Cloud base, a church spire and a 500-foot rule
You plan a VFR flight in a Cessna 152 with one passenger from a grass strip (elevation 200 ft AMSL) to a farm strip 40 NM away. Your planned cruise is 1,600 ft AMSL. The route crosses a town of about 40,000 people whose highest structure is a 180-ft church spire standing on ground 250 ft AMSL; the highest terrain off-route is 300 ft AMSL, and a 250-ft radio mast stands 3 NM east of track. The forecast gives scattered cloud base 2,000 ft AMSL, visibility 8 km, QNH 1022, temperature 18 °C. You hold a PPL(A) with no instrument rating, and the aircraft is VFR-equipped only. You intend to fly the direct track at 1,600 ft AMSL and to descend only if the cloud base forces you down.
1. Is the planned cruise of 1,600 ft AMSL lawful in Class G? Show the numbers.
Read the worked answer
No - the cruise level fails the VMC distance-from-cloud requirement.
In Class G below 3,050 m (10,000 ft) AMSL the VFR minima are a flight visibility of 5 km and a distance from cloud of 1,500 m horizontally and 300 m (1,000 ft) vertically. Visibility of 8 km is fine; the vertical distance is not.
- Cloud base: 2,000 ft AMSL.
- Planned cruise: 1,600 ft AMSL.
- Vertical distance to cloud: 400 ft.
- Required: 1,000 ft.
So the aircraft would be legal in visibility terms but illegal in cloud-clearance terms, even though it is clear of cloud and in sight of the surface. 'Clear of cloud' is only one part of the test.
The trap is that the natural fix - fly lower - runs straight into the minimum-height rule over the town, which is examined next.
2. You consider descending to 1,000 ft AMSL to get the cloud clearance. What stops that, and what are your real options?
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What stops it: the minimum-height rule over a congested area. Under the ICAO Annex 2 standard, flight over a congested area requires at least 300 m (1,000 ft) above the highest obstacle within 600 m, and in any case a height from which an emergency landing can be made without undue hazard to people or property. SERA expresses the congested-area rule as sufficient height for a landing without undue hazard, and national rules commonly apply the 1,000 ft/600 m criterion.
- Spire top: 250 ft + 180 ft = 430 ft AMSL.
- Minimum over the town: 430 + 1,000 = 1,430 ft AMSL.
At 1,000 ft AMSL over the town you would be roughly 570 ft above the spire and below the minimum; at 1,430 ft AMSL you would be only 570 ft below cloud. Over the town the two rules cannot both be satisfied in this weather.
Real options:
- Wait for a higher cloud base. For a 1,600 ft cruise you need cloud at or above 2,600 ft AMSL.
- Reroute around the town over non-congested ground, where the minimum is 500 ft above the ground or water, and cruise at about 1,000 ft AMSL (about 700 ft AGL over 300 ft terrain), which also gives 1,000 ft to cloud. Keep well clear of the radio mast - it is not a congested-area obstacle, but a 500-ft-above-ground cruise leaves little margin and no reason to test it.
- Cancel or delay. With one engine and no instrument rating, that is a legitimate answer.
3. Halfway there the scattered cloud lowers and visibility drops to about 4 km ahead of you. What is the correct decision sequence?
Read the worked answer
Turn back or divert while you are still legal - do not press on to see whether it improves.
- Recognise the trigger. Below 5 km flight visibility, or with cloud forcing you below the required distance, you are no longer in VMC and cannot continue VFR. Descending to 'get under' is how VFR flights enter terrain.
- Fly the escape. A 180° level turn back to the last known good conditions is the standard escape manoeuvre; alternatively divert to the nearest aerodrome where the weather is reported good.
- Do not enter cloud. You hold no instrument rating and the aircraft is not equipped for IMC; entering cloud is both illegal and outside your competence. If you inadvertently enter IMC, concentrate on attitude, make a rate-one 180° turn, and tell ATS if you can.
- Special VFR is not a rescue here. It exists in control zones with ATC clearance, not in Class G, and it does not lower the cloud-clearance requirement in the way pilots sometimes assume.
- Land and reassess. A precautionary landing at a suitable strip is cheaper than a forced landing in a field.
Judgement point: the decision to turn back is made on the forecast and the trend, not on the hope that the next valley is clearer.