Boeing 737 Max – The illustrated example of technical debt

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What happens when Product Management, Finance and Engineering all seat at the same table and decide not to pay their debt? 346 lives are taken as collateral.

How Boeing got to this point ? Ryanair gave a hand…

The Boeing 737 Max is the 4th and latest interaction of the most successful airliner of all times, having sold more than ten thousand units since its introduction in 1968. That’s right, this airplane has its roots on a 51 year old base design, when airports and passengers were a lot less and jet engines were and a lot different, and smaller.

The very low cabin ground clearance of the Boeing 737 Credits by Raboe001 - selbst fotografiert DigiCam C2100UZ, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=524652 The very low cabin ground clearance of the Boeing 737
Credits By Raboe001 – selbst fotografiert DigiCam C2100UZ, CC BY-SA 2.5, Link

One such example is shown on the next image, from a Ryanair 737-800. Back on the 1960’s few airports had jet bridges, and most relied on (sometimes) motorised stairs to load and unload passengers. Even then, some had very few ground personnel. How is this relevant? Because of the unique features that the 737 has kept over the years is the embedded cabin stairs. However, these stairs don’t represent any problem, but in fact, are extremely welcomed by Ryanair, which extensively uses them to save costs on airport ground services, by using none. It may seem insignificant, but having its own stairs means that a passenger’s ticket costing 45€ may add an additional 5€ charge for using a 3rd party stair. For an 180 passenger jet plane, it makes for 900€ per flight. At the end of the day, it makes a huge difference for Ryanair. This is one of fundamental reasons why Ryanair exclusively uses the 737, and only the 737.

The relevance of the stair, is the reason why only the 737 is capable of carrying one, and not the competing model from Airbus, is the ground clearance. If you look closely at the picture, you’ll see that the distance from the ground to the cabin is extremely small, only 910mm on the worst case. It’ precisely this very low ground clearance which allows a small stair to be embedded onto the aircraft. Not that this characteristic brings inherent security drawbacks, but it’s simply what it is: a characteristic of the aircraft, albeit a fairly important one.

However, the business case for Ryanair to only use the Boeing 737 isn’t limited to this characteristic. Equally important for Ryanair is to streamline the fleet gaining significant scale effects. This not only applies to pure mechanical parts, but much more significantly, the cockpit crew. As we’ll focus latter on, cockpit crews are focused on a single specific aircraft model, or sometime a family. This is what applies to the whole Airbus A319/320/321 family. A pilot carrying such certification is qualified to fly any of those models, however, for that same pilot to be able to fly a Boeing 373, it requires a completely new certification. As pilot training and certification is tremendously expensive, there are few pilots carrying both certifications.



This is extremely important for Ryanair, as it affectively halves the number of flight crew on standby at each base. As only only single flight crew is needed per base, which is able to fly every single Ryanair aircraft. These kind of airline economics means that at the end of the day, pure fuel savings may not be the ultimate goal, specially in cases oil is far from its record values.

A small characteristic has stark effects down the line

A direct consequence of this very low ground clearance, is the ground clearance of the engines, this is has a direct consequences. Again, if you have a look at the picture, the distance between the engine cowling and the ground is extremely small, at a low 460mm. However is value was not always so small. The original 737-200 had a more manageable 510mm, but this doesn’t tell the whole story of how the engine clearance of the 737NG is so small, but the following photo starting telling the story.

CFM56 engine from an 737-400 Credits: By Davidelit – Own work, Public Domain

The distance between the CFM56 engine used on the 737-400 and the ground is so small notwithstanding the huge effort of shaving every millimetre from the much larger engine. What engineers could not foreseen on 1968 is that turbofan engines were on its infancy and were still tremendously inefficient, and soon to be replaced by much more effective high-bypass turbofan engines. The problem? High bypass engines have a far wider diameter then the old low-bypass engines. In fact, so larger, that fitting the unaltered engine would simply not fit the aircraft, making the airplane stand on the engines instead of the landing gear.

So, on 1979 Boeing decided to revamp the 737 with updated engines, but the math didn’t add up: the old JT8D engines used on the original 737 had a 101cm diameter main fan, whereas the new CFM56 engine uses a 152cm fan. It simply wouldn’t fit. At this stage Boeing had two alternatives: either redesign some critical parts, such as the wing box and the landing gear, and risk a lengthy and expensive aircraft certification process, or maintain the current basic design and get creative with the new engines. Boeing choose the latter. By buying technical debt, it delayed some more substantial and expensive changes.

It’s also worth noting that at this stage the world was changing fast: the second oil crisis was raging and airlines were grasping for every single drop of saved fuel, and them 737 was under pressure from order manufacturers, such as the McDonnel Douglas DC-9, so the pressure was to deliver quick and cheap results.



The solution was then a number of changes on the engine design, stripping every simple millimetre from the bottom of the engine, moving parts out of there at any cost and putting them elsewhere. This is why the new 737 engines are such a fun looking oval shaped, they’re simple not round.

The story of the engine modifications is barely started. In order to still fit the engine into the airplane, Boeing moved the engines from below the wings, to in front of the wings, something which is closely related to the current 737 Max problem.

On 1993, as Airbus’s grip on the airlines was starting to take hold, Boeing decided to launch the 737NG. This time the airframe structure design was again in front of the new and larger engines, but the cost of changing the aircraft design was now far larger. Not only the cost of redesigning those core components were higher, but there was another critical factor: pilot re training and type certification. One of the advantages of maintaining the same aircraft design is the fact that the aircraft behaves very similarly, which allowed type rating certification to be maintained, which meant pilots certified for the original 737 were also able to fly the 737 Classic, and then the 737NG. And by 1993, the number of 737 certified pilot represented a huge investment by the airlines, an investment Boeing wasn’t willing to throw away, as it represented the single biggest obstacle for airlines to switch to Airbus. So the decision not to change the airframe was again maintained.

So Boeing decided for a second time not to change the 737 design, but focused on improving the current design, presenting the 737-NG, from from which the 737-800 is based, was able to increase the fan diameter one single inch, to 155cm. The technical debt continued to pile up.

Even then, this problem causes most of the performance issues as compared with the Airbus A320 family. The problem is again, physics. For turbo fan engines, the bigger the fan diameter, the more efficient the engine is. This is so true that the Airbus A320, also using the CFM56 engine, uses a 173cm diameter main fan, some entire 18 cm larger than the much more recent 737-800, not to mention the 737-400.

Again, this underlines the fact that some airlines, fuel consumption is not the ultimate goal, but needs to be considered amongst other variables.

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