First, the DIM control — Page 244, Lesson 288

Let’s pick up right where the controls on the Control Display Unit, the CDU, leave off. We’ve already covered the waypoint sequencing and the data input keyboard, so now I want to walk you through the remaining annunciators and switches that sit on that panel, because each one has a very specific job in how the INS flies the aircraft. First, the DIM control. That’s the knob that governs the brightness of the LED displays and the panel lighting. So when you’re flying at night or in bright sunlight, you use DIM to set the intensity of those light-emitting diode readouts and the backlighting of the panel itself. Next, the ALERT annunciator. This is a warning light that tells the operator the aircraft is approaching the next waypoint. And here’s where the mode matters, because the behaviour is different in AUTO versus MANUAL. In AUTO mode, the alert light comes on steady, 2 minutes before you reach the waypoint, and it extinguishes as the track changes overhead the waypoint. In MANUAL mode, the alert light also comes on steady 2 minutes before the waypoint, but then it flashes 30 seconds before the waypoint, and it keeps flashing until the track is actually changed. So the flash is your cue that you, the operator, need to act. One important limitation: the annunciator will not illuminate below a set speed, typically either 100 knots or 250 knots. So below that threshold, you get no alert at all. Now the battery annunciator. This one illuminates when the INS is operating on internal power. In other words, if the aircraft’s main electrical supply has dropped out and the inertial system has switched to its own internal battery, this light comes on to tell you that’s what’s happening. Then the warning annunciator. This illuminates when a system malfunction occurs. So it’s your general fault indicator — if something inside the INS has failed, this light comes on. Now the auto/manual/remote switch. This determines the level of pilot intervention necessary to fly the aircraft. In the automatic mode, the INS will automatically switch from one track to the next as each waypoint is overflown — you don’t have to do anything. In the manual mode, the operator is required to update the waypoint from/to read-out as each waypoint is overflown. So you have to physically tell the system to move on to the next leg. The exact function of the remote position will depend on the complexity of the INS computer programme, and it’s outside the scope of this syllabus, but in general terms it allows for simultaneous insertion of waypoints into more than one INS from one CDU. So one control display unit can feed several inertial systems at once. Next, the INSERT push-button. This is used in conjunction with the data input keyboard to enter information into the system. So you type the data on the keyboard, then press INSERT to commit it into the INS. Finally, the HOLD push-button. This is used primarily for updating the INS position when overflying a reliable fix, such as a VOR overhead. The HOLD button is depressed as the fix is overflown. That’s the key action — you press and hold it at the moment you cross the fix, and that tells the system, “this is where I actually am,” so it can correct any drift in its computed position. So to tie it together: DIM controls your display brightness, ALERT warns you of the next waypoint with different behaviour in auto versus manual, the battery annunciator tells you you’re on internal power, the warning annunciator flags a malfunction, the auto/manual/remote switch sets how much you intervene in track changes, INSERT enters data, and HOLD updates your position over a known fix. Each one has a precise role, and in the cockpit you’ll read them in exactly these terms.