Performance-Based Navigation (PBN) Operations

062-07-04Pablo Asensio Martínez2026-04-012 min

PBN operations are based on the navigation system's ability to maintain a defined accuracy and, in the case of RNP, to monitor and alert on performance.

Navigation System Errors

The total system accuracy is defined as TSE (Total System Error). TSE is the geometric sum of three independent error components:

  1. PDE (Path Definition Error):
    • Occurs when the path defined in the RNAV system does not correspond to the desired path over the ground.
    • Considered negligible if the navigation database is intact and up to date (valid AIRAC cycle).
  2. FTE (Flight Technical Error):
    • Related to the crew's or autopilot's ability to follow the defined path.
    • Includes CDI centering errors.
    • Managed through crew procedures or the use of the autopilot.
  3. NSE (Navigation System Error):
    • The difference between the aircraft's estimated position and its actual position.
    • Depends on the accuracy of the sensors (GNSS, DME/DME, IRS).

TSE = PDE + FTE + NSE

Monitoring and Alerting (RNP)

In RNP specifications, the system must monitor performance:

  • ANP (Actual Navigation Performance) or EPE (Estimated Position Error): A statistical estimate of the current position error (TSE or NSE depending on context, but functionally represents position uncertainty).
  • RNP (Required Navigation Performance): The allowed error limit for the operation (e.g., 1.0 NM).
  • Alert: If ANP > RNP, the system alerts the crew (e.g., "NAV ACCUR DOWNGRAD"). This indicates that the position uncertainty exceeds the allowed limit.

Operational Procedures

  • Database: Must be updated to the current AIRAC cycle (every 28 days).
  • Deviation: During an approach, if the deviation exceeds half-scale deflection, a missed approach must be initiated.
  • Loss of Integrity/Accuracy: If an integrity alert (e.g., RAIM, LOI) is received or if ANP exceeds RNP before the FAF, the approach must be discontinued.
  • Communication Failure: Not an immediate cause for a missed approach in an RNP approach (standard communication failure procedures are followed).