The Boeing 737 MAX 8: How did we get here?

"ttyymmnn" (ttyymmnn) 03/11/2019 at 13:31 • Filed to: wingspan, Planelopnik

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On March 10, 2019, a factory-fresh Boeing 737 MAX 8 airliner flown by Ethiopian Airlines crashed shortly after takeoff from the capital city of Addis Ababa. The crash killed 157 passengers and crew, and came only five months after Lion Air Flight 610, another brand new 737 MAX 8, plunged into the Java Sea off Indonesia with the loss of 189 passengers and crew. The MAX 8 is one of Boeing’s newest airliners, and having two such technologically advanced aircraft go down in relatively rapid succession has the commercial aviation industry and regulators looking very hard at the latest evolution of Boeing’s most successful airliner. But just what is the MAX 8, and how might its design factor into a possible cause for these two disasters?

Building a World Beater

By the mid-1960s, Boeing was taking the world by storm with their four-engine 707 and three-engine 727, but they needed a smaller airliner to service shorter routes with fewer passengers. The design of the new 737 borrowed heavily from the 727, and had two engines rather than three. Its !!!error: Indecipherable SUB-paragraph formatting!!! turbojet engines were housed in long cigar-shaped pods under the wings, and the slender engines allowed Boeing engineers to employ shorter landing gear that kept the 737 nearer to the ground. This facilitated engine servicing and helped decrease turnaround times. While those short legs worked well with early turbojet engines, they became a problem when airlines transitioned to larger high-bypass turbofan engines.

The flattened engine nacelles are seen on this Southwest Airlines Next Generation Boeing 737-700 (Tim Shaffer)

Though initial sales of the 737 were slow, Boeing never gave up on its narrow body airliner, and the 737-200 was soon fitted with more powerful JT8Ds that increased takeoff performance and range. But the big change came with the 737-300 which introduced the type’s first high-bypass turbofan engine, the !!!error: Indecipherable SUB-paragraph formatting!!! . Unlike the earlier turbojet, the CFM56 had a large fan disk at the front, and the 737 had short legs. So the bulk of the engine was moved ahead of the wing, and the opening of the engine nacelle was flattened at the bottom to allow for necessary ground clearance. The fan itself was also restricted to a 60-inch diameter. These variants, the 737-300, -400, and -500 came to be known as the 737 Classic. The Classic was followed by the 737 Next Generation, which encompassed the 737-600, -700, -800, and -900. These stretched variants could accommodate anywhere from 108-177 passengers and were fitted with newer CFM56 engines which were larger still.

It’s all about the burn

Despite the 737 becoming the world’s best selling airliner, Boeing faced stiff competition from European competitor Airbus, particularly from the !!!error: Indecipherable SUB-paragraph formatting!!! (new engine option). With nobody willing to completely reinvent the airliner, the only way to compete for sales was to offer greater fuel savings. The neo featured newer and larger engines with decreased fuel burn, and Boeing found themselves in a bit of a bind. They could either design an entirely new aircraft, or they could keep trying to put bigger, more efficient engines on the venerable 737.

Rather than spend billions of dollars on a new airliner and chase after new type certificates from the Federal Aviation Administration, Boeing chose to do the latter, and re-engined the existing Next Gen airliners with !!!error: Indecipherable SUB-paragraph formatting!!! engines. They also added a few aerodynamic tweaks like split scimitar winglets, a tail cone taken from the 787, and fly-by-wire spoilers to save weight. In line with the branding of the 787, the re-engined 737-700 became the MAX 7, the re-engined 737-800 became the MAX 8, and so on until the as-yet-to-fly 737 MAX 10, which is stretched still longer than the MAX 9 and intended to compete with the larger A321. The MAX 8, basically the entry model, began flying in May 2017 for Malindo Air of Malaysia, a subsidiary of Lion Air.

But now Boeing was faced with the serious problem of not having enough room between the wing and the ground to fit the big new engines. They had already scrunched the nacelle of the Next Gen 737 into the so-called “hamster pouch,” so now, to accommodate the 69-inch diameter fan disk of the LEAP-1B, they had to move the engines even further forward and raise them slightly above the wing. They also had to lengthen the nose gear. The new engines fit, but their placement shifted the relationship between the center of gravity and the center of lift over the earlier Next Gen 737s, and also introduced some lateral instabilities. The engine nacelles themselves also created some lift of their own, and this caused a tendency for the nose to pitch up slightly under manual flight. But Boeing had an answer for that, too.

MCAS

Flying any airplane is a balancing act, and keeping the plane flying straight and level is a matter of fine adjustments, known as trimming the aircraft. For the 737 MAX series of airliners, Boeing introduced the Maneuvering Characteristics Augmentation System, or MCAS, which is designed to counter the lifting of the nose by moving the horizontal stabilizer on the tail to push the nose down. Their goal was to make the MAX feel like the older 737s in the hands of experienced pilots, but perhaps more importantly, to allow the new MAX to be grandfathered under the same FAA certification as older 737 models. Boeing designed the system to work in the background, and expected that pilots would never know it was there. Trouble is, Boeing never made a point of telling pilots about the system. In earlier versions of the 737, if the trimming system got of of whack (called runaway trim), or started to get bad data, pulling back hard on the control column would turn off the system. MCAS, however, would not shut off this way. It required the crew to disable the system using switches, then trim the plane manually.

The Angle of Attack Sensor on the 737 MAX. One of these sensors is mounted on either side of the aircraft. (Author unknown)

In order for MCAS to control the angle of attack, or how high the nose is pointing into the air, it requires data from the angle of attack sensor. There are two of them on the MAX, one on each side of the nose. But MCAS only gets data from one sensor at a time. Should that one sensor provide faulty data, it could cause MCAS to think the plane was in a stall and actively push the nose of the airliner down even if the plane is in level flight. If a pilot reacts as he was trained by pulling back on the yoke, the system will fight back, pushing the nose down every 10 seconds until it is disabled by the crew. Boeing, for their part, felt that following existing procedures for runaway trim would be sufficient. And the FAA agreed with them, as did regulators in Europe.

