Diversion into Deteriorating Weather with Fuel Reserve Pressure
You are flying a single-engine piston aircraft VFR from a coastal airfield toward an inland destination, planned flight time 2 h 10 min. You depart with 150 L usable fuel; the aircraft burns 34 L/h at your planned power setting. After 1 h 15 min you have 78 L remaining. A line of cumulus build-ups with embedded showers is developing across your planned track ahead, tops reported above your cruising level. Your destination has no instrument approach and its only runway is aligned across the gusting surface wind. The nearest suitable alternate is 25 min away with a paved runway and fuel, but it lies behind you. Sunset is in 55 min. You hold a CPL and are not instrument rated.
1. Assess your fuel situation against the VFR reserve requirement and decide whether continuing, diverting, or holding is defensible.
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Recompute usable fuel at the decision point. Planned burn for the remaining 55 min to destination at 34 L/h is about 31 L, leaving roughly 47 L on arrival - comfortable against a 30-minute (17 L) VFR reserve if the flight goes as planned.
But the weather ahead is convective with tops above cruise level, you are not instrument rated, and the destination runway is crosswind-limited with no approach aid. The realistic outcome is either a long detour around the line or a go-around and diversion anyway. Budget for the worst credible case, not the planned case.
Decision: divert now to the alternate behind you. It is 25 min away, has fuel and a paved runway, and keeps you clear of the cell. Continuing into embedded showers without an IR, with a marginal crosswind destination and a setting sun, converts a fuel-comfortable flight into a fuel-critical one. The reserve exists to protect against exactly this kind of unplanned deviation.
2. State the factors that make the 'press on' option legally and safely weaker than the diversion.
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- Weather vs. rating: Convective cloud with tops above cruise level is not VFR-compatible; without an IR you cannot legally or safely enter it.
- Destination limitations: No instrument approach and a crosswind-aligned runway reduce the chance of a successful landing first time.
- Daylight: Sunset in 55 min; a go-around and second approach would push you into night VFR without night planning.
- Fuel reserve erosion: Any holding or detour eats the 30-min reserve; the reserve is a minimum, not a target.
- Alternate availability: A suitable alternate behind you is the conservative, regulation-consistent choice under the precautionary principle.