Warm Front, Freezing Rain and GPS Outage over Water
A VFR flight in a Cessna 182 is planned from Niigata to Sapporo Okadama, 2 h 10 min over the Sea of Japan. The forecast shows a warm front with widespread cloud base 1200 ft, tops 12,000 ft, and freezing rain between 4000 and 8000 ft. The planned VFR altitude is 6500 ft; MSA is 4700 ft. The aircraft is not approved for flight into known icing. GPS RAIM prediction shows a 40 min outage on the route; the aircraft has VOR/DME and ADF, but the route is mostly over water beyond VOR/DME coverage. Fuel allows 30 min reserve at destination; an alternate on the coast is 25 min away. The pilot must decide whether to depart or revise the plan.
1. Interpret the freezing rain forecast and explain the icing risk at the planned altitude.
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Freezing rain means supercooled water droplets that freeze on impact. It is a strong indication of severe airframe icing potential, especially in the 4000 to 8000 ft band. The planned VFR altitude of 6500 ft is inside that band, and the aircraft is not approved for known icing. Ice can accumulate rapidly on the airframe, propeller and pitot system, increasing stall speed and reducing climb performance. Climbing above the band may not be possible and descending into warmer air may not be an option over water. The correct interpretation is to avoid the forecast icing band entirely, not to depart and hope to pick a gap. This is a no-go item for this aircraft.
2. Evaluate the navigation and VFR-over-water aspects of the plan.
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The cloud base is 1200 ft and tops are 12,000 ft, so a VFR cruise at 6500 ft would require flying in cloud or between layers, which is not VFR. Over water, there is little opportunity for a forced landing or diversion. The GPS outage removes the primary navigation source for 40 min, and VOR/DME/ADF coverage is limited beyond the coast. Dead reckoning over water with a 40 min outage and marginal VFR is not a safe plan. The MSA of 4700 ft does not solve the cloud problem. The route needs either a coastal VFR route with continuous visual reference and navigation backup, or an IFR aircraft and pilot appropriately equipped. As filed, the plan is not acceptable.
3. What decision would you make as PIC, and what revised plan would you consider?
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I would not depart on the planned route at 6500 ft. The freezing rain forecast, cloud tops, no icing approval, GPS outage and over-water exposure are cumulative hazards. A revised plan could be a coastal VFR route below the freezing-rain band but clear of cloud, with VOR/DME or visual navigation and frequent position checks; or delay until the warm front passes and the freezing rain ends. If the flight is time-critical, use an IFR-capable aircraft with de-icing equipment and a pilot qualified for the conditions. The decision should be based on the combination of hazards, not on the fact that the destination forecast alone is VFR.