Afternoon Convection and Density Altitude at a 1,200 ft Strip
You plan a 95 nm VFR cross-country in an ultralight. The departure strip is at 1,200 ft MSL, runway 08/26, 620 m dry grass. At 13:30 local the temperature is 28°C, dew point 19°C, QNH 1015 hPa, wind 220/12 kt. The forecast calls for scattered cumulus with bases at 4,500 ft AGL and isolated thunderstorms after 15:00, with gust fronts to 30 kt. Your aircraft has an empty mass of 260 kg and maximum takeoff mass 450 kg. Pilot 80 kg, passenger 70 kg, baggage 10 kg, fuel 30 L (density 0.72 kg/L). The POH gives a sea-level, 15°C, zero-wind takeoff ground roll of 180 m and says to add 12% per 1,000 ft of density altitude. Climb at sea level is 450 ft/min. You planned to return at 17:30. Decide whether to depart and how to plan the return.
1. Calculate the mass, density altitude, and adjusted takeoff ground roll. Is the departure legal and suitable?
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Mass: empty 260 + pilot 80 + passenger 70 + baggage 10 + fuel 30 × 0.72 = 21.6 kg, total 441.6 kg, below the 450 kg limit. Pressure altitude = 1,200 + (1013 − 1015) × 30 = 1,140 ft. ISA temperature at 1,140 ft = 15 − 1.98 × 1.14 ≈ 12.7°C. ISA deviation = 28 − 12.7 = 15.3°C. Density altitude ≈ 1,140 + 120 × 15.3 ≈ 2,976 ft, about 3,000 ft. Takeoff increase = 2.976 × 12% ≈ 36%. Ground roll ≈ 180 × 1.36 ≈ 245 m. The 620 m strip leaves margin, but climb at 450 ft/min sea level will be noticeably reduced and the grass may add drag. It is legal by mass but performance-limited.
2. Interpret the temperature/dew point spread and forecast. What convective hazards matter to an ultralight?
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Spread is 28 − 19 = 9°C. That is not automatically convective, but with afternoon heating and the forecast, isolated thunderstorms after 15:00 are credible. Hazards for an ultralight: gust fronts to 30 kt, low-level wind shear, downdrafts and microbursts, lightning, hail, and rapid visibility loss. Cumulus bases at 4,500 ft do not mean the storms are high; bases lower and rain shafts can reach the ground. Visual avoidance is unreliable because an ultralight is slow and has low wing loading. Watch for towering cumulus, dark bases, dust gusts, and a sudden wind shift. Do not launch into a building cumulus field.
3. Make a go/no-go and return-time decision.
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Planned return at 17:30 is inside the thunderstorm window, so the plan as filed is not acceptable. If you go, depart now and plan to be on the ground before 15:00, with a hard turn-back time and an alternate outside the storm area. Given the density altitude, passenger load, and isolated storm forecast, the safer decision is to cancel or shorten the flight. If the return is delayed, do not press on; land early. A 30-min reserve does not protect against a gust front or a blocked route.