Hot-and-High Load Limit on a Trellised Slope
You are operating a turbine helicopter at a strip 1,250 m above mean sea level. OAT is 32 °C, QNH 1004 hPa. Basic empty mass 1,050 kg, pilot 85 kg, fuel 140 kg, spray tank 300 L of water-based product (density 1.0), and boom/ancillary equipment 25 kg. The hover ceiling in ground effect for these conditions, from the performance chart, is 1,350 kg, and OGE is 1,180 kg. The target block is a trellised vineyard on a 12° slope with 3 m vine rows and a 4 m headland, and the only practical spray line is a downslope run at 55 km/h with a 15 m turnaround at the bottom. The site is 8 km from the strip and you must plan for a go-around and a diversion to a 900 m alternate.
1. Compute the take-off mass and assess it against the published limits.
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Sum the masses: basic empty 1,050 kg + pilot 85 kg + fuel 140 kg + product 300 kg (300 L × 1.0 kg/L) + equipment 25 kg = 1,600 kg.
Now compare with the charts. The IGE hover ceiling of 1,350 kg is exceeded by 250 kg, and the OGE ceiling of 1,180 kg is exceeded by 420 kg. The aircraft is therefore not merely marginal; it is outside the IGE limit before any consideration of slope, turbulence or go-around. Take-off in this configuration would be a performance violation and, more importantly, would leave no margin to arrest a sink or to hover out of ground effect during a turnaround.
A reduction of at least 250 kg is needed just to reach the IGE figure, and that figure itself is a ceiling, not a target. A sensible operating target is well below OGE, so the practical reduction is closer to 420 kg plus margin. The only realistic way to achieve that is to reduce product load substantially and/or reduce fuel to the minimum for the sortie plus diversion plus reserve.
2. How would you restructure the sortie to make it legal and safe, and what figures would you verify?
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Work backwards from the OGE limit of 1,180 kg and apply a margin. If the target operating mass is, say, 1,120 kg, the allowable payload is 1,120 - 1,050 - 85 - 25 = -40 kg before fuel, which is impossible. That shows the aircraft cannot carry meaningful product at this density altitude with this fuel load. Options:
- Reduce fuel to the minimum for 8 km out, spray, 8 km back, diversion to the 900 m alternate plus final reserve. If that minimum is, say, 90 kg, allowable product becomes 1,120 - 1,050 - 85 - 90 - 25 = -130 kg, still impossible. This indicates the aircraft is not suitable for the task at this DA.
- Reduce the number of spray loads and refuel/reload at a forward point at lower elevation, so the aircraft operates at a lower mass and lower density altitude.
- Defer to early morning when OAT is lower; at 20 °C the charts would give a higher ceiling.
Verify: pressure altitude from QNH 1004 (about 1,250 m + 90 m ≈ 1,340 m), density altitude from the chart at 32 °C, IGE and OGE limits at that DA, fuel burn for the sortie, and the go-around/diversion fuel. Document the final mass and the limiting chart value.
3. Independently of mass, what flight-technique hazards does the trellised slope present, and how would you mitigate them?
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The 12° slope with 3 m rows and only a 4 m headland creates several hazards. First, the downslope run at 55 km/h means the aircraft is descending with rising terrain behind it and a short 15 m turnaround at the bottom; an out-of-ground-effect hover or steep pull-up at the bottom may exceed power available, especially at this density altitude. Mitigation: fly the run so that energy is managed - begin the pull-up early, keep airspeed above translational lift, and use a turn that does not require a hover.
Second, trellis wires and posts are a snagging and collision hazard; the 3 m row spacing leaves little lateral margin, so the spray line must be flown along the row direction, not across it, and the pilot must maintain a constant height above canopy rather than above ground.
Third, the slope generates mechanical turbulence and a possible lee-side downdraft; approach and departure should be planned into wind and along the slope contour. Fourth, the 4 m headland limits the go-around; a rejected run must be briefed before entering. Finally, the pilot should fly a reconnaissance pass at altitude to confirm obstacles, wire heights and the turnaround, and set a personal minimum for abort.