Radar Corner

The importance of understanding the HSI range selections is that they determine what range the radar is operating in. This is turn determines several other items including AutoTilt, alternate scanning, and INAV range selections. Figure 2 shows a typical cockpit where you can have different ranges selected on each of the HSI and INAV displays. 

Figure 3 shows that on this system the PFD full-range selections can be from 5nm to 2,000nm. INAV map ranges can go from 250 feet to 750nm.The table shows the associated range selections the radar is capable of operating in for the RDR-2000/2060/2100 radar systems. Let’s look at some of the implications of this.

For scenario #1 assume both HSI range selections are 50nm full range. From the table we can see that the radar will be operating in 40nm range. If the INAV map display is set for 300nm the radar will only be able to supply returns out to 40nm. No radar returns should be expected beyond 40nm on the HSI or INAV displays.

For scenario #2 let’s reverse the range selections. Now we have 300nm selected on both HSI’s and 50nm on the INAV map. Now the radar is operating in 240nm range. On the Primus 880 radar system the Altitude Compensated Tilt (ACT) function adjusts the tilt based on altitude, range selection, and the earth’s curvature (Figure 4). ACT attempts to replicate the cruise ground park technique by setting tilt to provide ground returns at the outer edge of the display. The amount of ground returns that actually appear will be dependent on the type of terrain (water, crops, trees, mountains, etc.). The BendixKing systems operate differently, once a tilt value is selected it will maintain the same intercept angle with altitude changes. 

Figure 5 shows what happens in this situation. The ACT function adjusts the tilt to intercept the ground near the 240nm range based on the HSI range selection. But if you are looking at weather on the INAV map on 50nm range, the radar will be over scanning most of the weather close to the aircraft. The weather may not show at all, or it may appear less intense because the beam is narrow and only picking up less reflective frozen storm tops. In this scenario where cells are close to the aircraft a lower range selection should be used with ACT. 

Scenario #3 is much simpler. We set the HSI ranges on 10nm and 200nm range. This causes the radar to alternate scan, or update on the pilots display on the clockwise sweep and update the co-pilots display on the counterclockwise sweep. If flying single pilot, or when only one pilot is monitoring the weather radar, set both HSI range selections to the same value to avoid the delay caused by alternate scanning.

For our last scenario the HSI ranges are both 500nm (250nm half range) and the INAV map is on 50nm (25nm half range). On these radars anything >400nm sets either the 240 or 320nm radar range. In this case the display system takes the data from the radar but has to scale it (magnify) to the INAV range. Instead of 240 and 50 let’s use 250 and 50 to make the math easier. If we divide 250 by 50, we get 5. So, the display system is magnifying the data 5 times making it look blocky. 

Figure 7 shows a more extreme example with both HSI ranges at 100nm (50nm half range) and the INAV map at 2nm. In this case the radar is operating in 80nm range, and the display system is magnifying the INAV image 40 times (80 divided by two) causing a very blocky image.

Figure 8 shows the effect of magnifying an image 40 times. Setting a longer-range setting on the PFD range and a short-range selection on the INAV has the same effect.