PBN, RNAV and RNP performance

What error does a navigation specification constrain?

The aircraft can be off the intended path even with an accurate position.

Three contributions

Path-definition error is the difference between the intended path and the path represented in the navigation system. Navigation-system error is the difference between estimated and actual position. Flight-technical error is how well the aircraft follows the guidance. Together they contribute to total-system error; a centred display does not prove that the underlying position or encoded path is correct.

The laboratory shows signed components on one lateral axis. In a particular instant their algebraic sum gives the illustrated actual path error. This is not the same as combining independent statistical error distributions, and opposite signs in one example do not justify assuming permanent cancellation.

RNAV and RNP

Area navigation permits operation on a desired path within the applicable aid coverage or self-contained capability. PBN navigation specifications include performance and functional requirements. RNP adds onboard performance monitoring and alerting. A numerical designation describes a stated lateral accuracy requirement, commonly achieved for at least 95% of flight time; it is not an all-purpose obstacle-clearance margin.

A centred needle can follow a displaced path

Suppose the navigation system estimates the aircraft 0.4 NM west of its true position. The flight-control system faithfully centres its lateral guidance to the coded path. The aircraft can then be about 0.4 NM east of the intended path even with very small displayed tracking error, depending on the sign convention and other components. Good flight-technical performance does not remove navigation-system error.

Add a path-definition error of +0.1 NM and an instantaneous flight-technical error of −0.2 NM under the laboratory’s stated sign convention. The illustrated total becomes +0.3 NM. The algebra makes the relationship visible at one instant. A statistical requirement over time needs the appropriate distributions, containment and monitoring, so the apparent cancellation in this example is not a basis for planning to operate with known biases.

Does RNAV 1 capability automatically authorise every RNP procedure?

Read the whole specification, aircraft capability and operation.

A specification is more than a number

RNAV 1 and RNP 1 share a nominal numerical lateral-accuracy scale but do not have identical requirements. RNP includes onboard performance monitoring and alerting. Particular specifications can require functions, sensors, databases, training and operational processes beyond a single accuracy value.

Capabilities such as radius-to-fix turns, advanced leg handling or special approach authorisation cannot be inferred solely from a small displayed estimated uncertainty. The AFM, avionics documentation, operator approvals where required and published procedure establish the applicable capability.

After a sensor failure

An aircraft may remain within an apparently small error temporarily after losing GNSS. Whether it can continue a PBN operation depends on its remaining sensors, approved reversion, monitoring and required specification. The laboratory compares the magnitude of a synthetic instantaneous signed lateral error with an exercise value. It does not demonstrate 95% accuracy, monitoring availability or continuation authority.

The same number does not mean the same capability

A crew sees an estimated position uncertainty of 0.2 NM before a procedure requiring a particular RNP capability. The number seems smaller than 1 NM, but that comparison does not establish onboard monitoring, required functions, database suitability, training or any specific authorisation. The procedure may contain an RF leg that the installation is not approved to execute.

Similarly, RNAV 5 eligibility does not automatically establish RNAV 1, and an RNP capability does not authorise every other specification by numerical ordering alone. Read the actual approved capabilities and operational conditions. In the laboratory, change the supplied comparison value while holding the illustrated instantaneous error fixed; the displayed comparison changes, but the eligibility answer remains conditional on the full specification. This separates a useful performance indication from a much broader permission claim.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.