00
Air in, thrust out
Everything in this diagram is one process: take air, compress it, add fuel, burn it, and extract more energy from the exit than the compression cost.
Two shafts run the machine. The high-pressure shaft links the single-stage HPT to the nine-stage HPC. The low-pressure shaft runs the full length, connecting the four-stage LPT at the back to the fan at the front. Neither is driven directly — each turbine pays for the compression ahead of it.
The commercial reading
The engine is priced, leased, insured and removed as one unit. It does not deteriorate as one unit. Every judgement that follows depends on holding those two facts apart.
01
Fan
A single stage of wide-chord blades moving far more air around the core than through it, and producing the large majority of the thrust.
The fan is turned by the LPT at the far end of the engine through the inner shaft. Being first in the flow path, it also meets everything the airfield produces — birds, debris, ice, hail.
The commercial reading
Fan blades are serialised, individually expensive, and governed by defined blend and repair limits. A fan set close to those limits changes both the shop visit estimate and the redelivery position. On a lease return it is one of the first things worth establishing, because a claim discovered late always costs more than one planned for.
02
Booster · 3 stages
The low-pressure compressor sits behind the fan on the same shaft and takes the first bite of core compression.
Because it turns at fan speed rather than at the speed the core would prefer, its loading is a compromise inherent to the two-shaft architecture. Its condition sets what the HPC inherits.
The commercial reading
Booster hardware is generally less costly than the high-pressure section, but it carries life-limited parts and its efficiency affects everything behind it. Rarely the headline of a shop visit; frequently a line item inside one.
03
High-pressure compressor · 9 stages
Nine stages, each smaller than the last, driving pressure up by an order of magnitude before the air ever reaches fuel.
Compressing air heats it severely, so the rear stages run hot before combustion has even happened. Blade tip clearances here are measured in thousandths of an inch, and they open as the engine ages.
The commercial reading
Compressor deterioration is the quiet one. It does not fail — it drifts, and it surfaces as fuel burn and as lost exhaust gas temperature margin. An operator can carry it a long time without a fault message, which is exactly why it belongs in a technical evaluation and not in a defect report.
04
Combustor
An annular combustor where fuel meets compressed air and burns continuously. Gas leaves hotter than the melting point of the metal containing it.
The liner survives inside a moving film of cooler air and behind thermal barrier coatings. Combustion is not the hard engineering problem; keeping the surrounding hardware alive while it happens is.
The commercial reading
Combustor distress rarely arrives alone. It sits directly upstream of the most expensive hardware in the engine, so a borescope finding here should move the expected shop visit cost before the engine is even removed — and should change the conversation from restoration scope to inspection depth.
05
High-pressure turbine · 1 stage
The first thing the combustion gas touches. It extracts the energy that drives the HPC, and it runs hotter than anything else in the engine.
Blades here are typically cast as single crystals — no grain boundaries to creep along — hollowed for internal cooling and coated to insulate against gas temperatures beyond what the alloy alone could survive.
The commercial reading
This is where shop visit cost concentrates. HPT hardware dominates the material bill on a heavy visit and carries the hardest limits. In most CFM56 decisions the single most consequential question is what this module will need, and when. Everything else is arithmetic around that answer.
06
Low-pressure turbine · 4 stages
The last extraction stage. Larger, cooler, and responsible for turning the fan through the length of the engine.
By this point the gas has given up most of its pressure, so the blades grow to capture what remains. It is a physically large module with a substantial stack of life-limited parts.
The commercial reading
LPT life-limited parts are a slow, large, predictable liability — and predictable is what makes them plannable. Their remaining cycles are a material input to residual value and frequently decide whether an engine earns another shop visit or is worth more disassembled.
07
Accessory gearbox
Driven from the high-pressure shaft, it turns the pumps, generator, starter and controls that make the engine a usable system rather than a thermodynamic cycle.
It sits outside the gas path, so it is easy to overlook in a condition discussion. It is also mechanically busy, and its drives and bearings accumulate their own history.
The commercial reading
Gearbox findings are the ones that surprise a budget, because they sit outside the module everyone was watching. On an induction they are worth establishing early — a late gearbox finding can extend a turnaround that was already sold to an operator as fixed.
08
Eight modules, one asset
Separated, the engine stops being a single number and becomes a set of positions — each with its own remaining life, its own cost to restore, and its own effect on what the asset is worth.
No two CFM56s with the same total cycles are in the same commercial position. The distribution of life across these modules is the difference between an asset that should fly, one that should be repaired, and one that is worth more disassembled.
Where we come in
Atlas Aero reads the asset at this resolution and then evaluates every path against it — repair, exchange, purchase, sale, lease, life extension, part-out — without holding a position in any of them.
