RNP AR Approach With Cold Temperature and Degraded ANP
You are pilot flying a twin-jet on the RNP AR approach to Queenstown (NZQN) runway 05, in IMC, wind calm. The ATIS gives OAT -12°C and QNH 1022 hPa. The approach chart in use states a baro-VNAV temperature limitation of -8°C, an LNAV/VNAV DA of 890 ft and an LNAV MDA of 1,090 ft, both with RNP 0.30. On the initial segment the FMS annunciates ANP 0.42, the RNP 0.30 label turns amber and an UNABLE RNP message appears; the GPS status page shows that one satellite has dropped out and the geometry has changed. You are 15 nm from the airport, above the terrain, with 40 minutes of fuel holding. There is no other suitably equipped diversion within 30 minutes and the weather at the alternates is at their minima.
1. Explain why the baro-VNAV temperature limitation exists, and what it means for this approach at -12°C.
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Why cold matters. The baro-VNAV path is flown by reference to the pressure altimeter, which is calibrated to the International Standard Atmosphere. In air colder than ISA, the pressure at any true altitude is lower than standard, so the altimeter over-reads - the aircraft is lower than the indicated altitude. The error grows with height above the altimeter-setting source, roughly 4 ft per 1°C of ISA deviation per 1,000 ft. At -12°C, about 27°C below ISA at that level, the aircraft could be 100 ft or more below the nominal path in the final segment, eroding the obstacle clearance on which the RNP AR design depends.
What it means here. -12°C is below the chart's -8°C limit, so baro-VNAV must not be used. Descend using LNAV minima (1,090 ft MDA) and apply the published cold-temperature correction to the MDA, or to the DA if the operator's procedure provides one. Fly the lateral path in LNAV and treat the vertical path as step-downs checked against DME or waypoints. If the corrected minima cannot be met with the available visibility, do not conduct the approach.
2. Explain the ANP 0.42 against RNP 0.30 and the action the UNABLE RNP message requires.
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ANP versus RNP. ANP is the flight management system's current estimate of its own navigation performance - the radius containing 95% of the position solutions. RNP is the performance the procedure requires (0.30 nm here). While ANP is at or below RNP, the aircraft can be relied on to stay within the containment on which the procedure's obstacle clearance is calculated.
ANP 0.42 exceeds RNP 0.30, so the system can no longer guarantee containment. On an RNP AR approach the UNABLE RNP alert is a mandatory stop: the crew must not continue past the final approach fix, and if already inside it must go around. There is no 'continue and monitor' option, because the segment's terrain and missed approach protection assume containment to RNP.
Actions: advise ATC, discontinue the approach, fly the published missed approach (checking the missed approach can itself be flown to RNP), and then either hold to see whether the geometry recovers or divert. Record the event for the operator's navigation database and GNSS performance monitoring.
3. With both restrictions in force, set out your plan and the reasoning behind each decision.
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The two restrictions compound: baro-VNAV is unusable below -8°C, and lateral containment is degraded. The plan:
- Do not commence the RNP AR approach. Advise ATC and remain in the hold at a safe terrain-clearing level while assessing.
- Try to restore lateral performance. Check the satellite status page, allow the geometry to change - 10 to 20 minutes often restores ANP - and confirm no FMS fault. If ANP recovers to 0.30 or better, an LNAV-only approach with cold-temperature corrections applied to the MDA remains available.
- Set a bottom line. If ANP does not recover, or the corrected LNAV minima cannot be met, divert. With 40 minutes of holding fuel, decide early so you do not arrive at the diversion at minimum fuel.
- Ask why ANP rose. Satellite dropout and geometry, ionospheric disturbance, jamming or spoofing, or an FMS fault. If interference is suspected, abandon GNSS approaches entirely and use conventional aids.
The trap is continuing because the lateral path still looks flyable. The containment guarantee, not the look of the deviation indicator, is the basis on which the approach is authorised.