Climb, Cruise, Descent and En-route Performance
Climb
En-route climb requirements depend on the number of engines: the net gradient must be at least 1.2% for twins, 1.5% for three, 1.7% for four in the second segment configuration, and 1.2%/1.5%/1.7% respectively in the en-route configuration at 1,500 ft. Ceiling definitions matter: absolute ceiling (zero rate of climb), service ceiling (100 ft/min for twins, 500 ft/min for others), single-engine absolute ceiling and drift-down height. Accelerate-stop and accelerate-go distances are also climb-related.
Cruise
Cruise performance is a trade-off between specific range (nm per kg of fuel), long-range cruise (99% of max range, LRC speed), max endurance (minimum fuel flow, roughly VY) and max range (approximately 1.32 VY for propeller aircraft, or the tangent from the origin to the power-required curve). Cost index trades time against fuel. The crossover altitude (or crossover point) is where the cruise speed at one altitude equals the best speed at the next higher altitude.
Descent and en-route
Descent is normally at 3° (≈ 5.2%), giving about 320 ft/nm; the rule of thumb is 3 × altitude in thousands = nautical miles to lose. Top of descent distance = altitude to lose / 1,000 × 3, adjusted for wind and speed. Drift-down is the net flight path with one engine out; the net take-off flight path and net en-route path must clear terrain by 35 ft and 1,000 ft respectively.
Worked example
FL350 to 5,000 ft = 30,000 ft to lose, so TOD ≈ 90 nm at 3°, plus 10 nm for deceleration and wind. If the descent is flown at 2,000 ft/min and ground speed 420 kt, the required rate is 420 × 5.2% ≈ 22 kt per 1,000 ft... rather, 420 × 0.052 ≈ 21.8 kt per 1,000 ft, i.e. about 2,180 ft/min.
Traps
Confusing service and absolute ceilings; forgetting the 1,000 ft en-route terrain margin; using TAS instead of ground speed for TOD.