Class A Climb and Cruise - ATPL 032 — Performance

Climbing on IAS, climbing on Mach

Constant IAS — as the air thins, the TAS behind the fixed IAS rises and the Mach number rises with it; the climb gradient and rate of climb decay as the thrust falls away.

Constant Mach (below the tropopause) — the falling temperature drags the speed of sound down, so the TAS falls, and the IAS falls faster; gradient and rate continue to decay.

Constant Mach (above the tropopause) — the ISA temperature is constant, so the speed of sound and therefore the TAS hold constant while the IAS keeps falling.

The operational sequence for a jet transport strings these together: 250 kt below FL100, then acceleration to the scheduled climb IAS, a constant-IAS climb to the crossover altitude, and a constant-Mach climb from there to the cruise level. The crossover altitude is where the scheduled IAS and the scheduled Mach describe the same TAS: below it the IAS is the more limiting instrument, above it the Mach is. The same geometry governs the limits: climbing at constant IAS drives the Mach up toward MMO, while climbing at constant Mach walks the IAS down and away from VMO, which is why the envelope is VMO-limited low and MMO-limited high.

The range speeds, revisited at altitude

Module 2's tangent construction returns wearing Mach clothes. The maximum-range cruise sits at the best distance per kilogram; the long-range cruise (LRC) is scheduled slightly faster, at the speed on the fast side giving 99 per cent of the maximum specific range: about one per cent of range traded for several per cent more speed, and a bonus the exam likes: the faster LRC sits more firmly on the front side of the drag curve, so its speed stability is better than the maximum-range speed's.

The optimum altitude, and what caps it

The optimum cruise altitude is the level where the specific range peaks for the current mass. It moves with everything module 2 promised: lower when heavy, drifting upward as fuel burns, nudged by temperature deviation and by the speed regime the cost index selects. The maximum operating altitude is a different ceiling, with three guards standing over it:

Thrust — the engines must still sustain level flight up there; the classic ceilings live in this guard.
Certification — the certified maximum altitude stands, set by pressurisation and systems.
Buffet margins — the cruise level must conventionally keep a 1.3 g margin to buffet onset, protection for a 40° bank or the equivalent gust before the airframe shakes.

That last point is its own LO: the high-speed buffet can occur slower or faster than MMO. Light and low, the buffet boundary sits beyond MMO and the certified limit is what stops you; heavy and high, the buffet arrives first. The 1.3 g requirement turns the same chart into a step-climb gatekeeper: a step climb chases the rising optimum altitude for range, and is flown when the next available level's specific range beats the current level's; but the step is only permitted when the aeroplane, at its present mass, still holds the buffet margin up there. Bank and mass eat that margin, which is why the step waits until enough fuel has burned. And sometimes the step is rightly skipped:

A short remaining sector — too little cruise left to repay the climb fuel.
A better wind at the current level — a wind trade can beat the mileage gain of the higher level.
Turbulence reported above — ride quality and margins outrank a percent of range.
ATC — the level must actually be available on the airway.

The cost index

Fuel is not the only thing a flight burns: crews, maintenance and schedules cost money per hour. The cost index is the ratio of time costs to fuel costs the operator feeds the FMS, and it turns economics into speeds. CI zero says fuel is everything: the FMS flies maximum-range speeds. A high CI says time is everything: speeds push toward the fast limits. In between lies the economical cruise speed, the whole reason the concept exists: the cheapest total operation is usually faster than the cheapest fuel burn. The chosen CI colours the whole profile: a higher CI raises the climb, cruise and descent speeds together, buying minutes with kilograms across the flight.

Climbing on IAS, climbing on Mach

Check you understood

The range speeds, revisited at altitude

The optimum altitude, and what caps it

The cost index