Density Altitude Departure from a Confined Pad
You are flying a Robinson R44 Raven II (empty mass 683 kg, MTOW 1134 kg) from a mountain helipad at 1,650 m pressure altitude on a day when the OAT is +28 °C. The pad is 12 m wide with a downslope lip to the north and rising terrain beyond. Pilot 82 kg, one passenger 95 kg, baggage 20 kg, fuel 120 L (density 0.72 kg/L). The aircraft's performance chart gives a maximum gross mass for hover in ground effect at these conditions of 1,020 kg, and out of ground effect 940 kg. The pad is large enough for full ground effect. Wind is calm. The departure path requires a vertical climb to 15 m before any forward translation.
1. Compute the actual take-off mass and decide whether the departure as planned is legal and safe. Show your arithmetic.
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Fuel mass = 120 × 0.72 = 86.4 kg. Payload = 82 + 95 + 20 = 197 kg. Empty 683 + 197 + 86.4 = 966.4 kg. MTOW 1,134 kg is not the limiting figure here - the performance chart is. IGE limit 1,020 kg: 966 kg is below it, so a hover-taxi or ground-effect departure is within chart limits. But the required profile is a vertical climb clear of the surface to 15 m, which is OGE. OGE limit is 940 kg, and 966 kg exceeds it by 26 kg. The departure as planned is therefore not permissible. Options: offload 26 kg of fuel (about 36 L) or baggage/passenger mass, wait for a cooler part of the day, or reposition to a pad with a clear translational departure path. Note that the OGE figure already includes the chart's safety margin; do not erode it further by 'just trying it'. The rising terrain beyond the lip removes the option of trading height for speed.
2. The passenger asks why you cannot simply 'pull a bit less collective and drift forward' off the pad. Explain the aerodynamic and regulatory reasoning.
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Two separate constraints coincide. Aerodynamically, translational lift only develops once airflow through the rotor becomes significantly non-axial; below roughly 10–15 kt of forward speed the induced velocity is still high and power required is near the hover value. If you leave ground effect at low speed you are simultaneously losing the ground-effect reduction in induced power and gaining no translational benefit - the worst corner of the power-required curve. At 1,650 m pressure altitude and +28 °C the density altitude is well above 2,500 m, so power available from the normally aspirated engine is substantially reduced while power required is increased; there is no reserve to arrest a sink. Operationally, the pad lip and rising terrain mean any sink is unrecoverable - there is no runway or clear area to trade for height. The chart's OGE limit exists precisely to keep you out of this region, and departing over max gross mass for the profile invalidates the performance data you are relying on. The correct answer is to change the mass, not the technique.
3. After offloading fuel to reach 940 kg, what additional pre-departure checks and in-flight decision points should you brief?
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Brief: (1) re-verify mass and CG after offloading - removing fuel from a tank behind the mast affects CG, so recompute against the envelope, not just the total. (2) Confirm the fuel remaining gives a safe margin to destination plus reserve at the higher-than-planned consumption in dense air; a 36 L reduction may compromise range. (3) Note the density altitude trend - if the OAT is rising, the 940 kg limit itself falls, so depart early. (4) Brief the departure: vertical climb to 15 m, then a controlled transition to forward flight, calling out airspeed and manifold pressure limits. (5) Identify a rejection point - if manifold pressure is at the limit and climb rate is below a nominated value (e.g. 150 ft/min), land back on the pad rather than pressing on. (6) Confirm LTE avoidance: with calm wind, establish translation to the right or into wind as soon as height permits, and keep pedal margin in reserve. (7) Brief the engine-failure plan: below 15 m the only option is a cushioning landing on the pad; above it, a turn away from rising terrain.