First, the DIM control — Page 244, Lesson 288

Let's pick up with the control panel of the INS — the Control Display Unit, or CDU. We've already covered the basic display and the keyboard, so now we're looking at the specific controls that let you, the operator, manage the system in flight. First, the DIM control. This is straightforward — it governs the brightness of the LED displays and the panel lighting. So you use it to adjust how bright the screens are, both the main display and the backlighting of the panel itself. Next, the ALERT annunciator. This is a warning light that tells you the aircraft is approaching the next waypoint. Now, how it behaves depends on the mode you're in. 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 light also comes on steady 2 minutes before the waypoint, but then it starts to flash 30 seconds before the waypoint, and it keeps flashing until the track is changed. There's also a speed limitation here — the annunciator will not illuminate below a set speed, typically either 100 knots or 250 knots. So below that speed, you won't get the alert. Then we have the battery annunciator. This one illuminates when the INS is operating on internal power — meaning the system has switched over to its own battery, rather than aircraft power. The warning annunciator is next. This illuminates when a system malfunction occurs. So if something goes wrong inside the INS, this light comes on to tell you. Now, the auto/manual/remote switch. This determines the level of pilot intervention necessary to fly the aircraft. In 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 manual mode, you, the operator, are required to update the waypoint from/to read-out as each waypoint is overflown. So you have to actively confirm the change. The remote position's exact function depends 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 you could load the same route into multiple systems at once. The INSERT push-button is used in conjunction with the data input keyboard to enter information into the system. So when you type something on the keyboard, you press INSERT to confirm and enter it. 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 — you press and hold it at the moment you pass over that known position, which lets the system update its position reference. So, to tie it together: DIM controls brightness, ALERT warns you about approaching waypoints, battery annunciator tells you when you're on internal power, warning annunciator flags malfunctions, the auto/manual/remote switch sets your level of intervention, INSERT enters data, and HOLD updates your position over a fix. Each one has a specific role in managing the INS during flight.