Cruise and en-route
Cruise and en-route performance is about range, altitude and the one-engine-inoperative path. The exam tests specific range, optimum altitude, wind effects, step climb, drift-down and the en-route obstacle gradient.
Specific range and wind
Specific range (SAR) = TAS / fuel flow, in nm per kg (or per lb). Ground specific range uses ground speed: GS / fuel flow. A headwind lowers ground SAR and range; a tailwind raises them. For maximum range in a headwind, fly faster than the still-air optimum; in a tailwind, fly slower. Maximum endurance is not the same as maximum range: a jet's maximum-range speed is higher than its maximum-endurance speed.
Altitude
Optimum altitude is where SAR is greatest for the current mass. As fuel burns and mass falls, the optimum altitude rises, so a step climb keeps the aircraft near the optimum. The ceiling is limited by thrust available and by buffet onset; the gap between low-speed buffet and high-speed buffet is the coffin corner. Cruise climb is a slow continuous climb used when a step climb is not practical.
Drift-down
If an engine fails at cruise altitude, the aircraft may not be able to hold level flight. The drift-down procedure maintains altitude until speed decays to the drift-down speed, then descends at maximum continuous thrust to a drift-down altitude where level flight is possible. The net drift-down path must clear en-route obstacles; the CS-25 net en-route gradient requirement is 1.1% for a twin, 1.4% for a three and 1.6% for a four.
Worked example
TAS is 450 kt, fuel flow 2,400 kg/h, headwind 50 kt. Still-air SAR = 450 / 2,400 = 0.1875 nm/kg. Ground SAR = 400 / 2,400 = 0.1667 nm/kg. With 5,000 kg of fuel, still-air range is 5,000 × 0.1875 = 938 nm; with the headwind it is 5,000 × 0.1667 = 833 nm. The 50 kt headwind costs about 105 nm, or 11% of the still-air range.
Traps that cost marks
Using TAS instead of groundspeed for range; ignoring wind when comparing SAR; assuming optimum altitude is fixed; confusing maximum range with maximum endurance; and using the take-off net increment for en-route drift-down. Temperature affects both thrust and buffet margins, so a drift-down computed at ISA is not automatically valid at ISA + 15.