Though there is no proof yet from the airliner’s voice and data recorders, !!!error: Indecipherable SUB-paragraph formatting!!! from Lion Air Flight 610 shows the airliner pitching up and down before crashing and could indicate a struggle in the cockpit between a runaway system with faulty data trying to push the plane down pitted against pilots who may not have known how to disable the system and fighting to keep the plane in the air. And while it’s also far too early to blame MCAS for the crash of Ethiopian Airlines Flight 302, investigators and Boeing engineers are almost certainly looking at it. So far, 19 airlines around the world, though none in the US, have grounded their MAX 8 fleet until more is known about what is causing these brand new airliners to crash.

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Special thanks to !!!error: Indecipherable SUB-paragraph formatting!!! for providing information critical to my understanding of this topic. The post has been updated to reflect his excellent explanation of the MAX 8 flying characteristics.

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You can find more stories about aviation, aviators and airplanes at !!!error: Indecipherable SUB-paragraph formatting!!! .

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McMike > ttyymmnn 03/11/2019 at 13:43

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*raises hand*

Having watched at least seven episodes of Air Disasters, I believe I am well qualified to take this one.

New safety automation + lack of training on new safety automation = pilots fighting the computer all the way to the ground.

They should have a big-ass button on the dash that says “LMFTDA ” (let me fly the damn aircraft ) and hand it over to a pilot so they can manually do everything while the other pilot starts reading the manual they were never given to figure it out , or look it up on YouTube or something.

For Sweden > ttyymmnn 03/11/2019 at 13:44

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ATTN: Boeing

someassemblyrequired > ttyymmnn 03/11/2019 at 13:44

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They shouldn’t be flying without MCAS disabled until this gets figured out. The ADS-B data ends a few minutes before the crash, but there’s already high airspeed showing on the data that is available, indicating that they may have been fighting a false stall indication .

In control system design, you always fail safe. On a bad AOA indication, the system should be checking airspeed, climb rate and bank angle before it does anything. If it does act, it needs to prominently indicate that on the flight display along with instructions for turning it off. Think like traction control getting disabled on an implausible sensor reading or the little blinky lights for ABS and TC when they engage in your car.

It definitely looks like MCAS is not reality checking its inputs and communicating with the crew properly.

This makes me pretty angry - there’s no excuse for half- assing a control system in a passenger aircraft, especially when a $10K Kia can sort out bogus inputs and inform the driver when it intervenes .

ttyymmnn > McMike 03/11/2019 at 13:46

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I would heartily support the LMFTDA button. I did ask a 737 pilot if all of this automation was aimed at, shall we say, less experienced pilots in certain parts of the world. He said not necessarily. I do believe, though, that too much automation can lead to too much dependence on automation. 

ttyymmnn > someassemblyrequired 03/11/2019 at 13:52

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I’m certainly not an expert, and my goal with this piece was to put things in more layman’s terms, and teach myself along the way. Boeing thought the system would run in the background and pilots wouldn’t need to know about it. They said that, in the event of runway trim, existing protocols would cover it. But how can you put a system on board that is affecting flight and not tell the pilots? I think they are just trying to be too smart here. 

McMike > ttyymmnn 03/11/2019 at 13:56

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user314 > ttyymmnn 03/11/2019 at 13:57

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If this is a similar situation to the Lion Air flight, that’s troubling. Just my own semi-informed speculation, but f or two aircraft to have the same failure might indicate a problem with the whole run of AOA sensors, or maybe the MCAS software itself.

Highlander-Datsuns are Forever > ttyymmnn 03/11/2019 at 13:59

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That thing with the engine s being big and in front of the wing, air bus does that too.

For Sweden > ttyymmnn 03/11/2019 at 14:00

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But how can you put a system on board that is affecting flight and not tell the pilots?

You know the tech support saying that the problem is between the keyboard and chair? That drives much of aircraft design.

ttyymmnn > McMike 03/11/2019 at 14:00

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ttyymmnn > user314 03/11/2019 at 14:03

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In the Lion Air crash, the crew of the last flight before the crash indicated a problem with the AOA sensor. They plane probably should not have been allowed to fly, but that’s Lion Air. Ethiopian is a safe airline, one of the best in Africa. I haven’t seen any data on altitude fluctuations, but they were flying about 1ookts faster than usual for that part of the flight, which may indicate they were fighting a stall situation. Still way to early to cast blame, but two crashes of brand new airliners in similar conditions in 6 months might even warrant a global grounding, for now at least. 

ttyymmnn > Highlander-Datsuns are Forever 03/11/2019 at 14:04

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They’re not as high, though. I’m not an aerodynamicist, and I have tried very hard to understand the “why” of the problem with the shift in COG. For now, I just have to accept it as “that’s what happens.” The placement of the engines on the neo may also not affect COG in the same way.

Ash78, voting early and often > ttyymmnn 03/11/2019 at 14:17

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I hate to be That Guy, but the running joke 20+ years ago was “LOL Boeing just puts new engines and longer fuselages on the 737 instead of doing something new!” (pre-777/787)

This almost seems like the sad, yet logical, end to that stereotype. To say I’m just a little bit spooked about flying right now is an understatement. I’ve never, ever been scared of an airplane before.

ZHP Sparky, the 5th > ttyymmnn 03/11/2019 at 14:22

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Awesome write up, the amount of knowledge in here is amazing. I learnt a lot. But yeah its insane thinking they put a system in these planes that they figured pilots wouldn’t even need to know about as it would replicate what the older planes did by default. Which sure when it’s regular smooth sailing that sounds great – but in situations like this where the system may have been malfunctioning it might be helpful for those pilots to know these systems might exist and actually may be the culprit.

