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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...

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.

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.

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

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!

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.

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?

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.

"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.

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

I never finish a full video on UKposts 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.

If it’s a Boeing I ain’t going.

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.

It's not that it made it unstable, it is that it made it a aircraft with a different type rating. The problem is, they tried to hide that fact so that aircrews did not need to get a different type rating

As a programmer, it baffles me that they didn't do simple saturation arithmetic to control the inputs. If the needed adjustment was very slight as said, it should've been capped, in order to not force the aircraft down even if the algorithm gets completely haywire.

Your callout to Stefan Drury's channel & encouragement to help him out: that was kind & generous. Thank you for being a friend to him. That small gesture is infectious - encourages community amongst us all, is an example of how we can all help each other. We need more of that. I've occasionally enjoyed your channel's posts, but am now subscribed to yours & his.

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.

High, I’m a 737 veteran myself and quite familiar with the concept you are discussing. I flew most variants of the 73 and my flying came to a halt when the mcas mess came about a few years ago. I was in the middle of the type conversion in Singapore when it all happened. Anyways to the point, I firmly believe that it’s time for Boeing to say good bye to the 73 and maybe concentrate on the 75 comeback and making that model a better unit than the original was. I’m not an engineer but common sense tells us that shrinking an airframe is probably a little easier than trying to stretch it as in the max. The Russians have come up with an airframe around 210 pax and I think that’s a good size to have, which is why I think a redesign 75 makes a lot of sense. And knowing Boeing will do anything to screw this up, make it a totally new wing design instead of the bs job they did with the max which was the same airframe as the NG. A new design is a new aeroplane period.

In my 30 years at a US major I never heard our 737-300's called the "classic" that was a southwest term. Also the "Jurassic jet" was uniquely a term for the 727, never used for the 737-200. When I bid up from the 737-200 to the 737-300 I was trained on how at hi nose & low speed the addition of thrust does input a bit of nose up pitch unlike the 200, exactly was you have addressed. Also the NG series again had bigger more forward engines, higher mounted engines and would input some nose up pitch more so than the 300. Thank you so much for addressing this misinformation propagated by lax/sloppy media reports.

*You should drop BetterHelp as a sponsor as fast as possible*

If the engine upgrade didn't make the plane unstable, then why did they have to use a computer band-aid to make the plane stable and why were there two fatal crashes directly attributable to computer band-aid?

It didn't make it unstable. It made it different. So came the MCAS, which nobody knew about. The FAA didn't know about it, the pilots didn't know about it. Never mentioned in the Max manuals. THIS is the outrageous part. Profits were put above safety. Boeing should have worked on a new 737 since the 80s, or at least early 90s, to take full advantage of composite materials, more efficient engines, fly by wire technology... They're at least 30 years late.

Another major issue with MCAS is that it was not implemented as designed. The engineers designed its ability to change trim to be very minor. In this way tying it to a single angle of attack sensor was not a big deal as if it went off the rails due to bad input MCAS' ability to endanger a flight was almost nonexistent. Later on however management dictated that MCAS needed to be much more aggressive in order to ensure that they could tell airlines that no simulator training would be required for pilots of earlier 737s. It was a perfect storm. A system that was not designed to be redundant because it wasn't seen as necessary due to the relatively minor fight inputs it was capable of making, which was then altered to be able to make extreme changes, and then pilots were not informed about the system.

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.

All current MAX aircraft have an ongoing and known issue with the engine anti-ice system! Left on too long in excess of five minutes the engine could fail and blow apart. The FAA has now required to redesign of the systems on the MAX 7 and 10 aircraft. Clearly, this says the FAA does not think the aircraft that are currently flying are safe however, they continue to allow them to fly. It’s only a matter of time before there is a major and accident involving them. The MAX needs to be grounded until they can redesign and fix all of these aircraft!

I believe that the ME262 was the aircraft that inspired the original B737's wing & engine layout!

