The Inertial Navigation System in this panel is typical of the waypoint navigation systems in use in the 60s and 70s. The system can store up to 9 waypoints at any one time.

You will find an INS on the 727-200, 727-200 Advanced and 727 Super 27 panels.

THE MODE SELECTOR UNIT (MSU)

INTRODUCTION

The MSU, located on the second overhead panel, has controls used to prepare the INS for use before take-off. This panel loads up with the INS ready to use if the saved flight has the engines running. Otherwise the INS will be switched off, and must be prepared before use while parked on the ground.

THE MODE SELECTOR KNOB

• OFF - The INS is switched off.
• STBY - This mode does not permit the INS to be used for navigation, but switches on the CDU allowing the entry of ramp co-ordinate and waypoint data.
• ALIGN - This mode is used to align the INS's gyroscopes before use.
• NAV - This mode is used at all times while in flight. It can only be selected successfully if the accuracy performance index is at most 5.
• ATT - This mode is used only in the event of an INS malfunction. The INS gives attitude data only (this is not included in the simulation). Alignment is lost and the CDU displays are blank.

When moving between OFF/STBY and ALIGN/NAV, or to/from ATT mode, the mouse button must be held down for about 2 seconds before the switch moves. This simulates pulling the knob above its mechanical stops.

THE READY NAV LIGHT

When the Mode Selector is in ALIGN mode, the Ready Nav light illuminates when the gyroscopes are aligned and the unit is ready to be switched to NAV mode.

THE CONTROL AND DISPLAY UNIT (CDU)

INTRODUCTION

Most operations are carried out using the CDU (displayed above), which is displayed along with the ADF radios, transponder and WX radar.

WAYPOINT SWITCH

This switch, below the HOLD and REMOTE buttons, is used to select a waypoint to be edited in WPT mode.

FROM-TO DISPLAY

This displays the numbers of the FROM and TO waypoints of the current leg (except in WPT mode, where the waypoint selected by the waypoint switch is displayed - flashing if it is part of the current leg). The WPT CHG button is used to change the current leg – for example to travel from waypoint 3 to waypoint 5, enter

WPT CHG, 3, 5, INSERT

AUTO/MAN SELECTOR SWITCH

This switches between automatic and manual waypoint switching. The switch has a button on it which illuminates all lights on the CDU and MSU while pressed (for testing purposes) unless there is an error condition in the INS (see under WARN below for the button's operation in this case).

BUTTONS

• HOLD - This freezes the currently displayed position, and used for updating the co-ordinate readings for error-correction purposes.
• REMOTE - This is not used in this panel (where only one INS is fitted), but would be relevant in a dual or triple INS set-up.
• INSERT - This is used for the purposes of entering co-ordinates into the system.
• ALERT - Illuminates when the destination waypoint is less than 2 minutes away, and flashes once the destination waypoint is passed (if in MANUAL waypoint switch mode).
• BAT - Illuminates when the INS is operating from battery power, and also for 15 seconds during the gyroscope alignment process.
• WARN - Illuminates when an error occurs in the operation of the INS. To display the error code, switch to DSTRK/STS mode. The second and third digits of the right-hand display show the first half of the error code. Pressing the TEST button (above the AUTO/MAN switch) shows the second half of the error code, while pressing it again clears the error (if possible) or returns to the first half of the error code (if the error cannot be cleared).

