PPRuNe Forums - wx radar on 737NG question~

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- - wx radar on 737NG question~ (https://www.pprune.org/tech-log/426215-wx-radar-737ng-question.html)

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.

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

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

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?

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.

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

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

Quote:

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?

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:

Quote:

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.