"It wasn't dangerous, it just needed MCAS to bring it into full regulatory compliance[....] to keep the authorities happy" So the plane without MCAS wasn't dangerous, it just wasn't compliant with the safety regulations. You might be contradicting yourself just a tad.

I don’t agree with your conclusion. You have no explanation supporting your affirmation that the pitch up effect is only minimal and unimportant. The problem is not about weight distribution affecting the stability which is relatively constant in a small time frame, but it’s all about the change of pitching force during a pitch up manoeuvre at slower speed (higher angle of attack) and how it becomes out of trim in this situation. The 737 NG is already difficult after a go around. The pilots have to push the yoke forward after the initial pull up with a strong tendency to pitch up by itself due to increased thrust located lower from the CG and some pitch up effect due to “lift” of the engine nacelles at higher angle of attack as explained in your video, and raising the flap up one notch. 3 pitch up forces happening simultaneously and suddenly. And that’s the problem with the MAX if one of those pitch up force is also increasing. It’s cumulative. The pilots then was trimming the nose down to level of. If the pilots was “flying the trim” to level off, they were sometimes over trimming down and producing a loss of control and subsequently a crash (CFIT) immediately after the go around. The fact that the max have bigger and more forward engines increased the change in pitch up forces after a go around. The pilots have to push down with more forces after a go around, increasing the risk of pilot over trimming down. Boeing had to increase the efficiency of the MCAS (and hiding it to the FAA) during the conception the MAX for a reason. The reason is probably that a stronger MCAS WAS actually NEEDED due to the pitch effect being a bigger problem than expected. I don’t buy the idea that the MCAS was only to make if feels more like the previous versions of 737. It was to fix a pitch up effect problem that was getting too problematic. Reducing the effectiveness of the MCAS (after the 2 crashes) is only a compromise between two problems. It’s helping to fix one issue by making an other one worse.

"A little knowledge can be a dangerous thing." And lately, thanks be to internet - there are a ton of people running around repeating false/misunderstandings base on "a little knowledge." Thanks for seeking to counteract some of this with Quality Information, Mentour Now!

I have flown most 737 models except the 200 and the MAX and have thousands and thousands of hours on the type. For the last 15 years I have been flying the Airbus A320 series and the NEO. From being an absolute Boeing fan, after changing to the Airbus, as I see it, the 737 is a totally outdated product, and should’ve been scrapped years ago. (Sorry Petter). In my opinion, the A321 NEO is the best plane you can buy in the narrow buddy segment. Flying with 240 passengers, it burns less fuel than an old A319 with 156 passengers, on the same route. So quiet and so powerful. What a machine. Petter, work on one of these for a while and you would never want put your feet in a Boeing again, I promise, from experience. 😊

My son is passionate about pilot and aviation. So I started to search a good channel about aviation finally I found it. Sir, your are doing great work and your creating curiosity but you are explain about technical side. I am really impressed. Who wants become a pilot must follow this channel can learn lot. this channel should be dubbed in other languages also. (Consider)But English is the priority. I really appreciate your efforts. ❤

I haven't watched your main channel in years. This channel of yours has been really successful & amazing in terms of education. Your like my Aviation pilot teacher.

Fun fact. The 737-100/200 and the DC-9 had the same engines and reversers. The DC-9 engines were mounted quite a bit higher on the tail and the reverser buckets didn’t need to be canted. Thereby saving the tail of the DC9 a lot of unsightly soot on the fuselage. They both would hit the ground though if you deployed them with the nose wheel off the ground. 9:31

MCAS is all about one small paragraph in 14CFR25. The real problem was the single point of failure of the AoA vane. “25.203 Stall characteristics. (a) It must be possible to produce and to correct roll and yaw by unreversed use of the aileron and rudder controls, up to the time the airplane is stalled. No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls.”

To answer your question at the end: Did Boeing have any problems with its new LEAP engine mounting (aka MAX)? Yes. A lot! Did Airbus have any problems with its new LEAP engine mounting (aka NEO)? No. Not that I know of any. That says it all.

