Roughly 80% of the air entering a modern turbofan never touches a drop of fuel. It bypasses the combustion section entirely — and it still produces most of the thrust.
That single fact unlocks a large share of the gas turbine questions in EASA ATPL subject 021 (Airframe, Systems and Powerplant). This guide walks the machine from intake to nozzle: the two airflows, the two spools, and the pressure–temperature–velocity story examiners test relentlessly.
One engine, two airflows
Every turbofan is built around a gas generator: compressor, combustion chamber, turbine. The core produces hot, fast exhaust. But on its own, a narrow jet of hot gas is an inefficient way to push an airliner along at Mach 0.78.
So the turbofan adds a fan. The fan accelerates a huge mass of cold air around the core through the bypass duct. The bypass ratio compares the two streams: the mass flow through the bypass duct divided by the mass flow through the core. A CFM56 runs a bypass ratio of roughly 6:1; the newest geared and wide-chord designs reach 9:1 to 12:1.
Why bother? Propulsive efficiency. Thrust is mass flow times the acceleration given to the gas. You can produce the same thrust by accelerating a small mass of air by a large amount, or a large mass by a small amount. The second option wastes far less kinetic energy in the wake — and it is dramatically quieter. That is the entire case for the high-bypass turbofan, and it is a recurring 021 exam point.
Two spools, no gearbox between them
Most turbofans in airline service are twin-spool machines.
- The low-pressure (LP) spool — fan, LP booster stages and the LP turbine — rotates as one shaft. Its speed is indicated as N1.
- The high-pressure (HP) spool — HP compressor and HP turbine — spins much faster on a concentric shaft around the first. Its speed is N2.
Here is the trap: the two spools have no mechanical connection. They are coupled only aerodynamically, by the gas flowing through the engine, and each finds its own speed. (Rolls-Royce adds a third, intermediate spool — same principle, three shafts.)
Intake to nozzle: the pressure, temperature and velocity story
Examiners love asking what happens to pressure, temperature and velocity at each station. Walk it through once properly and the questions become free marks.
Intake. A diverging duct at cruise speed: the air slows slightly, static pressure rises (ram recovery), temperature rises slightly. No energy is added in the intake — it is only exchanged between velocity and pressure.
Compressor. Each stage pairs a rotor, which accelerates the air, with a stator, which diffuses it and converts velocity into pressure. Across the whole compressor, pressure rises sharply — overall pressure ratios of 30:1 to 40:1 are typical — temperature rises with it, and axial velocity stays roughly constant.
Diffuser and combustor. The diffuser just ahead of the combustion chamber is where static pressure peaks — not inside the combustor. That is one of the most reliable trick questions in the bank. In the combustor itself, fuel burns at essentially constant pressure (there is a small friction loss), temperature rises enormously, and velocity increases. Only about a quarter of the core airflow is burned in the primary zone; the rest cools the flame tube and dilutes the gas down to a temperature the turbine can survive.
Turbine. The turbine extracts energy to drive the compressor, the fan and the accessory gearbox — roughly two-thirds to three-quarters of the total energy in the gas. Pressure and temperature fall stage by stage, while velocity rises through each stator nozzle as the gas expands.
Propelling nozzle. A convergent duct: static pressure falls, velocity rises, temperature falls. At higher thrust settings the nozzle usually chokes — the gas reaches the local speed of sound at the throat, and further acceleration is no longer possible in the nozzle.
Setting thrust: N1, EPR and the temperature limit
There is no thrust gauge on the flight deck. Thrust is set by proxy, and the syllabus expects you to know both conventions:
- N1 — fan speed, shown as a percentage. CFM and GE engines, including the CFM56 pictured above, use N1 as the thrust-setting parameter.
- EPR, the engine pressure ratio — turbine exhaust total pressure divided by compressor inlet total pressure. Rolls-Royce, Pratt & Whitney and IAE designs traditionally set thrust with EPR.
The hard limit on any gas turbine is turbine inlet temperature. Measuring it directly is impractical at those temperatures, so a ring of thermocouples further downstream indicates EGT (exhaust gas temperature) as a proxy. FADEC — full-authority digital engine control — meters the fuel to deliver the commanded thrust while protecting the N1, N2 and EGT limits.
One more term worth knowing: flat rating. Most engines deliver full rated thrust up to a fixed outside air temperature; above it, thrust is progressively reduced so the EGT limit is respected.
Five turbofan traps in the 021 question bank
- "Highest pressure is in the combustion chamber." No — it peaks in the diffuser at compressor outlet, just before combustion.
- "Bypass ratio is core flow divided by bypass flow." Inverted. It is bypass (cold) mass flow divided by core (hot) mass flow.
- "N1 and N2 are geared together." The spools are mechanically independent; only the airflow couples them.
- "In the convergent nozzle, pressure rises." The opposite: velocity rises and static pressure falls.
- "Most thrust comes from the hot exhaust." In a high-bypass turbofan the fan stream produces the clear majority of total thrust — around 75 to 80%.
FAQ: turbofan basics for the EASA ATPL
What is the bypass ratio of a turbofan engine?
Bypass ratio is the mass flow of air passing through the bypass duct divided by the mass flow passing through the engine core. A CFM56 has a bypass ratio of about 6:1, while the latest generation of turbofans reaches 9:1 to 12:1.
What is the difference between N1 and N2?
N1 is the rotational speed of the low-pressure spool — the fan, booster stages and LP turbine. N2 is the speed of the high-pressure spool — the HP compressor and HP turbine. The two spools are not mechanically connected and are coupled only by the airflow.
Why do modern airliners use high-bypass turbofans?
Because accelerating a large mass of air by a small amount is more efficient than accelerating a small mass by a large amount. Higher bypass ratios give better propulsive efficiency at typical cruise Mach numbers, lower fuel burn and significantly less noise.
Is thrust set with EPR or N1?
It depends on the manufacturer. CFM and GE engines use N1 as the primary thrust-setting parameter, while Rolls-Royce, Pratt & Whitney and IAE engines traditionally use EPR — the ratio of turbine exhaust pressure to compressor inlet pressure.
Ready to test yourself against real examiner styles? Drill AGK questions with worked explanations like these — and track your weak areas across all 13 subjects — at atpltraining.io.
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