Radar Tilt Management

By Dave Gwinn

Reproduced by permission of the author

Pilots who have used both radar and sferics devices (Storm-scopes/Strike Finders) will tell although both are effective at detecting dangerous weather radar has more potential to tell you what’s inside that threatening black Cb up ahead. But to truly capitalize on radar’s powerful capabilities have to tell it where and how to look and then make sense of the information it displays.

A typical airborne radar has several controls, including range selection, gain, weather and mapping modes. But none of these controls are more critical – or more misunderstood – than the radar-tilt control. Deftly adjusted, the tilt control points the radar’s beam squarely at the weather you wish to examine.

You’re not likely to miss much. Misadjust or misunderstand tilt, and you could lose track of dangerous weather in the confusing clutter of ground return or fly right under a killer cell, without realizing it.

When explaining tilt, I like to use the flashlight analogy. Like a flashlight, radar has a similar focus and beam. The f1ashlight’s reflector acts upon light’s tiny wavelengths and concentrates its energy into a piercing beam. But whether it’s radar or light, a beam loses its ability to focus with distance, as the beam disperses. At great distance, the target is too poorly illuminated for us to know much about it.

Think too, of how you use a flashlight. You search for things in the darkness. You don’t simply “set and forget” it and hope that what you want to see will eventually wander into the beam. Instead, you pan the scene ahead, searching for hazards and items for interest. It’s exactly the same with radar. With a flashlight, you can scan vertically, horizontally and even behind you with unlimited ease. Obviously, we can’t do that with radar. But we can still see what’s coming and the tilt control offers all but unlimited ability to examine the scene ahead vertically. Here’s how you use it.

WHAT IS TILT?

Most airborne weather radar antennas sweep from left to right, through 60, 120 or 180 degrees in front of the aircraft. In typical general aviation radars this sweep is not controllable by the pilot. What the pilot can control is the vertica1 sweep or tilt. Most radar units permit plus to minus 15 degrees of tilt as they scan left to right

The typical airline or corporate radar is stabilized, meaning that this plus or minus 15 degrees is measured from a radar platform that’s always parallel with the earth’s surface How’s that done? It is usually through a dedicated gyro or from inputs from inertial navigation systems.

In unstabilized radars, zero-tilt equates to the longitudinal axis of the aircraft and that’s obviously going to change through climb, cruise and descent. If the airplane is 5-degrees pitch down and the antenna is unstabilized, then the plus 5 degrees up-tilt position is actually level with the horizon Therefore, a plus 5-degree tilt is actually “zero” for a leve1 beam center. So, in an unstabilized system, deck angle must always be applied first, before calculating true tilt. Most high-end GA aircraft have stabilized systems although some older installations may not. Check your radar manual to see which you’ve got.

Beam diffusion, or the tendency of the radar beam to widen with distance from the antenna, is directly related to antenna diameter; the smaller the diameter, the more the radar energy is diffused with distance. The typical general aviation radar has a 10-inch antenna and a 10-degree beam; radars found in corporate airplanes typically have 12-inch antennas with an 8- degree beam. The 30-inch antennas in airliners yield a 3-degree beam. At 80 miles, the three antennas have cones of energy that are 80,000, 64,000 and 24,000 feet respectively, top to bottom, left to right.

You’ll recall that the ability to accurately display weather was predicated upon beam-filling meteorology, meaning that all of the available energy the radar puts out hits the target of interest and enough returns to form a display that the pilot can see. When the energy misses the target – as it inevitably does at great distance from the antenna – the lost returned energy tends to lead the pilot to understate the strength of the target. Think of your neighbor standing on his patio at night, illuminated with only a diffuse, relatively dim outdoor light. You’d fail to see details that a powerful spotlight would instantly reveal.

A LITTLE MATH

Of course, in the above analogy, it matters where you point your spotlight. If you aim it at your neighbor’s feet, you’d know he was wearing black shoes but you’d miss his red shirt. If you aimed too high, you wouldn’t see a thing. The idea is to ”center your beam” to illuminate the target to best advantage. With a flashlight you can see where the beam center is, with radar you’ll have to calculate it.

Conveniently, the same formula that applies to beam diffusion works with tilt and beam-center calculation: 1- degree of tilt up or down moves the beam center 1000 feet up or down at a distance of 10 miles from the antenna. So, when you nudge the tilt 1 degree, the center of the beam moves 6000 feet (up or down) at 60 miles, 10,000 feet at 100 miles and 18,000 feet at 180 miles. Notice the pattern here? Simply add two zeros to the range and that will tell you how many feet you’re moving the beam up or down.

Okay, fine. So what? Now that you know where the beam center is, where are you supposed to put it? I can give you a pretty good idea. In the convective environment that gives birth to thunderstorms, the diagnostic altitudes are 18,000 to 25,000 feet. And what goes up that high, will usually come down. Sometimes with enough energy to bring down an airliner. Thunderstorms go up; non-hazardous rainshowers maintain their low profile. The FL180-FL250 altitudes are where the severe storm symptoms occur and it’s where the NWS looks when it’s forecasting storm intensity and hazards.

Simply stated, from low altitudes, we’d like to look (tilt) up into that area. From high altitude, we’d look (tilt) down.

PUTTING IT TO WORK

Now that you’ve grasped the math of the situation, let’s consider some basic tilt management strategies for various phases of flight, including take-off, cruise, approach maneuvering and so forth.

