Density Altitude and the Confined Helipad
A Bell 206B is scheduled to depart from a confined helipad at Jos, elevation 4,200 ft, with QNH 1013 hPa and OAT 31°C. The basic empty mass is 2,100 lb. Pilot 180 lb, three passengers 200 lb, 190 lb and 170 lb, baggage 40 lb. Fuel is 60 US gal at 6.7 lb/gal. The helicopter's maximum gross mass is 3,200 lb. The departure helipad is surrounded by trees and requires an out-of-ground-effect (OGE) hover to clear them. The flight manual shows that at a density altitude of 7,500 ft the maximum mass for OGE hover is 2,850 lb; for IGE hover it is 3,050 lb. The planned flight is 25 minutes to a site with fuel available. The pilot's normal reserve is 30 minutes at 20 gal/h. The pilot wants to take all three passengers and full baggage because the client has paid in advance.
1. Calculate the take-off mass and the density altitude, and compare them with the limits.
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Take-off mass = 2,100 + 180 + 200 + 190 + 170 + 40 + (60 × 6.7 = 402) = 3,282 lb. The maximum gross mass is 3,200 lb, so the helicopter is already 82 lb over the structural limit. Density altitude: ISA temperature at 4,200 ft is about 15 − (2 × 4.2) = 6.6°C. The OAT is 31°C, so the ISA deviation is about +24.4°C. Using the rule of thumb, DA ≈ 4,200 + (120 × 24.4) ≈ 7,100 ft; the flight manual chart for these conditions gives 7,500 ft. At 7,500 ft the OGE hover limit is 2,850 lb, so the aircraft is 432 lb over the OGE limit. The flight cannot depart as loaded; the IGE figure of 3,050 lb is not relevant because the trees require an OGE hover.
2. Propose a workable load that meets the OGE hover limit and reserve fuel, showing your arithmetic.
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One workable load is to offload the 200-lb passenger and reduce fuel to 25 US gal. New mass = 2,100 + 180 + 190 + 170 + 40 + (25 × 6.7 = 167.5) = 2,847.5 lb. This is 2.5 lb below the 2,850-lb OGE limit and well below the 3,200-lb maximum gross mass. Fuel check: 25 gal is available. The 25-minute trip at 20 gal/h uses 8.3 gal. The 30-minute reserve uses 10 gal. Total required is 18.3 gal, leaving a margin of 6.7 gal, or about 20 minutes. The pilot should also confirm that the destination has fuel, because the load depends on refuelling before the return. If the passenger cannot be offloaded, the flight must wait for cooler conditions or use a helipad that permits an IGE departure.
3. What operational risks remain and what should the pilot brief the passengers?
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The margin is only 2.5 lb at the OGE hover limit, so any change in wind, temperature, mass or aircraft performance could make the departure marginal. The high density altitude, trees, downdrafts and possible gusting require a careful departure into wind and a plan for an aborted hover. The pilot should brief the passengers on doors, seat belts, loose items, sterile cockpit during departure, and emergency egress from a confined site. The pilot should monitor torque and turbine temperature and avoid overtorque while clearing the trees. If the temperature rises further or the wind becomes unfavourable, the pilot should delay the departure rather than accept a negative performance margin.