Electronic Flight Information Systems — Page 289, Lesson 344

I want to walk you through the Electronic Flight Information System — specifically, how decision height and radio altitude are presented to you on the EADI, the Electronic Attitude Director Indicator. This is the display you'll be flying the approach on, so let's get the details exactly right. First, the decision height — DH — is selected on the ADI control panel. That's the panel you use to set up the attitude director indicator. Once you select it, that decision height is shown in two places: on the ADI itself, and back on the control panel where you set it. So you always have a cross-check between what you dialed in and what the display is showing you. Now let's talk about how radio altitude is presented on Boeing-based systems. Radio altitude is your height above the ground, measured by the radio altimeter. On these systems, it's displayed digitally — as a number — when the aircraft is between 2500 feet and 1000 feet above ground level. So in that band, you just read a number. But here's the key change: below 1000 feet, the display automatically switches format. It changes to a white circular scale, and that scale is calibrated in increments of 100 feet. So each tick mark on that circle represents 100 feet of altitude. Your selected decision height is then shown as a magenta-coloured marker on the outer scale of that circle. And the radio altitude itself still appears, but now as a digital read-out within the scale. Here's the clever part about how it behaves as you descend. As the aircraft goes down, segments of that altitude scale are simultaneously erased — so the scale continuously diminishes in length, and it does so in an anticlockwise direction. Think of the circle shrinking away counterclockwise as you get lower. That gives you an immediate, intuitive sense of how much altitude you have left before you reach your decision height. Now, the aural alerts. On the descent, at decision height plus about 50 feet, an aural alert chime sounds. And it doesn't just ring once — it sounds at an increasing rate as you get closer, until you actually reach decision height. So the urgency builds as you approach DH. When you reach decision height, two things happen visually: the marker and the scale flash, and they change colour from magenta to amber or yellow. That's your cue that you're at the critical point. Now, if you press the DH RST button on the EADI control panel — RST stands for reset — that will cancel the alert chime, stop the marker from flashing, and change the DH displays back to their normal colour. So you acknowledge the alert, and the system settles back down. There's also a deviation warning. If you deviate beyond the normal localizer and glide slope limits — that is, you drift too far left or right of the runway centreline, or too high or low on the glide path — the scales change colour to amber and the pointer flashes. That flashing and colour change ceases automatically if the aircraft returns to within limits. So it's a self-correcting warning: get back in the envelope, and the display clears itself. Now, I want to give you an important note about a different system. The A300 system currently in use differs from the Boeing approach — it uses digital read-out only. This system, which is 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 when you're above decision height, and amber, slightly bigger numbers when you're below decision height. So the colour and size change signal your position relative to DH. That digital read-out is enhanced by a voice warning system, which gives aural indications of height passing to the pilot. So you'll hear a voice telling you your height as you pass through significant altitudes. Those aural warnings are given at significant heights as decided by the manufacturer, working with the airline company, at the time the system is built. So the specific callout heights are set during system build, not something you configure in the cockpit. Let me also cover the Speed Error Display. This consists of a pointer which moves relative to a deviation scale. It shows the difference between your actual speed and your commanded speed. So if the pointer is centred, you're at the commanded speed; if it moves, you're off — and the direction and amount of movement tell you how far and which way. Finally, let's touch on the EHSI section of the control panel — that's the Electronic Horizontal Situation Indicator. The control panel has a section dedicated to it, and the first function there is the RANGE control. That's what you use to select the range of the map or navigation display — how far out you're looking. Let me show you the layout of the control panel and the EADI so you can see how these pieces fit together. And here's the multi-crew EFIS setup with the signal interfacing, so you understand how the units talk to each other. And the EFIS control panel itself. So to pull it together: you've got decision height selected on the ADI control panel, radio altitude presented digitally above 1000 feet and on a shrinking circular scale below 1000 feet, with a magenta DH marker that flashes and turns amber at DH, plus a chime that builds in rate from DH plus 50 feet. You've got the DH RST button to acknowledge it, deviation warnings that colour the scales amber and flash the pointer, and on the A300-style system, a digital read-out with green-to-amber colour change and a voice warning system. And the Speed Error Display and the EHSI range control round out the picture. That's the full presentation of decision height and radio altitude on the EADI.