Icing and the Alternate Minimum
You are planning an IFR flight from Porto Alegre (SBPA) to Chapecó (SBCH) at FL180 in a non-FIKI twin turboprop. The area forecast shows an extensive layer of supercooled liquid water between FL120 and FL200, with freezing rain reported at the destination and a temperature of -2 °C at the surface. SBCH has one ILS with a decision altitude of 200 ft and a cloud base forecast at 300 ft, visibility 2,000 m. The only alternate within range is Passo Fundo (SBPF), 110 NM east, with an ILS and a forecast cloud base of 800 ft.
1. Assess the icing threat and decide what route and altitude you will file.
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The aircraft is not FIKI, so any flight into known icing is prohibited. The forecast layer from FL120 to FL200 with freezing rain means the planned FL180 is inside the threat, and the destination is below freezing at the surface, so the approach itself will be in icing conditions.
Options: file below the layer, for example FL100-FL110 if terrain and minimum en-route altitude allow, and accept the higher fuel burn; or route around the layer if the forecast shows a break; or cancel the flight. Because the destination is at -2 °C with freezing rain, the approach and landing would also be in icing, so even a low-level cruise does not solve the destination problem. The correct decision is to cancel or delay the flight until the layer moves, because a non-FIKI aircraft cannot legally or safely complete the destination approach in freezing rain. The examiner is testing whether you treat the forecast as known icing and whether you check the destination, not just the cruise.
2. If you did dispatch, how would the alternate requirement change and why is SBPF the only realistic choice?
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With the destination forecast at 300 ft cloud base and 2,000 m visibility, the weather is at or near the approach minima, so an alternate is required and must be forecast to be at or above the alternate minima at the estimated time of arrival. SBPF at 800 ft cloud base with an ILS satisfies that; SBCH does not, because its forecast is at the decision altitude and any deterioration would make it unusable.
The alternate must also be reachable with the fuel required and must not itself be in the icing layer on the approach. SBPF is 110 NM east, within range, with an ILS, and its forecast is above the alternate minima. The judgement point is that the alternate is chosen against the forecast at the ETA, not against the current weather, and that a destination at minima with freezing rain is a strong argument for not dispatching at all.