Predictive Windshear Detection
 

The Airbus A330/A340 and probably other modern aircraft have a "predictive windshear detection system" PWS, to warn it's pilots about possible windshears or microbursts ahead after take-off or during final approach. (I am not talking about the "reactive windshear detection" which tells you that you are having an energy problem.)

While it is clear to me how to "use" this system, I would like to know how the PWS works and the physics-principles it is based upon. How is it using the weather radar??

NASA did quite a lot of work on windshear back in the mid 90s and there used to be some very interesting papers on their website.

I've not looked to see if they are still there.

MP;)

RZ,

I am not an expert on the technology but this is what I know!

The weather radar can 'see' reflective particles (usually water droplets) and graduate their size based on the microwave energy that is returned. With modern digital electronics the radar can also now detect moving targets using doppler shift principles. This target shift can be quantified on detection and since windshear will always result in movement (usually vertically but also sometime horizontally), the radar can now be used to detect it.

The 'alert' formula is this F= Wh/G - Vz/ TAS
where
Wh = rate of airspeed loss
G = Gravitational Force
Vz = Vertical downdraft in knots
TAS = A/C true airspeed.

F is the alert level and NASA/FAA studies found that this should be set to 0.15 if memory serves. Above this value the warnings are generated.

Note that in 'dry' conditions the wx radar cannot see a downburst.

During take-off the antenna scans in a figure of 8 pattern (laid on it's side) so that the antenna is always looking up and down and left and right to see if there's windshear about.


:ok:

Hallo idg!

• Are you sure that the "predictive windshear detection system" PWS, does not work in dry air without any water-droplets??
• If the WX-Radar is sweeping horizontal “eights”, why can you still use it independently for normal WX-detection?

RZ

RZ,

Indeed yes the PWS won't 'see' windshear in dry conditions. Sometimes it will get energy reflected from dust particles but this is very unpredictable (if you'll pardon the pun!).

You will still be able to use the radar with PWS on, but the rate of updating reduces because the antenna does a normal scan for the radar 'picture' followed by the '8' for the PWS followed by a normal scan again etc.

Cheers idg.

I wanted to share a practicle experience with you, well i work on the 737-NG and experimented by opening up the radome, and pushing the throttle levers full ahead, as most of us will be knowing, when the throttle levers are set beyond 53deg on the ng the radar automtically, begins a PWS Scan, well the scanner never did make the pattern of eight....

hello,

just google `egpws` and you will find loads of info on the system.
windshear detection is a part of the egpws system and works in what is called mode 7.

rgds Vincent

I'm often surprised at how many pilots I fly with who don't know that you have to have clouds or precipitation for PWS to work.