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.

What I've learned here, basically, a modern 737 is the aircraft equivalent to a 1967 car that originally came with a straight six, and later got retrofitted with a twin turbo big block. Also, I i was one of those people thinking "if they needed more clearance why didn't they just fit taller landing gear" but you talked about that.

You are wrong about the reasons for the engine placement. A wing in flight creates two forces - lift and drag. But it also creates a torque or twist. By placing the engines forward of the leading edge, they act as counterweights to the aerodynamic torque on the wing, allowing for a lighter structure. Boeing did this first on the B-47. It's a very clever thing to do, using the weight of the engines for something positive. Airbus copied it. The engine placement of the 'Jurassic' 737 is simply a matter of the engine being longer than the wing chord. They couldn't put it anywhere else. Also, placing the engine forward without a pylon would lead to heat issues on the wing - you don't want anything touching that hot exhaust. So the 737 wing was designed to handle the aerodynamic torque by itself, without counterweights. The plane is small and the weight penalty was probably not much. Fuel was cheap back then. The reason for the engine placement on the newer 737s is that the entire design is a kludge. By lining up the top of the cowling with the wing and squashing the lower part, they barely got the fan to fit, almost scraping on the runway. The only way to do this is to move the engine forward. Together with the shortness of modern fanjets, this places almost the entire engine forward of the wing, but not for the reason it was originally done. Boeing may have been able to lighten the wing structure a bit, I don't know. The alternative would have been to lengthen the undercarriage, but on the 737, that would amount to a complete redesign, might as start over clean. Incidentally, Boeing had an aircraft just like this, the wonderful 757, but the bean-counters killed it. Boeing is now reaping the rewards of decades of short-sighted accounting decisions. I wouldn't worry about them though, the military division can carry the wayward airliner group for a long time. Boeing didn't buy McD for their airliners. Every time I look at a 'fat-engine' 737 plowing down a runway, I wince. FOD must be a major problem that gets swept under the rug. How Boeing got away with such a mess is beyond me. They obviously have the FAA and NTSB in their pocket. As for the old thrust reversers being pointed at the fuselage, I can't imagine a good reason. I may have mis-remembered the part about the torque on the wing, or 'aerodynamic pitch moment'. It's been a long time. The discussions I find online only talk about straight wings, not swept. There are other reasons to hang the engines out front, the most important being to get clean undisturbed airflow, but also for flutter. And mounting the engines directly on the wings results in an overall lighter airframe. This gets pretty technical: agodemar.github.io/FlightMechanics4Pilots/mypages/pitching-moment/ At any rate, Boeing had good reasons for mounting the engines the way they did, even if my old brain is addled by covid. Engines are typically mounted below the wing for maintenance reasons. It was easier for ground crews, although modern engines are so huge that it is much less help today. The HondaJet mounts its tiny engines on pylons above the wing.

The most amazing aspect about the 737 is that they still fly with the original ceretification

As a retired aeronautical engineer and airline pilot I found this video to be an excellent explanation for the average person. Great job

Tnx for great analysis. When you talked about the different noise of various engines. This brought back memories when i was a flight dispatcher at O,Hare working for Aeroflot and later Air Ukraine with Iluyshun 62M aircraft. While waiting for the aircraft to land the loud engine sound told me that that was my aircraft. This was in the mid ninetys. Later Aeroflot switched to Boeing 757.The Illuyshin 62M had a tail hydraulic post so it would not tip over.

If the 737-MAX had a fly by wire system (which it doesn't it only has FADEC which is just engine control)), the pitch up tendency at low speed high thrust setting could have been simply dialled out. But it still uses a 1960s mechanical direct link control system which needs a supplementary control device (MCAS) to prevent a pitch up under those conditions. Airbus also have a system which prevents over-rotation/pitch up situations. But the key thing is its fully integrated into the electronic FBW system. Not only that, but the Airbus' always run two AoA sensors at the same time, and if there is a disagreement between them, the system disconnects automatically. It's just a smarter plane, not a Frankenstein 1967 design.

