Inertial Navigation Systems — Page 248, Lesson 289

Let's pick this up right where the function switch and the hold button left off, because that sets the scene for what I'm about to show you. When you punch the exact lat/long of a radio fix into the machine in POS mode and then release the hold button, you've just injected an external position. Now, here's the critical warning I want you to remember: if you do that, the radial error rate assessment — which I'll explain in a moment — becomes invalid, unless you account for the position update vector. That's the vector that represents the difference between where the INS thought you were and where the radio fix says you actually are. Ignore it, and your error model is corrupted. Now let's move to the warning lights, because this is where the system talks to you. I'll walk you through each light, what it means, and exactly what action you must take. First, the READY NAV light on the MSU — that's the Mode Selector Unit. It's a green light, and it indicates that alignment is complete. The action required is simple: select 'NAV'. So when you see green, the gyros and accelerometers have finished their alignment sequence, and the system is telling you it's ready to navigate. Next, the BATT light on the MSU. This one is red, and it indicates that battery power is too low for operation. The action required is to check your power supplies. This is a serious one — if the battery can't support the system, you could lose your inertial reference entirely. Now we move to the CDU — that's the Control Display Unit. The ALERT light on the CDU is amber. It indicates that you are approaching a waypoint — and note, if you're in MAN mode, it means you're overflying it. The action required is none, unless you're in MAN mode, in which case TK CHG is initiated — that's track change. So in manual mode, the system is flagging that you need to initiate the track change to the next leg. The BATT light on the CDU is also amber, but it means something different from the MSU one. Here it indicates that the INS is operating on back-up power. The action required is to check your power supplies. So you've lost primary power, and the system has switched to its internal battery. Finally, the WARN light on the CDU. This one flashes red, and it indicates a system malfunction. The action required is to set the selector to DSR TK/STS — that's Desired Track/Status — note the action code, and consult the user's guide for the appropriate action. This is your malfunction indicator, and the procedure is to read the status page to find out what's wrong. Now, let's move to the LED display. All of the following descriptions are based upon a Desired Track between waypoints of 060°. That's the reference — your desired track is 060° true. The first display is Track and Ground Speed. The INS derived aircraft track, shown in degrees true, is displayed to the nearest tenth of a degree in the left-hand window. So you get precision to a decimal point on your track. The INS derived ground speed is shown to the nearest knot in the right-hand window. In Figure 18.18, you can see the track is 040°(T) and the ground speed is 502 knots. So the left window reads 040.0, the right reads 502. That's the TK/GS display — track and ground speed. The key thing to appreciate is the precision: track to a tenth of a degree, ground speed to the nearest knot. That's the kind of accuracy the INS gives you, and it's why we trust it for long-range navigation.