RNP AR Approach: ANP Above RNP
You are conducting an RNP AR APCH RWY 23 into Queenstown (NZQN). The chart requires RNP 0.3 and specific crew and aircraft authorization. The aircraft is GNSS-based and the FMS shows ANP 0.42 nm. The crew notes that the alert limit is 0.6 nm and argues that because ANP is below the alert limit, the approach can continue. One of the two GNSS receivers is flagged invalid; the other is operating normally. Weather is VMC, but the approach is in mountainous terrain with a missed approach climb gradient. You are cleared for the approach and are 8 nm from the final approach fix. The tower asks your intentions. You must decide whether to continue, go around, hold, or divert, and explain the RNP containment logic. Do not rely on the alert limit alone.
1. Does ANP 0.42 nm meet the RNP 0.3 requirement?
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No. RNP 0.3 means the navigation system must achieve 95% position accuracy within 0.3 nm. ANP is the aircraft's estimate of its current navigation performance. ANP 0.42 nm is worse than the required 0.3 nm, so the required navigation performance is not met.
The approach should not be continued on that performance. The correct response is to discontinue the approach, go around or hold, and attempt to restore performance before considering another approach.
2. Explain the difference between RNP and the alert limit, and why the crew's argument is wrong.
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RNP is the operational accuracy standard for the procedure: 0.3 nm for this approach. The alert limit is the error threshold at which the system must alert the crew; it is not the accuracy required to fly the approach.
For RNP AR, the ANP must be at or below the RNP value before commencing and while continuing the approach. An ANP below the alert limit but above the RNP value means the system has not alerted, but the required performance is still not met. Using the 0.6 nm alert limit as a go/no-go criterion is therefore incorrect. The single valid GNSS also needs to be checked against the aircraft's RNP AR authorization and MEL; if the authorization requires dual GNSS, the approach is not permitted regardless of ANP.
3. What actions should you take?
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Discontinue the approach. Fly the published missed approach or go around, then hold or divert to an airport where the navigation performance is adequate. Do not attempt to continue to the final approach fix with ANP above RNP.
While holding, check satellite geometry, receiver status and the aircraft's RNP AR authorization. If performance recovers below 0.3 nm and the aircraft/crew configuration is authorized, a new approach may be considered. If not, divert. In mountainous terrain, do not accept a visual continuation that depends on the failed RNP path.