Inertial Navigation Systems — Page 248, Lesson 293

I want to walk you through the manual and automatic system checks for the INS, because this is where the operator's skill really matters. We've already seen how the INS computer assumes it will normally be coupled to the flight director or autopilot, so that across-track errors don't build up. Now let's look at what happens when you're setting the system up. First, the test function. When you put the function switch into the test position, all of the digits on the various displays illuminate, showing either a figure or letters. This lets you, the operator, check that all of the functions are operating. It's a simple lamp and display test — if a segment is dead, you'll see it right there. Now, the critical part: the initial setting up stage. The start position must be fed into the INS computer with a high degree of accuracy. Let me explain why this is so demanding. If the initial latitude is slightly in error, the platform will not remain earth horizontal once the equipment is switched into the navigation mode. Here's the mechanism: the torque motors tilt the platform to keep it level, but they're driven by computer calculations based on the latitude you entered. If that latitude is wrong, the torque motors tilt the platform at an inappropriate rate. So the platform drifts away from earth horizontal. And for the same reason, the platform will not remain directionally aligned with respect to north either. Now, here's the clever part. If the initial latitude setting is grossly in error — not just slightly, but grossly — the system will detect the error and warn you. This is one of the principal functions of the warning annunciator on the CDU, the Control Display Unit, whilst the equipment is in the align mode. How does it sense this? The equipment compares the apparent drift and topple rates sensed by the rate gyros against the corrections being applied by the torque motors. If those don't correspond, the system knows the latitude input is wrong and it warns you. But note the contrast: a slight latitude error won't trigger that warning — it just quietly degrades your platform alignment. Only a gross error gets caught. Now, longitude is a different story. An incorrect operator input of longitude will not affect the stability of the platform. The platform doesn't care about longitude for its levelling. But obviously, the track and distance from the departure point to the first waypoint will be incorrectly computed. And worse, all subsequent indications of longitude will be in error by the amount of the initial input error. So a longitude error propagates through the entire flight. Then we come to the waypoints. An incorrect input of the lat/long of any of the waypoints will have serious consequences. Here's the key point: the INS will navigate very accurately between waypoints, but it is incapable of detecting operator malfunctions — what we call finger trouble. The system has no way of knowing that the waypoint you typed in isn't the one you meant. So you have to check. There are two checks. The first: recall the waypoints from store onto the LED display, and recheck them before flight. The second check: call up the initial great circle track, shown as TK/GS, and the distances, shown as DIS/TIME, between consecutive waypoints. Then compare these values against those shown on the flight log, or the flight progress log, or the flight plan. If the INS's computed track and distance match your flight plan, you've caught any finger trouble before it becomes a navigation error. So the whole philosophy here is: the INS is extremely accurate once it's set up correctly, but it's completely blind to bad inputs. The system checks catch gross latitude errors automatically, but longitude and waypoint errors are yours to catch with these manual checks. That's the operator's responsibility, and it's why the pre-flight verification procedure exists.