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AFH · AFH Chapter 15

Jet Engine Fundamentals

Master jet engine basics: Brayton cycle, turbofan vs turbojet, N1/EPR, EGT limits, and thrust characteristics. FAA Airplane Flying Handbook Chapter 15.

CFI's Whiteboard Explanation

A jet engine is basically suck, squeeze, bang, blow — intake, compression, combustion, exhaust — happening continuously instead of in cylinders. Newton's third law does the rest: throw air backward, the airplane goes forward.

Key things to remember as a new jet pilot:

  • N1 or EPR is your thrust gauge, not RPM like a piston.
  • EGT is the limit that bites you — overtemp during start can cook a turbine in seconds.
  • Engines spool up slowly (5–8 sec from idle), so plan power changes early — especially on approach.
  • Thrust stays roughly flat with airspeed but drops with altitude and temperature.
Handbook Reference
AFH Ch 15

15.jet-engine-fundamentals. Jet Engine Fundamentals

The turbojet, turbofan, turboprop, and turboshaft engines are all variations of the gas turbine engine, which produces thrust by accelerating a mass of air rearward. The principle of operation is rooted in Newton's third law: for every action there is an equal and opposite reaction. Thrust is calculated by the basic equation:

F = m × (V2 − V1)

where F is thrust, m is the mass flow rate of air, V1 is the inlet velocity, and V2 is the exhaust velocity. A jet engine produces high thrust by either accelerating a small mass of air to very high velocity (pure turbojet) or accelerating a large mass of air to a moderate velocity (turbofan).

The Brayton Cycle

All gas turbines operate on the Brayton cycle, a continuous-flow thermodynamic cycle consisting of four phases:

  • Intake — ambient air enters the inlet and is delivered to the compressor at a controlled velocity.
  • Compression — the compressor increases pressure and temperature; modern engines achieve overall pressure ratios of 30:1 or higher.
  • Combustion — fuel is sprayed into the combustor and burned at essentially constant pressure, raising temperature to 1,700–2,500°F.
  • Exhaust — high-energy gases expand through the turbine (which extracts energy to drive the compressor) and then accelerate through the exhaust nozzle to produce thrust.

Unlike the four-stroke reciprocating (Otto) cycle, where these events occur sequentially in the same cylinder, the Brayton cycle performs them simultaneously in different sections of the engine, which is why a turbine produces smooth, continuous power.

Major Sections of a Gas Turbine

  1. Air inlet (intake) — delivers smooth, undistorted airflow to the compressor face. Inlet design is critical; distortion or ice ingestion can cause compressor stalls.
  2. Compressor section — either axial-flow (multiple stages of rotor and stator blades, used in most modern engines) or centrifugal-flow (impeller throws air outward, used in smaller engines and APUs). Axial compressors are often split into low-pressure (N1) and high-pressure (N2) spools.
  3. Combustion section — fuel nozzles inject atomized Jet A into can, annular, or can-annular combustors where ignition is initiated by igniter plugs and sustained by continuous combustion.
  4. Turbine section — hot gases drive turbine wheels that are mechanically coupled to the compressor (and to the fan or propeller in turbofan/turboprop designs).
  5. Exhaust section — directs gases overboard. In a pure turbojet, the exhaust nozzle accelerates flow to produce most of the thrust.

Types of Gas Turbine Engines

  • Turbojet — all thrust comes from the exhaust jet. Efficient at high altitude and high subsonic/supersonic speed but noisy and inefficient at low speed.
  • Turbofan — a large fan ahead of the core moves bypass air around the engine. High-bypass turbofans (bypass ratios 5:1 to 12:1) power most modern airliners and produce 75–85% of total thrust from the fan, yielding excellent fuel efficiency and low noise.
  • Turboprop — most of the energy drives a propeller through a reduction gearbox; only residual jet thrust is produced. Most efficient below about 25,000 ft and 350 knots.
  • Turboshaft — nearly all energy is delivered to a shaft (helicopters, APUs).

Engine Performance Parameters

Pilots monitor several key indications:

  • N1 — low-pressure spool/fan rpm, expressed as a percentage. On most turbofans, N1 is the primary thrust-setting reference.
  • N2 (and N3) — high-pressure spool rpm, used to monitor starting and bleed-air operations.
  • EPR (Engine Pressure Ratio) — ratio of turbine discharge pressure to compressor inlet pressure (Pt7/Pt2). Used as the primary thrust reference on many Pratt & Whitney engines.
  • EGT/ITT/TIT — exhaust, interstage, or turbine inlet temperature. The most critical limit during start and high-power operation; exceeding redline can cause turbine blade damage in seconds.
  • Fuel flow — pounds per hour, used for cruise planning and trend monitoring.

Thrust Characteristics

Jet thrust differs sharply from reciprocating-engine power:

  • Thrust is relatively constant with airspeed at a given throttle setting (unlike a propeller, where thrust falls off rapidly with speed).
  • Thrust decreases with altitude as air density falls, but specific fuel consumption improves, making jets most efficient in the high 30s to low 40s.
  • Thrust decreases with temperature; on a hot day, takeoff performance can degrade significantly.
  • Engines exhibit slow spool-up from idle — 5–8 seconds may be required to reach go-around thrust, which is why stabilized approaches and higher approach idle settings (or anticipating throttle inputs) are essential.

Operating Considerations

The pilot must respect EGT and N1/N2 limits during start to prevent a hot start or hung start. Compressor stalls — caused by disrupted airflow at the compressor face — can occur from rapid throttle movement, ice ingestion, or extreme angles of attack and are recognized by loud bangs, EGT spikes, and yaw. Smooth throttle handling, proper use of anti-ice, and adherence to AFM limits are fundamental to safe jet operation.

Oral Exam Questions a DPE Might Ask
Q1What are the four phases of the Brayton cycle, and how does it differ from the Otto cycle in a piston engine?
The Brayton cycle consists of intake, compression, combustion, and exhaust. Unlike the Otto cycle where all four events happen sequentially in one cylinder, the Brayton cycle performs them simultaneously in different sections of the engine, producing continuous rather than pulsed power.
Q2Why do high-bypass turbofans dominate modern airline service instead of pure turbojets?
High-bypass turbofans accelerate a large mass of air to a moderate velocity, which is more fuel-efficient, much quieter, and produces better thrust at typical cruise Mach numbers. Pure turbojets only beat them at very high speeds, which airliners don't fly.
Q3What is a hot start, and what should you do if you see one developing?
A hot start is an engine start where EGT exceeds the limit, usually due to too much fuel, weak starter, or tailwind. Immediately cut the fuel/start lever to OFF, continue motoring the engine to cool it, and consult the AFM — an overtemp typically requires a maintenance inspection before further flight.
More from AFH Chapter 15
High-Altitude Aerodynamics
High-Altitude Considerations for Jet Airplanes
Jet Airplane Speed Management
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Jet Engine Fundamentals: AFH Chapter 15 | GroundScholar