CDU DATA SELECTOR

The CDU data selector knob selects the data displayed by the CDU:

• TK/GS - Left display shows track angle (0.0° to 359.9° true). Right display shows ground speed (0 to 2400 knots). If HOLD is pushed down the right display indicates ATA (along track acceleration) in milligees, while the left display gives N if the acceleration is negative (or is blank for a positive acceleration).
• HDG/DA - Left display shows heading angle (0.0° to 359.9° true). Right display gives drift angle (180L° to 180R°). Zero drift angle is shown if ground speed is below 75 knots.
• XTK/TKE - Left display shows cross track distance in nautical miles (999.9L to 999.9R). Right display shows track error with respect to desired track angle (180L° to 180R°).
• POS - Left display shows aircraft latitude, right display shows aircraft longitude. Co-ordinates are frozen if HOLD is pushed down. POS mode is used to insert ramp and position updating co-ordinates.
• WPT - Left display shows latitude of waypoint selected on waypoint switch, right display shows waypoint's longitude. Waypoint 0 is the ramp position (or after a manual track leg change, the aircraft position at the time of the leg change). When HOLD is pushed down, the uncorrected inertial position of the aircraft is shown.
• DIS/TIME - Left display shows distance from aircraft to TO waypoint - or if HOLD is pushed down, from FROM waypoint to TO waypoint - in nautical miles (0 to 9999). Right display shows time from aircraft to TO waypoint - or if HOLD is pushed down, from FROM waypoint to TO waypoint - in minutes (0 to 999.9).
• WIND - Left display shows wind direction (0° to 359° true). Right display shows wind speed (0 to 606 knots). Both display show zero if TAS if below 115 knots or above 606 knots, or if ground speed is below 75 knots. If HOLD is pushed down, the right display shows the longitudinal wind speed component in knots, while the left display shows N for headwind or S for tailwind.
• DSTRK/STS - Left display shows desired track angle (0° to 359° true), or if HOLD is pressed down the program ID number. Right display shows operation mode for first digit (1 for NAV mode, 0 otherwise), accuracy performance index (API) for the penultimate digit and mode index (MI) for the last digit. If there is an error, the second and third digits show the error code.

SETTING UP THE INS FOR USE

If a flight is loaded with the aircraft parked and the engines stopped, the INS will be switched off. The following procedure should be followed to get the INS working.

WARNING: The aircraft must not be moved until the setup procedure is complete and the aircraft is in NAV mode. Otherwise, error 04-57 will be generated and the setup procedure must be restarted.

1. Switch the MSU to STBY mode.
2. Switch the CDU to POS mode, and enter your current latitude and longitude – for example, the co-ordinates N051°28'6 W000°29'3 would be entered using the keypresses
   
   N, 5, 1, 2, 8, 6, INSERT, W, 2, 9, 3, INSERT
   
   This may cause error condition 06-41. If the entered position is correct, this error can be cleared by pressing TEST twice.
3. Switch the CDU to DSTRK/STS mode MSU to ALIGN mode. Alignment takes about half an hour. You may switch the CDU to WPT mode in order to enter waypoint data during this time (using the same format as for setting the initial position).
   
   A hidden switch at the top-right of the MSU permits alignment within a few seconds.
   
   
4. Once the accuracy required performance index has decreased to 5 or less, the MSU green light will come on, indicating that the gyroscopes are now aligned. Switch the MSU to NAV mode.

ERROR CODES

The WARN light illuminates if an error is encountered. To view the error code move the CDU switch to DSTRK/STS. The first half of the error code is immediately visible in the second and third digits of the display, while pressing the TEST button (on the AUTO/MAN waypoint change switch) displays the second half of the error.

• 04-45 - Entered ramp position is incorrect, or outside operating limits (80°N to 80°S). The aircraft must be stopped and the MSU switched to STBY to clear this error.
• 04-57 - Caused by taxying during gyroscope alignment. The aircraft must be stopped and the MSU switched to STBY to clear this error before alignment is restarted.
• 06-41 - Inserted ramp position more than 3 NM miles away from last inertial position. This happens often in the simulation environment, and can be cleared by pressing TEST twice if the inserted ramp position is correct.
• 06-56 - Unacceptable discrepancy between Inertial and Displayed position. Position updates must be eradicated by selecting DSTRK/STS mode, switching autopilot out of INS mode, then pressing
  
  1, INSERT.
  
  This changes the MI from 4 to 5.