Cold Battery Over a River
You plan a 1.2 km out-and-back flight over a river to photograph a bridge with a 900 g C1 quadcopter. Ambient temperature is 1°C; the battery was stored in a warm car and shows 100% and 16.8 V. Wind at 60 m AGL is forecast 9 m/s from the north, gusting 12 m/s; your route is north then south. The drone's maximum wind resistance is 10 m/s. The manufacturer's cold-weather note says capacity may drop by up to 20% below 0°C and voltage sag can trigger early landing. Your RTH height is set to 50 m. The river is 150 m wide; the nearest safe landing area on the far side is a field, but it is soft after rain.
1. Assess whether the planned flight is within the aircraft's performance limits, showing your reasoning with the wind and temperature numbers.
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It is not within limits. The aircraft's maximum wind resistance is 10 m/s, but the forecast is 9 m/s gusting 12 m/s. A gust above the maximum means the aircraft may be unable to hold position or make headway, especially at 60 m where wind is often stronger than at the surface. The outbound leg is into a northerly wind, so groundspeed and range are reduced; the return leg is downwind but the battery will already be depleted.
The cold adds a second margin problem: at 1°C the manufacturer warns of up to 20% capacity loss and voltage sag. A warm-car 100% reading is optimistic once the pack cools in flight. Combined with a 150 m-wide river and a soft-field landing option, the flight has no reliable diversion. The correct assessment is to wait for lower wind or shorten the route over land.
2. If you still fly a shortened 400 m out-and-back route along the near bank, what battery and contingency planning would you apply?
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Plan around a 30% landing reserve, not the displayed 100%. Derate the usable capacity for cold: if up to 20% is lost, treat the pack as roughly 80% usable before reserve, and keep the route over land so a forced landing is survivable. Warm the battery before launch, start with a low-voltage alarm, monitor cell voltage under load rather than only the percentage, and set a hard turn-back point at 400 m. Do not cross the river; the soft far-side field is not a dependable diversion.
Contingency: if voltage sags earlier than expected, land at the nearest safe land point rather than climbing for RTH. A 50 m RTH height may put the aircraft into stronger wind; manual flight at low height may be safer. The plan should state the abort criteria before launch.
3. During the flight the battery voltage drops from 15.8 V to 14.9 V under load and the aircraft warns ‘low voltage’. You are 300 m from home at 60 m. What do you do?
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Treat the warning as real. Reduce load immediately: stop climbing, stop aggressive manoeuvring, and fly the shortest safe route to a landing point. Do not trigger an RTH that climbs to 50 m and adds time; descend deliberately while maintaining control, and land at the nearest safe ground-even the soft field-rather than trying to reach the original home point over water.
Reasoning: the voltage drop under load is the signature of sag, and it can precede a sudden loss of power. The remaining 300 m is not a reason to gamble; a controlled landing away from people is preferable to a forced landing in the river. After landing, do not reuse that pack without testing, and record the event.