Performance-Based Navigation (PBN)

Performance-based navigation (PBN) changes the question asked of an aircraft. Conventional procedures are tied to particular ground aids, so the approval is for a receiver: an NDB approach needs an ADF, a VOR route a VOR receiver. PBN instead states the navigation performance required on a route, a procedure or in an airspace, and any area navigation system that can show it meets that performance may be used, whatever its sensors.

ICAO sets out the concept in the Performance-based Navigation Manual (Doc 9613), and PBN now shapes airspace design worldwide. In the United States the AIM places PBN under the umbrella of RNAV. In Europe, Commission Implementing Regulation (EU) 2018/1048 obliges air navigation service providers to implement PBN, including RNP approaches at every instrument runway end, and allows conventional procedures after 6 June 2030 only as a contingency, for example against a GNSS outage.

From sensor-based to performance-based

ICAO describes the PBN concept as three components. The navigation specification sets the requirements for aircraft and crew. The navaid infrastructure is what is available on the ground and in space: GNSS, DME, VOR. The navigation application is the use of the two on a particular route, procedure or airspace. Together they serve an airspace concept, which describes what the airspace must deliver, such as route spacing or access to a runway.

The gain is flexibility. A route can be placed where the airspace needs it rather than where the beacons stand, parallel routes can be spaced by the demonstrated accuracy, and a procedure can be flown by any approved system. The price is that navigation depends on data and equipment the crew cannot see directly, which is why PBN specifications set requirements for navigation databases, functions and crew knowledge as well as accuracy.

Navigation specifications

A navigation specification (NavSpec) is the set of aircraft and aircrew requirements needed to support a navigation application within a defined airspace concept. Its number is the lateral navigation accuracy in nautical miles that the population of aircraft is expected to achieve for at least 95% of the flight time. Beyond accuracy it can require functions, such as radius-to-fix (RF) turns or parallel offsets, specific sensors, and crew training.

Specifications are different from one another, not better or worse. The FAA gives the example that an RNP 1 eligibility does not automatically confer RNP 2 or RNAV 1 eligibility, so each is listed separately in the flight manual or avionics documents. Some do overlap by rule: an aircraft eligible for RNP 4 is also eligible for RNP 10, and one eligible for Advanced RNP (A-RNP) is typically eligible for RNP APCH, RNP and RNAV 1 and 2, RNP 4 and RNP or RNAV 10. The FAA requires navigation databases to hold only the procedures for which the aircraft is eligible, so a missing procedure is a hint that the aircraft cannot fly it.

RNAV versus RNP specifications

PBN has two families. An RNAV specification sets an accuracy. An RNP (required navigation performance) specification adds on-board performance monitoring and alerting (OBPMA): the aircraft itself must monitor the navigation performance it achieves and tell the crew whether the requirement is being met. The sensors may be identical; the difference is the monitoring. Because the aircraft monitors itself, an RNP specification needs less reliance on ATC intervention or procedural separation to reach the same level of safety.

RNP 10 is the anomaly in exam questions. It predates PBN and has no monitoring and alerting requirement, so it is in fact the RNAV 10 specification under its old name; an aircraft eligible for RNP 10 is deemed eligible for RNAV 10. The rule is to read the specification, not the letters. Approaches are covered in RNP approaches.

Oceanic and en-route specifications (RNP 10, RNP 4, RNAV 5)

In oceanic and remote airspace there is little ground infrastructure, and accuracy buys separation. RNP 10 routes in the North Pacific allowed tracks to be spaced 50 NM apart. RNP 4 requires GNSS and, with ADS-C and CPDLC, supports 30 NM lateral and longitudinal separation; PANS-ATM also allows 23 NM lateral separation where the required communication and surveillance performance (RCP 240 and RSP 180) is met. Operators in such airspace typically carry two independent long-range navigation systems (see oceanic and North Atlantic operations).

RNAV 5 is the continental en-route specification, the successor to European B-RNAV. It requires the total system error to stay within 5 NM for 95% of the flight time and, being an RNAV specification, carries no monitoring and alerting requirement.

