GNSS outage on an overlay approach with a minimum that depends on it
You are flying a published area-navigation (RNAV/GNSS overlay) non-precision approach to a regional aerodrome. The approach minima table shows two lines: LNAV minima of 480 ft and 1,600 m visibility, and LNAV/VNAV minima of 380 ft and 1,200 m. Your aircraft is approved for LNAV only; the vertical guidance is advisory and not approved for use. The final approach fix is 5.4 NM from the threshold. Reported weather: ceiling 500 ft broken, visibility 2,000 m, wind 210/12, surface temperature +2 °C, dewpoint +1 °C. The missed approach is a climbing right turn to a holding fix 6 NM away.
1. Which minima apply, and are you legal to commence the approach?
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You are approved for LNAV only, so the LNAV line applies: 480 ft and 1,600 m. The LNAV/VNAV line is not available to you even though your equipment displays a glidepath - using unapproved advisory vertical guidance to fly to lower minima is not permitted.
Weather check: ceiling 500 ft is above 480 ft, visibility 2,000 m is above 1,600 m, so the approach may be commenced. But the margin is only 20 ft of ceiling, which is within the accuracy of a reported ceiling and within your own altimetry tolerance. Treat it as marginal.
Temperature: at +2 °C with a dewpoint of +1 °C, the spread is 1 °C - fog or low stratus is likely to form or thicken. The reported weather may deteriorate between the report and your arrival. That, not the minima, is the real risk.
2. At 2.0 NM from the threshold you lose GNSS. What do you do, and what does the missed approach require?
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With GNSS lost inside the final approach fix, you no longer have the navigation accuracy the approach was designed around, and you have no approved vertical guidance. Go around immediately: apply power, establish a positive climb, and fly the published missed approach.
The missed approach is a climbing right turn to a fix 6 NM away. Without GNSS you cannot navigate precisely to that fix, so:
- Fly the turn and climb on the published heading and altitude, using heading and time as your primary means.
- Advise ATC immediately that you have lost GNSS and are going around; ask for vectors and a climb clearance.
- Do not attempt to re-fly the approach without GNSS. Request a surveillance-radar or vectored approach, or divert to an aerodrome with a conventional approach.
- If no radar is available, hold as published using timing and turns, or proceed to your alternate on a DR/radio-navaid plan.
3. How does the temperature affect your decision, and what would you do differently if the temperature were −5 °C?
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At +2 °C with a 1 °C spread, the risk is fog and rapid visibility loss, not altimetry. The decision should be to have a firm alternate with better weather and enough fuel to reach it, and to set a bingo point before starting the approach.
If the temperature were −5 °C, altimetry becomes the issue. Cold temperatures make the altimeter over-read, so the aircraft is lower than indicated. On an approach with a 480 ft minimum, the true height above the threshold could be significantly less than 480 ft - enough to matter for obstacle clearance on a non-precision approach with a step-down. The correction must be applied to all published altitudes, and the decision height effectively raised. Also check whether the aerodrome has published cold-temperature restricted minima; if the temperature is below the published limit, the approach is not authorised without the correction table.
In both cases the answer is the same shape: identify which hazard the temperature creates (fog versus altimetry error), apply the appropriate mitigation, and set a hard decision point before commencing.