Especially amongst smaller airlines or with less experienced pilots (heard the copilot on this most recent flight had 200 flying hours) they might be fine when a flight is going along without issue, but may severely lack the experience or knowledge to deal with obscure issues that may arise like these – especially those made even more obscure by the manufacturer trying to be too smart for their own good. What a shitty ending for all those innocent people – heard a lot of them were UN aid workers too. Hits a bit close to home given that my mom used to be one and actually worked in Nairobi (where this plane was headed) until she retired a few years ago.

ttyymmnn > Ash78, voting early and often 03/11/2019 at 14:26

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I have long been a big fan (not fanboi, there’s a difference) of Boeing. They always seemed more pilot-centric than Airbus and they’re planes are just prettier (not that aesthetics counts towards safety or reliability) . But the continuous delay of anything genuinely new, and the absurd stretching of a 50-year-old design, looks to be coming back to haunt them. Hell, to fill the gap left by the 757 they are talking about the 737 MAX 10. And they have tabled their decision on the 797 for yet  another year. I’m not sure what sort of signal they are waiting for. Then again, maybe 346 dead people may be just the signal they need.

Ash78, voting early and often > ttyymmnn 03/11/2019 at 14:30

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Yeah...I’m not ready to place the blame on Boeing just yet (I mean, there about 10 other stakeholders here, from training to individual pilots to national certification boards) but the similarities are already too eerie for comfort.

It’s already reminding me of the 787 battery issues from several years ago. Prior to that point, I wasn’t ever really aware of model-specific design flaws resulting in major accidents. Most of that stuff was behind us (from an aerodynamics standpoint) a few decades back. Now it just seems to be new technologies causing all the havoc. The fundamentals of flying are unchanged.

OPPOsaurus WRX > McMike 03/11/2019 at 14:31

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ummmm , passengers, this is the captain speaking, ahhhhh , please stand by while I google this shit, ahhhh, in the mean time, ahhhh, brace for impact.

ttyymmnn > ZHP Sparky, the 5th 03/11/2019 at 14:34

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Awesome write up, the amount of knowledge in here is amazing.

Thank you. I assure you, there was a lot of homework here, since these were all questions that I had myself. I just wanted to write it down in a way that might be clearer for the layman.

Especially amongst smaller airlines or with less experienced pilots (heard the copilot on this most recent flight had 200 flying hours)

When planes crash, news outlets seem to go immediately to the Wiki and find out how old a plane is, thinking that age equals danger. That is not always the case, even of you are flying in a WWII-era B-17. And the flight time of a junior officer doesn’t necessarily indicate a lack of skills. The PIC on the Ethiopian flight was very experienced, but you don’t get there without starting out with zero hours. Any pilot gets his first hours on a revenue flight.

And I find it hard to believe that the captain of ET302 hadn’t studied the Lion Air crash to know about MCAS. Which makes this all the more troubling. It may well turn out that these events are totally unrelated, and I am trying very hard not to say, “THIS IS WHAT HAPPENED” because we just don’t know. Yet. 

Turbineguy: Nom de Zoom > ttyymmnn 03/11/2019 at 14:37

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I’d think the loss of Lion Air 610 would’ve resulted in changes to the 737 crew ops manual if it was determined to be a problem with MCAS. (i.e. how to disable in case of a runaway trim situation)

ttyymmnn > Ash78, voting early and often 03/11/2019 at 14:38

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T he 787 was grounded over the battery incident, and not a single a/c had crashed, nor was there a single fatality. Why 346 dead people doesn’t warrant a global halt is beyond me. Admittedly, the problem with the 787 was obvious. However, they didn’t fix the battery problem, or move away from Li-Ion batteries. They just made a better containment structure for when they fail. 

ttyymmnn > Turbineguy: Nom de Zoom 03/11/2019 at 14:42

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Boeing issued an emergency airworthiness directive back in November after Lion Air.

Boeing Company today published an update to the flight crew operations manual for its 737 Max 8, warning of a possible fault in the aircraft’s angle of attack system that could cause the aircraft to violently pitch nose down, according to the FAA. The AOA measures the angular difference between the direction the aircraft is moving and the pitch of the aircraft’s wing. The agency followed the ops manual update with an emergency airworthiness directive against the aircraft, warning operators the pitch-over threat exists even when the aircraft is being hand flown by pilots. The agency said operators have less than 30 days to comply with the AD. (Flying)

BahamaTodd > ttyymmnn 03/11/2019 at 14:45

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I’m on a United M ax 9 flight from IAH to PHX next month. Let’s see what happens.

ttyymmnn > BahamaTodd 03/11/2019 at 14:47

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My prediction is that it will be entirely uneventful. You won’t even know that MCAS is trying to kill you.

Just kidding. Really, you’ll be fine.

user314 > Highlander-Datsuns are Forever 03/11/2019 at 14:59

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Yeah, but the A320 has always been like that, there’s little change in the engine position between an A 320-200 and an A320NEO:

Compare that to the difference between a 737-100 and a 737 Max8:

The engine went from being under the wing to in front of it. 

Mercedes Streeter > ttyymmnn 03/11/2019 at 15:04

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The Ethiopian flight was all over the place. Not as dramatic as the Lion Air flight, but something definitely went wrong almost immediately:

Eyewitness reports say the plane was smoking, making erratic maneuvers,   and had just dodged a mountain before taking its final dive.

someassemblyrequired > ttyymmnn 03/11/2019 at 15:13

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Yeah it’s almost if Boeing forgot that the aircraft is part of the CRM loop in that case.  It’s not hard to provide additional information to the pilot and instructions for disabling the system - this is especially important if crews won’t be trained on the new system.

I mean this would work for bad sensor inputs:

MattHurting > ttyymmnn 03/11/2019 at 15:14

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I was hoping you’d have a piece about these events, as they appear to similar to be a coincident. Also for selfish reasons, as I just booked my wife and children on a flight from DEN to BWI in a couple weeks, and just double checked to see what plane they’d be on (Airbus A 320, phew).

WilliamsSW > ttyymmnn 03/11/2019 at 15:16

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This may help a bit.

https://www.wingsbywerntz.com/1014-stall-speeds-and-center-of-gravity-c-g

Moving the engines forward would move the CG forward (moving them upward doesn’t really have an impact).

Another thing not covered here is that the engine thrust line was moved upward - that *could* have some significant impact on how the airplane flies, but the thrust is very close to the Center of Lift on the 737, I believe.

someassemblyrequired > For Sweden 03/11/2019 at 15:18

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Seems like the keyboard/chair problem is especially acute in Renton, WA at the moment.

