One-engine-out obstacle clearance at Rotorua
Rotorua (elevation 936 ft) on a warm afternoon: OAT 22°C. You are flying a twin-engine aeroplane with MTOW 1,900 kg. The AFM gives gross one-engine-inoperative (OEI) climb gradients at sea level ISA, gear up, flaps up, propeller feathered: 9.2% at 1,700 kg, 7.8% at 1,800 kg, 6.4% at 1,900 kg. For a two-engine aeroplane the net takeoff flight path is the gross gradient minus 0.9%. The departure is runway 36; an obstacle 90 m above aerodrome elevation lies 1,200 m from the end of TODA. Screen height is 35 ft. The aircraft is loaded with full fuel and four passengers to 1,880 kg. V_MCA is 71 KIAS and V_YSE is 88 KIAS. The only alternate runway, 18, has a tailwind of 10 kt but a lower obstacle.
1. Calculate the net and gross OEI climb gradients required to clear the obstacle.
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Screen height = 35 ft = 10.7 m. Height to be gained = 90 − 10.7 = 79.3 m. Required net gradient = 79.3 / 1,200 = 0.0661 = 6.61%.
For a two-engine aeroplane the net takeoff flight path is the gross gradient minus 0.9%, so: Required gross gradient = 6.61 + 0.9 = 7.51%.
This is the gradient that the AFM gross OEI figures must meet or exceed at the planned weight. It must be flown from the end of TODA at the screen height with the gear and flaps up and the propeller feathered; any delay in feathering or configuration change reduces the margin.
2. Determine the OEI climb-limited weight and compare it with the planned 1,880 kg.
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Interpolate between 1,800 kg (7.8%) and 1,900 kg (6.4%):
Weight = 1,800 + (7.8 − 7.51)/(7.8 − 6.4) × 100 = 1,800 + (0.29 / 1.4) × 100 = 1,800 + 20.7 = 1,821 kg.
At the planned 1,880 kg the aircraft is 59 kg above the climb-limited weight. The available gross gradient at 1,880 kg is about 6.4 + (100 − 80)/100 × 1.4 = 6.68%, net about 5.78%, which is less than the 6.61% required. The obstacle would not be cleared on one engine.
The takeoff at 1,880 kg is therefore not acceptable on runway 36, even though the aircraft is below MTOW and the all-engines performance may look satisfactory.
3. Recommend what to do and explain the operational factors that affect the decision, including V_MCA and V_YSE.
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Actions: reduce weight by at least 59 kg (fuel or payload), choose a departure with a lower obstacle, or wait for cooler air. The AFM figures are ISA; at 22°C the gradient will be lower than the sea-level ISA table, so the real limitation is likely worse than 1,821 kg and a larger reduction is prudent.
Runway 18: the lower obstacle helps, but a 10 kt tailwind increases the ground distance to the obstacle and reduces the time available to climb, so it is not automatically better; it must be recalculated.
V_MCA and V_YSE: V_MCA is 71 KIAS and V_YSE is 88 KIAS. If an engine fails after the decision point, the aircraft must be at or above V_YSE and the calculated gradient flown accurately; below V_MCA directional control may not be maintainable. The safe band is narrow, so do not accept a weight that leaves no margin for temperature, engine deterioration, or a late feather.