One thing comes to mind is flying down the north coast of New Guinea at midday listening to the Goon Show in a 707 and watching the engines contra rotating on their fairings.

I love the way you make your explanation as simple as possible for the average layman. I also like that you aren’t afraid to admit that you maybe wrong on something.

It's so stable, they incorporated a software system to correct the pitch when an inevitable stall happens.

I am of the belief that the 737 MAX should have its own type rating. When a plane doesn’t behave like its predecessors, it needs to be treated a new design entirely with pilots specifically trained for it, not using a system as a crutch for poor design. It may not be “unstable” if we really want to split hairs here, but it behaves differently in the most critical stages of flight: Takeoff and Landing.

There were known issues with the 737MAX before Boeing delivered the aircraft. For sales purposes, Boeing didn't want to require the customer have their pilots go through any extensive training over their experience with the current 737s. This required FAA approval. Coincidently during this time FAA personnel reported that they shouldn't require this extra training from higher up the food chain in the DOT. Boeing had made a significant donation to one of Senator McConnell's super PACS, and the SecTrans at that time was, wait for it, McConnell's wife, Elaine Chou. Hmm.

If MCAS was needed to correct a pitch up problem in some situations, it means the aircraft has reduced stability in some parts of the flight envelope. The normal solution would be to increase the tail size. Using a computer system with trim inputs to do the same thing is a low-cost work around. In its original form implementing MCAS with a single point failure mode (inputs from one angle of attack sensor) is simply against the most basic principles of engineering 101 for any transport category aircraft. It's hard to believe why it was done that way.

What drives me crazy is that they got away with it by paying, but nobody has truly faced personal consequences. The principle has been established that if you work for a big company, you can do whatever you want in the company's interest and get away with it. But responsibilities are personal; someone decided that the plane was safe, both at Boeing and the FAA, especially after the first incident. These individuals should have been held accountable, to be lenient for gross negligence. Instead, we don't even know their names, and they're probably advancing in their careers right now, in a position to cause more disasters.

Great video again Petter! Don't know about the jurrasic 737 but the Classic already had the Speed Trim System which would act on the Stab position in Low speed, high AOA+High Thrust conditions, i.e. Take off and Climb phases. According to our course notes and instructors this was to counter the nose up tendency the aircraft would have in such conditions. This was a direct result of engine shape and placement. In other words, the issues with the Max were not exactly new ones. However, the course notes never went into depth about the exact characteristics of the systems (one in each FCC) and which inputs were actually used to control the Stab. Neither did the Aircraft Maintenance Manual. We just knew the system was there with a fault light on the overhead panel to prove it. To add a little more, the CFM56-7 wasn't the only upgrade on the NG, it too had a taller main gear (and re-designed wing) to accomodate the larger nacelle diameter compared to the CFM56-3 on the Classic. In other words, the issues with the Max were not exactly new ones.

I'd say a clean design is long overdue ;) and I always understood what MCAS was designed for. The 737 Max would have been a safe aircraft to fly on even if MCAS was never installed. But the system was designed to prevent the requirement of simulator training for pilots, which was also the reason why it also was using the input of a single AOA sensor because to be dependable, it requires all AOA sensors which would have made it a safety critical system which would have brought back the simulator training even though Boeing was selling the aircraft to airlines with the promise of not needing sim training. At the end of the day, the problem with the entire thing is that Boeing sold an aircraft with a promise that it couldn't keep. What many people also don't know is that the 737 Max was not the first aircraft to get an MCAS system. A variation has been used on the KC-46 refueling aircraft as the balance of the aircraft shifts during refueling. While the system is also called MCAS, it does serve a different, but similar function.

Thank you for showing the photograph of the Air California 737 (time 5:02 in the video). My father was vice president of Air California starting in 1968 for several years. As a child I loved flying on those 737-200 airplanes. The paint job was golden yellow (for the Golden State) as were the flight attendant's uniforms (then known as stewardesses). I loved the sunflower on the nose of the airplane. It was a great logo. Air California ceased to exist when American Airlines purchase the company in 1987. They had one crash at John Wayne Airport in 1981. Thankfully everybody survived.

