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Radar Corner - The use, and misuse of weather radar – Part 2

Radar Corner

The use, and misuse of weather radar – Part 2

This article will look at another of the radar questions that we address on a regular basis. Antenna stabilization is often misunderstood and can cause pilots to set tilt incorrectly. To understand how stabilization works we’ll turn it off. When stabilization is off tilt is referenced to the longitudinal axis of the aircraft (Figure 1). Assume we’re flying along straight and level with 0-degrees pitch, and the tilt set at 0-degrees. 

Figure 1 – Longitudinal Axis of the Aircraft

Take your hand and hold it flat (0-degrees pitch) and put a laser pointer under your hand pointed at the wall ahead of you. Scan the laser back and forth +/-90º and you’ll see a straight line on the wall at the same height as your hand (Figure 2). 

Figure 2 – Laser Pointer at Zero Degrees Pitch

In cruise flight we are slightly pitch up so emulate that by tilting your hand up some and do the same thing – scan the laser back and forth at the wall. Instead of a straight line you now see something that resembles a frown (Figure 3). Compare that to the original green line. This is what stabilization must correct for in flight on a continuous basis. In this pitch attitude the stabilization has to push the antenna up and down by different amounts to follow the green line.

Figure 3 – Pitch Up Pattern Compared to 0-Degree Pitch

Now put your aircraft/hand into a bank and scan the laser back and forth. This time the image on the wall is a diagonal line. These examples show what the antenna would do if you didn’t have antenna stabilization. 

Figure 4 – Turning Pattern Compared to 0-Degree Roll

The purpose of stabilization is to maintain your selected tilt angle “in reference to the horizon”. In other words, if you select 0-degrees tilt we want to see a straight line on the wall in front of you regardless of your pitch and roll attitude (Figure 4).

Figure 5 – A Stabilized Antenna

There are limits beyond which the antenna cannot maintain stabilization but if these limits are exceeded weather information is probably not your primary concern at that point. Typical stabilization limits are +/-30º including the selected tilt angle.

Many pilots incorrectly believe that if they are in cruise at say 2º pitch up that they must account for the 2º when selecting a tilt value. If stabilization is on and working that is not correct. The system will maintain the selected tilt angle in reference to the horizon. If stabilization has failed or is turned off, then tilt is referenced to the longitudinal axis of the aircraft and the 2º would need to be considered. Generally, there is no reason to turn stabilization off unless it has failed.

That finishes up this month’s article. Next time we’ll look at some additional items.

 

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Program Pilot Stephen Hammack supports Honeywell Apex and radar for Flight Technical Services. He can be reached via email at stephen.hammack@Honeywell.com.