Ridge clearance with marginal climb performance
You are departing a valley airport at 2,000 ft MSL in a 2,400 lb normally aspirated aircraft. The OAT is 20 °C and QNH is 29.92 inHg, so pressure altitude equals elevation. A terrain ridge 8 NM ahead rises to 5,500 ft, and you require 500 ft clearance over it. The climb chart gives 620 ft/min at 2,000 ft and 250 ft/min at 6,000 ft, decreasing linearly with altitude. Climb TAS is 85 kt; the forecast headwind gives 75 kt ground speed. The ridge cannot be flown around, and the only alternative is a 25 NM diversion to lower terrain. You have 40 min of fuel reserve. Decide whether a straight departure can clear the ridge and what you would change.
1. Calculate the required climb gradient and required rate of climb to clear the ridge by 500 ft.
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Height to gain = (5,500 + 500) − 2,000 = 4,000 ft.
Horizontal distance = 8 NM × 6,076 ft/NM = 48,608 ft.
Required gradient = 4,000 ÷ 48,608 = 0.0823 = 8.2%.
Ground speed 75 kt = 75 × 101.3 = 7,598 ft/min.
Required rate = gradient × ground speed in ft/min = 0.0823 × 7,598 = 625 ft/min.
The aircraft must average about 625 ft/min over the whole climb to the ridge, not merely achieve it for a few seconds after liftoff.
2. Compare the available climb performance with the requirement, allowing for the decrease with altitude.
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At 2,000 ft the book rate is 620 ft/min, already just below the 625 ft/min requirement. The rate then falls linearly to 250 ft/min at 6,000 ft.
Average rate between 2,000 and 6,000 ft = (620 + 250) ÷ 2 = 435 ft/min.
Average gradient = 435 ÷ 7,598 = 0.0573 = 5.7%.
Even using the initial 620 ft/min, the gradient is 620 ÷ 7,598 = 8.16%, which is slightly below 8.2%, and it decreases immediately. At the ridge, the aircraft would have gained only about 3,500–3,600 ft, reaching roughly 5,500–5,600 ft - near the ridge elevation with little or no required clearance. The straight departure cannot reliably meet the 500 ft clearance requirement.
3. What is your decision, and what practical change would most improve the margin?
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I would not depart straight out over the ridge. The performance is marginal at the initial altitude and deteriorates as the aircraft climbs, so the required 625 ft/min average is unattainable.
The most effective change is to reduce weight: offload fuel or passengers, provided the fuel plan still meets reserve requirements. Weight reduction improves both rate of climb and the altitude at which the aircraft can maintain it. A cooler departure time also helps by lowering density altitude, but a few degrees alone will not close a shortfall of roughly 190 ft/min in average rate. Climbing overhead the airport before setting course is not a solution unless the aircraft can reach 6,000 ft before the ridge, which the available rate does not allow. If weight cannot be reduced enough, take the 25 NM diversion or postpone the flight.