This is really basic stuff - there’s really no excuse for not sanity checking parameters before intervening. 

WilliamsSW > Highlander-Datsuns are Forever 03/11/2019 at 15:23

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It’s not the placement of the engines per se, but rather the placement of the engines relative to all of the other weight on the aircraft, mainly in relation to the center of lift (which is roughly over the center of the wing).

ttyymmnn > Mercedes Streeter 03/11/2019 at 15:25

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I’m not going to trust eyewitness reports just yet. I’ve seen so many that claimed an airplane was on fire when it wasn’t. Still, this is very troubling.

ttyymmnn > MattHurting 03/11/2019 at 15:29

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My family is flying SWA in June. With them, you fly a 73 or you don’t fly. It remains to be seen which flavor it is. Maybe I should check. They should be grounded though.

I was hoping you’d have a piece about these events,

I actually wasn’t planning on it. But I was reading about the situation with the MAX last night and I realized that I didn’t really know what a 737 MAX 8 is and bingo, I had a piece in mind. I t’s not just enough to say “up-engined 737-800.” You have to know how we got here, because it matters. History must have context.

Mercedes Streeter > ttyymmnn 03/11/2019 at 15:32

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Indeed, eyewitness reports often incredibly unreliable (and people like to lie). IIRC, when TWA 800 went down there were a lot of reports of people seeing a streak of light leading up to the aircraft, suggesting a missile had been fired at it. Still, every once in a while the witnesses do get it right, so I guess we’ll see.

The good news is that the CVR and the FDR were both recovered so the investigation is already off to a head start.

ttyymmnn > WilliamsSW 03/11/2019 at 15:35

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As you move the center of gravity forward, you lengthen the “arm” over which the center of gravity operates – that means to stay balanced, the tail must generate a greater downforce (since you can’t change the length of its arm). That extra downforce increases the effective load borne by the wings. You haven’t increased the weight of the airplane, but you have increased the load on the wings (much like a high G maneuver might do). The only way to continue level flight is to increase angle of attack in order to increase lift – that means that for the same aircraft weight, a forward C.G. will cause you to fly at a higher angle of attack and thus closer to the critical angle of attack, and therefore you increase the stall speed.

Bingo. Thanks.

If only EssExTee could be so grossly incandescent > user314 03/11/2019 at 15:35

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The A320 was designed and built twenty years  after the 737. Engines had already started getting bigger and I assume they would have been able to plan for future upgrades. 

WilliamsSW > Mercedes Streeter 03/11/2019 at 15:36

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I think it’s more that people are unreliable than that they actually lie.

In the case of TWA 800, the way that the airplane broke up could have easily led people to *think* that they saw a streak leading up to the aircraft (the tail separated, which led to the rest of the aircraft climbing rapidly after the explosion - because contrary to popular belief, the tail pushes the rear *down*, not up, on the vast majority of aircraft ).

WilliamsSW > ttyymmnn 03/11/2019 at 15:43

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No problem! You can almost see where this would lead designers - you could move the wings forward on the fuselage to offset this, but you’d be moving the engines with them, so it might take a fair amount of movement in order to get the effect you wanted.

By the time you did that, you’d be looking at a lot more fuselage behind the wings - and behind the landing gear. Which creates a whole new set of problems on takeoff and landing (trying not to scrape the tail when rotating), so now you have to lengthen the landing gear.

By the time you’re finished with all of that, you’ve designed an entirely new airplane.  Which will look suspiciously like an A320NEO.

Mercedes Streeter > WilliamsSW 03/11/2019 at 15:44

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Oh, I know! That particular crash is one I still like to read up on today.

To clarify, my remark about lying is more for more recent crashes. I remember when MH370 happened it seemed a lot of people were trying to milk the tragedy for all the attention they could get for as long as they could.

ttyymmnn > WilliamsSW 03/11/2019 at 15:48

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By the time you’re finished with all of that, you’ve designed an entirely new airplane. Which will look suspiciously like an A320NEO.

LOL. But only too true.

Everybody shakes their head about the demise of the A380, but Boeing is getting their ass handed to them by the neo and the 321. And that was after getting brilliantly outplayed on the C Series debacle. I only hope the Boeing execs are sitting around a table in Chicago or Renton or wherever and saying, “How the hell did we get ourselves into this mess?” I think the answer is by being too cautious, waiting too long to make any sort of decision, and staking the future on a 50-year-old design. I certainly wouldn’t want to be making billion-dollar decisions, but that’s why these guys make the bank that they do.

ttyymmnn > WilliamsSW 03/11/2019 at 15:59

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...... using wrong weights for the passengers (adults instead of children) may move the CG to a wrong location, not balanced by the trim, and create strong difficulties during the takeoff and the initial climb. ( source )

Which is fascinating, since both of these crashes occurred minutes after takeoff, while the airliners were still climbing. I’ve read a lot about how the CG is different between the MAX and the NG, and I wonder if this could be throwing pilots off. Or if there is some sensor that is just getting everything wrong.

WilliamsSW > ttyymmnn 03/11/2019 at 16:02

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Designing a new jetliner from the ground up is, and always has been, a ‘bet the company’ decision. Boeing and Airbus are big enough now that a failure won’t destroy the company completely, but it will still put a huge dent in it.

Boeing’s best opportunity might have been with the NG. That one got a new wing - maybe it should have been a clean-sheet airplane instead.

This may force their hand on the 797 - which might just be a good thing.

Spamfeller Loves Nazi Clicks > ttyymmnn 03/11/2019 at 16:06

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Southwest has only just started taking delivery of the 737 MAXes, at least.

You missed my money shot elsewhere, when someone brought up the MD80-family versus the 737MAX. Of the two, one has 100% of accidents due to manufacturer design defect, and a 100% fatality rate in 100% of accidents, with multiple threat-to-life airworthiness directives in the first 12 months .

The other is the MD80 with more than 4 0% of total hull loss accidents resulting in zero fatalities .

Spamfeller Loves Nazi Clicks > ttyymmnn 03/11/2019 at 16:10

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They’re spinning the 737MAX as a “continuation” of the 737, so they’re rolling it’s stats in with the “classic” 737's. “346 dead people in 12 months? Nah! We’ve flown 60.8M times with this aircraft, everything is fine!”

