FMS Operations and Limitations: Initialization, CI, LNAV/VNAV, and Accuracy

022-11-03Pablo Asensio Martínez2026-03-273 min

Initialization Sequence

Setting up the FMS for a flight typically follows a logical flow of pages on the CDU:

  1. IDENT Page:

    • The first page displayed after power-up.
    • Used to verify Aircraft Model, Engine Type, and Navigation Database validity (Active dates).

  2. POS INIT (Position Initialization):

    • Initializes the Inertial Reference System (IRS).
    • Typically uses the GPS position or the Airport Reference Point (ARP) coordinates.

  3. RTE (Route):

    • Entry of Origin and Destination airports.
    • Selection of a Company Route or manual entry of waypoints and airways.

  4. PERF INIT (Performance Initialization):

    • Entry of Zero Fuel Weight (ZFW), Fuel Quantity, and Cruise Altitude.
    • Cost Index (CI): A crucial parameter determining the economic profile of the flight.

Cost Index (CI)

The Cost Index is a strategic management tool that balances Fuel Cost against Time-Related Operating Costs.

$ CI = \frac{\text{Time Cost}}{\text{Fuel Cost}} $

  • CI = 0: Minimum Trip Fuel (Maximum Range Speed). The aircraft flies slowly to save fuel.
  • High CI: Minimum Flight Time. The aircraft flies fast, disregarding higher fuel consumption.

Position Computation and Accuracy

The FMS calculates a "System Position" by mixing data from multiple sensors using a Kalman Filter.

Inputs & Priority

  1. GPS (GNSS): The most accurate and primary source.
  2. DME/DME: Radio position derived from multiple DME distances.
  3. VOR/DME: Bearing and distance from a station.
  4. LOC: Localizer for lateral alignment during approach.
  5. IRS: Inertial position (drifts over time).

Degradation & Limitations

  • GPS Loss: If GPS signals are lost, the FMS reverts to Radio/IRS updating. The position accuracy may gradually degrade or "shift" (Map Shift).
  • RAIM (Receiver Autonomous Integrity Monitoring): Essential for RNP and GPS-based approaches. Loss of GPS implies loss of RAIM and the inability to fly these approaches.
  • Runway Update: On takeoff, applying TOGA power often triggers a position update to the runway threshold coordinates.

Navigation Modes

Lateral Navigation (LNAV)

  • Command: Follows the horizontal flight plan.
  • Autotuning: The FMS automatically tunes VOR and DME frequencies for position updating without crew intervention.
  • Overfly vs. Fly-by:
    • Fly-by: Standard. The aircraft anticipates the turn to cut the corner.
    • Overfly: The aircraft must pass directly over the waypoint before initiating the turn.

Vertical Navigation (VNAV)

  • Command: Manages the vertical profile (Climb, Cruise, Descent).
  • Barometric Reference: VNAV logic relies on Barometric Altitude form the Air Data Computer (ADC).
  • Autothrottle: Essential for full VNAV operation (managing speed/thrust interactions).
  • Modes:
    • VNAV SPD: Pitch maintains speed.
    • VNAV PTH: Pitch maintains the geometric path (speed controlled by thrust/drag).
    • VNAV ALT: Level off at an intermediate constraint or MCP altitude.

Performance Predictions

  • The FMS predicts fuel usage and arrival times (ETA).
  • Limitation: Predictions assume the aircraft is in a normal configuration. If the aircraft flies with slats/flaps extended or gear down for extended periods (abnormal config), drag is higher than modeled, and fuel predictions will be inaccurate.