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relax.jet
28th Jan 2008, 01:52
What is the trigger to activate B737NG wind shear warning? I’m not talking about predictive wind shear warning but a reactive one.

We were experiencing severe turbulence, speed changes +/- 20kts....and no WS warning at all. I'm going to belive there is none on B737 :}

Thank you.

Happy landings.

Capt Chambo
29th Jan 2008, 10:26
On the 737NG.
Windshear is a function of the GPWS. Predicitive windshear is a radar function.
To answer your question: FOM Vol. 2. Section 15 "Warning Systems" says that "The GPWS provides a warning when the aeroplane is in windshear".
It goes on to say you will get the two tone siren followed by "WINDSHEAR", the red "WINDSHEAR" on both attitude indicators, and this is caused by "excessive" windshear in the current aeroplane position detected by the GPWS, and that it is enabled below 1500' RA. GPWS windshear detection begins at rotation.
In your case you were either too high (>1500'RA) or the machine decided you were not in windshear!
Hope that helps.

The Real Slim Shady
29th Jan 2008, 10:32
It does work!!

We had a windshear warning taxiing out at Valencia as we approached the hold for 12 with a huge thunderstorm at 5 miles on the approach.

BOAC
29th Jan 2008, 11:27
relax - I have no idea what the 'triggers' are - like you I have been in pretty hefty w/s in 200/300/400/500 and 700 and not had a warning. The tech manual studiously avoids any mention which leads me to believe it may not 'be fitted' on the NG? If it is, I suspect that like the Classic is is only enabled at lift-off and below 1500' as mentioned here. Even the Classic manual does not mention parameters.

Ka8 Flyer
29th Jan 2008, 11:37
@Slim, that warning was the work of the PWS and not the reactive windshear alert function as it is only activated once airborne (has to be able to measure differences in velocity)

RMC
30th Jan 2008, 22:12
So how does PWS work off the radar...movement of water droplets?

250byThePark
31st Jan 2008, 00:11
That's right - the predictive windshear system uses the doppler data from the weather radar. i.e. it detects horizontal movement of water droplets relative to the aircraft position.

250byThePark
31st Jan 2008, 00:26
Point of discussion though - would you wait for the system to say it's actual windshear? In the interest of safety if you consider the aircraft to be in windshear would you wait for it to say so - how do you know it ever will? Worth thinking about...

BOAC
31st Jan 2008, 08:48
In my experience airline Ops manuals define 'severe w/s' and this is the 'guide' we use. I believe the automatic system is just there as a backup.

Not sure whether your post includes the 'predictive' but if so, again, that is a mandatory take-off 'STOP' call in my experience.

sani-com
1st Feb 2008, 11:33
According to the BMM, windshear warning from the GPWS is a function of rate of change of headwind and amount of updraft/downdraft. There's a picture of a graph in the manual but I'll try using less than 1000 words - x-axis is increasing/decreasing headwind in knots per second, y-axis is updraft/downdraft in fpm. Draw a line passing thru the points 1500fpm updraft and 2 kts per second increasing headwind, anything to the right of this line is the windshear caution area and there appears to be no cockpit indication for this. Draw another line parallel to the first passing thru 1500fpm downdraft and 2 kts per second decreasing headwind, anything to the left of this line is the windshear warning area and gives you the "WINDSHEAR WINDSHEAR WINDSHEAR" aural and red "WINDSHEAR" visual indication.

Cheers :ok: