Step climb
A Socratic walk-through of the step climb — reasoned out one step at a time, not lectured.
The question we started with
THE QUESTION #Why does an airliner climb higher partway through a long flight instead of settling at its best altitude from the start?
Four hours into a transatlantic flight, the aircraft climbs two thousand feet and settles again. Nothing has gone wrong; the weather is unchanged; this was planned before departure.
Two things need explaining. Why is the best altitude not simply chosen at the start and held? And if a higher altitude is better later, why was it not better earlier — the aircraft is the same machine, flying the same route, at the same speed.
Reasoning it through
REASONING #Begin with what "best altitude" means. Air thins with height. Thin air means less drag for a given speed, which is why airliners fly high. But thin air also means the wing must work harder to generate the lift holding the aircraft up, and beyond some point the extra work needed for lift outweighs the drag saved. The efficient altitude is where those two effects balance.
Now the key move: the balance point depends on how much the aircraft weighs. Lift must equal weight. A heavy aircraft needs a lot of lift, and generating it in very thin air is expensive; it does better a little lower where there is more air to work with. A light aircraft needs less lift and can afford to go higher, collecting the drag saving without paying much for it.
Follow that through the flight. An airliner leaving on a long sector is at its heaviest — it is carrying the fuel for the whole crossing. Hours later, having burned a large fraction of that fuel, it may weigh tens of tonnes less. It is a different aircraft from a performance point of view, and its efficient altitude has risen.
So the optimum is not a fixed number. It drifts upward continuously, all flight, as fuel burns away. The aircraft was at its best altitude at the start; it simply did not stay best.
That gives the ideal: a slow continuous climb, tracking the optimum exactly — a cruise-climb. And that is very nearly what an aircraft alone in the sky would do.
The reason it does not is the last piece, and it is not aerodynamic at all. Aircraft are separated vertically by assigning discrete flight levels, conventionally a thousand feet apart in the airspace where this matters, with direction-of-flight conventions determining which levels are available to whom. A controller cannot keep a drifting aircraft separated from everything else; separation depends on aircraft being at stated levels. So the continuous optimum has to be approximated by a staircase: hold a level while the optimum climbs past you, then request the next one and step up.
Now the shape is complete. The step climb is the intersection of a continuously rising physical optimum with a discretely quantised airspace. The physics wants a ramp; the traffic system supplies stairs.
One implication is worth drawing out. Since the aircraft is at exactly the optimum only at the instants it crosses it, most of the cruise is spent slightly off — a little too low just before a step, a little too high just after. The penalty is small, which is why the arrangement survives; but it is not zero, and it is the price of shared airspace rather than of the aeroplane.
The analogy
THE ANALOGY #Think of a hiker following a contour path around a hill where the comfortable walking line rises steadily as the ground steepens, but the only paths cut into the slope are at fixed heights, twenty metres apart.
The ideal is to drift upward continuously. What is available is to walk one path until it is clearly too low, then climb the short connector to the next. The hiker is rarely on the perfect line and never far from it, and the connectors are brief.
The hiker's paths are fixed by whoever cut them, whereas flight levels exist to keep aircraft apart from one another — so the constraint is not the terrain but the presence of other traffic, and a genuinely empty sky would permit the continuous climb the analogy cannot offer.
Clarifying the model
THE MODEL #"Higher is more efficient" is only true relative to weight, and the unqualified version is wrong. Fly too high for the current weight and the aircraft ends up in a narrow band between the speed at which the wing stalls and the speed at which it meets compressibility limits — and it burns more fuel, not less, because it must fly at a high angle of attack to make the lift. A step taken too early costs money. The rule is not higher but higher as you get lighter.
The optimum is not calculated from weight alone. Temperature matters, because it changes air density at a given altitude. Wind matters a great deal and often dominates: a level two thousand feet lower with a materially better tailwind can beat the aerodynamic optimum comfortably, which is why flight plans on some routes deliberately fly below the performance optimum. And on busy oceanic tracks the level a crew wants may not be available, so real flights step when the airspace permits rather than when the arithmetic says.
The steps are not always up. Where a route crosses long stretches with a strong wind gradient, or where a lighter aircraft would be optimal above the certified ceiling, the profile can be flat or even involve descending into better wind. Presenting the step climb as a universal staircase overstates it.
The falsification test. If the mechanism is the weight-dependence of the optimum, then the timing of the steps should track fuel burned rather than distance or elapsed time — a lightly loaded aircraft on the same route should reach a given level sooner, and a full one later. If aircraft stepped at the same points regardless of load, the account would be wrong and the steps would be an artefact of route structure or controller convention instead.
A picture of it
THE PICTURE #How to readThe line is the continuously rising efficient altitude as fuel burns off; the bars are the discrete levels actually flown. Read the vertical gaps between them as the cost of quantisation — the aircraft is a little low just before each step and a little high just after, and exactly right only where the two cross. The values are illustrative of the shape rather than measurements from any flight. Note that the line never jumps: nothing physical happens at the step, which is the point.
What became clearer
WHAT CLEARED #There is no single best altitude for a flight, because the aircraft's weight changes by tens of tonnes en route and the best altitude depends on weight. The optimum rises continuously; the sky is divided into fixed levels so that aircraft can be kept apart; and the staircase is what you get when a smooth curve has to be flown in discrete steps. The climb four hours in is not a correction to an earlier mistake — it is the aircraft catching up with an optimum that has been quietly rising since departure.
Where to go next
ONWARD #- Why wind routing can beat the aerodynamic optimum, and how flight planners trade the two.
- How reduced vertical separation minima halved the spacing between levels and what that bought.
- Why the same weight-dependence sets the maximum altitude an aircraft can reach at all.