Four Miles and a Two-Thousand-Five-Hundred-Foot Ceiling
Departure 2130Z, 1 hour 45 minutes en route, ETA 2315Z. Destination TAF:
KXYZ 211720Z 2118/2224 18010KT P6SM SCT050 FM212300 28015G25KT 4SM -SHRA BKN025 OVC050 FM220400 32012KT P6SM SCT030
Destination field elevation 1,200 ft MSL; the airport lies in a Class E surface area. The minimum en route altitude over the ridge 15 nm west of the field is 9,500 ft MSL. AIRMET Sierra is current for mountain obscuration along the route. The airplane is a normally aspirated single. You and your student both hold private pilot certificates with no instrument rating. The student says, "Four miles and a 2,500-foot ceiling - that's VFR, we're good."
1. Which forecast conditions govern your flight, and what are the ceiling and visibility in both AGL and MSL terms?
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The ETA is 2315Z. The governing group is FM212300, which becomes prevailing at 2300Z:
- Wind 280 at 15 gusting 25
- Visibility 4 SM in light rain showers
- BKN025 OVC050
The first group (2118Z) expires at the changeover and is irrelevant to your arrival. The third group (FM220400) does not take effect until 0445Z - an hour and a half after you land - so it is equally irrelevant, and reaching for it is the classic student error.
Ceiling: BKN025 means 2,500 ft AGL. Field elevation is 1,200 ft MSL, so the ceiling is at 3,700 ft MSL. Visibility 4 SM satisfies the 3 SM required in Class E at night below 10,000 ft MSL.
2. Is the flight completable under VFR? Identify the specific barrier, and explain why the visibility number is a red herring.
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No - not as a VFR flight. The barrier is the descent, not the visibility.
The route must be flown at 9,500 ft MSL to clear the ridge. The cloud bases are at 3,700 ft MSL. To reach a 1,200 ft MSL airport you must pass through a solid broken-to-overcast deck with no forecast break in it, and AIRMET Sierra is already warning of mountain obscuration. There is no altitude at which you can remain VFR and also get down.
The student focused on the visibility because 4 SM passes the 3 SM Class E requirement and that is the number stamped on the rule. But visibility governs whether you can see; the ceiling governs whether you can get in. VFR flight above a cloud layer is lawful in the United States only if you can maintain cloud clearance above the deck and can descend VFR - which requires a hole - and no hole is forecast. Planning around an unforecast hole, over rising terrain, at night, with a student aboard, is not a plan.
3. What options do you present, and what do you drill into the student so this analysis becomes automatic next time?
Read the worked answer
Options, in order of preference:
- Move the flight to daylight. The forecast improves to P6SM SCT030 after 0445Z, and daytime Class E minimums plus a visible horizon and visible terrain change the risk substantially.
- Pick a different destination with a TAF showing a forecast break, and pre-brief the divert.
- Do not go. With gusty surface winds and obscuration over a ridge, the honest answer for two non-instrument-rated pilots is to stay on the ground.
Drill: make the student compute ETA first and write it next to the TAF before reading any group. Then have them read only the group in force at that time. Then have them convert every ceiling to MSL and ask the two questions in order - "can I see?" and "can I get down?" Then have them look for the next change group and ask whether it arrives before or after the ETA, because students reliably pick the best-looking line on the page. Finish by having them say out loud what would have to be forecast for the flight to be a yes - that is what turns a weather briefing into a decision.