Another (2nd) 737 Max 8 crash

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#2214044 10-Apr-2019 05:19

The trim motor (pdf here) drives the jack-screw through a reduction gearbox, on the side of which is the pulley for the manual (cabled) trim wheel. A torque clutch between the motor and gearbox allows the manual trim pulley to have priority in the system.

It does indeed operate at two speeds – faster with flaps down (when low speed requires faster trim movements) slower (1/3 that rate) with flaps retracted (higher airspeed assumed - smaller adjustments required)
(Edit: controlled by a speed change relay in the STM)

The slower speed is the one MCAS operates at – as MCAS only operates in manual, flaps up flight.
MCAS in operation moves the horizontal stabiliser trim .27° per second, up to 2.5° and 9.26 seconds per activation.
(Edit: and repeats after 5 seconds if activation conditions remain)

The yoke mounted electric trim switches (theoretically) move the trim at the same speed.

In comparison (I read on PPRuNe forum that) one turn of the manual wheel equals about 0.07 trim units, so requires 15 rotations per unit of trim.
So MCAS trims down at 0.27 units per second, or 3.5 rotations of the manual wheel per second.

That's a lot of manual winding to reverse every 9 second MCAS activation.

In addition, compared to earlier iterations Boeing had reduced the trim wheel size on the NG & MAX (to clear the new display panels) reducing it's mechanical advantage.

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#2214124 10-Apr-2019 08:48

Summary: The corporate bosses did everything they could to avoid the expense of recertification, and the bother with retraining pilots.

Another great example of playing the system, and short term gains overriding everything else

From the NY Times:

Pilots start some new Boeing planes by turning a knob and flipping two switches.

The Boeing 737 Max, the newest passenger jet on the market, works differently. Pilots follow roughly the same seven steps used on the first 737 nearly 52 years ago: Shut off the cabin’s air-conditioning, redirect the air flow, switch on the engine, start the flow of fuel, revert the air flow, turn back on the air conditioning, and turn on a generator.

The 737 Max is a legacy of its past, built on decades-old systems, many that date back to the original version. The strategy, to keep updating the plane rather than starting from scratch, offered competitive advantages. Pilots were comfortable flying it, while airlines didn’t have to invest in costly new training for their pilots and mechanics. For Boeing, it was also faster and cheaper to redesign and recertify than starting anew.

But the strategy has now left the company in crisis, following two deadly crashes in less than five months. The Max stretched the 737 design, creating a patchwork plane that left pilots without some safety features that could be important in a crisis — ones that have been offered for years on other planes. It is the only modern Boeing jet without an electronic alert system that explains what is malfunctioning and how to resolve it. Instead pilots have to check a manual.

The Max also required makeshift solutions to keep the plane flying like its ancestors, workarounds that may have compromised safety. While the findings aren’t final, investigators suspect that one workaround, an anti-stall system designed to compensate for the larger engines, was central to the crash last month in Ethiopia and an earlier one in Indonesia.

The Max “ain’t your father’s Buick,” said Dennis Tajer, a spokesman for the American Airlines pilots’ union who has flown the 737 for a decade. He added that “it’s not lost on us that the foundation of this aircraft is from the ’60s.”

The Max, Boeing’s best-selling model, with more than 5,000 orders, is suddenly a reputational hazard. It could be weeks or months before regulators around the world lift their ban on the plane, after Boeing’s expected software fix was delayed. Southwest Airlines and American Airlines have canceled some flights through May because of the Max grounding.

The company has slowed production of the plane, putting pressure on its profits, and some buyers are reconsidering their orders. Shares of the company fell over 4 percent on Monday, and are down 11 percent since the Ethiopia crash.

“It was state of the art at the time, but that was 50 years ago,” said Rick Ludtke, a former Boeing engineer who helped design the Max’s cockpit. “It’s not a good airplane for the current environment.”

The 737 has long been a reliable aircraft, flying for decades with relatively few issues. Gordon Johndroe, a Boeing spokesman, defended the development of the Max, saying that airlines wanted an updated 737 over a new single-aisle plane and that pilots were involved in its design.

