Astana to Karaganda: Convective SIGMET, Falling QNH and Marginal Fuel
You are planning a VFR flight from Astana to Karaganda, 250 km, in a Cessna 172, on a July afternoon. The route forecast gives scattered cumulus at 1,200 m, tops to 3,500 m, isolated thunderstorms with heavy rain and hail, surface wind 240°/25 kt gusting 40 kt, QNH 998 hPa and falling, temperature 28 °C, dewpoint 19 °C. A SIGMET for embedded thunderstorms is current for the area. Your aircraft has 110 litres usable, the engine burns 40 litres per hour, taxi and run-up use 8 litres, and company practice is a 45-minute reserve. Forecast headwind gives 150 km/h ground speed. At destination the only instrument approach is an NDB, and there is no alternate within 60 km with a paved runway; the next suitable alternate is 120 km away. The aircraft is not approved for flight in known icing and has no weather radar.
1. Compute the fuel required and say whether the flight can carry a 30-minute hold.
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Usable 110 L. Trip: 250 km at 150 km/h ground speed = 1.67 h; at 40 L/h that is 66.7 L. Taxi and run-up 8 L. Reserve 45 min at 40 L/h = 30 L. Total 104.7 L, leaving 5.3 L - about 8 minutes - before the reserve is touched.
A 30-minute hold is 20 L, so it cannot be carried: the tanks would be dry before the hold ended. Note also that the 45-minute reserve is a planning minimum, not a contingency for a thunderstorm diversion. A realistic plan for convective weather would carry fuel to reach the 120 km alternate (about 48 minutes plus approach) or would not launch. On the numbers alone the flight is marginal; combined with the weather it is not viable. If it were attempted, diversion fuel must replace the reserve, not supplement it - the figure to compare is total usable fuel against trip plus taxi plus diversion plus final reserve.
2. Given the meteorology and the destination/alternate situation, state your decision and defend it.
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Do not depart now. Thunderstorms with hail, an embedded-CB SIGMET, tops to 3,500 m, a 15 kt gust spread, QNH 998 and falling, and a temperature/dewpoint of 28/19 °C (a 9 °C spread over a moist surface) all say convection is active and growing. A Cessna 172 has no radar and no icing approval, and “see and avoid” does not work against embedded cells - the first clue is often the hail. The destination has only an NDB approach, and the only alternate is 120 km away, so a missed approach in a gust front leaves no realistic diversion.
Delay until the afternoon convection has passed or go in the morning. If you must go, plan a route with 20 NM clearance from the cells and a firm turn-back point, and keep enough fuel for the diversion. The fact that the aircraft could physically fly the leg is irrelevant; the flight has no safe outcome if the forecast is even partly right.
3. You are already airborne and the cell builds across track. What do you do?
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Turn away early. 20 NM is the usual planning clearance, and at 150 km/h that is 8 minutes of flying; do not press on hoping for a gap. If ATC is available, ask for a heading and, where possible, radar information about the cells - a controller's echo description helps, but it does not replace your own visual decision. Climbing is rarely the answer with tops at 3,500 m and your service ceiling, and descending into a downdraught is worse.
Divert to the alternate while you still have the fuel, and land before the gust front arrives: the strongest shear is at the leading edge, where a 40 kt gust on final can become a 25 kt airspeed loss in seconds. If caught, fly attitude and power, accept the go-around, and do not chase the glideslope. Afterwards, note that a flight that ends in a diversion is a successful flight; a flight that ends inside a cumulonimbus is not.