This is what happens when you gradually replace all the engineers in your management structures with financial people....😑😒😔

Your videos are always clear, fascinating, and watch-friendly. I have to compare the Max issues with my experience in military aircraft, where we were instructed that we would compensate for wild changes in handling characteristics in various routine flight regimes (not just takeoff and landing) simply by learning to anticipate that the plane would attempt to surprise, annoy, resist, or depart from controlled flight. We also had cutout switches for multiple automated systems in case we suspected the automation had somehow taken on a life of its own...including stab trim, anti-skid, stability augmentation, autopilot (of course) and others. Finally, we also had many, many restrictions on engine anti-icing, which became an onerous addition to workload when it was in use. I can really appreciate the comparatively modest enhancement that MCAS makes, and I hope more of your viewers come to understand that the terrible results stem from Boeing's corporate (Chicago) decisions regarding the profit implications of how they addressed transition training, rather than from the engines or the MCAS engineering (Everett).

I really just think the 737 isn't a great aircraft. Every variant isn't really something i'd want to fly on. The NG isn't my favorite to fly on comfort wise, and would prefer to fly on a 320 series everytime its offered. Regardless of the reasons for failures, I will never fly on a MAX. I would pay hundreds more to not fly it, and would drive 8+ hours before ever getting on one.

I was always under the impression that MCAS was there to readjust the C.G. which was impaired by moving the new engines forward and upward. I also did not realise they had lengthened the nose wheel leg. Thank you for explaining, so I even I, just an enthusiast understands. Really great content as usual. Stay safe. All the best. Mick🇬🇧

Retired American Engineer here: Sigh...I despair that in Boeing board room, a VP has written on the board "1) Gov't assist 2) Airbus merger 3) Military group split off. ...pick 1 or 2 and 3". Whenever we saw these kind of continuing systemic problems, we knew there were holes in the corporate structure and procedures that would be hard to fill short order. If the reserves were not there to fix it, writing appeared on the wall.

I remember that I read that tendency of the aircraft to increase the input of desired maneuver without increasing control input is also called "instability". Instability in yaw, pitch, roll, etc. Now, there is no perfect airframe and those instabilities exist almost in any airplane, and such non-linear characteristic are carefully implemented in flight controls, speeds placarded, pilot trained for such regimes etc. This way or another, those are dealt with in almost any airplane. The problem of MCAS was that discovered "instability" (because by the upper definition, it is THE instability), one that was acceptable by itself IF pilots got warned and trained, got to get "smoothed out" to approximate previous behaviour by totally inappropriate methods from the standpoint of safety and relability.

Thorough as usual Mentour. I flew all versions of the 737 from the 200 thru the 800. With each new variant, Boeing increased the size of the horizontal tail surface. The result was a jet that was fun to fly. After the McDonnell Douglas merger, Boeing broke with tradition. No longer were engineering decisions made be engineers. The Bean Counters had taken over. They thought the flight characteristics could be "smoothed out" with software and refused to mod the tail. Yes, the MAX can be flown without MCAS, but MCAS caused the two accidents.

I will never ever feel safe if flying a 737 MAX. It has been too many issues with that plane. Time to start working on a new (and safer) airplane. My favourit Boeing is Dreamliner, i flew it many times to Bangkok when Norwegian operated it. A beautiful airplane.

Thank you for your excellent discussion of the technical (and piloting) issues surrounding the 737 MAX. (I worked at Grumman Aircraft over 60 year ago, on the A-6 Intruder and the Gulfstream 1 Turboprop). It is refereshing to hear a knowledgeable and dispassionate discusion of the issues, My early experiences as a 737-100 passenger is that the pilots loved it (especially former military) b/c it was one "hot" aircraft.

