Radio Altimeter — Page 270, Lesson 317

I want to walk you through the radio altimeter, and I want to start with how it presents its information to you in the cockpit, because that's where this chapter begins. On Boeing-style EFIS displays, you get a digital read-out of your height. But here's the clever part: when you descend below 1000 feet, the system draws a pictorial image of an altimeter dial on the screen. That icon actually shrinks in size as you go lower, which gives you a quick visual sense of your height changing. The colour of this icon changes from white to flashing amber as you approach decision height. That's your cue that you're getting close to the point where you must decide whether to continue the approach or go around. There's one more function I want you to note: the radio altimeter desensitizes the autopilot and flight director response to the ILS glidepath in the latter stage of an approach. In plain terms, as you get low, the system deliberately makes the autopilot less twitchy so it doesn't overreact to small glidepath deviations near the ground. Now, on Airbus-style systems, like the A300, the presentation is different. The height indication sits at the base of the EADI or PFD attitude display. As you pass decision height, that indication changes colour from green to amber, and the numbers actually grow in size. So you get both a colour change and a size change to grab your attention. And I want to stress one critical point: the radio altimeter is a major component of the Ground Proximity Warning System, the GPWS. That's the system that warns you if you're getting dangerously close to terrain. So this instrument isn't just for approach guidance — it feeds the safety systems too. Let's look at the range and accuracy now. The instrument can be used between zero and 2500 feet above the surface. The overall expected accuracy is ±3% of indicated height, or ±1 foot, whichever is the greater. So if you're at 100 feet, 3% is 3 feet, which is greater than 1 foot, so your accuracy is ±3 feet. If you're at 10 feet, 3% is only 0.3 feet, so the ±1 foot figure takes over. Those figures include various error contributors — principally Doppler shift, step error in the digital counting circuits, and height lag. Doppler shift is the frequency change from the reflected signal, step error comes from the digital counting circuits, and height lag is the delay in the reading catching up with reality. Now here's a really important practical issue: aircraft installation delay, or cable length compensation. The radio altimeter is required to indicate zero height AGL — that's above ground level — as the main wheels touch down on the runway. Because of that, it has to be extremely accurate, and in practice it's designed to perform to an accuracy of plus or minus one foot. But here's the problem: a single manufacturer's product may be found in multiple aircraft types, from the very large Boeing 747 down to a much smaller corporate commuter jet. That variation must be catered for. At either extreme, the aircraft weight and oleo compression will vary. The oleo is the shock absorber strut in the landing gear. This leads to the need for compensation. The height difference between the antennas on the fuselage and the bottom of the trailing wheels on the main landing gear bogeys, on the approach to touchdown, is known as the Residual Height. That's the gap you need to account for so that zero indicated equals wheels on the ground. In addition, the different physical sizes of the aircraft create differences in cable run length between the avionics bay and the antennas on the underside of the fuselage. In a larger aircraft, that distance may be as much as 100 feet; in a smaller jet, as little as 6 feet. If you don't compensate for cable length in the larger aircraft, you'd generate an error — potentially up to 100 feet of height error in that example. So the aircraft installation delay is adjusted to compensate for both residual height and cable length. Let me tie that together. The radio altimeter measures height above the ground, but it must read zero exactly when the wheels touch. So you have to subtract the residual height — the distance from the antenna to the wheel bottom — and you have to account for the time the signal takes to travel through the cables. Both of those are built into the installation delay adjustment. That's why the same radio altimeter unit can work accurately on a 747 and on a small commuter jet — the installation is compensated for each aircraft. That figure shows you the Airbus-style presentation at the base of the EADI, with the height indication and its colour change.