wx radar on 737NG question~

[citizensun]

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

[BOAC]

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!

[Denti]

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.

[alistomalibu]

Hi

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

[citizensun]

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.

[alistomalibu]

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.

[FCS Explorer]

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

[citizensun]

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~

[STBYRUD]

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.

[citizensun]

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?

[zkengr]

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...

[STBYRUD]

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.

[citizensun]

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

[Checkboard]

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

[citizensun]

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~;)

[BOAC]

surely? - 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.......................

[ImbracableCrunk]

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? ;)

[maggot738]

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

[Graybeard]

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

[Denti]

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.

[STBYRUD]

Graybeard: The WX700 has a peak output of 10 kW, if even the standard kitchen microwave magnetron has a kilowatt of power (and can do plenty of damage) I surely do not want to be in front of any weather radar when its operational.

In any case, the critical point is that it often enough takes less than 12 seconds from setting takeoff power to reaching 80 knots before the first warnings are inhibited.

[Graybeard]

Don't know where you get the 10KW figure STBYRUD, but the WXR-700X is 150 watts peak power, typical 1 microsecond pulse width, 180 to 1440 PRF, pulse repetition frequency. You can do the math to see it's less than one watt average power.

Here's the spec from Collins on the WXR-2100 Multi-Scan Weather Radar:


Transmitter
PRF 180 (up to 3,000) pp/s
Pulse widths 1 to 25 microseconds
Frequency (direct 9.33 GHz digital synthesis)
Peak power 150 watts nom
SPECIFICATIONS SUBJECT TO CHANGE WITHOUT NOTICE



3000x25=75000 microseconds. Divide by a million, times 150 = 12.5 watts average radiated power, max.

OK, I'll concede the 12 seconds to get to 80 knots, but I doubt it takes that long for alerts to be active. If true, it's no doubt worst case. Automatic antenna calibration is four seconds; the rest is solid state, and the first active sweep will identify doppler shifts.

I still can't fathom why Boeing would change from oil pressure plus transponder to TLA (alone?) One less wire?

GB

[EW73]

Our 737-700 fleet FCOMs carry a warning regarding the advancement of the thrust levers beyond the T/O config setting, due to the automatic initiation of the PWS, with or without engines running. It even goes on to mention trying to limit breakaway thrust during taxi to avoid the PWS initiating!
This is designed as a safety measure for external personnel.

EW73

[citizensun]

zkengr, what you said:"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."
this afternoon i tried this,(with IRS not aligned) but there was no PWS FAIL flag showed up。。。。
why?

[STBYRUD]

Graybeard - I stand corrected, I had the rated transmitting power of the transmitter in mind, havent seen the actual power that the radar is using.

[Captain Smithy]

Well, to get a Tx output of anything like 10kW you'd need a whacking great old-fashioned microwave tube, e.g. a TWT, Magnetron, CFA etc., which would certainly cook you nicely... :uhoh: I might be wrong but I think the WXR-2100 is a phased array system equipped with solid-state Tx/Rx modules which tend to produce a much lower power, certainly along the lines of the quoted 150W peak.

Added to the tiny pulse width, you have a very low average power. Still not a good idea to stand in front of one IMO, but there again I want to err on the side of safety so I can have kids when the time's right... :uhoh:

Microwave tubes tend to be a thing of the past these days, all solid-state now.

Funny how it's always the same folk who post on radar threads... :)

Smithy

[Graybeard]

You guys ever give a thought to being near the DME antennas?
500 watts peak power
1.1 GHz.

Same goes for TCAS and transponder.

GB

[Spendid Cruiser]

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.
I fly the Classic so I'm not entirely sure, but I seem to remember reading in the Bulfer guide that with PWS enabled, there are four radar sweeps of 4 secs each, something along the lines of WX capt, PWS capt & f/o, WX f/o, PWS both again. So it takes 16 secs to complete the cycle, but PWS can be detected on the second sweep (max 8 secs).

So I'm wondering if this 12 secs relates to how long the F/O has to wait for the WX paint on his side (if PWS is active) and not to how long it takes for PWS to be effective?

[Denti]

It seems the 12 seconds is simply a resultant thing from the fact that at least the WXR-2100 needs 11.2 seconds anyway to complete a sweep cycle in windshear mode. By the way, display sweep and antenna sweep are nowadays independent as the display sweep is done out of memory.

The following is what the manual says about some of that topic:

The total time required to complete one cycle of the MultiScan process in all modes except windshear is 8 seconds. When in windshear mode, the total cycle time for both MultiScan and windshear is 11.2 seconds. Thus, there is no significant change to observed weather during one cycle of the MultiScan process. What does change is the relationship of the aircraft to the weather. To compensate for this, MultiScan translates (figure 3-8) and rotates (figure 3-9) the stored digital image to compensate for aircraft movement.

The result is that the Collins MultiScan updates all radar displays every four seconds in all modes except windshear, in which case the displays update every 5.5 seconds. One interesting element of this process is that the antenna scan is no longer tied to the display sweep. This frees the antenna to perform multiple functions without interrupting the pilot’s weather presentation.