Electronic Flight Information Systems — Page 289, Lesson 344

We’re now into the Electronic Flight Information System, and I want to walk you through how the decision height and radio altitude are presented on a Boeing-style EADI — that’s the Electronic Attitude Director Indicator, the primary attitude display in front of you. First, the decision height, abbreviated DH, is selected on the ADI control panel — the Attitude Director Indicator control panel — and that same value is shown both on the ADI itself and back on the control panel, so you always see what you’ve set. Now, the radio altitude presentation. On Boeing-based systems, radio altitude is displayed digitally while the aircraft is between 2500 feet and 1000 feet above ground level. So in that band, you get a straight digital readout of your height above the ground. But here’s the key change: below 1000 feet, the display automatically switches to a white circular scale, calibrated in increments of 100 feet. On that outer scale, your selected decision height appears as a magenta-coloured marker. The radio altitude also appears within the scale as a digital read-out, so you have both the marker and the numeric value together. As the aircraft descends, segments of that altitude scale are simultaneously erased, so the scale continuously diminishes in length in an anticlockwise direction. Think of it as the scale physically shrinking as you get lower — the remaining arc represents your remaining height. Now, on the descent, at decision height plus about 50 feet, an aural alert chime sounds, and it sounds at an increasing rate until you actually reach decision height. So the closer you get, the faster the chime. When you reach DH, the marker and the scale flash and change colour from magenta to amber or yellow. That’s your visual cue that you’re at the decision point. To clear it, you press the DH RST button on the EADI control panel — RST stands for reset. Pressing it cancels the alert chime, stops the marker flashing, and changes the DH displays back to their normal colour. There’s also a deviation warning: if you deviate beyond the normal localizer and glide slope limits, the scales change colour to amber and the pointer flashes. That flashing and colour change ceases once the aircraft returns to within limits. Now, a note on the A300 system currently in use — it differs from the above by using digital read-out only. This system, also found on some other aircraft types, displays radio altitude at the base of the PFD centre display — that’s the Primary Flight Display. It uses green numbers above decision height, and amber, slightly bigger numbers below. So the colour and size tell you where you are relative to DH. The digital read-out is enhanced by a voice warning system that gives aural indications of height passing to the pilot. Those aural warnings are given at significant heights, as decided by the manufacturer working with the airline company at the time of system build. Finally, the Speed Error Display — that consists of a pointer which moves relative to a deviation scale, showing the difference between actual speed and commanded speed. So it’s a direct visual of how far off your target speed you are. And that brings us to the EHSI section of the control panel — EHSI is the Electronic Horizontal Situation Indicator — and that’s where the RANGE control lives, which we’ll pick up next.