Composite Mode Radar Scare

The mission was a short one. Fly across Lake Michigan from Milwaukee to my home base at Muskegon on the Michigan shore. The trip is less than 70 miles. But looking at the Nexrad radar picture before takeoff made that short flight look like it would be a big challenge.

For days thunderstorms have been pounding the upper midwest and Great Lakes. Many of the storms contained straight line winds powerful enough to flatten trees and knock out power. But heavy rain has been an even greater threat with slow moving storms dropping several inches of rain on the same spot in an hour or two.

Nexrad radar has done a good job of showing where the storms are, and where they are headed. You can see Nexrad radar continuously on TV, on hundreds of online sites, and in our airplanes. People who don’t know a cumulonimbus from a cirrus cloud are constantly checking the radar on their phone or iPad. And I don’t know a pilot who takes off without checking radar if there is a cloud in the sky or a chance of precip in the forecast.

Nexrad images are compelling. The color coding that ranges from a cool blue through a magenta to a dark red appear to instantly show where the trouble lies. We have been conditioned in all areas of life to take green for granted, start to pay attention when something is yellow, and go on high alert at red.

Weather radar didn’t always work this way. Before the late 1970s radars used a display storage tube (DST). Radar echoes appeared on the DST but immediately began to fade. Radar operators worked in darken rooms so they could more accurately interpret the brightness, thus the strength, of radar returns on the DST. Pilots couldn’t darken the cockpit so many radar displays had a hood over them. You pressed your forehead against the hood to block the light so you could see the dim radar image on the DST.

Bendix changed all of that when it perfected the digital weather radar display in the 1970s. What Bendix engineers did was to chop up the radar returns into three levels of strength. Each of the three levels were then shown as a distinct brightness of green on the display. Because the three levels were distinct they could be shown continuously without fading.

Whala. With the Bendix radar all of us pilots were radar interpretation experts. No need to see how bright a target is, or how quickly it fades. With digital radar we worried only a little about the dim level one, stayed away from middle brightness level two, and gave level three returns a wide berth.

RCA was first to make the next step and assign a color to each radar return. Color made it even easier to see where the heavy stuff was but it didn’t do a thing to eliminate the ability of a weather radar to mislead. And Nexrad is no different. Nexrad appears to present an unambiguous picture of the weather, but it often isn’t.

That was the case for my flight from Milwaukee to Muskegon. The Nexrad image I looked at in the FBO, and then again on the satellite system in my cockpit, showed almost the entire route covered by medium intensity yellow returns. There was no rain at Milwaukee and the clouds were starting to break, but just a few miles offshore the radar showed significant precipitation.

I took off and climbed to the assigned altitude of 5,000 feet. The air was smooth, and there was no precip. Muskegon approach controllers, who look at the same Nexrad returns I was seeing, asked for my flight conditions, I’m sure wondering why I wasn’t complaining. I told them I was in the clear with an overcast that looked to begin at about 8,000 to 10,000 feet. Smooth ride. They were surprised. I flew the visual approach and landed, all with yellow Nexrad targets over the airport.

Nexrad wasn’t lying, only misleading. I was seeing “composite” mode where the Nexrad antenna scans at multiple levels up and down. There was precip up there, somewhere, and the Nexrad composite mode could see it. In “base” mode the Nexrad antenna scans at a fixed tilt angle just above the horizon. When I looked at the Nexrad online and could select to see either composite or base mode, the precip disappeared when I looked at base because the precip was not falling low enough to get to the base antenna angle sweep.

The controllers wondered how I was doing, Stancie, who had been looking at the radar, asked how rough the ride was, but I sat there flying in good VFR.

We can’t and shouldn’t ignore Nexrad but we need to understand how composite mode works. If you can stay out of the clouds it really doesn’t matter what the composite mode shows. If you can’t stay clear of clouds and the Nexrad shows precip, then it’s time to deviate.

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7 Responses to Composite Mode Radar Scare

  1. Roger Halstead says:

    Saying Red is bad is not necessarily so. In composite mode the color merely indicates the degree of precipitation. You really need to look at the winds at various altitudes(tilts), relative winds (in relation to the RADAR site, and lightning. Stability or lifting factor is handy too. Torrential rain does not necessarily mean a dangerous or even rough ride.

    One of the smoothest and picturesque rides I’ve had was an IFR training hop some years back, from Midland (KIKW) to Lansing (LAN)where I shot the ILS, then on up to Alma AMN for the NDB and back to Midland for the VOR 06

    Most of the trip was through a large area of RED and Yellow and we were flying through those echos, not over or under. Only the departure from Midland was VFR while the approaches were at or near minimums except the ILS at lansing. We were between layers from Lansing to Alma which was beautiful. Outbound on the NDB gave an unbelievable sense of speed as we were skimming the top of the lower layer and shooting through the occasional extension. Inbound and we immediately descended into very dark clouds with heavy rain. We broke out right at minimums with the runway dead ahead in moderate rain.

    On the missed we headed back to midland where MBS told us to expect the visual. I replied that I didn’t think that would be possible as we were seeing a lot of lightning just to the West of Midland.

    We were able to pick up the VOR-06 with vectors and broke out just a few miles from the airport just above minimums. Fortunately the Lightning was far enough West, not to be a problem

    The point is that although RADAR was painting mostly Red and Yellow, we had a smooth ride albeit the Deb was thoroughly washed. The only lightning we saw was the one cell to the West of Midland.

    Although Midland is less than 12 miles from MBS the conditions were quite different than what MBS was reporting.

  2. Drew Steketee says:

    Good, educational insight, but “Whala?”
    You are kidding, aren’t you, Mac?


  3. Cary Alburn says:

    Yeah, Mac can’t spell too well.


  4. Ron Maimon says:

    Excellent article! Really enjoyed it. Thanks!

  5. 60AV8TOR says:

    Halstead, sincerely hope you were the trainee and not the CFII on your ‘hop’. I understand the point of Mac’s article – and it is a good one – composite NEXRAD doesn’t distinguish altitude/flight levels. The logic (if I can liberally use the word logic) of your comment, however, is ridiculous – essentially, you walked through a mine field and happened to not step on a mine; therefore’ walking through a mine field is not dangerous..? Going “through a large area of red” on the NEXRAD in VMC is one thing (the point of the article), but your comments of “descended into very dark clouds with heavy rain” and “Fortunately the Lightning was far enough West, not to be a problem” – all on a training flight – WTH? So how many times do you plunge into very dark clouds and red returns, until your wx expertise and good fortune run out? I usually do not respond to such things, and I do not mean to sound unnecessarily critical – perhaps I am inferring things you are not implying from your article – but I just find your whole story an example of poor ADM. Who knows how many NTSB reports were only a few minutes too late to end with an internet write up such as yours…

  6. Bill Simpson says:

    Voila is an established French expression translating as “there it is”. “whala” and “walla”? Ugh…

  7. Tim Kramer says:

    I have found that what produces the yellow level precipitation and the gradient spacing of the next higher (Red) or lower (green) precipitation is important. Convective activity with abruptly rising and sinking air makes for unpleasant flying. It needs to be avoided. But a large low pressure system can lift moisture to high levels where it gives a moderate radar return but there is smooth flying and light drizzle below. I once flew four hours (650 nm) in solid yellow returns under a winter low in smooth air with only very light drizzle just above freezing. This was from southeast Michigan to Kansas City. Clearly all the moisture and water droplets were well above my 4000′ cruising altitude.

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