How to - INS System, Operation & Procedures

[mattY]

The system initializes with three independent INS units. During power-up, the sensors stabilize and the system establishes initial attitude in ATT mode. Once the attitude is valid and aligned, navigation initialization and alignment with the aircraft’s actual position must be performed. This full alignment process takes approximately 17 minutes.

Important: Alignment must be performed with the aircraft on the ground and engines off. Any movement during alignment introduces position offset errors.

The artificial horizon can use either the vacuum system or the INS attitude reference. It is strongly recommended to keep the INS attitude source active as a backup.

INS Initialization

• Verify that the DC Standard  Bus or AC Standard Bus is powered.
• Check relevant circuit breakers are in.
• Set INS Mode Selector to STBY.
  • Note: While the displays are powered by DC STBY BUS (battery or DC ESS BUS), the inertial platform itself requires AC ESS BUS power (normally fed from AC BUS 3 or via AC ESS BUS SELECTOR).
• Set Display Selector to POS.
• Verify display matches the expected format (example shows “72” or “73” – in our case “72”).

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At this point the INS units are powered and initialized avionics-wise, but navigation is not yet active.

Display Test

• Should be performed in STBY, ALIGN (preferably avoid during alignment), or NAV modes.
• Set Display Selector to TEST.
• All characters, digits (including HSI), and warning indications must illuminate fully.

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Navigation Initialization & Alignment

• Alignment starts when Mode Selector is moved to ALIGN (after entering present position).
• The alignment timer counts down from ~17 minutes (DSR TK / STS page display shows accumulator from 90 → 01 on the right side).

Present Position Entry Procedure

The Present Position Entry is the known coordinate where the aircraft is currently located. It must be derived from a valid and accurate source to ensure reliable data for system initialization.

During the initialization process (typically on the ground), this is usually the gate or airport coordinates obtained from the airport chart, parking stand diagram, or airport reference point in the navigation database.

In mid-flight (or when reinitializing due to loss of position), it can be a calculated intersection of two VOR-DME radials, visual references, or any other reliable navaid or method where the aircraft's position is already known with high confidence.

Key Guidelines:

• Always use the most accurate position available (e.g., GPS if available, followed by reference airport/gate, last known position, or manual entry from charts).
• Verify the entered position against multiple sources (e.g., airport chart, or navaid cross-check) to avoid errors that could affect IRS alignment or navigation accuracy.
• An incorrect or unreasonable present position entry may cause serious navigation drifts and errors that could lead to dangerous loss of situational awareness.

Procedure:

• Mode Selector → STBY
• Display Selector → POS
• Enter latitude:
  • Press N or S
  • Verify “N” / “S” appears on left display and INSERT button illuminates
  • Enter degrees/minutes/decimal using numeric keypad (DDMM.S - e.g. N32477 which is N32°47.7’)
  • Press INSERT
• Enter longitude (same procedure):
  • Use E or W
  • Longitude has one extra digit (max 180°) (DDDMM.S - e.g. W072234 which is W072°23.4’)
  • INSERT light should remain on after both lat & lon are entered
• To correct: press CLEAR and re-enter.
• Move Mode Selector to ALIGN to start alignment process.

Notes on coordinates:

• IMPORTANT: The initial position is the point where the system ties the four-dimensional navigation frame to the NED (north-east-down) frame and is where the aircraft will be placed on it - any mistake during this process (like unaccurate or mistaken data) could lead to dangerous navigation estimations
• Format: degrees, minutes, decimal minutes (no negative signs – use N/S, E/W)
• Latitude limited to 90°, longitude to 180° - if input goes beyond this limit the system will normalize it and calculate unexpected results.
• A valid coordinate must have both a latitude or longitude component different than 00”00.0 or 000”00.0 respectively.

Waypoint Entry

• Set AUTO/MAN/RMT switch to AUTO or MAN
• Display Selector → WPT
• WPT Selector → desired waypoint number (1–9)
• Enter lat/lon same way as present position.
• INSERT light extinguishes after each coordinate is accepted (unlike POS mode).
• Waypoints must be entered sequentially (1→2→3…); gaps (e.g. 1 & 3 but not 2) will cause navigation discontinuities.

Alignment Monitoring & Status Codes

• Set Display Selector to DSR TK/STS to monitor alignment progress.
• Alignment status starts at 90 and counts down to 01.

