seeking help on wx radar

[eole]

I use a neat little rule of thumb when using the wx radar in the cruise so that the radar looks forward and up, but I don't really know what to do on the climb.
Can anyone share their wisdom on how to set the tilt angle of the radar on the climb? Thanks

[Sick Squid]

eole

Digging deep in my notebooks, I've scribbled down a couple of formulae for wx radar in the past, but don't use them myself... can't remember who gave them to me either, or when, but it was at least 8 years ago!

Radar tilt for climb:

Rate of Climb (fpm)/Groundspeed x2 = 4 x tilt angle

Tilt angle against range;

Delta Alt (100's of feet)/ range (nm) = tilt angle

If anyone can explain these, I'd be grateful too! Me, I just play the wx radar by ear...

Good luck!

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[QAVION]

"I use a neat little rule of thumb when using the wx radar in the cruise so that the radar looks forward and up,...."

Being relatively ignorant in this area, I'm just wondering why you're aiming the radar upwards? Don't you want to look at the weather directly in front of you(along track, vertically and horizontally).

I understand that sophisticated wx radar systems auto-adjust for _pitch_ but not inertial vertical path. Would you be able to use your FPV(Flight Path Vector) display, if fitted to your aircraft, to help you determine your inertial vertical path and adjust your radar accordingly?

Rgds.
Q.

[EchoTango]

If I understand you correctly, you want to tilt the unit DOWN in a climb, so that it looks along your vertical track, rather than along your heading. That is, where you are going, rather than where you are pointed.

So you would tilt it down by the same amount that your angle of attack has increased over your normal cruise attitude.

If this is correct, then Sick Squid's 1st law, if you rewrite it as Angle = fpm/(2 * knots) is a rule of thumb for Angle of Attack. So this gives (at 200kt) 1000 fpm 2.5 degrees; 2000 fpm 5 degrees; 3000 fpm 7.5 degrees. Does that sound close ?

Sick Squid's 2nd Law for looking forward and down umpteen thousand feet is a very good one, except for almost underneath you. 10,000 feet down, 25nm ahead, Sick Squid says 4 degrees down. Trigonometry says 3.76. Same level of accuracy holds in range 2,000 to 30,000; 15 to 50nm

ET

[Shore Guy]

Pretty cool - from honeywell's "avionics zone" website

Terrain Based AutoTiltTM

Airlines are now able to select the RDR-4B with an automatic setting of antenna tilt angle - also known as AutoTilt.

By interfacing the RDR-4B, with the EGPWS, the system will automatically select the optimum tilt setting for weather detection, based on the plane's geometric altitude and terrain conditions ahead. This set-up does away with the over-scan and under-scan associated with manual tilt control. It also overcomes the introduction of ground clutter that can occur when using a tilt setting that fails to account for the terrain ahead.

[HPSOV]

Well, the way I was taught to use the radar was to takeoff with 5 degrees uptilt. Gradually decrease this so that the radar is level at 20,000ft. Still decreasing so that its at basically 2-3 degrees downtilt in the cruise.
The reason you need to have downtilt during cruise is that the radar only reflects off water, not ice crystals. And the top of Thunderstorms are usually made up of ice crystals. So if you were to go along with it level you could fly into something. Another rule of thumb, with 2-2.5 degrees downtilt if a target dissapears inside 80nm you will fly over it, if its still there inside 80nm you will fly into it.
The reason for the uptilt during initial climb is twofold. One the aircraft path is going up, so you want to see where you are going. And secondly it eliminates ground returns which would otherwise obscure genuine targets.

[Sick Squid]

Excellent, ET, many thanks for working those examples! I usually do almost what HPSOV does, start at 5 before the t/o roll, but go straight down to about 1 or 0, then have 2.5or so down by the cruise portion. I always leave it as a bare minimum with the weak ground returns at 80 miles or so, and use the mark one eyeball to fine tune the search area. And have p2 on a different range as well.


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