The displays associated with the various rotary display selector switch positions are listed and explained in the following paragraphs:
TK/GS (track and groundspeed)
The INS computed track, usually referenced to magnetic north, is displayed to the nearest tenth of a degree in the left display and the groundspeed in knots in the right display. For example, a current track of 1358M and a groundspeed of 467 knots would appear as 135.08 and 0467.
HDG/DA (heading and drift angle)
The heading obtained from the angle between the platform frame and north reference is displayed to the nearest tenth of a degree in the left display. The angular difference between heading and track (drift angle) is displayed to the nearest tenth of a degree in the right display, preceded by the letter R or L to indicate whether drift is right or left. Thus, a heading of 1378M on a track of 1358M would be presented as 137.08 and L 02.08.
TK CHG INSERT 1 2 3 4 5 6 7 8 9 0 N E W S HOLD CLEAR
ALERT BATT WARN
W P T MAN AUTO RMT TK/GS HDG/DA XTK/TKE POS WPT DIS/TIME WIND DSR TK/STS TEST D I M 4
XTK/TKE (cross track distance and track error angle)
Cross track distance is the distance by which the aircraft is displaced right or left of the desired great circle track and is displayed in the left display to the nearest tenth of a nautical mile. The track error angle is the angular differ- ence, right or left, between the desired great circle track and the actual track being made, to the nearest tenth of a degree. If the aircraft were displaced 11
2nm to the left of the desired track of 1358M, the left display would read L 01.5. If the track being made good happened to be 1308M, the right display would read L 005.08.
POS (present position)
The aircraft's current latitude and longitude are shown to the nearest 6 minutes of arc in the left and right displays, respectively. Suppose it happens to be at 51.15.7N, 04.23.6W, this would appear as 51815.7'N in the left display and 04823.6'W in the right display. This is the situation illustrated in Figure 3.10.
WPT (waypoint positions)
The position of each inserted waypoint is shown as latitude in the left dis- play and longitude in the right display by selecting WPT on the rotary selector switch and scrolling through the waypoint numbers with the waypoint selector wheel.
DIS/TIME (distance and time to the next waypoint)
The distance in nautical miles from the present position to the next waypoint is shown in the left display and the time at present groundspeed to the nearest tenth of a minute in the right display.
WIND (wind speed)
The INS is able to compute wind direction and speed and these are dis- played in the left and right displays, respectively, to the nearest degree of arc and knot.
DSR TK/STS (desired track and status)
The great circle track from one waypoint to the next changes as the aircraft progresses between the two and the INS computes the present desired magnetic track based upon distance from the waypoints, magnetic variation and the assumption that the aircraft is on track. This will appear in the left display to the nearest tenth of a degree and the right display will be blank. The status function is for use only whilst the INS is in ALIGN mode and it shows a numerical display in the right window that indicates the status of the alignment procedure. The display typically shows 99 at the start of
alignment and counts down to 0, when alignment is completed and READY NAV is illuminated.
Other CDU controls
It will be noted from Figure 3.10 that there are additional controls and annunciators and their functions are as follows:
The waypoint selection controls are situated immediately below the left LED display. A thumbwheel is rotated to select the number of a waypoint that is to be inserted or amended. An LED display indicates the current from/to situation; the illustration depicts the display that would appear when flying between waypoints 2 and 3. The track change (TK CHG) push button is used when altering the pre-planned sequence of waypoints. Con- ventionally waypoint 0 is the aircraft's current position. Suppose ATC has cleared you to fly direct to, say, waypoint 5 then the TK CHG button is pressed until 0±5 appears in the display. The DIM thumbwheel adjusts the brightness of the LED displays.
The AUTO/MAN/RMT rotary switch is used to select the type of flight control to be used in flying from waypoint to waypoint. In AUTO the INS will automatically change the from/to display as each waypoint is over- flown and would normally be used in conjunction with automatic flight. In MAN (manual) the pilot is required to enter the from/to display as each waypoint is reached. The RMT (remote) position is used when two or more INS are fitted and enables the waypoint information to be transferred from one INS to the other(s).
The three annunciators situated below the right LED display serve to draw attention to specific events. The ALERT annunciator illuminates amber as the aircraft approaches the next waypoint, typically when about 2 minutes short of it, and will continue to flash until either cancelled by the pilot or, when in AUTO mode, by overflying the waypoint. The BATT (battery) annunciator will illuminate amber when the INS is operating on battery power, reminding the pilot that the system will only operate for a limited time on internal power. The WARN annunciator illuminates red in the event of a system failure. At the same time the status display will show a number that cross refers in the system manual to the nature of the failure.