Inertial Navigation Systems — Page 244, Lesson 287

I want to walk you through the Inertial Navigation System control panel — the INS, as we call it. Now, before we touch a single knob, I want to set your expectations straight. These notes are not an operator's brief; they're written to get you through an examination. So please appreciate that, although other INS panels may look dissimilar to the one we're about to study, the information the system gives you will be basically the same. Don't get hung up on cosmetics — learn the logic, and you can fly any panel. Let's start at the top left-hand corner of the panel and work our way through each display and control. The two large windows at the top of the panel — labelled left and right numerical displays — are the principal outputs of the system. These are your primary readouts, the two big LED windows that tell you what the system is doing. Now glance down at the bottom left-hand corner. That's the function selector. We'll start with the selector in the seven o'clock position, which is the TK/GS position, and work anticlockwise through the functions, considering the values shown in the two LED windows as we go. TK/GS stands for Track and Ground Speed — that's your first function. As you rotate the selector anticlockwise, you'll step through the other functions, and each one changes what those two big windows display. Next, the waypoint selector switch. This is a thumbwheel you thumb to the appropriate waypoint number, and that number shows in the window to the left of the thumbwheel. You use this when loading the waypoint latitudes and longitudes before flight, when reloading new waypoints in flight, or when checking that waypoints are correctly loaded. So it's your way of telling the system which stored waypoint you're working with. Then we have the FROM/TO waypoint display. This shows the two waypoints between which the INS assumes it is flying. And here's the critical part — all digital readouts, all flight director displays, and all autopilot commands will be based on this information. So you can imagine the consequences of either giving the equipment the wrong TO/FROM waypoint numbers, or feeding the system with the wrong waypoint latitude and longitude to begin with. Garbage in, garbage out — and in an INS, garbage out means the autopilot is steering you somewhere you don't want to go. The track change push-button enables you to tell the system between which two waypoints the aircraft is required to fly. This matters when the system is not set up to fly sequentially through the loaded waypoints. Normally it would step through waypoints in order, but if you need to skip ahead or jump to a different pair, this button lets you override the sequence. Finally, the DIM control. That governs the brightness of the LED displays and the panel lighting. Simple, but essential — you need to be able to read those windows in bright sunlight and in a dark cockpit at night. Now, I want to pause on the FROM/TO display because that's where pilots get into trouble. The system computes everything — every readout, every flight director command, every autopilot input — based on the assumption that you're flying between those two specific waypoints. If you load the wrong TO waypoint, the system will happily steer you toward it, and you'll be flying the wrong leg. If you load the wrong latitude or longitude for a waypoint, the system will steer you to a point in space that isn't where you think it is. So the discipline here is absolute: verify your waypoint entries, verify your FROM/TO selection, and understand that the entire navigation solution hangs off that one piece of information. Let me also make sure you understand the function selector logic. It's a rotary switch, and the seven o'clock position is your starting point — TK/GS. As you rotate it anticlockwise, you cycle through the available functions, and each function changes what the two LED windows display. So the same two windows serve multiple purposes; the selector tells you what those windows are showing at any given moment. That's the panel layout. The key takeaways for your exam: the two large windows are the principal outputs, the function selector determines what they show, the waypoint selector loads and checks waypoints, the FROM/TO display defines the active leg and drives all flight director and autopilot commands, the track change button overrides sequential waypoint flying, and the DIM control handles display brightness. Get those relationships straight, and you've got the panel logic down.