Marginal VFR into Rising Terrain
You are flying at 2500 ft AMSL toward rising ground that peaks at 3100 ft, 12 NM ahead. Visibility is 6 km in haze, cloud is broken at 3500 ft with bases lowering to 2800 ft over the hills. QNH is 1018 hPa. You have 45 minutes of fuel to reserve. Your route has a valley option to the east that adds 8 NM but stays below 2000 ft terrain. The destination is 30 NM beyond the hills and its ATIS reports 10 km visibility and cloud at 4000 ft. You have no transponder and are not in controlled airspace.
1. Assess the legality and safety of continuing direct, and compare it with the valley route.
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VFR minima in Class G below 3000 ft AMSL in this kind of airspace are typically 5 km visibility and clear of cloud and in sight of the surface. At 6 km you meet the visibility figure, but the cloud base over the hills at 2800 ft with terrain to 3100 ft means you cannot remain clear of cloud and in sight of the surface while crossing the ridge. That makes the direct route unlawful and unsafe, regardless of the destination's good weather.
Fuel: 30 NM to destination plus 12 NM to the hills at, say, 80 kt is about 32 minutes to the ridge, leaving roughly 13 minutes before reserve - and the valley adds 8 NM, about 6 minutes. The margin is thin but workable if you commit now.
The valley route keeps you below 2000 ft terrain with cloud at 2800 ft or higher, giving about 800 ft of clearance and better visibility in haze at low level, though you must watch for obstructions and rising ground in the valley itself. The correct decision is to divert east now, not to press on and hope the cloud lifts.
2. What would make you turn back entirely rather than take the valley?
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Turn back if any of the following applies: the valley cloud base falls below about 2500 ft, reducing clearance to under 500 ft; visibility in the haze drops below 5 km at valley level; the fuel margin shrinks below 30 minutes to reserve given the extra 8 NM and any headwind; or rising terrain in the valley forces a climb into cloud.
A useful test is the escape route. In the valley you should always have a turn-back option to lower ground behind you. If the valley narrows and you cannot reverse without climbing, you have passed the decision point and should not have entered.
Also consider the destination forecast: if the 4000 ft cloud is a temporary improvement and the hills are lowering, the return leg may be worse. The disciplined answer is to set a hard limit - cloud base, visibility, and fuel - before entering, and to turn back the moment one is reached rather than renegotiating with yourself in the air.