Performance - climb, cruise and descent
Performance - climb, cruise and descent examines segment gradients, climb rates, cruise altitude capability, step climbs and descent planning. The exam wants you to separate rate of climb from climb gradient and to use ground speed, not indicated or true airspeed, for obstacle work.
Climb and cruise
- Vx is the best angle of climb; Vy is the best rate. Obstacle clearance uses gradient, while terrain separation over time uses rate.
- Gradient (%) = rate of climb (ft/min) ÷ [ground speed (kt) × 101.27] × 100.
- Net gradient is gross gradient minus the required margin; use the net figure for obstacle clearance.
- Cruise altitude is limited by thrust, buffet margin, MSA and driftdown terrain clearance.
Worked example
Rate of climb is 1,200 ft/min and ground speed is 150 kt. Gradient = 1,200 ÷ (150 × 101.27) × 100 = 7.9%. Over 5 NM, height gained is about 7.9% × 30,380 ft = 2,400 ft. If the obstacle requires 2,000 ft, the margin is about 400 ft. For descent, a 3° path is about 318 ft/NM, so from 30,000 ft the top of descent is about 30,000 ÷ 318 = 94 NM before the point.
Traps
Using TAS instead of ground speed changes the gradient whenever wind exists. Do not mix Vx and Vy. A tailwind increases descent distance for the same gradient but reduces climb gradient. Do not use gross climb performance for obstacle clearance, and remember that a step climb may be needed as mass decreases.