Definition
A required capability of an RNP (Required Navigation Performance) system in which the aircraft's navigation equipment continuously checks how accurately it is determining its position along the flight path and warns the crew if that accuracy falls below the level required for the procedure being flown. It is the feature that distinguishes RNP operations from ordinary RNAV operations.
Plain English
The aircraft watches its own navigation accuracy in real time and tells the pilots if it is no longer accurate enough for the route or approach they are flying.
Context Anchor
Seen in RNP procedures, especially where the aircraft must prove it can stay within a stated navigation limit during the operation.
Derivation
Three plain words doing exactly what they say: 'onboard' (in the aircraft itself, not from a ground station), 'performance monitoring' (watching how well the system is doing its job), and 'alerting' (telling the crew when something is wrong). The phrase is built to emphasise that the check happens inside the aircraft, automatically, without relying on ATC or ground equipment to spot a problem.
Why Pilots Care
It gives the crew immediate notice when navigation accuracy no longer meets RNP requirements, allowing corrective action before the aircraft leaves protected airspace.
Grounding Statement
The key idea is that the aircraft is not just following a route; it is also checking whether it is accurate enough to keep using that route.
Intuition Check
Do not read this as the pilot simply watching the instruments more carefully. In RNP, it means the aircraft’s navigation system automatically checks its own ability to stay within the required limits and warns the crew if it cannot.
Example Sentence 1
Because the aircraft was equipped with onboard performance monitoring and alerting, the crew were notified the moment navigation accuracy dropped below the level required for the RNP approach.
Example Sentence 2
Because the aircraft lacked onboard performance monitoring and alerting, it could not be cleared for the RNP 0.3 procedure.