WilliamsSW > ttyymmnn 03/11/2019 at 16:16

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I suppose that it’s possible, flying a Cessna at a forward CG does feel different from the same airplane with an aft CG, though you use elevator trim to reduce that.

FWIW, a forward CG is more ‘stable’ - meaning that it takes more force to raise the nose.

That said, most CG issues I’ve heard of on airliners are with smaller turboprops, not big airliners.  And the pilot/plane system *has* to be able to adjust to CG changes - just burning fuel while airborne moves the CG.  They actually calculate a takeoff CG and landing CG to make sure they’re within limits for the entire flight.

WilliamsSW > Mercedes Streeter 03/11/2019 at 16:20

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MH370 is a weird one - ripe for lying and conspiracy theories, since we haven’t found the aircraft.  As an added bonus, the Malaysian government hasn’t been entirely forthcoming with data - which really isn’t all that uncommon in many countries.

WilliamsSW > ttyymmnn 03/11/2019 at 16:22

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Keep in mind that Airbus has something like that - although it’s more of a ‘I dunno what to do so here’s the airplane, best of luck to ya’ abdication...

Snuze: Needs another Swede > ttyymmnn 03/11/2019 at 16:24

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People in my place of employment travel a lot. In fact I’m leaving on Sunday (my wife’s birthday no less) and I’m not going to lie, I’ve already checked my travel itinerary to see if any of the aircraft are a MAX series. Fortunately all the aircraft are -700 or -800.

It appears t he Lion 610 preliminary report by the Indonesian National Transportation Safey Committee has explicitly called out MCAS in that crash. It was also   indicated that 2 days prior to that crash a faulty AOA sensor was replaced. On the day before the flight the aircraft suffered another altitude/speed incident and the crew performed the runaway stabilizer checklist and took manual control of trimming the aircraft.

I’m not an air accident investigator or anything of the sort, but my gut feeling is the Lion and Ethiopian crashes are linked and it’s related to the MCAS system in some way (AOA sensors, software, crew training or lack there of, etc.)

ttyymmnn > Spamfeller Loves Nazi Clicks 03/11/2019 at 16:25

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Not entirely fair, since the MAX8 hasn’t been flying for 39 years. But point taken.

ttyymmnn > Spamfeller Loves Nazi Clicks 03/11/2019 at 16:25

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Amortization, with the emphasis on “mort.”

Jetstreamer > ttyymmnn 03/11/2019 at 16:27

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MCAS is necessary not so much because of a CoG shift. It is necessary because at high angles of attack, the engine nacelles actually generate enough lift to affect longitudinal stability to the point where it would not be able to be certified under the grandfathered 737 type certificate anymore. More a center of lift issue. And b asically, no MCAS, no 737 MAX certification. Therefore, it is also not a simple case of disabling MCAS as has been suggested. Doing that basically means your airplane is not airworthy anymore outside of non-normal situations.

Another issue with MCAS is that from what we know so far, it doesn’t seem to have enough redundancy to deal with bad sensors. While Boeing may think following the standard trim runaway checklist will fix any MCAS issue, the Lion Air case seems to proof otherwise. That case looks like bad data from an angle of attack sensor, leading to unreliable instruments, stick shaker going off all the time and in the background this new system MCAS is acting on bad data instead of having the redundancy to be able to filter it out. That’s a lot of stuff to deal with in the cockpit. And nothing initially points at trim runaway, so using the cutoff switches for MCAS also doesn’t sit high on the priority list.

I don’t want to jump to conclusions, but so far, a lot of stuff points at MCAS being a software fix to fix flaws of an ancient design. And the software fix does not seem to have gone through enough testing to make sure it works for the pilot in every scenario, instead of against them.

It also points towards Boeing being so averse to having to recertify the 737, that a system that can command full nose-down trim, something that requires a lot of force from the pilot to overcome by pulling on the yoke, gets introduced to the airplane, yet lacks redundancy and can get fooled by one malfunctioning sensor.

Basically, the signs ain’t pretty so far. Now, this Ethiopian crash may very well have absolutely nothing to do with MCAS. Or it may be the very same issue. But it is two very fatal crashes of a “ new” airliner in good weather conditions... the risk is too great not to have a good look at it and maybe keep them on the ground for a while until we know more.

There is always the argument “we haven’t seen any issues on our planes” but that doesn’t mean there aren’t any. China’s grounding may be partially (or wholly) politically motivated, but I am honestly glad they took that step as it sends a message and allowed others to join in. Don’t keep planes flying that may have a latent design flaw...

ttyymmnn > WilliamsSW 03/11/2019 at 16:29

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I need to figure out more of exactly what is going on here, which means more homework. Most of my sources are more general knowledge sites, which don’t go into much detail. That stack exchange article I read was the wonkiest by far. My only sense is that the MAX is more “tippy” than the others, maybe closer to stall than the NG. I’d be interested to see a CG/CL graphic for the MAX 8 (and 9 and 10). Because even if moving the engines forward would make it nose heavy, Boeing was concerned about it pitching up. Or else there’s just something horribly wrong with the AOA sensors. And I doubt Boeing make those.

ttyymmnn > Snuze: Needs another Swede 03/11/2019 at 16:32

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It appears the Lion 610 preliminary report by the Indonesian National TransportationSafey Committee has explicitly called out MCAS in that crash.

I have not seen that. Do you have a link?

The pilots of the last flight before Lion Air reported issues with the AOA sensor. In my unprofessional opinion, that plane never should have left the ground again. But maybe they didn’t even know about MCAS, and figured they could handle any issues like they used to on the NG? I don’t know.

What I find boggling though is that the MCAS relies on just a single sensor (I’d love to be wrong on that, but it’s what I’ve read). So one single bad  sensor can bring down an airliner. It shouldn’t be that easy. 

