down, and the baffling arithmetic works out fine.

It has always been my understanding (but have no way of really backing it up by rules or regulations), that the area under the nose (the obscured segment) is already accounted for.

The EU-OPS requirement is that (a.o.) "A 90 m visual segment is available from the cockpit at the start of the take-off run". To me, this means that you have to be able to actually see 90m from the cockpit - and the only way to be sure of that is seeing 7 lights (6 x 15m).

This would also make sense of the difference between the requirement for an RVR of 125m (CAT A/B/C) or 150m (CAT D), and the required visual segment of 90m. The difference (35m resp. 60m) would serve to account for (an approximation of) the obscured segment.