Area Navigation
RNAV concepts
Area navigation lets an aircraft fly a defined path between waypoints rather than directly between ground stations. Waypoints are defined by latitude/longitude or by rho-theta, and the FMS builds a leg from each pair. ARINC 424 path terminators decide how the aircraft joins and leaves each waypoint: a TF leg tracks to a fix, a CF leg tracks a course to a fix, an RF leg follows a constant-radius arc, and a fly-by waypoint uses turn anticipation while a fly-over waypoint is crossed before turning.
RNP and containment
Required navigation performance adds on-board performance monitoring and alerting to RNAV. The aircraft continuously estimates its actual navigation performance (ANP); if ANP exceeds the RNP value, an alert is raised. RNAV specifications (RNAV 10, 5, 2, 1) set an accuracy requirement without mandatory on-board alerting, while RNP specifications (RNP 4, 2, 1, RNP APCH, RNP AR APCH) do require it. Containment is normally quoted at 95% for accuracy and 99.999% for integrity.
Worked example
At 240 kt groundspeed and 25° bank, turn radius = V²/(g tan φ) = (123.5 m/s)²/(9.81 × 0.466) = 3,340 m ≈ 1.8 nm. For a 90° turn the arc is about 2.8 nm, and the FMS starts the turn roughly 1.8 nm before the waypoint - the turn-anticipation distance.
Traps
Assuming every waypoint is a fly-by; forgetting that turn anticipation grows with speed and with the size of the course change; treating ANP as a guarantee rather than an estimate; and mixing RNAV and RNP specifications.