Electronic Flight Information Systems — Page 289, Lesson 342

Let’s pick up with the colour logic on the EFIS displays, because that’s the foundation for everything else we’ll talk about. On the EADI, the colours are not just decorative — they carry meaning. Cyan is used for sky shading on the EADI, and for low-priority information such as non-active flight plan map data. Yellow is used for ground shading on the EADI, and for caution information — that includes failure warning flags, limit and alert annunciators, and fault messages. Red is reserved for the display of the heaviest precipitation levels as detected by the weather radar, which we call the WXR. So remember the hierarchy: cyan is low priority, yellow is caution, red is the most severe weather return. Now, the Remote Light Sensor. This is a photodiode device — a photodiode is a light-sensitive electronic component. It responds to ambient light conditions on the flight deck, and it automatically adjusts the brightness of the CRT displays to an acceptable level. So if the cockpit is bright, the displays get brighter; if it’s dark, they dim down. That’s the automatic brightness control. Next, the Control Panel. A control panel is provided for each system — one per side, typically. As you can see in Figure 22.2, the switches are grouped for the purpose of controlling the displays of their respective EADI and EHSI units. So the panel is split into sections, one for the EADI and one for the EHSI. Let’s go into the EADI section of the control panel. There are four controls here. First, the Brightness Control, labelled BRT. This is used to adjust the brightness of the ADI display to the desired level — that’s the manual override on top of the automatic sensor. Second, the Decision Height Selector, labelled DH SEL. This is used to select the desired decision height for DH alerting. Third, the Decision Height Reset Switch, labelled DH RST. When pressed, it resets a DH alert on the associated ADI, and it changes the RA display — that’s radio altitude — from yellow to white. So after you acknowledge the alert, the colour returns to normal. Fourth, the Decision Height Reference Indicator, labelled DH REF. This displays the selected decision height on the controller itself, and also on the EADI. So you can see your selected DH in two places. Now, what does the EADI actually present? Look at Figure 22.3. The EADI displays normal pitch and roll attitude indications, plus a whole list of additional data. Attitude data comes from an Inertial Reference System — the IRS. You also get flight director commands. You get localizer and glide slope deviation — that’s your lateral and vertical guidance relative to the ILS. You get ground speed. You get radio altitude. You get decision height. You get Automatic Flight Control System and Autothrottle modes — that’s the AFCS and the autothrottle. And you get a speed error scale, which is the difference between commanded and actual speed. One important note: the autoland status, and the pitch, roll-armed and engage modes, are selected on the AFCS control panel — not on the EFIS panel. The EADI just displays them. Finally, let’s define decision height precisely, because this is a critical operational term. Decision height is the wheel height above the runway elevation by which a go-around must be initiated unless adequate visual reference has been established, and the aircraft position and approach path have been assessed as satisfactory to continue the approach and landing in safety. So it’s a height above the runway — measured at the wheels — and it’s your last point to commit to landing or to go around. If you don’t have the required visual reference by that height, you go around. That’s the DH, and that’s what you select with the DH SEL control and see on the DH REF indicator.