Inertial Navigation Systems — Page 248, Lesson 291

We're starting a new part of the inertial navigation chapter now — the control display unit, the pilot's interface with the system. This is where you actually read the aircraft's position and manage the flight plan. Let's work through the display pages in order. First, present position. The aircraft's present latitude is shown to the nearest tenth of a minute of arc in the left-hand window. Present longitude is shown to the nearest tenth of a minute of arc in the right-hand window. So a typical readout looks like 34°31.5'N 117°11.3'W — that's 34 degrees, 31.5 minutes north, and 117 degrees, 11.3 minutes west. Notice the precision: tenth of a minute of arc, which is a very fine resolution for navigation. Next, waypoint positions. These are also shown in latitude in the left window and longitude in the right, again to the nearest tenth of a minute of arc. In the system we're considering, there are ten possible waypoint selections, numbered 0 through 9. Waypoints 1 through 9 are simply selected turning points, normally entered by the operator before the flight. Waypoint 0 is special — it represents the aircraft's position at the last time a track change from present position to a specified waypoint was selected by the operator. So waypoint zero is reserved for the computer to establish a track from the aircraft's present position, and it will not accept operator-entered waypoint coordinates. You can't type a lat/long into waypoint zero; it's the computer's own reference. Let me give you a practical scenario to make this useful. Suppose you're halfway between waypoints 3 and 4, and air traffic control clears you direct to waypoint 6. You select a track change from waypoint 0 — which is your present position — to waypoint 6, and insert it. If the aircraft is coupled to the flight director or autopilot, it will fly you directly to waypoint 6. That's the whole point of waypoint zero: it lets you go direct from wherever you are right now. Now the distance and time page. Distance to go from the aircraft's present position direct to the next selected waypoint is shown to the nearest nautical mile in the left window. The elapsed time from present position to the next waypoint is shown to the nearest tenth of a minute in the right window. So a readout of 140 NM and 16.7 minutes means 140 nautical miles to go, 16.7 minutes at current speed. Next, wind speed and direction. The INS-derived wind direction is shown in degrees true, to the nearest degree, in the left window. INS-derived wind speed is shown to the nearest knot in the right window. So a wind readout of 155°(T)/85 kt means the wind is blowing from 155 degrees true at 85 knots. The system derives this from the difference between your track and your heading — it's computed, not measured directly. Finally, desired track and status. The desired track — assuming the aircraft is on the direct great circle track between the two selected waypoints — is shown in degrees true to the nearest tenth of a degree in the left window. The right window will normally be blank, because the status check is generally only available with the equipment in the alignment mode. So during normal flight you see the desired track on the left, and nothing on the right; the status function only appears during alignment. That's the full set of display pages: present position, waypoint positions, distance and time, wind, and desired track with status. Each one has its own precision and its own purpose, and waypoint zero is the key to understanding how the system handles direct clearances.