BahamaTodd > ttyymmnn 03/11/2019 at 16:33

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I know I’ll be fine. Just interested to see what comes of the investigation and whet her we’ll see any more groundings.

ttyymmnn > Jetstreamer 03/11/2019 at 16:40

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Thank you for the lengthy reply. I am more of an armchair historian and certainly no pilot nor aerodynamicist, so I’m learning as I go here. This piece was more of an exercise in teaching myself and hopefully providing others with an understandable explanation . So I really appreciate your input. I was admittedly a bit nervous about the MCAS portion of the article, because my understanding is still quite shallow on that point.

WilliamsSW got me going in the direction of CG vs CL, and yes, it did seem to me that therein lay the real issue. Thanks for the explanation.

!!! UNKNOWN CONTENT TYPE !!!

I’m reminded of what ForSweden said the other day, “ Too many operators think the absence of a failure is the absence of a hazard.”

Snuze: Needs another Swede > ttyymmnn 03/11/2019 at 16:47

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There are 2 AOA sensors installed on the aircraft, but I’m not sure how they are operated... i.e are both polled and averaged or is one a primary and the other a backup? 

Regarding the report, I’ve included a link but I have had trouble opening it. The information above was from a summation of the report.

Report link:

http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/pre/2018/2018%20-%20035%20-%20PK-LQP%20Preliminary%20Report.pdf

Another link with good info:

https://leehamnews.com/2018/11/28/indonesian-authorities-release-preliminary-lion-air-crash-report/

Jetstreamer > ttyymmnn 03/11/2019 at 16:52

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Apologies for the length and possible lack of coherence in the my post, in my defens e, I have had a long day and then will sometimes just ramble on :P. But yeah, it’s too easy to just say “our planes are fine” when that same type of plane is dropping from the sky on fair weather days.

I honestly feel the MAX is a plane that shouldn’t have happened. It should have been a clean-sheet design that they should have started working on straight after the NG. I’m sure there are all kinds of market forces and reasons not to do that, but hey, I’m just a simple pilot.

You won’t see me getting on a MAX soon though , not until more is known about this crash. Hope they can get some decent data of the recorders in a timely manner.  

Rusty Vandura - www.tinyurl.com/keepoppo > ttyymmnn 03/11/2019 at 16:54

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The new engines fit, but their placement shifted the center of gravity forward over the earlier Next Gen 737s, and this arrangement tended to cause the nose of the MAX to pitch up slightly in flight

Is it possible you have this backwards?

ttyymmnn > Snuze: Needs another Swede 03/11/2019 at 17:01

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The data confirms that a sensor that measures the plane’s angle of attack, the angle between the wings and the air flow, was feeding a faulty reading to the flight computer. The two angle-of-attack sensors on either side of the jet’s nose differed by about 20 degrees in their measurements even during the ground taxi phase when the plane’s pitch was level. One of those readings was clearly completely wrong.

On any given flight, the flight computer takes data from only one of the angle-of-attack (AOA) sensors, apparently for simplicity of design. In this case, the computer interpreted the AOA reading as much too high an angle, suggesting an imminent stall that required MCAS to kick in and save the airplane.

When the MCAS system pushed the nose down, the captain repeatedly pulled it back up, probably by using thumb switches on the control column. But each time, the MCAS system, as designed, kicked in to swivel the horizontal tail and push the nose back down again.

The data shows that after this cycle repeated 21 times, the captain ceded control to the first officer and MCAS then pushed the nose down twice more, this time without a pilot response.

After a few more cycles of this struggle, with the horizontal tail now close to the limit of its movement, the captain resumed control and pulled back on the control column with high force.

It was too late. The plane dived into the sea at more than 500 miles per hour.

Seattle Times

WilliamsSW > ttyymmnn 03/11/2019 at 17:02

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I think that will be a challenge, for a few reasons. You can easily find CG limitation charts online, though they’re a challenge to interpret (%MAC is % of Mean Aerodynamic Chord, for a start :))

Also, my impression (which may be wrong) is not that the CG limitations changed with the MAX, but that the CG of the empty aircraft itself changed. Those charts don’t show that - you’d need the inputs for a detailed weight and balance calculation.

Also, the MAX CG charts would include whatever impact MCAS has - which maybe was installed in order to extend the forward CG limitations?

Anyways, this sort of talks about the philosophy on CG limits.

http://www.smartcockpit.com/docs/Center_Of_Gravity_Limitations.pdf

ttyymmnn > Jetstreamer 03/11/2019 at 17:02

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No apologies necessary. You’re helping me understand something. Thanks again.

ttyymmnn > Rusty Vandura - www.tinyurl.com/keepoppo 03/11/2019 at 17:04

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Not backwards, just not entirely correct. Admittedly, I was good to go on the history part, but quite shallow in my understanding of the MCAS discussion. There have been some excellent comments to this article, from WilliamsSW, JetStreamer, and others. It’s really more of an issue of a change in the center of lift issue than with the center of gravity. I’m going to edit the piece tonight to reflect what I’ve learned from commenters. 

ttyymmnn > WilliamsSW 03/11/2019 at 17:07

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(%MAC is % of Mean Aerodynamic Chord, for a start)

I actually kind of vaguely understood that. And for me, it’s fascinating to see just how much math is involved in making an airplane fly, and fascinating to see the accrual of knowledge over the history of aviation.

Take a look at the comments in this article from JetStreamer. He confirms what you said about it being a CL issue rather than simply a CG issue, and the MCAS is required by the FAA for the MAX to be certified under existing 737 parameters. Good stuff.

Snuze: Needs another Swede > ttyymmnn 03/11/2019 at 17:12

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Wow. C hilling to read.

But it does answer the question as to how it receives input from the AOA sensors.

It boggles my mind, though, that an  aircraft taxiing on the ground could have a 20 degree difference in the AOA sensors and there’s not alarm or warning or any other indication that something is jacked up.

ttyymmnn > Snuze: Needs another Swede 03/11/2019 at 17:17

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It boggles my mind, though, that an aircraft taxiing on the ground could have a 20 degree difference in the AOA sensors and there’s not alarm or warning or any other indication that something is jacked up

This.

