RNAV Approach with a Database and Monitoring Trap
You are cleared for an RNAV (GNSS) approach to runway 27 at an airport with a published LNAV/VNAV line and a circling minimum. The approach has a final approach course of 270 degrees and a missed approach climb to 3,000 ft. Your GNSS receiver is approved for LNAV/VNAV. During briefing, you notice the chart shows a note that the final approach segment is not authorised when the temperature is below -15 degrees C. The current temperature is -18 degrees C. The airport has a remote altimeter setting and the current QNH is 1013 hPa. ATC clears you to descend to 2,500 ft until established. Your autopilot is coupled in approach mode.
1. What is the significance of the temperature note, and what should you do before continuing?
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The note means the final approach segment is not authorised at -18 degrees C because the barometric altimetry and the vertical path may be affected by cold-temperature altimeter errors. The crew must not fly the final segment as published. Options are to discontinue the approach, request a different approach that is authorised, or, if the operator's procedures permit, apply cold-temperature altitude corrections to the published altitudes. Because the note specifically prohibits the segment below -15 degrees C, the crew should treat the approach as unavailable and request an alternate approach or diversion. The QNH of 1013 hPa does not resolve the temperature limitation. The key judgement is that a chart note is an operating limitation, not advisory text.
2. How should the crew manage the autopilot and the missed approach if the approach is continued?
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If the approach were authorised, the crew would monitor the autopilot in approach mode, cross-check the vertical path against the published altitudes, and be ready to disconnect. The missed approach must be flown promptly at the decision altitude or missed approach point, with the climb to 3,000 ft and the published track. In this case, because the approach is not authorised, the crew should not continue. If ATC has already cleared the descent to 2,500 ft, the crew should advise ATC of the inability to fly the approach and request a revised clearance. The autopilot should be monitored for mode confusion, particularly if the approach mode arms but the vertical path is invalid. The safest action is to discontinue and reassess.