Terminal specifications (RNAV 1/2, RNP 1)

RNAV 1 is the usual specification for RNAV SIDs and STARs: total system error within 1 NM for 95% of the flight time, typically with GNSS or DME/DME/IRU. RNAV 2, with 2 NM, is used en route in the United States for T- and Q-routes. RNP 1 requires 1 NM with monitoring and alerting for arrivals and departures, and for the initial and intermediate segments of conventional approaches with PBN elements, such as an ILS with an RNAV feeder or missed approach. RF capability is optional in RNP 1, so an eligible aircraft may still be barred from a procedure's RF legs.

Charts state what is required. In the United States a PBN box lists the specification and, where needed, sensors, additional functions and the minimum RNP value. French STAR headers read, for example, "RNAV 1, GNSS or DME/DME (IRU required)" at Lyon and "RNAV 1, GNSS only" at Toulouse. A charted RNP value of 0.30 or below is written with two decimals, such as RNP 0.15, which distinguishes it from the RNP 0.3 specification, which applies initially only to helicopters.

Total system error and its components

The accuracy of a specification applies to total system error (TSE), the difference between the aircraft's true position and the desired path. TSE combines three components:

• Path definition error (PDE): the difference between the path the RNAV system defines and the path intended, for example through a coding error.
• Flight technical error (FTE): how closely the aircraft follows the defined path, whether flown by the autopilot or by hand from the deviation display.
• Navigation system error (NSE): the difference between the aircraft's true position and the position the system estimates.

A perfect GNSS position does not guarantee compliance if the aircraft wanders off the line. FTE is why the tighter specifications and RF legs call for the autopilot or at least the flight director.

View full schematic

RNP containment

The accuracy value is statistical: the aircraft must be within it for at least 95% of the flight time, so brief excursions beyond it are expected. RNP applications also account for errors at a multiple of the accuracy value, typically twice. That is the containment limit at which the system must alert: an RNP 4 system when TSE exceeds 8 NM, an RNP 0.3 system at 0.6 NM. On RNP AR approaches the obstacle evaluation area extends only twice the RNP value either side of the path, with no secondary area or buffer, so the performance must be sustained, not merely achieved on average.

On-board performance monitoring and alerting

The FMS estimates the uncertainty of its own position, called estimated position uncertainty (EPU), actual navigation performance (ANP) or estimated position error (EPE) depending on the manufacturer. Boeing defines ANP as the radius of a circle around the FMS position within which the aircraft lies with 95% probability. The system compares it with the RNP for the phase of flight:

• Boeing 737: UNABLE REQD NAV PERF-RNP on the CDU and the navigation display after the time to alert, with amber FMC lights.
• Airbus: NAV ACCUR DOWNGRAD when the EPE exceeds the required accuracy; on an A320 approach with GPS PRIMARY available, no separate accuracy check is required.
• Embraer E1: DEGRAD when the EPU exceeds the RNP or the position integrity exceeds the alarm limit.

Receiver autonomous integrity monitoring (RAIM) is an example of the monitoring a GNSS receiver performs. When an alert appears, the crew tells ATC and reverts to other means: conventional navigation, radar vectors or another procedure. On an approach, an alert before the final approach fix means the approach is not started; after it, a missed approach is flown.

Vertical RNP

The accuracy value of every PBN specification is lateral. The vertical path of an approach comes from barometric VNAV or from SBAS, each with its own limits: baro-VNAV is restricted by temperature, and SBAS bounds its vertical error through its integrity data (see SBAS and GBAS augmentation). Some FMSs add a vertical counterpart to RNP. The Boeing 737 FMC uses a vertical required navigation performance (VRNP), by default 400 ft in the oceanic, en-route and terminal phases, and computes a vertical ANP, the estimated maximum altitude error with 99.7% probability, from the barometric altitude. That estimate is valid only with the correct altimeter setting, the one from the ATIS or the approach clearance.

In Europe the aircraft is certified for PBN under CS-ACNS, and RNP AR APCH is the only PBN specification for aeroplanes that needs a specific operational approval; in the United States RNP AR procedures require an authorisation issued under AC 90-101A.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.