How to Interpret Low-level Wind Shear Alert System’s (LLWAS) Warning Data

a case of Mircrostep’s LLWAS

Muhammad Ryan
Apr 19 · 5 min read
Photo by Marcus Ng on Unsplash

Hi, actually this is my resume for the Low-level Wind Shear Alert System (LLWAS) system. LLWAS is the system to detect the Low-level Wind Shear (LLWS) phenomenon. I write this primary for myself so I can “bind” this knowledge because this is very important for me.

WARNING! Almost what in this article is my opinion.

Here I try to grasp the information in the LLWAS’s warning data using available resources around me. So, feel free to correct me if you see something wrong. I will update this article when I gained new knowledge about this or there is something wrong here.

Ok, to the point. If you have LLWAS warning data (in my case, I have Microstep LLWAS data), it will be like this.

LLWAS warning data sample

It’s CSV data with some beaders that are

So there are a time and any other header that are the state of the runway (RWY). FYI, a runway naming is based on compass bearings where the runway extends.

Runway naming is consist of 01 to 36 where 01 mean 10 degree and 36 is 360 or 0 degree.

You may think that the ender side of the runway like below will be named like this

But no, this is wrong

The right answer is

Err, what? this is counter-intuitive (at least for me). The naming system is based on what direction the aircraft will be heading when it wants to take off or landing when using the runway.

source: https://en.wikipedia.org/wiki/Runway

In the LLWAS’s warning data here, 1 of the header is RWY 07LA. We know RWY is an abbreviation for “Runway”, 07 is the name of the runway, but what is LA? So A is Arrival (Landing) the opposite of D for Departure (Take-off). What’s left is L! Before that, I have to tell you that the airport that we observe here has 2 Runway, so L and R (the opposite of L) is the abbreviation for Left and Right. Wait, left and right is different depending on the perspective. Whose perspective did we use here? The perspective of the aircraft! So the LLWAS data here is the data of the airport with 2 runway. This convention is consistent with the naming system. The naming is based on what direction the aircraft will be heading to if it takes the runway from the perspective of the aircraft.

Now why there is A (Arrival) and D (Departure) in the information of the LLWAS? Because what part of the runway the aircraft cares about the most when in landing and takeoff is different. I will elaborate on this later in this post. The point is, the warning information from the LLWAS is intended to help the pilot in the aircraft.

Here I give a simple illustration of our runway.

The area that the LLWAS will cover is divided into 4 quadrants, that is 07L, 07R, 25L, and 25R. But we can’t choose who is where if we don’t have a fixed aircraft perspective. So for the sake of this example, here we assume the aircraft comes from runway 07 like below so we can divide the quadrant area.

In the figure of the LLWAS warning data previously, we can see the value of NIL followed by the name of the runway. NIL means there is no LLWS occur at that moment so I will give you new sample data. Now for example there is an LLWS warning at 15-02-2020 07:39:50 UTC.

First, let’s assess in what quadrant that LLWS (WSA) happens. The value of this row except NIL is 25RA WSA -15KT 1MF and 07LD WSA -15KT RWY. So, this is happening in quadrant II.

Yeah, it says 25RA but why not quadrant I cause it also mention 07LD? Cause when you landing (arrival) and take-off (departure), what part of the runway you care about the most is different. When landing, you care about the first half of the runway that the aircraft will first touch on the ground.

And on the other hand for the take-off, what the aircraft care about the most is the second half of the runway where the aircraft will ascend to the sky.

Now 1MF and RWY are referring to where exactly the LLWS happen. 1MF means it happens from 0 to 1 nautical mile from the start of the runway and RWY means the LLWS happens on the runway. Now it says RWY in the data, why I am sure it happens in quadrant II while runway 07 is covering quadrant I too? Because in the column of RWY 25RD and RWY 07LA is NIL. The opposite of MF is MD which means is that it happens at the end of the runway. 1MD means it happens 0–1 nautical mile from the end of the runway.

Now we will try to decipher the intensity of the LLWS? it says WSA -15KT, what the meaning of this? WSA means Windshear Alert. This is the phenomenon. The other phenomenon that LLWAS can detect is MBA which means Microburst Alert. -15KT means there is a headwind loss of 15 knots for the aircraft. This means a tailwind. Headwind is the wind that comes from the front of the aircraft and tailwind is the wind that comes from behind the aircraft.

That is for now. Feel free to tell me if you find something wrong or confusing with my explanation.

References

https://en.wikipedia.org/wiki/Runway accessed on 19 April 2021

Microstep’s LLWAS manual book

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Muhammad Ryan

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Meteorologist | Coder | Problem Solver | Writer | Daydreamer

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