“Listening to pilots is an important aspect of our work. Their experienced input is front-and-center in our mind when we develop airplanes,” he said in a statement. “We share a common priority — safety — and we listen carefully to their feedback.”

Boeing’s chief executive, Dennis Muilenburg, said in a statement on Friday that the crashes in Indonesia and Ethiopia appeared to have been caused by the Max’s new anti-stall system. “We have the responsibility to eliminate this risk, and we know how to do it,” he said.

At a factory near Seattle on Jan. 17, 1967, flight attendants christened the first Boeing 737, smashing champagne bottles over its wing. Boeing pitched the plane as a smaller alternative to its larger jets, earning it the nickname the “Baby Boeing.”

Early on, sales lagged Boeing’s biggest competitor, McDonnell Douglas. In 1972, Boeing had delivered just 14 of the jets, and it considered selling the program to a Japanese manufacturer, said Peter Morton, the 737 marketing manager in the early 1970s. “We had to decide if we were going to end it, or invest in it,” Mr. Morton said.

Ultimately, Boeing invested. The 737 eventually began to sell, bolstered by airline deregulation in 1978. Six years later, Boeing updated the 737 with its “classic” series, followed by the “next generation” in 1997, and the Max in 2017. Now nearly one in every three domestic flights in the United States is on a 737, more than any other line of aircraft.

Each of the three redesigns came with a new engine, updates to the cabin and other changes. But Boeing avoided overhauling the jet in order to appease airlines, according to current and former Boeing executives, pilots and engineers, some of whom spoke on the condition of anonymity because of the open investigations. Airlines wanted new 737s to match their predecessors so pilots could skip expensive training in flight simulators and easily transition to new jets.

Boeing’s strategy worked. The Federal Aviation Administration never required simulator training for pilots switching from one 737 to the next.

“Airlines don’t want Boeing to give them a fancy new product if it requires them to retrain their pilots,” said Matthew Menza, a former 737 Max test pilot for Boeing. “So you iterate off a design that’s 50 years old. The old adage is: If it’s not broke, don’t fix it.”

It did require engineering ingenuity, to ensure a decades-old jet handled mostly the same. In doing so, some of the jet’s one-time selling points became challenges.

For instance, in the early years of the 737, jet travel was rapidly expanding across the world. The plane’s low-slung frame was a benefit for airlines and airports in developing countries. Workers there could load bags by hand without a conveyor belt and maintain the engines without a lift, Mr. Morton said. In the decades that followed, the low frame repeatedly complicated efforts to fit bigger engines under the wing.

By 2011, Boeing executives were starting to question whether the 737 design had run its course. The company wanted to create an entirely new single-aisle jet. Then Boeing’s rival Airbus added a new fuel-efficient engine to its line of single-aisle planes, the A320, and Boeing quickly decided to update the jet again.

“We all rolled our eyes. The idea that, ‘Here we go. The 737 again,’” said Mr. Ludtke, the former 737 Max cockpit designer who spent 19 years at Boeing.

“Nobody was quite perhaps willing to say it was unsafe, but we really felt like the limits were being bumped up against,” he added.

Some engineers were frustrated they would have to again spend years updating the same jet, taking care to limit any changes, instead of starting fresh and incorporating significant technological advances, the current and former engineers and pilots said. The Max still has roughly the original layout of the cockpit and the hydraulic system of cables and pulleys to control the plane, which aren’t used in modern designs. The onboard computers have the processing power of 1990s home computers. A Boeing spokesman said the aircraft was designed with an appropriate level of technology to ensure safety.

When engineers did make changes, it sometimes created knock-on effects for how the plane handled, forcing Boeing to get creative. The company added a new system that moves plates on the wing in part to reduce stress on the plane from its added weight. Boeing recreated the decades-old physical gauges on digital screens

As Boeing pushed its engineers to figure out how to accommodate bigger, more fuel-efficient engines, height was again an issue. Simply lengthening the landing gear to make the plane taller could have violated rules for exiting the plane in an emergency.

Instead, engineers were able to add just a few inches to the front landing gear and shift the engines farther forward on the wing. The engines fit, but the Max sat at a slightly uneven angle when parked.

