SID climb gradient versus ATR performance
You are captain of an ATR 72-600 departing NZWN RWY 16 on a published SID. The SID requires a minimum climb gradient of 6.0% to 3,000 ft. Takeoff weight is 21,000 kg, OAT 24°C, QNH 1008, runway elevation 40 ft. Performance charts give a maximum climb gradient of 4.8% in the takeoff configuration at V2, and 5.6% after gear and flap retraction. The first segment obstacle is a 1,200 ft hill 3.2 NM from the departure end of runway. You plan a reduced-thrust takeoff. ATC offers an intersection departure with 2,100 m runway remaining. The aircraft has no special departure procedure approval. You must decide whether the SID can be flown, what options exist, and what to tell ATC.
1. Calculate the required climb rate at 150 kt ground speed for the 6.0% SID and compare with the aircraft's 4.8%.
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150 kt = 150 x 6076/60 = 15,190 ft/min along track. At 6.0%, required rate = 15,190 x 0.06 = 911 ft/min. The aircraft's 4.8% gives 15,190 x 0.048 = 729 ft/min. The aircraft cannot meet the SID at the planned weight and thrust. Even the 5.6% after cleanup gives 851 ft/min, still below 911, and the obstacle is close at 3.2 NM, so waiting for cleanup is not enough. A reduced-thrust takeoff makes the shortfall worse. Do not accept the SID unless performance is recomputed.
2. What options do you have?
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Options include reducing weight (fuel or payload), increasing thrust if allowable, waiting for a cooler temperature, using a different runway or SID with a lower gradient, or requesting a departure procedure with ATC vectors and obstacle clearance. An intersection departure with 2,100 m may change obstacle distance; check the specific procedure. If no option meets 6.0%, do not depart on that SID. You could accept a vectored departure only if ATC can provide terrain separation and the procedure is authorised.
3. What would you tell ATC and what would you monitor?
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Tell ATC: 'Unable SID due climb gradient; request ...' before taxi or at the holding point. If departing, monitor VSI, airspeed, engine torque, and obstacle distance; call 'positive rate' and ensure the required gradient by the departure end of runway. Do not rely on the VSI alone because it lags; use pitch attitude and computed rate. If the gradient is not met, abandon the departure and return or land. The key is to make the performance decision on the ground, not after takeoff.