Let’s look at takeoff first. With their sweep-the-horizon capability and long range, sferics devices have an edge on radar when it comes to making pre- takeoff decisions. But radar is not entirely blind from the ground. If you point the airplane in the direction you plan to fly and use the tilt control correctly, you can get a coarse assessment.

Here’s the limitation, however: From the runway it’s only possible to elevate the center of the beam 7500 feet at 5 miles and 15,000 feet at 10 miles. Evaluation of weather nearby the departure end of the runway is not practical with airborne weather radar alone. A good preflight NWS radar picture provides better information. Don’t hesitate to ask approach controllers for assistance, especially if ASR-9 radar is in use. And don’t forget to ask about pireps, too. It’s quite possible that an inbound aircraft encountered vicious shear or turbulence but that tower or approach just forgot to pass it on to you.

While climbing to cruise altitude, use the correct tilt for the altitude you happen to be at. Remember, you want to centre the beam between FL180 and FL250, so until you get to those altitudes, you’ll be looking up. Once above them, you’ll be looking down on whatever weather happens to be in your flightpath.

If there’s one “set and forget” tilt setting, it’s what I ca11 “low-level park.” Being vectored in the terminal area (where most mid-air’s occur) is not the time to be a head-down, full-time radar manipulator. Additionally, your decision to commit to a low-altitude operation should have been made much farther from the aerodrome. So, at altitudes between 12,000 feet and down to 8,000 feet, use low-level park.

If you have an 8-degree beam – that’s a 12-inch antenna – low-level park is a tilt up of 4 degrees. This places the bottom of the beam at the aircraft’s altitude and it eliminates all ground returns. Also, anything that appears in the beam must be equal-to-or-higher than the present altitude. With a 10-degree beam width (10-inch antenna) tilt-up 5 degrees. When something appears in the beam, it’s definitely weather and merits further investigation.

Now, how about terminal operations or on an approach? This is where radar runs up against one of its trickiest limitations. Low-level rainshowers can be highly reflective, but represents no danger whatever. In previous issues I have discussed ”Radar Red” in Florida and concluded that sufficient heavy status rain creates red returns, but represented no hazards.

Red in arid Kansas, however, always means an invitation to Oz. But we know that thunderstorms have height, and radar determination of a hazard is validated by how high those clouds are. If they’re low-level, red usually means very little. If they’re at 20,000 feet or above, watch it. Height is often a good indication of severity.

With 15-degrees of tilt-up and short-range distances of 5 miles or so, you can readily see the problem. (Multiply the range by the tilt and add two zeros. It’s always the same math.) So, 5 X 15 equals 75 and adding two zeros gives us a beam center altitude of 7500 feet above ground.

The outer marker is approximately 5 miles from the airport. The airplane is 1500 feet AGL at the marker. With full- scale 15-degree tilt-up, the beam is 7500 feet above the aircraft or centered at 9000 feet AGL. This is inadequate elevation to assess any convective threat.

Remember the horrific Delta L-1011 crash at Dallas in 1985? It’s believed that the aircraft’s radar never painted the actual threat. At best, the Captain saw Level 2 (moderate) returns that correlated with “rainshower” reports he’d received. Actually, Level 4 (horrid!) returns existed above FL200, well beyond the radar’s reach from 1ow altitude.

Conclusion: Committing to a low-altitude operation must be based upon radar inspection of terminal weather from at least 15 miles from the airport when tilt versatility still exits. The airplane will be at lower altitudes at 15 miles, around 7000 to 9000 AGL. At 15 miles, you can skew the beam up to 22,000 feet and you’ll have a better chance of seeing what could kill you at the outer marker long before it has a chance to get at you.

DEFENSIVE RADAR

With the exception of “defensive radar use,” setting and forgetting the radar beam is an opportunity to miss seeing weather. Scan, scan, scan. Defensive radar use is appropriate in IMC weather, when no hazard is forecasted and you want to be alerted to one getting born. Your suspicions are not high, but preparation is in place.

Defensive radar use would be to set the beam at the appropriate altitude about 40 to 60 miles in front of the aircraft and scan it occasionally. Set it at the FL200 level at an appropriate distance. Another method is to lower the beam, creating ground returns painting across the top 1/2 inch or inch of the radar display.

Oncoming weather will first be obscured in the ground returns, but any- thing advancing toward you and out of the ground returns is weather. With an 8-degree beam, you’d be observing from ground level to 48,000 feet MSL, with ground returns at 60 miles. With a 10-degree beam, ground returns would begin at 40 miles and the top of the beam would be at 40,000 feet MSL. Whatever moves out of the ground returns and toward the airplane has to be radar-detectable weather.

Without lengthy commentary, I consider the following uses of tilt control to be nonsense and impractical:

·  estimating the height of a thunderstorm,

·  assessing safe terrain over-flight,

·  navigating with airborne weather radar,

·  calibrating beam and receiver sensitivity with ground returns and attempting to fly the aircraft using ground returns in the radar display as an artificial horizon.

I’ll be happy to discuss any of those with an inquiring reader.

The interpretation of what radar sees comes with experience, education and yes, even instinct. But if the beam isn’t pointed toward the right area or is so altitude-limited as to make effective scanning impossible, all the experience in the world won’t yield the correct decision. You have to see it to avoid it, visually or on radar.

Dave Gwinn gives world-class seminars on radar and has an audio-tape course on the subject available. For information, contact him at Flight Operations Training, P.O. Box 6423, Kansas City, Kansas M106.