Radio Altimeter — Page 270, Lesson 317

Let's pick up with the radio altimeter's display formats and then move into its accuracy and installation considerations. First, the Boeing-style digital read-out. On some EFIS displays, you get a digital read-out of height, and when you're below 1000 feet, a pictorial image of an altimeter dial is drawn on the screen. That icon shrinks in size below 1000 feet to indicate height change, and its colour changes from white to flashing amber as decision height is approached. One further function of the radio altimeter is to desensitize the autopilot and flight director response to the ILS glidepath in the latter stage of an approach. Now the Airbus-style EFIS indicator. On some more modern systems, such as that used by the A300, the indication of height is given at the base of the EADI/PFD attitude display. This height indication changes colour from Green to Amber, and the numbers also grow in size as Decision Height is passed. It's also important to note that the Radio Altimeter is a major component of the Ground Proximity Warning System, or GPWS. Now let's talk about range and accuracy. The instrument can be used between zero and 2500 feet above the surface, with an overall expected accuracy of ±3% of indicated height, or ±1 foot, whichever is the greater. Those figures include various error contributors, principally Doppler shift, step error in the digital counting circuits, and height lag. Finally, aircraft installation delay, or cable length compensation. The radio altimeter is required to indicate zero height AGL as the main wheels touch down on the runway. Because of this, it has to be extremely accurate, and in practice is designed to perform to an accuracy of plus or minus one foot. However, a single manufacturer's product may be found in multiple aircraft types, from the very large Boeing 747 to a much smaller corporate commuter jet, and this must be catered for. At either extreme, the aircraft weight and oleo compression will vary, leading 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. In addition, different physical sizes of aircraft create differences in cable run length between the avionics bay and the position of the antennas on the underside of the fuselage. In the larger aircraft, this distance may be as much as 100 feet, or in the smaller jet, as little as 6 feet. If compensation for cable length is not catered for in the larger aircraft, an error would be generated, seen as an error of height, perhaps up to 100 feet. The aircraft installation delay is therefore adjusted to compensate for residual height and cable length.