Is the Boeing 737MAX Really Unstable?! The 737 Engine Saga.

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Using a single sensor for MCAS even though two were available and it really should have been three was criminally stupid. And not mentioning MCAS to the pilots seems actually criminal to me. Why was nobody sentenced to prison?

Imagine an alternate world where Boeing didn't merge with Mcdonald Douglas. And instead just made a successor to 737 in 90s.

RIP John Barnett, you will be remembered for standing up for what's right...

I’ve flown on a 737 with gravel kit. Canadian North retired them only after the pandemic. Air Inuit still has one.

I am a retired aerospace engineer and a sport pilot. Your videos provide the absolute best explanations of these issues. Thank you so much for making them.

As a control engineer (we are the guys mostly obsessed with stability), I am horrified by how B737 was modified and the control system was augmented. The way this passed regulatory approval is scary. I will never willing fly B737 again.

My understanding is that the simulator test pilot for the max very pointedly made clear that the plane naturally pitches up way more than “slightly”. The MCAS was not there to simply make the plane more pre-Max-like, but to address a serious pitch up issue.

You are absolutely right when you say that using heavier engines or moving the engines forward increases static longitudinal stability, but may create longitudinal controlability problems. However, the main problem with the 73 max is an inversion of the static longiitudinal stability at high angles of attack, probably caused by altered airflow patterns aft of the engines toward the horizontal tail. It is a solvable broblem. However the way Boeing implemented a solution was a disaster, and we do not understand why the FAA did not catch it: 1) MCAS based on a single alpha vane. An alpha vane has a probability of failure somewhere around 1 failure in 40000h. But the MCAS in a way affects primary flight controls. Failure of primary controls are catastrophic and the requirement is that catastrophic failures can only happen once in 1000000000 hours. (10-9), as per FAA AC 25.1309. Had they used two alpha vanes (deactivating the system if there is a discrepancy between them) the probability of both failing at the same time would be once in 1600000000h, which would meet the safety requirement. 2) The MCAS actuated the stabilizer at a very high speed, turning the stabilizer into a kind of primary control. 3) PIlots were totally unaware of all those differences.

Not exactly to keep regulators happy, was it... it was to persuade regulators to keep the same type rating so that airlines didn't have to do more pilot training. Or am I wrong?

I can't believe I am hearing the conclusions I am hearing from the Mentor Pilot 😮 The positions of the engines on NG were already enough of a problem during go-arounds (not some abstract "high angle of attack" scenarios mentioned in the video) as, depending on the configuration, the pitch up tendency was sometimes very difficult to handle, and this is part of the reason why pilots on 737 hate go-arounds, btw. And this is precisely the reason for the introduction of MCAS, as Boeing was sure that the number of incidents during go-arounds related to this issues will only increase with more powerful engines. As for choosing to rely on what the name of the system says —"augmentation" —rather than what it actually does — move the stabilizers — as Mentor Pilot does — well, this is reliance is exactly what killed those pilots on the two fatal flights. To repeat this now once we know all the facts is something unbelievable!

"If it's Boeing, I'm not going."

Together with Jean-Claude Malroux of Snecma, my father was part of the original marketing team for the CFM-56. The big break was the DC-8, which was an obvious easy re-engine, followed by the KC-135 and other 707 military variants. My father always felt that the 737 re-engine program was a rush job as Boeing tried to keep small Airbus variants from eating their lunch. Still, the 737-300 broke the Pratt stranglehold with Boeing and proved to be a launching point for both CFM and GE. That clever engineering you mentioned paid off hugely. Although long-retired by the time that the MAX line came out, Dad was still connected enough to know that one of Boeing’s major pitches for the LEAP-engined planes was to that they were to require minimum pilot retraining. And therein lies the rub and at least some of the design errors.

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Actual aircraft designer and propulsion engineer here. Any change that results in a nose-up tendency at slow speeds is inherently less stable, and MCAS was intended to address it by inducing elevator deflections to keep the nose down. That's exactly what lead to the 737 Max 8 crashes. Additionally, the CFM56 redesigns also led to a greater risk of uncontained blade loss which also resulted in passenger injuries. Lastly, the worst problem is Boeing's repeated failure to report known safety issues, starting in the early 90s with a known issue with the rudder controls that eventually led to two fatal accidents, and ending up with unreported issues with the MCAS and it's single-point failure mode (which violates regulations for safety critical systems) and finally the recent quality escapes on the Max 9 and potentially other models. The 737 has become increasingly less safe with each generation and Boeing has repeatedly cut corners on safety.

I never finish a full video on YouTube or never watch a full tv show or movie. However with your videos I have never stopped it early and always watch it in its entirety. Thank you for the awesome consistent content.

About placing the engines forward and under the wing. Some good points were mentioned. Throw in a few more. I think it causes the engine to aerodynamically interfere with the wing less. Which makes the wing more efficient and allows a smaller wing, less drag etc. Higher bypass engines also have different airflow characteristics than lower bypass engines, with the air going in, its got to be carefully designed so wing and engine don't interfere with each other too much. Placing the engine forward allows the weight of the engine to act as a mass balance which helps to reduce aerodynamic/aeroelastic flutter. Which helps to save structure weight. And the possibility of fire was mentioned. Also if theres a uncontained failure such as compressor or turbine blade it will be less likely to penetrate wing structure and damage crucial structure or fuel tank or system. So theres lots of benefits. As for the instability caused by larger engines. Yes its not because of the added weight forward of the wing I agree. It comes from a combination of things. Compared to previous 737 types. The engine has a larger diameter. Which means the thrust line tends to be further below the wing. It produces more thrust. The shape of the nacelle causes it to produce lift. And the engine is further forward. So all this means that the engine and nacelle are going to generate more pitch up moment. Thats means if you're in a high angle of attack situation with high thrust setting. It mite be harder to pitch the nose down. Hence taking away from the effect of natural longitudinal I think stability. Hence the need for all this MCAS crap to compensate. Not just to avoid potentially dangerous high angle of attack situations. But also to make the plane handle like previous 737s to please airlines and regulations and certification etc. etc. But ya the new engines do kinda make it unstable, but not because of weight and cg, because of thrust and aerodynamic effects and moments. Thanks for listening to my 2 cents good nite.

I never knew they extended the gear length. Every other source concentrated only on engine placement. Thank you.

You are correct regarding the reuction of the fan diameter on the CFM-56 engines powering the 737NG, but your explanation was incomplete. In addition to lowering its efficiency, this lowered its power too. The engine was derated from 24,000 lb of thrust to 20,000 of thrust. Also, your explanation of the MCAS is great. I would like to add that MCAS is not new to the 737MAX. An MCAS was implemented on the KC-46 (767) air refueling tanker. This much more robust MCAS was needed because a tanker's center of gravity is frequently changing during flight as the tanker offloads fuel to recipient jets requiring it, and as such, MCAS is more active on the refueling tanker. The KC-135 tanker and KC-10 Extender tanker had a Second Officer (flight engineer) who was in charge of fuel transfers between tanks (to manage the center of gravity) but the 767 tanker does not, and requires more robust computer functions to fly safely. The 737's MCAS was less robust, lacking sensor redundancy, to reduce its cost, because Boeing calculated that airline 737s would not need it often, only in specific situations. Boeing's assumption, while correct, led to a problem which not only caused two crashes, but also caused some incidents on US airlines that could have resulted in crashes, but did not.

I'm a private pilot, (love the decathlon for the upside down silly stuff), and obviously never flown the big guys but I love hearing the technical stuff about how these huge metal boxes fly.