Inertial Navigation Systems — Page 248, Lesson 289

All right, let's pick this up right where we left off. We've just finished the position update procedure, and I want to make sure you understand one critical consequence before we move on. When you place the function switch in the POS mode and punch in the exact latitude and longitude of the radio fix — in this case, the lat/long of the VOR — you are telling the INS exactly where you are. Then you release the hold button. But here's the catch: if you do this, the radial error rate assessment will be invalid, unless you account for the position update vector. Let me unpack that. The radial error rate is how quickly your position error is growing over time. If you manually inject a new position, you've changed the error picture, so that assessment no longer holds true unless you factor in that update vector — the direction and magnitude of the correction you just applied. Now, let's move to the summary of the INS warning lights. This is a table you need to know cold, because it tells you what each light means and what action to take. There are five entries. First, the READY NAV light on the MSU — that's the Mode Select Unit. It's a green light, and it indicates that alignment is complete. The action required is to select NAV. So when you see that green light, your inertial system is aligned and ready, and you switch it into the navigation mode. Second, the BATT light on the MSU. This is a red light, and it indicates that battery power is too low for operation. The action is to check your power supplies. This is a warning that your backup battery can't sustain the system. Third, the ALERT light on the CDU — the Control Display Unit. This is an amber light, and it indicates that you are approaching a waypoint, or overflying one if you're in MAN mode. The action required is none, unless you're in MAN mode, in which case TK CHG — track change — is initiated. So in manual mode, the system will initiate the turn to the next leg. Fourth, the BATT light on the CDU. This is amber, and it indicates that the INS is operating on back-up power. The action is to check power supplies. Notice the difference: the MSU BATT is red and means battery too low; the CDU BATT is amber and means you're running on backup. Fifth, the WARN light on the CDU. This is a flashing red light, and it indicates a system malfunction. The action is to set the selector to DSR TK/STS — that's the desired track/status page — note the action code, and consult the user's guide for the appropriate action. So a flashing red WARN is serious; you need to pull up the status page, read the code, and follow the manual. Now, let's move to the LED display. All of the following descriptions are based on a Desired Track between waypoints of 060°. That's the reference for the examples. The first display is Track and Ground Speed. The INS-derived aircraft track, in degrees true, is shown to the nearest tenth of a degree in the left-hand window. So track is your actual path over the ground, in degrees true, and it's displayed with one decimal place. The INS-derived ground speed is shown to the nearest knot in the right-hand window. Ground speed is your speed over the ground, in knots, rounded to the nearest whole knot. In Figure 18.18, you can see an example: the track is 040° true, and the ground speed is 502 knots. So the left window reads 040.0, and the right window reads 502. Let me show you that figure. So to summarize what we've covered: the position update invalidates the radial error rate unless you account for the update vector; the warning lights — READY NAV, BATT on the MSU, ALERT, BATT on the CDU, and WARN — each have a specific indication and required action; and the TK/GS display shows track to the nearest tenth of a degree true on the left and ground speed to the nearest knot on the right.