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Alignment Requirements (must all be true):

• Aircraft on ground
• Aircraft completely static (no movement)
• AC power available (AC ESS BUS or AC STBY BUS)
• DC power available (DC STBY BUS)
• Valid present position entered (not 0° 0.0')

Critical notes:

• Once aligned (01) and switched to NAV, do NOT move mode selector out of NAV unless on ground and static → otherwise alignment is lost permanently until next ground static alignment.
• Loss of AC/DC power or movement during alignment will also force full restart on ground.

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Initial Desired Track (Leg 0–X)

Must be set after at least one waypoint is entered and alignment reaches 01, otherwise it wont let enter any TK.

• Ensure no active data entry is in progress.
• Press TK CHG button (both TK CHG and INSERT lights illuminate).
• Enter 0 + waypoint number (e.g. 0-1 to fly direct to WP1 from present position).
• Verify display shows 0–X.
• Press INSERT → lights extinguish if valid.

Invalid entries (0-0, non-existent WP) cause 0-0 and lights remain on.

NOTE: You can only set the Initial Desired Track once for navigation initialization or to restart navigation once you reach latest waypoint on previous flight plan.

Waypoint / Leg Change (TK CHG)

After navigation is active, change active leg anytime:

• Press TK CHG
• Check both TK CHG and INSERT lights are illuminate
• Enter FROM–TO (e.g. 3-5 or 0-7 for direct-to WP7)
• Press INSERT if valid, otherwise press CLEAR and repeat the process
• Check both lights extinguish

Behavior:

• 0–X → Direct-to waypoint X from present position
• A–B → Intercept A→B leg at ~30° angle
• If significantly off-track and far from leg, system may fly direct to B until intercept conditions are met
• Small overshoot possible at high speeds (>300 kt)

ATT REF Mode

Provides pitch & roll reference independent of full navigation.

• Gyro valid after:
  • Mode = ALIGN, NAV or ATT REF
  • >5 min powered from cold start, or >3 min if recently powered
• ATT REF mode disables navigation (status → 90); recovery only possible on ground, static.
• If mode selector moved to OFF in flight → attitude reference corrupted until next ground static realignment.

RMT (Remote) TK CHG Calculations

While in RMT, TK CHG can be used to query distances without affecting active navigation.

• A–B → great-circle distance between two waypoints
• 0–X → along-route distance from present position to waypoint X (passing all intermediate WPs)
• X–0 → direct distance from present position to waypoint X

Note: Below 100 kt groundspeed, system assumes 512 kt for time calculations.

Cross-Track (XTK) / Offset Mode

Accessed via XTK/TKE selector position.

Creating an offset parallel to active leg:

1. Valid leg must be active
2. Set selector to XTK/TKE
3. Press 1 (left field clears, INSERT illuminates)
4. Press L7 or R9 → L / R appears
5. Enter offset distance in NM (one decimal) e.g. R 34 → 3.4 NM right
6. Press INSERT

Display shows:

• Left: current cross-track distance to original leg
• Right: track angle error (max ~30° during capture)

Offset cancels when:

• Enter zero / blank offset (e.g. press 1 → INSERT)
• Waypoint transition occurs
• New leg selected via TK CHG
• Any waypoint coordinate changed

WAYPOINT SEQUENCING

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• ALERT light illuminates when time to your next waypoint is below 2 minutes and flashes when you are 30 seconds to reach it, then it extinguishes.
• WARN light illuminates for 30 seconds after the system automatically switches from last waypoint to the first one as per normal waypoint cycling whether is 9-1 or X-1.
• You could notice that in some cases you won't be able to see the flashing ALERT light depending on the track change angle, for closed angles the track change threshold starts a bit earlier than 30 seconds.

Note that INS ETA are in decimals of seconds, so 30 seconds would be 0.5 minutes.

• The system automatically advances to the next valid waypoint (1 to 9) when the transition threshold is reached.
• If the next waypoint does not exist or is invalid:
  • If ≥ 3 valid waypoints are present in the current batch → automatically cycles to waypoint 1 (leg: last → 1).
  • If 1 or 2 valid waypoints only → continues straight ahead indefinitely on the current track (no cycle).
• When reaching waypoint 9 and a next batch is available (when using waypoints import):
  • Current waypoint 9 is saved.
  • Next batch is loaded.
  • Saved (old) waypoint 9 is restored → leg changes to 9 → 1 (overlap transition).
  • After passing the new waypoint 1 and advancing (e.g. to waypoint 2), the new batch’s original waypoint 9 is restored.

For more information on other Navigation methods, please refer to How to - GPS Navigation (INS + GPS) & How to - Simbrief Integration