WilliamsSW > ttyymmnn 03/11/2019 at 17:21

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Brilliant stuff from Jetstreamer - needless to say, he knows a hell of a lot more about this than I do. I never thought about the large nacelles providing lift at high AoA, but it all starts to make sense with that in mind.

And yeah, if the aircraft loses longitudinal stability without MCAS, then it’s not airworthy without MCAS.

We’ll see what comes out of the two accident investigations, but it’s easy to envision the MAX getting grounded for a bit, then coming back with a more redundant MCAS that also makes it easier to disable it.

WilliamsSW > Jetstreamer 03/11/2019 at 17:22

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This is brilliant Oppo- thanks! 

ttyymmnn > WilliamsSW 03/11/2019 at 17:23

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I have edited the post to reflect the input from JetStreamer. And yes, it is all starting to make sense now, with both your help and his.

Rusty Vandura - www.tinyurl.com/keepoppo > ttyymmnn 03/11/2019 at 17:40

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I’ve been reading along. If the engines are hanging out front, that would make the airplane nose heavy and the MCAS would be there to help raise the nose. As I understand things.

ttyymmnn > Rusty Vandura - www.tinyurl.com/keepoppo 03/11/2019 at 17:45

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Read JetStreamer’ s comment . Yes, the engine placement moves the CG forward, but there is some relationship to the center of lift that changes, and the nacelles themselves provide a certain amount of lift. This works to move the nose upward when flying manually. I’m still working on my own understanding of it. I have edited the post to reflect JetStreamer’s comments.

WilliamsSW > ttyymmnn 03/11/2019 at 18:22

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Keep in mind that one of the benefits SWA gets from not having assigned seats is that they can swap out one type of 73 for another at the drop of a hat.

Which may come in handy for them if their handful of MAX’s get grounded.

Rusty Vandura - www.tinyurl.com/keepoppo > Jetstreamer 03/11/2019 at 18:29

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That was excellent commentary, and points to what I can only suspect and not put into words. I suspect, and it’s only a guess or a suspicion, that there are engineers and bean counters at Boeing who are not sleeping well right now. If any of what you said proves true, then you know that some engineer(s) at Boeing have voice those concerns and hopefully have documented it.

I’d never considered the nacelles providing lift in a high angle of attack. And if that’s what’s being countered, the little bit of data I’ve seen suggests some sort of oscillation or porpoising that would be taking the aircraft in and out of that zone of what the system is meant to correct.

Like you said, wait to see what comes out, but it does not look good for Boeing.

Rusty Vandura - www.tinyurl.com/keepoppo > ttyymmnn 03/11/2019 at 18:29

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I replied.

Spamfeller Loves Nazi Clicks > ttyymmnn 03/11/2019 at 18:58

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I’ll certainly concede that it isn’t a wholly fair comparison without reservation. But yeah, the real point is that the MD80 family never had any fatality incident that was the result of poor design.

I got curious last night, and went looking on aviation-safety.net. And I found this:

Total loss of power during initial climb due to improper de-icing . 2250kg of fuel per wing. Contact with trees at over 130kt while attempting to land gear-down in a field. Broke off most of a wing. Hard impact. Fuselage broken in three pieces. 129 occupants.

Zero fatalities.

I never liked flying MD80's - because I was always stuck on ones that still had CRTs - but if I’m going to be in an aircraft incident?

I want the one where an accident and fire like THAT leaves 134 survivors out of 1 45.

Spamfeller Loves Nazi Clicks > ttyymmnn 03/11/2019 at 18:58

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OUCH.

P5guy now GTIguy > ttyymmnn 03/11/2019 at 18:59

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Awesome post as usual. What are you thoughts on the focus being in the -8 models and not the -7 or -9 ? Are they so much less common? Or are their shorter and longer bodies less affected by the cg shift? When and if Boeing grounds the -8 I think it'll be the whole MAX family.

ttyymmnn > Spamfeller Loves Nazi Clicks 03/11/2019 at 19:05

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Plus, you can fly them inverted when the jack screw gets jammed. Or so I learned in this documentary.

ttyymmnn > P5guy now GTIguy 03/11/2019 at 19:15

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Well, I can tell you that what I know is in the article above. This is the result of a lot of research, since I needed to teach myself about the MAX 8. The MAX 8 has more than 2,600 orders right now, while the MAX 10, which hasn’t even flown yet, has about 580. The 7 and 9 are far less popular. Not sure why, unless customers are sticking with the Next Gens. Half of the 7's on order are slated for SWA, but there are thousands more orders that haven’t been announced as to variant. I think UA has the most MAX 9s, like this one that I spotted in AUS a couple of months ago.

As to which variants are more or less affected, I can’t say. All the attention is on the 8 right now because of the crashes. If this does turn out to be an issue with the MCAS, I would not be surprised if they ground all the MAXes, since they all h ave the system.

ttyymmnn > P5guy now GTIguy 03/11/2019 at 19:15

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And thank you!

Spamfeller Loves Nazi Clicks > ttyymmnn 03/11/2019 at 20:09

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Fuck, man, if they flew them like that I’d demand  to fly MD82 everywhere!

ttyymmnn > Snuze: Needs another Swede 03/12/2019 at 00:12

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On 28 October 2018, a Boeing 737-8 (MAX) aircraft registered PK-LQP was being operated by PT. Lion Mentari Airlines (Lion Air) as a scheduled passenger flight from I Gusti Ngurah Rai International Airport (WADD), Denpasar to Jakarta as LNI043. During pre-flight check, the PIC [Pilot in Command] discussed with the engineer of the maintenance actions that had been performed including replacement of the AoA sensor and had been tested accordingly.

The aircraft departed at 1420 UTC (2220 LT) at night time, the DFDR [Digital Flight Data Recorder] showed the stick shaker activated during the rotation and remained active throughout the flight. About 400 feet, the PIC noticed on the PFD the IAS [Indicated Airspeed] DISAGREE warning appeared. The PIC handed over control to the SIC [Second in Command] and cross checked the PFDs with the standby instrument and determined that the left PFD had the problem. The PIC noticed the aircraft was automatically trimming AND [Aircraft Nose Down]. The PIC moved the STAB TRIM switches to CUT OUT and the SIC continued the flight with manual trim without auto-pilot until the end of the flight.

