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Boeing 737 MAX-8: Deception of an Engineering Disaster Revealed, Part 4

The evolving turbofan technology and its 737 MAX culture

This is Part 4 of an in-depth investigative report by Aviation Reporter and Author of aviation books Thomas E. Gardner. In Part 4 Mr. Gardner discusses how a turbofan operates, the evolving turbofan technology, and its upgrade culture. Understanding how a turbofan operates is important, to be able to understand the philosophy as to which turbofan engine was chosen for the 737 MAX.

You can read Part 1 of the story HERE, Part 2 HERE, Part 3 HERE, Part 5 HERE, and Part 6 HERE.

Turbofan Design Overview

In order to understand why turbofan engines are so special, one needs to gain a better understanding of how a turbofan operates.

A turbofan or “fanjet” is simply a fan placed in front of the engine nacelle (a nacelle is the engine cover/housing).

Boeing 737 MAX engine
The engine and engine nacelle of a 737 MAX. Photo: Wikimedia.

The fan is powered by an axial-flow turbojet (in the core of the engine).

The most distinctive physical characteristic of a turbofan is its diameter.

diameter, evolving turbofan technology upgrade

The larger the engine diameter, the bigger the fan is. This is due to its unique design operating characteristic.

Jet engine bypass ratios

This relates to the thrust achieved through the bypass ratio the engine is designed for. There are principally two types:

They are high bypass ratio and low bypass ratio turbofans.

The bypass ratio is important for the type of work and performance the engine is designed for, for example, airliner performance versus military fighter performance.

Bypass ratio is defined by the mass flow of a) air bypassing the engine core, divided by the mass flow of b) air passing through the core engine itself. (Figure 1).

how a turbofan operates, 737 max
Figure 1: Schematic diagram illustrating a low-bypass turbofan engine: GE.

These two combined flow ratios not only establish the ratio but provide the total thrust (power) output from the engine itself. This ratio is reasonable for determining the overall operational efficiency of the engine regarding its power and fuel burn.

Low bypass turbofan engines (where most of the intake air goes into the combustible engine core) are used primarily for military applications such as fighters. This is due to the bulk of the thrust being produced in the core engine and facilitated by the higher thrust required for fighter operation.

High Bypass Ratio engine design is important for cost-conscientious airlines

However, the higher the bypass ratio, the more efficient these engines become, providing lower fuel burn (fuel cost savings) in the subsonic speed range of flight.

This high bypass ratio evolving turbofan technology upgrade became an important part of the upgrade of the Boeing 737 to the MAX model.

Animation of how a jet engine works

Turbofan gif
Source: Wikipedia.

A) Low-pressure spool. B) High-ressure spool. C) Stationary components.

  1. Nacelle
  2. Fan
  3. Low-pressure compressor
  4. High-pressure compressor
  5. Combustion chamber
  6. High-pressure turbine
  7. Low-pressure turbine
  8. Core nozzle
  9. Fan nozzle

Engine energy

The kinetic energy (energy which a body possesses by virtue of being in motion) expended through the exhaust nozzle from the core engine core is carefully balanced from the kinetic energy from the fan’s mass flow through the bypass ducts surrounding that engine core.

The more energy extracted from the engines’ cores’ turbine in rotating the fan produces more thrust, especially at subsonic flight speed, increasing the overall mechanical/aerodynamic efficiency from the rotating fan blades further reducing the fuel used.

Thrust capability

This is further facilitated by the available thrust growth found by increasing the engine core’s power. There are two accepted routes available.  They are:

  • Hot route.  This is achieved by an increase in the high-pressure turbine inlet temperature.
  • Cold route.  This is accomplished by increasing the engine core’s mass flow.

Pratt & Whitney and GE Safron

Pratt & Whitney and GE Safron (CFM) are developing larger and wider engines to increase thrust capability.

The future of jet engines. Bloomberg/YouTube.

Pratt & Whitney and G.E. Aviation in conjunction with Safron (formerly Snecma) have developed over this last decade the PW 1000 G geared and the CFM Leap-1 (Leading Edge Aviation Propulsion) turbofans.

The PW 1000 engine

The PW 1000 G had significant teething problems requiring a bit of tweaking on Pratt & Whitney’s behalf.

The planetary reduction gear system behind the PW 1000 G design was chosen in order for the low-pressure turbine located at the rear of the engine core in order to be in sync with the fan.

The gear reduction 3:1 employed behind the fan turbine 1/3 as fast as the low-pressure turbine. This ensures maximum undisturbed airflow around the core engine and its total thrust output as well.

The GE CFM engine

However, G.E./Safron (CFM) did not use a geared fan approach to achieve these same results. CFM used new advances in aerodynamic modeling through computational fluid dynamics (CFD), as well as composite fan blade materials.

Due to the special materials used in their fabrication, this allows for the fan blades to flex or untwist at higher rotational speeds. This flexing, untwisting action enables the blades angle of attack to change, admitting a significant increase in airflow around the engine core. This directly reduces the overall engine’s complexity.

737 LEAP
737 MAX LEAP engine. Source: Boeing.

Both engine types

In any event, by gearing or enhancing the fan blade design and its fabrication both control the mix of subsonic, transonic and supersonic flows acting on a single fan/gas compressor blade found in these newest turbofan designs.

Photo of the GFM Leap-X, or -1 engine currently used on the Airbus A320 N.E.O.

737 LEAP engine

Leap-X. Source: Boeing.

Engines placed on different B737 models

Here we can see the progression of engine sizes through continued upgrading starting from the original 737 to 737 Max. It is clear that the Boeing 737 could not utilize its current turbofans:

737 models jet engines
Schematics of the evolvement of engines placed on the different 737 models, leading up to the 737 MAX. Source: Boeing.

How Boeing lost its way

Here is Bloomberg’s take on Boeing:


This was an explanation of how a turbofan operates and its evolving turbofan technology upgrades. But, was the engine chosen for the 737 MAX correct for the wings and the frame of the aging 737 design? The reporter will continue revealing more explosive facts about the 737 MAX in the next article.

Featured Image: Boeing 737 MAX, Farnborough Airshow. Wikimedia.


  • Boeing’s Re-Engining Worry by Thomas C. Hayes, Nov. 27, 1981, New York Times.
  • oliverwyman.com.
  • planestats.com.
  • FAA Form 41.
  • AVAC Database.
  • Wikipedia.com.
  • Boeing.com.
  • Pratt&Whitney.com.


Story: ©2020 Thomas E. Gardner. Renowned Aviation Journalist & Author Thomas E. Gardner has spent months on his investigative report. The author has based his story on facts that he has learned in regard to the 737 Max-8 engineering disaster during his in-depth investigation process of the article.

Disclaimer: Captain Jetson is a news-publisher. Views and opinions stated on Captain Jetson Aviation & Travel News are strictly the ones of the many individual Captain Jetson independent contributing journalists, and in no way represent the opinion of any other person, company, airline, the Captain Jetson News Publication, or any third party. For further information, please read our TOS & Privacy page.

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