INS/IRS Alignment and Operation: NAV-ATT Modes and Drift Monitoring

Alignment Process

The Inertial Reference System (IRS) must be aligned on the ground before flight to establish a navigation baseline. This process is critical for accurate operation.

Requirements

• Stationary Aircraft: The aircraft must not move during alignment. Movement disturbs the sensing of gravity and Earth's rotation.
• Position Entry: The pilot must enter the current position (Latitude and Longitude) via the FMS CDU or ISDU.

Phases of Alignment

1. Levelling (Determination of Local Vertical):
   • Accelerometers sense the gravity vector.
   • The system establishes the Local Vertical (down) and creates a horizontal plane (mathematically in IRS, mechanically in INS).
2. Gyrocompassing (Alignment to True North):
   • The system detects the Earth's rotation (rate approx 15°/hour).
   • Since the Earth rotates about the True North axis, the system can determine the direction of True North by measuring the horizontal component of Earth's rotation.
   • Latitude Calculation: By measuring the magnitude of Earth's rotation vector, the system can estimate its own Latitude. It compares this calculated latitude with the pilot's entry to check for gross errors.
   • Longitude: The system cannot determine Longitude; it relies entirely on the pilot's input.

Alignment Time

• Duration: Typically 10 minutes at mid-latitudes.
• High Latitudes: Alignment takes longer as latitude increases (Earth's rotational velocity vector is more vertical, making the horizontal component harder to detect).
• Limit: Alignment is generally impossible above 82° Latitude (Polar regions) due to insufficient horizontal Earth rate.

Operating Modes (MSU - Mode Selector Unit)

The mode selector typically has the following positions:

1. OFF: System is unpowered.
2. ALIGN: Initiates the alignment and levelling process. Used for quick realignment or when full shut-down is not desired.
3. NAV: Normal operating mode for flight. Provides all data (Attitude, Heading, Position, Speed).
4. ATT (Attitude): Reversionary mode.
   • Used if the Navigation mode fails or alignment is lost in flight.
   • Provides Attitude (Pitch/Roll) and Heading only.
   • Heading Entry: In ATT mode, heading is not essentially reliable and often needs to be entered/updated manually periodically (e.g., every 15 mins) or synchronized.
   • Navigation Lost: Position and groundspeed data are not available.

Performance Monitoring (Drift)

Errors in inertial systems accumulate over time (Drift). Pilots check the accuracy of the system after the flight.

Post-Flight Check

• Condition: Aircraft parked and stationary.
• Residual Groundspeed: The Groundspeed (GS) readout should ideally be zero. A high residual groundspeed indicates excessive error/drift.
• Position Drift: Comparing the calculated position with the known gate coordinates.
  • The drift is measured in NM/hour.
  • Calculation: Total Position Error (NM) / Flight Time (Hours).
  • Limits vary by unit, but typically drift should be low (e.g., < 2 NM/hr for INS, significantly better for IRS).

Key Points & Warnings

• Moving Aircraft: If the aircraft moves during alignment, the process restarts or fails.
• Latitude Mismatch: If the entered latitude differs significantly from the sensed latitude, the system will reject the alignment (flashing align light).
• Longitude Error: Entering the wrong longitude is not detected by the system (as it cannot sense longitude) but shifts the entire navigation map, resulting in a position error for the whole flight.
• In-flight Realignment: Not possible for calibration of position/north. Only Attitude (ATT) mode can be engaged if NAV is lost.