RAIM Outage and Cold-Temperature Altimetry En Route to Bandar Abbas
You are in a Boeing 737 at FL180 en route from Tehran to Bandar Abbas. QNH at destination is 1002 hPa; the minimum safe altitude in the sector is 12,000 ft. The outside air temperature is ISA-20°C. Your altimeter is set to standard pressure 1013 hPa. The aircraft has GNSS, two IRS, and VOR/DME. The destination's RNAV (GNSS) approach requires RNP 0.3 and RAIM availability at the ETA. The latest RAIM prediction shows a 20-minute outage starting 10 minutes before your ETA. The weather is VMC, but the airport has no conventional approach. The nearest suitable alternate, Shiraz, is 35 minutes away and has an ILS.
1. Estimate the altimeter error and true altitude at FL180. Is the minimum safe altitude compromised?
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Altimeter set to 1013 while QNH is 1002: the altimeter thinks sea-level pressure is 1013, so at a given pressure level it indicates higher than true altitude. Pressure error ≈ 11 hPa × 30 ft/hPa ≈ 330 ft. Cold temperature ISA-20 causes further over-reading; using 4 ft per 1,000 ft per °C: 20 × 18 × 4 ≈ 1,440 ft. Total true altitude is roughly 18,000 − 330 − 1,440 ≈ 16,230 ft. MSA 12,000 ft is still protected by a wide margin at FL180, but the error is significant for lower altitude and approach procedure design; use QNH and apply cold-temperature correction when below MSA or on approach.
2. How does the RAIM outage affect the RNAV approach, and what are your options?
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The RNAV (GNSS) approach requires RNP 0.3 and RAIM. A predicted 20-minute outage starting 10 minutes before ETA means the aircraft cannot legally plan to conduct that approach at the ETA. Options: hold until RAIM is available (if fuel permits), delay departure/descent, divert to Shiraz ILS, or request a visual approach if VMC and ATC permits. Because the destination has no conventional approach, the safest plan is to divert to Shiraz rather than rely on a visual approach at an unfamiliar airport with no instrument backup. Do not assume IRS can substitute for RAIM for RNP 0.3.
3. If RAIM fails during the approach, what immediate actions would you take?
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If RAIM fails during the approach, go around immediately and inform ATC. Do not continue on GNSS without integrity. Revert to conventional radio navigation if available, or request a visual approach only if the airport is in sight and terrain/obstacle clearance is assured. If neither is possible, divert to the alternate. The failure may be annunciated as a loss of integrity or RAIM warning; treat it as a navigation degrade and use the published missed approach.