VFR transit clearance with a lowering ceiling
You are pilot-in-command of a Cessna 172 with two persons on board, departing Karaj (OIIK) VFR for Kashan at 10:20 local. Fuel endurance is 3 h 10 min and the planned trip is 1 h 15 min. Karaj ATIS reports wind 300/08 kt, visibility 6 km, few cloud at 1,200 ft, scattered at 2,500 ft, broken at 4,000 ft, OAT 18°C, dew point 12°C, QNH 1016. Tehran Approach clears you: leave the Karaj CTR via the south gate, maintain 6,500 ft, squawk 7000, report 5 NM south. Fifteen nautical miles south, visibility falls to 4 km in haze and the ceiling is 2,000 ft. Rising terrain lies to the west. Your PPL has no instrument rating and the aircraft is not IFR equipped. Your briefing minima for controlled airspace are 5 km visibility and 1,500 m horizontal, 300 ft vertical cloud separation. The nearest suitable aerodrome behind you is 20 min away; an en-route aerodrome 15 min ahead reports 8 km visibility and broken cloud at 3,000 ft.
1. At 15 NM south, can you accept the ATC clearance and remain VFR? State the controlling minima and the action you must take with ATC if you cannot.
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The clearance does not override the visual meteorological conditions you must maintain. At 15 NM south the reported visibility is 4 km, which is below your briefing minimum of 5 km for controlled airspace. The cloud ceiling of 2,000 ft is not automatically the deciding factor at 6,500 ft, but the reduced visibility and rising terrain to the west make continued VFR unsafe. You should not simply continue and hope the haze clears. Tell ATC clearly: 'Unable to maintain VFR due visibility, request turn back or amended clearance.' If ATC can offer a special VFR clearance, consider it only if the aircraft, pilot and route meet the applicable conditions and you can remain clear of cloud and terrain. Otherwise turn back toward Karaj or divert. The key judgement is that the PIC, not the clearance, remains responsible for staying VFR.
2. Using the fuel and weather data, compare continuing, returning to Karaj, and diverting to the en-route aerodrome. Which do you choose and why?
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At 15 NM south you have used roughly 10-15 min of the 3 h 10 min endurance, leaving about 2 h 55 min. Continuing to Kashan might take about 60 min, plus a 45 min reserve, which is within fuel but exposes you to a further hour of deteriorating haze with no instrument rating. Returning to Karaj takes about 15-20 min; Karaj was 6 km visibility, so that is viable but still marginal. Diverting to the aerodrome 15 min ahead is the best option: it reports 8 km visibility and broken cloud at 3,000 ft, which meets the briefing minima, and it leaves about 2 h 35 min of fuel after landing. Choose the diversion. Continuing is not justified by the weather trend, and returning adds exposure when a suitable aerodrome is closer ahead.
3. If visibility drops to 2 km before you reach the diversion aerodrome, what immediate actions and radio calls should you make? Include the possibility of a precautionary landing.
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Do not press on into 2 km visibility. Reduce speed, configure the aircraft for controlled flight, and turn toward the last area of better visibility or toward the diversion aerodrome if it is still safely within reach. Make a clear radio call: 'Mayday, Mayday, Mayday' if you are in immediate danger, or 'Pan Pan, Pan Pan, Pan Pan' if urgency is developing; give position, altitude, heading, intention and persons on board. If ATC radar coverage is available, request vectors to the nearest suitable aerodrome. If you cannot maintain VFR and cannot reach the aerodrome, select a suitable field for a precautionary landing, fly a low inspection pass if time and terrain allow, and land into wind. Squawk 7700 if an emergency exists. The priority is to avoid entering cloud or terrain while trying to salvage the original clearance.