Even in aircraft conceptual design stage, a commercial aircraft is always designed for inherent static and dynamic stability. It means the design itself tends to return to stable equilibrium in air. Then comes controllability - where we size stabilizers, control effectors, to be able to takeoff and land properly; and non-ideal cases. So, they're never designed to be unstable under any scenario. Even the wings are slightly angled upward to stabilize in roll. The way I understand it, the new larger engines created a larger thrust force that caused the aircraft to pitch up slightly from the OG design. Now I'm intrigued to look more into it. Thank you for the informative video, loved it!

IMO, the bigger forward engines didn't make the aircraft unstable, but it changed the flight characteristics from previous 737s. Boeing tried to hide the changed flight characteristics with software but kept it a secret (first hole in the cheese) so that current 737 pilots could fly it without getting a new type certification. So when the single AOA sensor (there were two but only one was active during a given flight, second hole in the cheese) failed, the software went haywire and the pilots had no training to recover (third hole in the cheese).

Thanks for this. Have to fly on these and your clear explanation is helpful, anxiety-wise

You mentioned open fan engines and I was wondering how they are figuring on dealing with safety issues compared to turbofans that have a scatter shield built into the cowling? Thanks for sharing.

Mentour Pilot's students must be the luckiest to have such an instructor. They get to learn History, Engineering and Navigation all at the same time. There's never a boring class when he's the tutor.

Boeing has anchored itself to the 737 and it is now pulling them under. Especially since management is more interested in stock buybacks and profit margins. They needed to bite the bullet and completely redesign the 737.

Here is the thing: Aviation has to adhere to a coverage factor 'k' of 4 when it comes to its engineering standards. To put it short, it essentially means everything has to be in order ~99.98% of the time (this is an extreme over-simplification, but it's the quickest way to get across how high of a standard that is). Different fields all use a coverage factor of either k=1 or 2, maybe 3. This goes for other types of engineering (except aeronautics related fields) and every type of research you can think of, from Physics to chemistry, to Psychology to Genetics. Even pharmaceutical research only has a coverage factor of k=3. Anything above k=4 is reserved for discoveries completely reshaping a scientific field (the discovery of the Higgs-Boson had one of k=5.9 for example). Aviation has the highest safety standard you can reasonably attain, of every field there is. You can skip this part, I'm just laying out how the k-factor works with in regards to measurements: If you measure a certain part, that measurement has an uncertainty (uncertainty is always present, by definition, and it's either calculated or fixed, depending on the tools you use, but it can't be flaked). In aviation, 99.98% of the measurements of the parts have to fall within this uncertainty, this where the k-factor of k=4 comes from (k=1.96 is 95%, to give another examample). And here is the mind-blowing part, and why I'm writing this comment: In a recent hearing in the US Congress, a leading Boeing engineer stated that the percentage of improperly installed parts are corrected only 2.3% of the time. This makes it impossible to attain a k-factor of 4. Remember the 2007 financial crisis (this is going somewhere trust me)? The problem was "collateral debt obligation", where they put shit loans that will likely go bang bang together into one folder, and said "we good now" because, if one loan goes bust and is not paid back, the rest carries the load. Problem is they all went bust. But this is not finance. This is aviation. The reason for a coverage factor of 4 is because other parts can't carry another broken part. If one part dips, the plane does too. This isn't just using faulty measurement systems with too low of a resolution (using a tool which measures in cm instead of mm), they are purposefully not making necessary adjustments. This is criminal.

Maybe I'm the odd one out here, but I really enjoyed flying the DC-9/MD-8X/MD-9X/717 family of aircraft. I always found them to be smoother in flight and less affected by turbulence, the major negative being the noise level if you were seated in the rear, adjacent to the engines.

Very swenglish centimeters, but i love the work going in to the channels. Firm knuten näve i bröstet, and the Robert Redford nod meme

Just to add to the points made: the mcas worked by moving the trim leadscrew in the horizontal stabilizer. This system would be way to slow to actually correct any aerodynamic instability.

