### Learning never stops

#### Eddie sez:

Proper radar tilt management can save your hide, you need to understand what the tilt does and how to best use it to detect weather.

Everything here is from the references shown below, with a few comments in an alternate color.

20131128

### An Introduction to Radar Tilt

Figure: Radar beam illustration high altitude 18-inch radiator, from Honeywell Primus 880 Pilot's Guide, figure 5-5.

[Honeywell Primus 880 Pilot's Guide, pgs. 5-5 to 5-6]

• The pilot can use tilt management techniques to minimize ground clutter when viewing weather targets.
• Assume the aircraft is flying over relatively smooth terrain which is equivalent to sea level in altitude. The pilot must make adjustments for the effects of mountainous terrain. The [figures] help to visualize the relationship between tilt angle, flight altitude, and selected range.

The Primus 880 Pilot's Guide illustrates beam widths for 12" and 18" radar plates, not for the 24" plate in the G450. The larger the plate, the narrow the beam. The 18" plate has a 5.6° beam, the 24" plate narrows that to 4.2°. The illustration shows the beam width off center, really just half the beam width. For the G450 in this illustration would show a distance of 11,119' at 50nm and 22,269" at 100 nm.

Figure: Radar beam illustration low altitude 18-inch radiator, from Honeywell Primus 880 Pilot's Guide, figure 5-7.

Here again the numbers need to be compensated for the narrower beam of the G450's 24" plate:

• 2,200' at 10 nm (instead of the 3,000' shown)
• 5,600' at 25 nm (instead of the 7,400' shown)
• 11,100' at 50 nm (instead of the 14,000' shown)

Table: Approximate tilt setting for minimal ground target display 18-inch radiator, from Honeywell Primus 880 Pilot's Guide, table 5-2.

[Honeywell Primus 880 Pilot's Guide, pgs. 5-7 to 10]

• The range at which ground targets can be observed is affected by the curvature of the earth, the distance from the aircraft to the horizon, and altitude above the ground. As the tilt control is rotated downward, ground targets first appear on the display at less than maximum range. To find the ideal tilt angle after the aircraft is airborne, adjust the TILT control so that ground clutter does not interfere with viewing of weather targets. Usually, this can be done by tilting the antenna downward in 1° increments until ground targets begin to appear at the display periphery. Ground returns can be distinguished from strong storm cells by watching for closer ground targets with each small downward increment of tilt. The more the downward tilt, the closer the ground targets that are displayed. When ground targets are displayed, move the tilt angle upward in 1° increments until the ground targets begin to disappear. Proper tilt adjustment is a pilot judgment, but typically the best tilt angle lies where ground targets are barely visible or just off the radar image.
• The following table gives approximate tilt settings for a 24-inch radar required for different altitudes and ranges. If the altitude changes or a different range is selected, adjust the tilt control as required to minimize ground returns.

You can estimate the tilt needed to paint the ground using level flight tilt as a starting point:

### Level Flight Tilt.

Figure: Beam width level flight tilt, from Eddie's notes.

If you know the width of your beam in degrees, you can figure it in feet with a little math.

In the case of the G450, we know our 24" plate produces a 4.2° beam that equates to 44,537' at 100 nm. Armed with this knowledge, if we were flying along at 44,537', we could adjust the tilt so the ground paints right at 100 nm. Raising the tilt 2.1° from that point will point the radar along level flight. With a little more math we could come up with the correct distance to paint the ground for various altitudes:

 Aircraft's Altitude (FL) Distance to paint edge of ground clutter 200 45 250 56 300 67 350 79 400 90 450 100

Table: G450 distance to pain ground clutter for level flight tilt adjustment, from Eddie's notes.

So let's say you are in your G450 at FL 350. Adjust your range scale to put 100 nm on the outer ring and adjust your tilt to show the ground clutter just starting at around 80 nm. Raise your tilt half the beam width, 2.1°, and know that the center of your beam is now point along level flight.

Figure: Ideal tilt angle, from Honeywell Primus 880 Pilot's Guide, figure 5-8.

[Honeywell Primus 880 Pilot's Guide, pgs. 5-11 to 15] Tilt management is often misunderstood. It is crucial to safe operation of airborne weather radar. If radar tilt angles are not properly managed, weather targets can be missed or underestimated. The upper levels of convective storms are the most dangerous because of the probability of violent windshears and large hail. But hail and windshear are not very reflective because they lack reflective liquid water.

The ideal tilt angle shows a few ground targets at the edge of the display.

Figure: Convective thunderstorms, from Honeywell Primus 880 Pilot's Guide, figure 5-10.

Convective thunderstorms become much less reflective above the freezing level. This reflectivity decreases gradually over the first 5000 to 10,000 feet above the freezing level:

Figure: Low altitude tilt management, from Honeywell Primus 880 Pilot's Guide, figure 5-15.

At low altitude, the tilt should be set as low as possible to get ground returns at the periphery only:

### Tilt and Beam Width Impact on Look Down Angle

[Honeywell Primus 880 Pilot's Guide, pg. 5-15] The antenna size used on the aircraft alters the best tilt settings by about 1°. However, tilt management is the same for either size:

[Honeywell Primus 880 Pilot's Guide, pg. 5-15] A 1° look down angle looks down 100' per mile and the bottom of the beam is 1/2 beam width below tilt setting.

At 40,000', setting 0° tilt on a 24-inch radar grazes the ground at 200 nm.

At 30,000', setting 0° tilt on a 24-inch radar grazes the ground at 150 nm.