Cruise · range, cost and buffet
Choose a level and speed that fit both the economic goal and the aircraft envelope.
Why is the cheapest cruise not always the slowest?
Maximum-range cruise seeks the greatest specific range at the stated condition. Long-range cruise is a manufacturer-defined faster schedule, commonly near 99% of maximum specific range in the classical definition. The small range penalty can buy a meaningful speed increase; use the actual aircraft definition rather than treating 99% as an operating command.
Trade a little range efficiency for time
Maximum-range cruise seeks the greatest specific range at the stated condition. Long-range cruise is a manufacturer-defined faster schedule, commonly near 99% of maximum specific range in the classical definition. The small range penalty can buy a meaningful speed increase; use the actual aircraft definition rather than treating 99% as an operating command.
Cost index compares time-related cost with fuel-related cost in the aircraft’s defined units. A higher CI generally commands faster climb, cruise and descent within the managed envelope; a lower CI prioritises fuel. It is not the ticket price divided by fuel price, and its numerical scale is not transferable between aircraft types.
Economy depends on the whole mission
Economic speed balances additional fuel against time-related costs. Winds, route restrictions, expected delay, altitude changes, maintenance and operational policy all affect the result. A higher CI does not override an ATC speed, VMO/MMO or a buffet margin. A holding delay may make a fast en-route segment economically pointless if it just arrives at the queue sooner.
Why does the usable speed window narrow at altitude?
Optimum cruise altitude is the level providing the desired best performance/economy for the actual mass, conditions and speed objective. It can be below the maximum permitted altitude. Available thrust, certified altitude limits, pressurisation, temperature, turbulence, buffet margin and ATC constraints can all restrict the usable level.
Optimum and maximum are different
Optimum cruise altitude is the level providing the desired best performance/economy for the actual mass, conditions and speed objective. It can be below the maximum permitted altitude. Available thrust, certified altitude limits, pressurisation, temperature, turbulence, buffet margin and ATC constraints can all restrict the usable level.
As fuel burns and mass decreases, the aerodynamic optimum often rises. A step climb approximates a continuous climb by moving between permitted levels. The fuel cost of climbing, distance remaining, wind change, traffic clearance and turbulence can make staying lower preferable for that sector.
Two buffet boundaries
Low-speed buffet follows high required lift coefficient/AoA; high-speed buffet is associated with compressibility and shock-related flow separation. At altitude the two boundaries approach, narrowing the manoeuvre speed margin. High-speed buffet can occur below MMO; MMO is an operating limit, not a guarantee that buffet cannot occur below it.
In a level coordinated turn, load factor n = 1/cos bank and stall speed rises approximately by √n. Higher mass and a colder-than-reference or otherwise changed atmosphere can alter the available margin and speed relationship. The buffet onset boundary chart must be read for the actual mass, Mach/IAS and required manoeuvre capability. A straight-and-level point that barely fits is not automatically suitable for turbulence or a turn.
Can you make the performance decision?
Four original questions. Work from the stated conditions and review the reasoning.