Confined-Area Approach with Rising Density Altitude
You are tasked with a VIP transfer to a mountain helipad at 1850 m pressure altitude, ambient temperature +28 °C, QNH 1013 hPa. The helipad is a 15 m × 15 m concrete pad surrounded by 12 m trees on three sides, with an approach path over a ridge that requires a steep 12° approach. Your helicopter (empty 1450 kg) has a maximum performance take-off mass chart showing 1900 kg at 1850 m and +28 °C in ground effect, and 1750 kg out of ground effect. Fuel and crew plus two passengers total 380 kg. Forecast wind is calm, and the pad has no wind indicator. The operator's manual prohibits operations into confined areas when the calculated out-of-ground-effect hover ceiling is below the pad elevation.
1. Calculate the density altitude and determine whether the planned landing mass is within limits for an out-of-ground-effect hover.
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Pressure altitude 1850 m. ISA temperature at 1850 m = 15 − (1.98 × 1.85) ≈ 11.3 °C. Actual +28 °C, so ISA deviation ≈ +16.7 °C. Density altitude ≈ pressure altitude + 120 ft per °C deviation × (deviation in °C) - using the rule of thumb: 1850 m + (16.7 × 36.6 m) ≈ 1850 + 611 ≈ 2460 m. (More precisely, DA ≈ PA × (1 + 0.0037 × deviation) ≈ 2460 m.)
Planned landing mass = 1450 + 380 = 1830 kg.
OGE limit at 1850 m/+28 °C = 1750 kg. 1830 kg > 1750 kg, so the aircraft is 80 kg over the OGE limit. The manual prohibits the operation. Even the IGE limit of 1900 kg leaves only a 70 kg margin, which is insufficient for a confined pad with trees and a steep approach.
Decision: the landing cannot be made at 1830 kg. Either reduce mass by 80 kg (offload fuel or a passenger) or select an alternative landing site.
2. Explain how the 12° approach and the surrounding trees affect the decision beyond the mass calculation.
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A 12° approach is steeper than the normal 6–8° and requires a higher approach speed and a more aggressive flare, which increases power demand at the bottom of the approach. Trees on three sides create turbulence and reduce the effective headwind, so the helicopter may need to hover OGE during the final transition - exactly the condition where the 1750 kg limit applies.
The pad is 15 m × 15 m, which leaves little margin for translational error. With no wind indicator and calm forecast, you cannot confirm the wind direction, and mechanical turbulence from the trees may make the approach unpredictable.
Therefore, even if the mass were reduced to 1750 kg, the operation would be marginal. The prudent action is to conduct a high reconnaissance, confirm the wind, and if conditions are uncertain, land at a nearby open area and complete the transfer by ground transport. The mass calculation is necessary but not sufficient; the approach geometry and turbulence must also be within the aircraft's limiting performance.