RNAV GNSS approach with cold temperature and baro-VNAV limit
With reference to the RNAV (GNSS) RWY 23 approach at fictional Tiko (elevation 1,200 ft), you are cleared to conduct the LNAV/VNAV approach. The published LNAV/VNAV minima are 1,560 ft AMSL; the LNAV MDA is 1,620 ft AMSL. The chart note states: 'Baro-VNAV not authorised below −15°C.' ATIS gives OAT −18°C, QNH 1013 hPa, wind 230/12, visibility 8 km. You are 12 NM from the initial approach fix at 3,000 ft, GNSS RAIM is available, and the aircraft has approved baro-VNAV. The crew has no cold-temperature correction table on board, but the EFB contains the operator's approved altimetry correction card. ATC asks whether you will accept the approach.
1. Can you fly the LNAV/VNAV approach as cleared? Explain the limitation.
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No. The chart prohibits baro-VNAV below −15°C, and OAT is −18°C. The restriction is not advisory: the barometric vertical guidance is not validated at that temperature and can place the aircraft below the intended path. An ATC clearance does not override an aircraft or procedure limitation. Advise ATC: 'Unable LNAV/VNAV due temperature, request LNAV.' If you are already inside the final approach fix, do not continue on the vertical path while debating it - go around and rebrief.
2. If cleared for LNAV, what must you apply before using the MDA and step-down altitudes?
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Apply the operator's approved cold-temperature altimetry correction to the LNAV MDA and any step-down fixes. The correction is additive - fly higher than published - because cold air makes the altimeter over-read relative to true altitude. Use the EFB correction card with OAT −18°C and aerodrome elevation 1,200 ft; do not use a generic rule of thumb. The corrected MDA becomes the new minimum. If the correction card is unavailable, or the corrected MDA cannot be safely flown with the available visual segment, divert rather than improvise.
3. ATC cannot offer LNAV and you are 2 NM inside the FAF. What do you do?
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Fly the published missed approach immediately and advise ATC. You cannot continue to LNAV/VNAV minima because the temperature limit is exceeded, and you cannot invent a local LNAV clearance inside the FAF. Climb on the missed approach track, hold or divert, and rebrief a legal approach. If fuel becomes a factor during the diversion, declare it early rather than pressing on to an unsuitable approach.