I flew on one yesterday, it was really nice flight but neos are just more comfortable

Going back in time, way back, I grew up in Chicago and remember some interesting sights. This was back in the early and mid+ 60s. I remember seeing a DC-8 fly over our house (going East) and one could hear it for 20 minutes or so but see the black exhaust for at least half an hour or so. Another thing was I saw a DC-3 painted with orange stripes or tips and Dad told me it was the FAA doing checks on the O'Hare landing radios. I did have a DC-3 right in front of me at Palwaukee, coming right at me but then turned and scared me to death. I was about 5. After that, I end up getting a license in a 152, and was in aviation for 45 years until retirement. Love you channels!

To me, the Max is equivalent to a lifted Monte Carlo with 22’ spinning rims, so it’s no wonder there are design issues related to it.

I would love to see you make a video on the History of pilot training. Who was the first pilot that the wright brothers trained? How did they train early pilots? When was the first flight simulator used? What was it like? How has pilot training evolved? Sure someone else might be able to read the Wikipedia article on it but you have the insider insight into this specific topic that I think would be extremely interesting to have that viewpoint.

Your videos have really helped with my previous fear of flying. I am no longer gripping the seats like a madman with sweat pumping from my cold forehead

At the 21:14 point, suffice to say that the wobbly (change in slope) of the Cm-alpha curve line near/at stall makes MCAS necessary from a human factors standpoint. Without MCAS nose-down trimming, it's trickier to fly near stall, meaning you can be in a full stall before you know it. The pilot would need to know to change back pressure on the stick as stall nears, which is possible, but a little startling. Never startle a pilot near stall !!! Actually, technically the part where Cm-alpha has a slight positive slope (the no-MCAS line) near/at stall, is the definition of "unstable". It is slightly unstable near stall, not dramatically, but slightly.

When 737 MAX returned to service after being grounded for two years, its stab trim cutoff switches were not rewired to the NG configuration. As a consequence, in case of runaway trim, it is not possible to turn autopilot/STS/MCAS off and still use servos controlled with the buttons on the control wheel. It is much harder to recover the MAX compared to the NG if runaway trim happens.

This video is probably more in-depth than the official iPad conversion training. Very good work Mentour

I seem to remember that when the NG came out it had a elevator tap working apposing the movement of the elevator to keep it from starting to rotate by it self during takeoff, this was the consequence of the new engine (more thrust and further in front of center of gravity) and this is the problem with moving engines too far in front of the wing as it will increasingly make the plane pitch up as you apply thrust(moving center of thrust changes the behaviour)

Whether its a stable design or not, is probably the wrong question. They installed a feature that could crash the plane, relied on a single sensor, and told noone about it. Dosnt really matter how stable the aircraft is.

It's probably in Boeing's best interest to make the Max their final variant and make an all new aircraft to replace it. They can design it in a way where they can retrofit taller landing gears on it in the future to accomodate bigger more efficient engines and also to minimize the risk of tail strikes when they eventually stretch it out again for the 2nd and 3rd generation of the aircraft

I think Boeing needs to focus on excellence again, and not on maximizing profit. It's great to make a profit, but when you start sacrificing safety and excellence for profit, that's going to eventually sink your company. Then there aren't any profits. They used to have a good balance but not anymore.

as long as the 737 design is as fuel efficient as the other aircraft coming out they should keep making them. it has been a reliable aircraft for decades now

This is one of the best articles/videos I've come across concerning the MAX tragedies. It makes the 2 fatal crashes even more tragic because it's clear that if the MCAS system had been developed, certified and deployed with complete transparency and pilots properly trained, it would have resulted in a perfectly safe airplane with a big increase in performance and efficiency. But because Boeing tried to conceal the MCAS system and boasted of a seamless transition for pilots from the NG to the MAX in the name of cost savings 346 people payed the price.

