wx radar on 737NG question~
 

the following words is quote from my company fcom:
"The weather radar automatically begins scanning for windshear when:
• thrust levers set for takeoff, even if engine is off or IRS not aligned, or
• in flight below 2,300 feet RA (predictive windshear alerts are issued below 1,200 feet RA).
737-800
Alerts are available approximately 12 seconds after the weather radar begins scanning for windshear. Predictive windshear alerts can be enabled prior to takeoff by pushing the EFIS control panel WXR switch."
my question are:
a. thrust set for takeoff means the TLA >53° or the thrust is set at predetermined N1?
b. alerts are available only 12 seconds after the weather radar begins scanning for windshear, does that mean without the wx radar turned on during takeoff roll we only have windshear alerts till that 12` passed? and within that 12` we may already in the air!!!
someone know about this?
thanks~:D:D:D

TLA I'm pretty sure

"we only have windshear alerts AFTER that 12` passed?"

"and within that 12` we may already in the air!!!" - somewhat unlikely!

If you suspect there is windshear-prone weather in the area it might be a good idea to switch to wx-radar before you start you take off roll, that way you have immediate PWS function upon reaching that TLA.

Hi

12 seconds? What type of wx radar are we talking about? Collins WXR-2100 MultiScan radar?

some of my 737-800 are equiped with the wxr-2100 and wxr-700x, but the fcom doesn`t state what kind of wx radar has the 12` characteristic .so i think it refers to both type.
and i don`t think it`s reasonable~

i will turn on the wx radar before takeoff no matter the weather is ,just in case.

From the Collins WXR-2100 manual:

BOEING philosophy

Takeoff:
Boeing alert activation regions for takeoff: Warnings and Cautions are
enabled from the beginning of the take off roll (0 knots) until the aircraft
reaches 80 knots. From 80 knots until the aircraft passes 400 feet, only
Warnings are enabled. From 400 feet through 1,200 feet, Warnings and
Cautions are enabled. All alerts are disabled from the time the aircraft
passes 100 knots until it reaches 50 feet. Boeing aircraft do not display
Advisories.

Landing:
Boeing alert activation regions for landing: Warnings and Cautions are
enabled from the time the aircraft passes 1,200 feet until 400 feet.
From 400 feet until 50 feet, only Warnings are enabled. From 50 feet
until touchdown (0 feet), all alerts are disabled. Boeing aircraft do not
display Advisories

Maybe the "power up" is related with those 12 seconds??:

Windshear alerts are active in the cockpit below 1,200 feet AGL.
However, the WXR-2100 radar actually enters the windshear scanning
mode at 2,300 feet AGL to provide time for the system to power up
(if necessary) and update the displays before the aircraft reaches the
1,200 feet AGL level.

just look how long it takes the engines to spool up...

Warnings and Cautions are enabled from the beginning of the take off roll (0 knots) until the aircraft reaches 80 knots.
so the speed> 0 kts is the trigger for Warnings and Cautions activiation, not TLA>53 with the wx radar on? i prefer the TLA.
during the approach from 2300` to 1200`, there is a power up , but during the takeoff, the 12`s takes too long~

The key statement is that warnings are available 12 seconds after the thing starts scanning - which it does automatically when you set takeoff thrust. Since this can be too late for a timely warning at low speed hit WXR as soon as you line up, then the radar is fully operational once you need it.

STBYRUD
i always do this during the before takeoff check.

for example: if there is a windshear 3 miles in front of the aircraft, the crew is going to takeoff without wx radar turned on, refer to the fcom, it must be 12 seconds after the takeoff thrust set can the wx radar autoscan function provide the windshear Alerts( including the caution and warning) .
but if after that 12 second ,the speed is already passed 80 kts and the new caution alerts are inhibited, or after 12 seconds ,the speed is passed 100 kts which the new warning alerts are inhibited.
in this senerio there will be no alerts at all.
and without the alerts the crew will continue the takeoff roll and finally find that they are in or very close to the windshear area which can be avoided.
is this auto scan take too much time to work or there is some misunderstanding in there?
someone shed some more light?

The PWS is triggered from a microswitch in the Throttle Stand. It shares the same switch as the T/O Config Warning trigger. In other words, when the throttles are approx vertical, independent of WXR switch position.

Next time you get in the cockpit with IRS turned off, advance the throttles until the T/O warning sounds. You will also see a 'PWS FAIL' on the ND.

Without investigating, I've always thought the IRS had to be aligned, so the a/c could identify the windshear in a relative position - but could be wrong.

Point to note; while taxiing onto the ramp, if you use so much power that the T/O warning sounds (however unlikely), you're frying the ground staff with PWS, even if WXR turned off...

citizensun: Nope, thats how I understand it as well - the radar needs 12 seconds to warm up, adjust the tilt (or rather its scan pattern) to provide the first usable results... And yes, most likely you are already going faster than 80 knots at that point (or even 100) so that you won't get much out of it.

thank you STBYRUD~
if it is really the truth,
I think both boeing and collins should improve their philosophy ~

If the weather is bad enough for windshear to be a factor, you'll be scanning the area looking for storm cells and escape paths during the taxi out & line-up in any case. (surely?) :8

thank u all for your efforts~~
someday when i meet some boeing guy and i will ask him this question, if the anwser is different from what we discussed here, i will tell you guys~;)

- well, you or I might - juggling the radar, planning the departure, talking to the cabin etc etc, but in my experience a lot of the F/Os do not even notice the weather some times, until.......................

.........until their coronation into the Left Seat? [trumpet flourish]

Is it the fourth stripe or the scepter that grants this mystical power? ;)

Clear to line up, Weather Radar on. Problem solved. This is the SOP in my outfit.
Cheers
Maggot

12 seconds from Throttle advance to liftoff? You'd have to be flying that 737 real empty, or with a 60 knot headwind. Typical is what, 30-35 seconds?

I don't know about the NG, but the retrofit Forward Looking Windshear systems were turned on by oil pressure on at least one engine, and Transponder out of Standby. I can't imagine why they would have changed it.

Weather radar nuking ground crew? Forget it. The average power from a modern Wx radar is less than one watt, as opposed to your home nuke with about 700 watts. The DMEs, ATC transponders and TCAS all radiate equivalent power to the Wx radar.

GB

Not 12 second to lift off, but from 80kts on all new caution alerts are suppressed (until 400ft) and from 100kts all new warning alerts (until 50 ft) so that is the main issue.

PWS is activated by thrust lever angle above a certain value (53°?) if weather radar is not selected active before that. It even begins to scan if only the TLA is above the required threshold, doesn't matter if IRSs are aligned or engines are off.