Density Altitude on the Benguet Plateau
At 07:15 you launch from a plateau in Benguet at 1,850 m elevation. The aerodrome altimeter setting is 1008 hPa and the site thermometer already reads 28 °C, with relative humidity at 84 percent. Your 9 kg multirotor is loaded to its maximum take-off mass with a survey payload. The flight manual gives a maximum density altitude of 3,000 m and warns that hover power rises steeply above 2,700 m. The mission is a 55-minute corridor survey flown in four 18-minute sorties, with the far end of the corridor 300 m below the launch point in a valley that funnels anabatic wind from mid-morning. The forecast is 12 kt upslope wind and convective build-up after 14:00. Your client wants all four sorties completed before noon so that the data can be processed overnight.
1. Calculate the pressure altitude and density altitude for the launch site and say what the result means for the mission.
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Pressure altitude
Use roughly 30 ft per hPa: (1013 − 1008) = 5 hPa × 30 = 150 ft. PA = 6,070 ft + 150 ft = 6,220 ft (the elevation of 1,850 m converts to about 6,070 ft).
ISA temperature at that pressure altitude
ISA = 15 °C − (1.98 °C per 1,000 ft × 6.22) ≈ 15 − 12.3 = 2.7 °C.
Density altitude
DA ≈ PA + 120 × (OAT − ISA) = 6,220 + 120 × (28 − 2.7) = 6,220 + 3,036 = 9,256 ft, about 2,820 m.
What it means
2,820 m is below the 3,000 m absolute limit, so the flight is legal on paper - but it is 120 m above the manufacturer's 2,700 m caution threshold, where hover power rises steeply. Two additional factors eat into that margin: 84 percent humidity slightly degrades performance, and the corridor ends 300 m below the launch point, so every recovery involves a climb back to a site that is already at high density altitude. The practical read is: flyable now, but only just, and the margin will disappear as the surface heats.
2. At what point in the day does the mission become non-viable, and how would you structure the sorties to protect the margin?
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Finding the wall
Run the same calculation forward. At 32 °C: DA = 6,220 + 120 × (32 − 2.7) = 9,736 ft, about 2,970 m. Still inside the limit, but only 30 m below it, and well above the caution band. At 34 °C: DA = 6,220 + 120 × 31.3 = 9,976 ft, about 3,040 m - over the limit. So the aircraft is over its certified density altitude somewhere between 32 and 33 °C surface temperature, which on a Benguet plateau in the dry season can be reached before 11:00.
Structuring the sorties
- Fly the two heaviest, longest legs first, while the temperature is lowest and the upslope wind has not yet built.
- Set a hard decision temperature, not a clock: if the site thermometer hits 32 °C, the remaining sorties are cancelled and the data is delivered partially. Tell the client this at 07:15, not at 10:30.
- Keep a 30 percent battery reserve and treat it as non-negotiable; high DA means the reserve is consumed faster than the percentage suggests.
- Plan the down-valley legs so that the recovery climb is made early in the sortie, not with a depleted pack.
Delivering three sorties and a clear explanation beats four sorties and a forced landing on a slope.
3. What flight-technique and monitoring changes does high density altitude require on this mission?
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Expect less performance than the numbers suggest
Hover power rises, climb rate falls, and the aircraft will feel sluggish and slow to arrest a descent. Do not interpret a poor climb as a battery problem and keep pushing; interpret it as the atmosphere and act on it.
Technique
- Fly a flatter, faster profile. Translational lift is your friend at high DA; hovering is the most expensive thing the aircraft does. Minimise hover time at waypoints.
- Fly approaches with a higher ground speed and a shallower angle, and be ready to go around early rather than late. A high-DA go-around from a deep approach may not be available.
- Avoid descending deep into the valley unless you have the climb performance to get out; identify an alternative landing area on the valley floor before you commit.
- Use the upslope wind: launch and recover into wind, and use the anabatic flow to help the return leg rather than fighting it.
Monitoring
- Watch cell voltage under load, not just the percentage. Voltage sag under a high-power climb is the earliest warning you will get.
- Track the site temperature and QNH through the morning and recompute the density altitude at each battery change; a two-degree rise is meaningful here.
- Brief the observer on the abort criteria: any unexplained power warning, any inability to hold the planned climb rate, any convective build-up earlier than forecast. Land, do not troubleshoot in the air over a valley.