GPWS Windshear Mode

[Bally Heck]

Does anyone have any info on the parameters which trigger the windshear mode on a GPWS?

I refer to the steam driven type rather than the new fangled predictive ones.

Ta

[Slasher]

Yeh theres a graph I read once. It depends on sink rate at the time vs rate of speed loss in knots/sec. Heres some WS-threshold figures I extracted and comitted to memory for Mode 7:

In level flt the WS alert will activate with 2.3kt/sec rate of decreasing headwind or higher.

In a 1000fpm sink, WSA activates with 1.0kt/sec rate of decr HW or higher

At 2000fpm sink, WSA activates with ANY sustained rate of HW loss.

At 3000fpm sink, WSA activates with anything less than +1kt/sec rate of decr HW or higher (figure THAT one out!)

These thresholds themselves can vary according to available climb perf, FPA, and SAT fluctuations (rapid colder rather than rapid hotter). Exact figures on these items I dont know nor give a rats about but I notice they do make a diference at times.

That graph had the new-fangled predictive WINDSHEAR CAUTION range for EGPWS but you said Bally your not interested. And neither am I to be honest. WS alerts make enough damn racket without this bloodey CAUTION thing thrown in too.

[Bally Heck]

Thanks Slasher. There is no information that I could find in a couple of hours googling about this, and an equal amount in Boeing manuals.

I assume that to have windshear mode, the aircraft must have IRS or GPS or similar to measure headwind?

Impressive memory by the way!

[OzExpat]

Bally ... the fact is that there's very little that Slash forgets!

[Kennytheking]

Another question

Hi guys,

Have searched this forum for info on windshear detection in order to understand. I have now seen that we have predictive detection which I believe uses the weather radar.

Is this a new concept? I fly an older aircraft(DC9) and am embarrased to admit that I do not know all that much about windshear detection. How does the re-active windshear detection work? I gather it calculates it from the air data comp, based on thresholds mentioned above.......so what exactly are the inputs that are required?

There is a theory going around our company that the radar must be on for the windshear detection to work. I don't believe it.......but am hesitant to dispell it until I know the facts. Being an old aircraft I doubt we have predictive windshear detection.

Any takers?

KTK

[brain fade]

The W/S mode is really helpful on our a/c type but I've seen pilots 'induce' a w/s caution due clumsy thrust lever handling. Also, gigantic windshears felt 'through the arse and seen on the ASI' go unmentioned by the WS system

ME? never had one but............ always wondered about the SOP which states 'leave config as is'.....why not at least take in the gear or the drag flap.

[NoseGear]

EGPWS

The modern W/S systems have 2 modes, Alert or Caution and Warning. In the first case, the EGPWS will detect an increasing headwind or updraft, thru the EGPWS inputs that it uses for all modes, VSI, ASI, RA, Barometric ALT, Gear and Flap position and aircraft position to the EGPWS database, which has GPS input to it as well. This will give an amber alert and not usually escape guidance, as its not really required with an alert, more to let you know whats happening, if you don't feel it thru your arse as mentioned! The warning comes with a red visual warning, audio alerts, and Flight director escape guidance. The older GPWS systems should give the same warning, but no cautions/alerts. However, I can't quite remember when I last used that system. Getting older so had to "download" that info so I could remember all the new items!

Kenny, I stand to be corrected, but I believe the WX radar has nothing to do with it as it only "sees" precipitaion and converts that to rainfall rates thru color. I don't know of any radars that can see wind direction and velocitys.

Slash, looking at this graph in our PRM, the rates you've mentioned look to be the same, so perhaps its an accepted industry standard.

Nosey

[Continuous Ignition]

Some good info here.....Honeywell EGPWS Website