Man and Machine — Page 278, Lesson 456

I want to walk you through a topic that sits right at the heart of how we interact with the aircraft: the design of the displays we use to fly it. We're going to look at the strengths and weaknesses of different display types, and why the way information is presented can directly affect your performance and safety. Let's start with the compass. A conventional compass card — the kind with a rotating circular dial — gives you a much better picture of the aircraft's orientation. You can see at a glance where north is relative to the nose. A digital readout for heading, on the other hand, makes it more difficult to determine things like the shortest way to turn onto a new heading. However, a digital readout is ideal for displaying quantitative information — that is, precise numbers like an exact heading of 273 degrees. Now, it is possible to combine both types of information in a single instrument. This is called a combination of analogue and digital displays. You can see an example in Figure 14.3. In that instrument, the thousands and hundreds of feet are displayed digitally — as numbers. But the hundreds of feet are also shown by a single moving pointer against a fixed scale. That pointer gives you a much better mental picture when you are approaching the end of the scale — for example, when you are approaching the ground. This form of display is also excellent for showing small changes, such as when you are levelling off, or if you inadvertently depart from your selected altitude. The pointer's movement makes those small deviations easy to see. Moving on to the modern flight deck, we have the 'glass cockpit'. Here, the basic presentation of the traditional instruments is maintained to some extent, but the instruments are displayed on a Cathode Ray Tube, or CRT — that's the screen technology. The attitude may still be presented in the traditional way, but other items, such as speed and altitude, may be displayed on moving tape displays. You might also see a conventional compass card, or a digital display readout. This is shown in Figure 14.4. The use of those moving tapes does present some specific problems, particularly during climb and descent. If we maintain the convention of having the higher figures at the top of the presentation, then during a climb you have a decreasing speed and an increasing altitude. The speed tape is moving down as the numbers get smaller, and the altitude tape is moving up as the numbers get larger. This can create a perception that the aircraft is rolling. Other displays have the higher values at the bottom of the tapes instead. Because there is no industry-wide standard for the presentation of this information, it is possible that when you transfer from one aircraft type to another, you might encounter presentations working in opposite senses to what you have become familiar with. Such a state of affairs is plainly undesirable and a recipe for possible errors. Finally, let's look at a promising development for future displays: the Head Up Display, or HUD. In this system, the information the pilot needs is projected onto a translucent screen positioned between the pilot and the forward windscreen. Because the display is projected at infinity, it allows you to view the outside world through the display without refocusing your eyes. The HUD has been in use for many years in military aircraft and is now being incorporated into commercial aircraft.