Definition
A required capability of Required Navigation Performance (RNP) systems in which the aircraft's navigation equipment continuously checks its own position accuracy against the performance standard required for the procedure being flown, and warns the flight crew if that accuracy can no longer be assured.
Plain English
The navigation system watches itself. It keeps measuring how accurately it knows where the aircraft is, and if that accuracy drops below what the procedure needs, it tells the pilots so they can take action.
Context Anchor
Seen in Required Navigation Performance discussions, especially for routes and instrument procedures that depend on the aircraft meeting a stated navigation accuracy.
Derivation
Two ideas combined: 'monitoring' (Latin monere, to warn) means keeping a continuous watch, and 'alerting' means raising a flag when something is wrong. The FAA pairs them because watching alone is useless without telling the crew, and alerting alone is useless without something watching first.
Why Pilots Care
It confirms the aircraft can safely stay inside the protected airspace of an RNP procedure; without it, crews may unknowingly exceed limits and must abandon the approach.
Grounding Statement
The system is not only showing where the aircraft is going; it is also checking whether its own position information is trustworthy enough for that procedure.
Intuition Check
Do not read this as the pilot simply watching the navigation display. In this FAA context, it means the aircraft system itself checks navigation performance and gives an alert when the required accuracy may not be met.
Example Sentence 1
Because the FMS provides onboard navigation monitoring and alerting, the crew was cleared to fly the RNP approach into the mountainous airport.
Example Sentence 2
When GPS signals weakened, the onboard navigation monitoring and alerting immediately displayed an RNP alert and the crew executed the missed approach.