While that design solved one problem, it created another. The larger size and new location of the engines gave the Max the tendency to tilt up during certain flight maneuvers, potentially to a dangerous angle.

To compensate, Boeing engineers created the automated anti-stall system, called MCAS, that pushed the jet’s nose down if it was lifting too high. The software was intended to operate in the background so that the Max flew just like its predecessor. Boeing didn’t mention the system in its training materials for the Max.

Boeing also designed the system to rely on a single sensor — a rarity in aviation, where redundancy is common. Several former Boeing engineers who were not directly involved in the system’s design said their colleagues most likely opted for such an approach since relying on two sensors could still create issues. If one of two sensors malfunctioned, the system could struggle to know which was right.

Airbus addressed this potential problem on some of its planes by installing three or more such sensors. Former Max engineers, including one who worked on the sensors, said adding a third sensor to the Max was a nonstarter. Previous 737s, they said, had used two and managers wanted to limit changes.

“They wanted to A, save money and B, to minimize the certification and flight-test costs,” said Mike Renzelmann, an engineer who worked on the Max’s flight controls. “Any changes are going to require recertification.” Mr. Renzelmann was not involved in discussions about the sensors.

The Max also lacked more modern safety features.

Most new Boeing jets have electronic systems that take pilots through their preflight checklists, ensuring they don’t skip a step and potentially miss a malfunctioning part. On the Max, pilots still complete those checklists manually in a book.

A second electronic system found on other Boeing jets also alerts pilots to unusual or hazardous situations during flight and lays out recommended steps to resolve them.

On 737s, a light typically indicates the problem and pilots have to flip through their paper manuals to find next steps. In the doomed Indonesia flight, as the Lion Air pilots struggled with MCAS for control, the pilots consulted the manual moments before the jet plummeted into the Java Sea, killing all 189 people aboard.

“Meanwhile, I’m flying the jet,” said Mr. Tajer, the American Airlines 737 captain. “Versus, pop, it’s on your screen. It tells you, This is the problem and here’s the checklist that’s recommended.”

Boeing decided against adding it to the Max because it could have prompted regulators to require new pilot training, according to two former Boeing employees involved in the decision.

The Max also runs on a complex web of cables and pulleys that, when pilots pull back on the controls, transfer that movement to the tail. By comparison, Airbus jets and Boeing’s more modern aircraft, such as the 777 and 787, are “fly-by-wire,” meaning pilots’ movement of the flight controls is fed to a computer that directs the plane. The design allows for far more automation, including systems that prevent the jet from entering dangerous situations, such as flying too fast or too low. Some 737 pilots said they preferred the cable-and-pulley system to fly-by-wire because they believed it gave them more control.

In the recent crashes, investigators believe the MCAS malfunctioned and moved a tail flap called the stabilizer, tilting the plane toward the ground. On the doomed Ethiopian Airlines flight, the pilots tried to combat the system by cutting power to the stabilizer’s motor, according to the preliminary crash report.

Once the power was cut, the pilots tried to regain control manually by turning a wheel next to their seat. The 737 is the last modern Boeing jet that uses a manual wheel as its backup system. But Boeing has long known that turning the wheel is difficult at high speeds, and may have required two pilots to work together.

In the final moments of the Ethiopia Airlines flight, the first officer said the method wasn’t working, according to the preliminary crash report. About 1 minute and 49 seconds later, the plane crashed, killing 157 people.

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#2214131 10-Apr-2019 09:12

Sidestep:

The trim motor (pdf here) drives the jack-screw through a reduction gearbox, on the side of which is the pulley for the manual (cabled) trim wheel. A torque clutch between the motor and gearbox allows the manual trim pulley to have priority in the system.

It does indeed operate at two speeds – faster with flaps down (when low speed requires faster trim movements) slower (1/3 that rate) with flaps retracted (higher airspeed assumed - smaller adjustments required)
(Edit: controlled by a speed change relay in the STM)

The slower speed is the one MCAS operates at – as MCAS only operates in manual, flaps up flight.
MCAS in operation moves the horizontal stabiliser trim .27° per second, up to 2.5° and 9.26 seconds per activation.
(Edit: and repeats after 5 seconds if activation conditions remain)

The yoke mounted electric trim switches (theoretically) move the trim at the same speed.