During certification, to stall an a/c, the pilot has to keep pulling on the yoke. This is mandatory. Otherwise a pilot might stall the thing inadvertantly. In the MAX (without MCAS) it became 'stick light' during the stall- so not acceptable. MCAS was the fix-trimming the a/c nose down, so the pilot still had to pull. MCAS augmented the manoevering characteristics during the stall, thus the name.

Yes, retire them because consumer confidence is on a downward spiral on the plane and is affecting the company too. Consider the Hindenburg. One horrible crash pretty much stopped LTA transport for a hundred years. Now even though airships are made that can not go up in a ball of flame, the collective memory has deemed them unsafe, so the industry remains sidelined. Perception is everything.

Continue to love your work!! An interesting tidbit of information for you: The very first jet aircraft in the world (while definitely NOT a commercial airliner) the Messchershmidt ME-262 fighter/interceptor had its engines mounted directly underneath the wings without pylons. They extended both fore and aft of the wings. Hope this helps! Looking forward to your next instalment. Cheers from Canada!

Moe to the point about MCAS, it was put into the MAX to make the aircraft fly as if were a standard 737 to eliminate any requirement for pilots be retrained and recertified as if it were a different aircraft. Boeing did this to make the aircraft more marketable to the airlines And they didn't bother to disclose that to the airlines and their pilots. And who in their right mind would design a system that could fail from ONE malfunctioning sensor, the AOA sensor? Military aircraft the Boeing makes have flight control computers that use multiple sensor inputs to mitigate against this precise scenario.

The Max responded to this video by dropping a wheel when this one aired….

Boeing Engineer 1: Hey I think I fixed the problems with the Max's engine displacement. Boeing Engineer 2: WOW, how did you fix it? Boeing Engineer 1: I added extra bolts to each engine. Boeing Engineer 2: Nice, where did you get the extra bolts from? Boeing Engineer 1: Ummmmmm!!!!

A part in this video reminded me of a flight when I was a kid. Then that reminded me of another part of that flight or another flight. I remember seeing the bucket reversers, as you just called them, deploy on landing and I thought the plane was falling apart. On a flight from DTW to San Diego when I was somewhere around 5 years old I think, I was afraid to use the toilet because I thought it was going to suck me out of the plane. Since then, I have been on many flights, including a single engine 4 seater prop plane around 3rd grade and one commercial flight in a puddle jumper flight. It was a VERY loud twin engine prop plane that I think were turbo props. I couldn't even stand up in it and they told us where to sit for weight distribution. Another commercial flight was in a more modern twin engine turbo prop that I could stand up in. It had just one seat on either side of the aisle and a curtain to the cockpit that was left open for the night flight. That was cool seeing into and through the cockpit while flying. And one memorable flight was Alaskan. It was a flight from California to Oregon and it had a capacity of around 50-100. It had top mounted wings and there was only 4 passengers on board. We got to drink for free. I remember getting a Craft Beer that was an Alaskan Brewery. I still have that can, empty of course. But with all the problems Boeing is having now and the whistle-blower taking his own life, allegedly, during his deposition, I will NEVER fly on a Boeing ever again. I just can't trust them anymore.

You should create a copy of the INOP shirt that says "Anti-ice 5 minutes"

One thing I've never heard in any video is what the pilots think about the actual differences between the NG an MAX, the differences MCAS was supposed to eliminate. It really feels like Boeing was more concerned with satisfying the bureaucrats than the actual pilots. If the pilots are/were unaffected by the difference, then it's the FAA that's responsible for the crashes.

I am very glad you pointed out the differences in the engines that Airbus and Boeing put under their wings. The plane you fly (and I live directly under the approachroute to Schiphol runway 06) is one of the noisiest, the 737-8. The newer models are already quieter, but the difference in noiselevel is remarkable. The lowpitched noise that the engines on the 737-8 or 737-9 make is thunderous. Where as the newer engines are highpitched and less noisy, mostly on Airbus neo-planes. I love the RR Trent engines, with their high bypass high pitched sound. Especially on the 350 and the 380.

Another great, informative performance. Wondering when you, Kelsey, Juan, and others will get together and collaborate.