The Climb That Would Not Climb
A light twin departs a coastal aerodrome at 1,200 ft elevation on a summer afternoon. Surface temperature is 33 °C; the QNH is 1003 hPa. The aircraft has been loaded to its maximum certificated mass, and the pilot plans to climb at VY to 9,500 ft. The aircraft's two normally aspirated engines develop rated power at sea level ISA conditions. Passing 5,000 ft the rate of climb has fallen to 180 ft/min, and the pilot notices the cylinder head temperatures are high. There is no terrain above 4,000 ft on the planned track, but the aircraft must reach 9,500 ft to clear a ridge 40 nm ahead. The pilot has one hour of fuel reserve at planned consumption.
1. Compute the density altitude at the departure aerodrome and explain its effect on the aircraft's take-off and initial climb performance.
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Pressure altitude = elevation + 30 × (1013 − QNH) = 1,200 + 30 × (1013 − 1003) = 1,200 + 300 = 1,500 ft.
ISA temperature at 1,500 ft = 15 − 1.98 × 1.5 ≈ 12 °C. The actual temperature is 33 °C, so the ISA deviation is about +21 °C.
Density altitude ≈ pressure altitude + 120 × (OAT − ISA temp) = 1,500 + 120 × 21 ≈ 1,500 + 2,520 = 4,020 ft.
So the aircraft is effectively operating from an aerodrome 4,000 ft higher than its surveyed elevation. Both engine power and propeller thrust fall with density, while true airspeed for a given IAS rises. Take-off ground roll increases substantially (typically 30–50% for a 4,000 ft density altitude rise at maximum mass), the initial climb gradient is reduced, and the single-engine climb gradient after a hypothetical engine failure may become negative. The pilot should treat the take-off and climb performance figures for 4,000 ft density altitude, not 1,200 ft, and consider a cooler departure time or reduced load.
2. Explain why the rate of climb deteriorates so sharply by 5,000 ft and what the high CHT indicates.
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Normally aspirated engines lose power with altitude because manifold pressure falls roughly 1 inHg per 1,000 ft; at 5,000 ft the engine can produce only about 75% of sea-level power, and the propeller also absorbs less power in thinner air. Rate of climb is proportional to excess power divided by weight, so the combination of reduced power, high density altitude and maximum mass produces a steep fall-off in climb rate. The high CHT is a warning: at a high density altitude the mixture is rich, the engine is working hard at low airspeed with poor cooling, and the pilot may be tempted to lean aggressively to recover power. Over-leaning at high power settings raises CHT toward the detonation margin. The correct response is to enrich slightly, reduce power or accept a lower climb rate, and monitor CHT.
3. Given the remaining fuel and the ridge ahead, what decision should the pilot make and what is the reasoning?
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The aircraft is climbing at 180 ft/min at 5,000 ft; extrapolating, the climb to 9,500 ft would take roughly 25 minutes even if the rate held, and it will continue to decay. At a typical twin consumption of 20–25 US gal/h per engine, 25 minutes of climb is a significant fraction of the one-hour reserve, and the aircraft would arrive at the ridge with little margin if any headwind or further performance loss occurs. The safe decision is to abandon the direct track, turn back or divert to lower terrain while fuel and altitude margin remain, and replan at a lower cruising level where the aircraft can maintain a positive climb gradient. Continuing to a ridge with decaying climb performance, high CHT and a shrinking reserve is the classic 'get-there' trap. The pilot should also consider that terrain clearance must be based on the aircraft's actual achievable gradient, not the planned one.