The PIC declared “PAN PAN” to the Denpasar Approach controller due to instrument failure and requested to maintain runway heading. The PIC performed three Non-Normal Checklists and none contained the instruction “Plan to land at the nearest suitable airport”.

The remainder of the flight was uneventful and the aircraft landed Jakarta about 1556 UTC. After parking, the PIC informed the engineer about the aircraft problem and entered IAS and ALT Disagree and FEEL DIFF PRESS problem on the AFML.

The engineer performed flushing the left Pitot Air Data Module (ADM) and static ADM to rectify the IAS and ALT disagree followed by operation test on ground and found satisfied. The Feel Differential Pressure was rectified by performed cleaned electrical connector plug of elevator feel computer. The test on ground found the problem had been solved.

At 2320 UTC, (0620 on 29 October 2018 LT), the aircraft departed from Jakarta with intended destination of Pangkal Pinang. The DFDR recorded a difference between left and right AoA of about 20° and continued until the end of recording. During rotation the left control column stick shaker activated and continued for most of the flight.

During the flight the SIC asked the controller to confirm the altitude of the aircraft and later also asked the speed as shown on the controller radar display. The SIC reported experienced „flight control problem.

After the flaps retracted, the DFDR recorded automatic AND trim active followed by flight crew commanded ANU [Aircraft Nose Up]  trim. The automatic AND trim stopped when the flaps extended. When the flaps retracted to 0, the automatic AND trim and flight crew commanded ANU trim began again and continued for the remainder of the flight. At 23:31:54 UTC, the DFDR stopped recording.

Sounds like MCAS was doing its job, but with faulty data. It may well be a combination of a bad sensor, or batch of sensors, and poorly written software.

pip bip - choose Corrour > ttyymmnn 03/12/2019 at 05:43

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quite a few countries now grounding them.

not good. 

ttyymmnn > pip bip - choose Corrour 03/12/2019 at 06:35

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I’m guessing this will turn into a combination of bad sensors and bad software. The sensors may not be Boeing’s fault. The software, however, is all on them.

pip bip - choose Corrour > ttyymmnn 03/12/2019 at 06:37

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my bet is all software

ttyymmnn > pip bip - choose Corrour 03/12/2019 at 06:42

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The flight data recorder of the Lion Air flight showed that the two AoA sensors disagreed by 20 degrees while the plane was taxiing for takeoff. It seems that MCAS read the faulty one, and the pilots experienced a stick shaker immediately after takeoff. A stick shaker is the plane waring the pilots that a stall is imminent, even though they were simply taking off. The pilots of the previous flight had similar erroneous readings, as well as with airspeed. Sounds to me like MCAS was doing its job, but it was doing it with faulty data. However, there should be some way for the system to get data from more than one source alone. 

thejustache > ttyymmnn 03/12/2019 at 12:51

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As a software engineer (and 2 time passenger on a MAX 8 last year) I’m pretty interested in this as well. We know there were two AOA sensors, but we don’t know (or at least I don’t know) how they were used - averaged together or as a primary and backup. The first way 1 bad se ns or could effect the reading by as much as 50%... which seems bad. The second way seems better, but would require some way of determining that one is malfunctioning. I’m assuming there must be some other system also feeding data into the loop that would lead it to question the primary and switch the secondary - but that’s probably a big assumption. If there’s just 2 sensors and no outside data, how can they know which one is right and which one is wrong? I don’t know enough to do anything but guess, but I’m interested to find out more.

At the end of the day, this does seem like trying to fix  a physics problem with a software solution rather than a redesign to avoid the physics problem. If that’s the case I’m not surprised Boeing chose that over certifying a new airframe, but as someone that writes software I’m also a little concerned that that’s even an option... or at least an option so easily taken without being clear about it, offering extra training, and a manual override. 

ttyymmnn > thejustache 03/12/2019 at 13:55

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At the end of the day, this does seem like trying to fix a physics problem with a software solution rather than a redesign to avoid the physics problem.

This is it exactly. In order to get the MAX certified under existing 737 certificates, the FAA required the implementation of the MCAS system since the design of the airliner affected its flight characteristics enough to warrant it. We have fighter jets that are inherently unstable and need FBW computers to keep them in the air. We shouldn’t have that sort of thing in a civilian airliner. But something else that is getting a bit overlooked here is the role of the pilots and their ability, or inability, to fly the plane in an emergency situation.

From a recent NYT article :

Experience is one factor behind regulators’ decisions.

Regulatory standards for pilot experience vary widely from country to country. They also differ considerably from airline to airline within countries.

Pilots in the rapidly expanding aviation markets of East Asia and in developing countries tend to be much less experienced than their counterparts in the West. Li Jian, the deputy director of China’s Civil Aviation Administration, said the agency — the first to ground the 737 Max after the accident Sunday — worried about the challenges that could face pilots if an aircraft had unexpected difficulties.

The biggest worry involves possibly inaccurate signals from key flight instruments, Mr. Li said on Monday. Many pilots with less experience depend heavily on automatic systems to help them fly planes, and such systems in turn need reliable data.

“We are facing uncertainties about whether pilots have the courage or the capability to fly” if an aircraft has difficulties, Mr. Li said.

I think one of the reasons that US regulators have held back is that they believe that US pilots can handle the emergency should it arise. But the FAA are also playing the dual role of aviation regulator and aviation promoter.

gmporschenut also a fan of hondas > Rusty Vandura - www.tinyurl.com/keepoppo 03/12/2019 at 23:58

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777

Airbus patent

https://aviation.stackexchange.com/questions/6399/what-is-this-flange-on-the-engine

Rusty Vandura - www.tinyurl.com/keepoppo > McMike 03/13/2019 at 00:53

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That would be the Omega 13:

gmporschenut also a fan of hondas > ttyymmnn 03/16/2019 at 14:40

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https://theaircurrent.com/aviation-safety/southwest-airlines-is-adding-new-angle-of-attack-indicators-to-its-737-max-fleet/

what is interesting is how Southwest and american have added additional indicator notifying the pilot if there is a likely error with the AOA sensor