Boeing 737 Max – The illustrated example of technical debt

Entering the 737 MAX

Fast forward to 2011. If the fuel consumption of the 737NG was not breaking the bank on Ryanair, the same could not be said for other airlines. Underscoring this, were the initial sales of the revamped Airbus A320Neo, corresponding the fastest selling aircraft ever. What distinguishes the A320Neo from its predecessor? Increased fuel efficiency due to the revamped engines, amongst other smaller aerodynamic improvements. The effect the Airbus A320Neo announcement was extremely noticeable on the 737NG sales, or lack thereof. There was however a marking event: on July 20, 2011, American Airlines announced the order of 130 Airbus A320Neo. It was the first time on recent history that any of the US airlines bought any non US manufactured aircraft, and represented a huge punch on the trenches Boeing had build for decades around the US market. For Boeing, that was a defining moment, which required an adequate answer, at the risk to totally putting the single aisle market on Airbus’ lap.

Now, on the aircraft market, when your competitor announces a re-engined aircraft, the easy answer is: launch your own re-engined aircraft. However, this was far from simply on Boeing’s case. Re-engining the Airbus A320 included a couple of new engines: the CFM Leap 1A and the P&W 1000G, both significantly increased the main fan diameter, to 198cm and 206cm respectively, representing an increase of 25cm to 30cm respectively. The simple fact of the matter is that the original Airbus A320 was so far away from the ground that increasing the engine size simply does not represents any issue, something which cannot be said regarding the Boeing 737.

Boeing was left with three options: make critical changes on those core components it had avoided twice before, designing a brand new aircraft from scratch or working around it, again. This time the decision was painfully harder. First, on 2011 Boeing was still trying to wrap up the development of the first totally new aircraft design on decades: the 787, with disastrous results. It took Boeing 8 years and 32 billion US$ to develop, just to be sold at a loss per unit when production started. So this option neither fit the need for a response to Airbus, nor the economics of the current 737NG.

Then, redesigning the old airframe represented the same challenges of the past, cost, pilot re certification and now, time.

So Boeing went back to the same old recipe : was there any engine Boeing could work on to make it fit the 50 year old original 737 basic design?

The 2m+ wide P&W 1000G engine was a non starter, leaving only the new CFM Leap as an option. Having only single engine supplier is far from optimal for both Boeing and the airlines, but there’s simply no other alternative. Even then, Boeing still need to make that new fan fit the aircraft. In order to even come close of the promised fuel savings promised by Airbus, Boeing and CFM agreed on a new version of the CFM Leap, the 1B version. Even then this variant is only 176cm wide, a whooping 22cm smaller than the version equipping the Airbus. However, this still meant finding a way to fit an engine 22cm wider onto the 737 design….


This is where things went very, very bad. Debt is a bitch, and it always comes with interests….

Going to August, when the 737 Max was announced, Boeing was desperate. It really needed to make the new larger engine fit the ageing design. Then it got very creative.

Boeing decided it had the room it need to fit the engine simply by pushing the new engine forward even more. There was just a problem: this heavier, longer and more powerful engine was moved even further to the front of the wings edges and away from the aircraft’s center of gravity, which for an aircraft is kind of a bummer. When the engines are not aligned with the aircraft’s center of gravity, it creates a torque, pushing the aircraft nose up every time the engines are set at full power, which is terribly dangerous, as the exact time the engines are use at full power is precisely during take off and ascent, where pitch is already quite high. Leaving this pitch up effect uncontrolled would cause the aircraft to raise the nose even higher resulting on a catastrophic and unrecoverable stall and unavoidable crash.

Again, Boeing decided to get creative, again, which by aircraft design standards is pushing your luck. So the solution for the second problem was: create a system whose only intent is to push the aircraft nose down, automatically.

I’ll let you ponder on the last sentence for a few moments.

From that point on, it was all downhill for Boeing, as all the creativity was spent, and now only stupidity was left. So, you design a system which only purpose is to push the aircraft nose down, but then you need to feed this system with data, in this case, using Angle of Attack (AoA) sensors. How many of them? Two. Only two. As a reference, Airbus always uses at least three. Then, how does Boeing create redundancy between them? Does the sensor’s data get averaged? There’s no such thing. The captain’s sensor is used by default, and that’s it. There’s no automative redundancy, voting system, no nothing.


Boeing left a system whose only purpose is to push the nose down, depending on one single sensor? What could go wrong? Well, two planes and 346 souls were lost, because Boeing got cheap, and then dumb.

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