Density Altitude, a Full Tank, and a 300-Foot Climb
You are hired to fly a 46.5 lb spray-and-survey hexacopter (28.5 lb empty, 18 lb of tank and product) over a 40-acre field. Launch point elevation is 5,350 ft MSL, altimeter setting 30.02 inHg, outside air temperature 33 °C at 1430 local. The profile needs a 300 ft AGL transit over terrain that rises 120 ft at the far end, and the aircraft must hold a 250 fpm climb after each turn. The manufacturer's chart shows a hover thrust margin of 2.0:1 at sea-level standard conditions, degrading to about 1.35:1 at 9,000 ft density altitude, where maximum sustained climb falls below 200 fpm. Your last flight was at 400 ft MSL on a 15 °C day with the same load, and it performed fine. Winds light and variable; sky clear; visibility 10 SM.
1. Compute pressure altitude and density altitude for the launch point, and explain what those numbers mean for this aircraft.
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Pressure altitude = field elevation + (29.92 − altimeter setting) × 1,000 = 5,350 + (29.92 − 30.02) × 1,000 = 5,350 − 100 = 5,250 ft
ISA temperature at 5,250 ft = 15 − (2 × 5.25) ≈ 4.5 °C
Density altitude ≈ PA + 120 × (OAT − ISA) = 5,250 + 120 × (33 − 4.5) = 5,250 + 3,420 ≈ 8,670 ft (call it ~8,700 ft)
Meaning: the air is about as thin as it would be at 8,700 ft on a standard day. Rotors and propellers produce less thrust for a given power setting, so the machine must run at higher RPM and higher current to hover. Climb rate, service ceiling, and endurance all fall, and motor/ESC temperatures rise. The aircraft's 46.5 lb mass has not changed, but the air's ability to support it has - effectively, the machine is heavier today than it was at sea level.
2. Compare the computed density altitude to the manufacturer's data and decide whether the mission as planned is feasible. If not, what is your plan?
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The computed ~8,700 ft is roughly 97% of the 9,000 ft point where the chart says maximum sustained climb drops below 200 fpm. The mission requires a sustained 250 fpm climb. That is outside the aircraft's demonstrated capability at this density altitude and this weight - the chart's degradation is not a cliff at 9,000 ft, so being 'just under' it does not buy you the margin you need, especially over rising terrain.
Options, in order of preference:
- Reduce weight. Offload product (e.g., fly with 8–10 lb rather than 18 lb) and split into two sorties. Lower weight buys back thrust margin more directly than anything else you can change on site.
- Change the time. Every 1 °C cooler buys roughly 120 ft of density altitude. An early-morning launch at, say, 15 °C gives a density altitude near 6,500 ft, which restores performance.
- Relax the profile. Lower the AGL transit and keep more terrain clearance margin at the far end, or launch from lower ground.
What you do not do is fly the full-load profile and 'see how it goes.' The failure mode is not a warning light - it is an inability to arrest a descent over rising terrain with a full tank.
3. Once airborne, what indications would you treat as abort criteria, and what reserve would you hold?
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Set the abort criteria before launch so that the decision is not made while you are busy:
- Climb performance: if the aircraft cannot hold 250 fpm with margin - or is at full power to hold altitude rather than to climb - the weight/conditions combination is wrong. Land and offload.
- Electrical: voltage sag under climb and current draw near the manufacturer's limit are the signature of high density altitude. Set a hard floor (e.g., the manufacturer's minimum cell voltage under load) and land when it is reached, not after.
- Thermal: motor and ESC temperatures climbing past the published limit, plus a battery that is hot to the touch at landing, indicate you are asking for more power than the air can support.
- Control response: sluggish yaw or altitude hold, or oscillation that was not present at sea level, means the flight controller is at the edge of its authority.
- Energy reserve: land with a substantial reserve (a common planning figure is 20–30% remaining) because a full-load machine at high density altitude needs more power, not less, on the return leg - and you must never be descending toward the ground with an empty tank and rising terrain at the far end.