Boeing When using a flat plate antenna for the first time, some pilots have expressed doubts about the lack of weather targets displayed, pointing out that they could see clouds that were not shown on the radar. Since radar display of clouds is dependent upon moisture content, clouds with low moisture may not have enough reflectivity to be displayed. This misunderstanding has been aggravated by use of flat plate antennas.
The flat plate antenna incorporated in modern weather radar systems exhibits characteristics different from the conventional parabolic antenna. A great reduction in sidelobe energy results in the tilt setting being very sensitive, and its adjustment critical to effective weather detection. It is recommended that the pilot not flying coordinate tilt and range selections with the pilot flying to detect and avoid weather as follows.
Just prior to takeoff, a. short range should be used, such as the 10 nm scale. Tilt the antenna full up, then return the tilt to approximately 7° up.
Shortly after takeoff, tilt the antenna full up and then down to where ground clutter appears, then back to 7° up. If course changes of 45° or more are made, repeat this procedure. As climb angle shallows, reduce the tilt approximately 1° per 2° reduction in airplane pitch.
During cruise, start with a range of 10 nm or 20 nm and adjust the tilt so that ground clutter is just visible on the display perimeter. Select the next higher range and adjust the tilt so that ground returns are just visible. Repeat this procedure for each range up to the longest range to be used. This procedure assures that weather at short range, which would normally lie undetected beneath the radar beam, will be detected. A range greater than 160 nm usually displays significant ground clutter, even at the optimum tilt setting.
Tilt should be readjusted each time the radar range is changed.
When descending, tilt the antenna 1° up for each 10,000 feet of descent. After descending to approximately 15,000 feet it may be necessary to tilt the antenna up for each 5,000 feet of descent to keep the display free from ground clutter.
Boeing Flight Operations Technical Bulletin 757-03, 15th December 1982