Newbie learning to use a weather radar

loco wrote:

As nokicky, I also wasn’t able to find a good resource. There’s difference in antenna size / beam width / effective range between GA and airliner radars. That makes some rules of thumb published online not applicable. An airliner can see farther than a GWX70 with 10 inch antenna and uses different tilt settings.

Yes antenna sizes and power outputs differ but most radar techniques are about knowing your beam width (a function of antenna size) and doing basic trig using the 1 in 60 rule.

If you have an 10” antenna your beam width is approx 10 degrees. 12” = 8 degrees.

In order to use the radar you need to calibrate the tilt. Your beam width in feet is 2 x d x tan(angular width/2) where d is distance from the antenna in nm.

So at 20nm, a 10“ antenna has a beam width of 10 degrees = 3.5nm or 21000 ft.

You first work out how to centre the beam ie have it so that it is aligned with your relative horizon and half of the beam is above you and half below. To do that you tilt the radar so it just gets ground clutter at 20nm (for example). At 20nm, a degree is 2000ft wide. If you are painting the ground at 20nm and you are at 10000agl, the bottom of your beam must be angled down 5 degrees from the horizontal.

An easy way to get the bottom of the beam horizontal is to get ground clutter at 20nm, divide your altitide AGL in thousands of feet by two then tilt the radar beam up by that number of degrees. So painting the ground at 20nm when you are at 10000ft, means a tilt up of 5 degrees puts the bottom of the beam at your 10000ft level. Trammell calls that the TIP position (threat identification position). Anything painted is weather at or above your altitude. Let’s say this is at a tilt of +6 degrees.

You then lower the tilt by half your beam width and this the the neutral position where the centre of your beam is level with your aircraft. With a 10” radar this is +1 degree in this example.

You only have to do this once and then you know the two tilts for your installation.

I would suggest reading the following article for lots of other ways to play with this stuff.

https://www.theairlinepilots.com/forumarchive/aviation-weather/wx-radar-antenna-tilt.pdf local copy

Agreed, basic rules for finding tilt are the same. Using fixed values is not possible and one has to find them for their setup.

Then there’s the tactical part. Once you get a proper image, what to do about it – when to go between, go around, go back. I think it’s useful to learn from someone experienced. Flying solo and trying out things learned online could turn out badly. For me things always happen too fast in such situations.

By the way, how is the radar in your current aircraft compared to typical SET?

loco wrote:

By the way, how is the radar in your current aircraft compared to typical SET?

The Collins 800 radar is very good. It also has a large antenna at 12” so gets a more focussed beam. As a result resolution and range seems a lot better.

No Ice/Ice Crystals on WX Radar!
Water drops yes, ice no!

Snoopy wrote:

No Ice/Ice Crystals on WX Radar!
Water drops yes, ice no!

Sure. But icing isn’t caused by pure ice right? Must have liquid water.

Obviously yes but if the icing is in a low layer, perhaps the pilot will not see it above the ground return, if some 30-50nm away and at FL200?

Never used the radar to avoid icing but I see what you mean Jason and of course thats correct. There is still the misconception out there though that wx radar will steer one clear of hail/ice/ice crystals and that notion can lead to bad surprises without auto/3d radars.

As always the more you pay for the kit the less work you have to do, having attended a very comprehensive lecture by a guy from the manufacturers who supply the radar for the B737NG the conclusion is to leave the radar in auto and the only time take it out of auto is for a brief glance down at TOD.

The radar knows your GPS position and adjusts the picture you get to provide returns based on the average turbulence within that Lat/long grid square and adds graphics for predicted wind shear , there is also an automatic wind shear mode.

This having been said there is more than a little “ crying wolf “ by these Systems and during daylight hours visual observation of the clouds can help to determine if the turbulence is likely.

Radar is not a good icing detector and some of the worst low level icing I have had has been in stratus cloud with the OAT between -5 & -10.

JasonC wrote:

I would suggest reading the following article for lots of other ways to play with this stuff.
https://www.theairlinepilots.com/forumarchive/aviation-weather/wx-radar-antenna-tilt.pdf

Nice article. However, it’s shame it doesn’t contain the figures which text is referring to.