In comparison (I read on PPRuNe forum that) one turn of the manual wheel equals about 0.07 trim units, so requires 15 rotations per unit of trim.
So MCAS trims down at 0.27 units per second, or 3.5 rotations of the manual wheel per second.

That's a lot of manual winding to reverse every 9 second MCAS activation.

In addition, compared to earlier iterations Boeing had reduced the trim wheel size on the NG & MAX (to clear the new display panels) reducing it's mechanical advantage.

The "manual winding" is only required when the CUTOUT switches are activated. Operation of the electric trim by using the control wheel switches would have reversed any of the MCAS inputs in the same time frame as it took the MCAS to apply those inputs.

The more I read about this accident the more I'm convinced the MCAS is being used as a bit of a scapegoat. There are other issues that are also significant contributing factors.

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#2214227 10-Apr-2019 10:03

kingdragonfly

Not a bad article considering it was obviously written by a someone with a lack of aviation experience. Some of the terminology used would not be used by anyone with aviation experience, even if they were putting it in laymens terms.

The reader should also remember many of the quoted comments by ex Boeing employees are just their opinion. There will be equally opposing opinions. I'm not making a judgement on which party is correct, just to point out some of the comments are not fact, they are opinion.

There are some parts of that article which deserve comment.

To compensate, Boeing engineers created the automated anti-stall system, called MCAS, that pushed the jet’s nose down if it was lifting too high. The software was intended to operate in the background so that the Max flew just like its predecessor. Boeing didn’t mention the system in its training materials for the Max.

MCAS is not an anti stall system.

MCAS is fitted to meet design stability requirements. The positioning of the engines and the shape of the engine cowls meant that at high angles of attack the stick forces would become lighter than previous 737 models and also if the stick pressure being applied by the pilot was released the nose would not drop as is required by the design rules, (the aircraft wasn't dangerous or difficult to control but didn't meet the longitudinal stability requirements specified in the design rules).

MCAS was installed to restore stick pressure and meet longitudinal stability requirements.

It is the only modern Boeing jet without an electronic alert system that explains what is malfunctioning and how to resolve it. Instead pilots have to check a manual.

Electronic systems don't necessarily bring advantages. These electronic systems have their achilles heel as well. Ask the guys flying QF32.

Electronic checklists do not do away with knowing your systems, knowing your memory items and remembering basic flying skills.

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#2214340 10-Apr-2019 12:37

Re. the ease of starting engines in the new Boeings...

Some of the new tech in planes makes the pilots' jobs a lot more pleasant. For example, I was watching a cockpit view video of a A380 landing at Heathrow. After turning off the runway one pilot is heard saying 'After landing checklist' then just a few seconds later you hear 'After landing checklist complete'. No calling out of items at all. So it seems that the after landing checklist in a A380 is either totally automatic or close to being automatic.

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#2214350 10-Apr-2019 13:00

kingdragonfly:
The onboard computers have the processing power of 1990s home computers.

While I found the majority of that article reasonably interesting, the above quote annoyed me for some reason.

In itself, it may be 100% accurate, but without any context it feels as is the author is implying, perhaps emotively, that the computers should have more processing power.

Purely my opinion, but if '90s era processing power is enough to fly the plane, why do they need to have any more?

From the perspective of someone who isn't involved in the aviation industry, it's very interesting to read the commentary here, but that sentence irked me for some reason.

(fully acknowledging that it was a quoted article from the NY Times)

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#2214464 10-Apr-2019 16:17

amiga500:
Re. the ease of starting engines in the new Boeings...

Some of the new tech in planes makes the pilots' jobs a lot more pleasant. For example, I was watching a cockpit view video of a A380 landing at Heathrow. After turning off the runway one pilot is heard saying 'After landing checklist' then just a few seconds later you hear 'After landing checklist complete'. No calling out of items at all. So it seems that the after landing checklist in a A380 is either totally automatic or close to being automatic.

By their very nature checklists are not automatic. However automation and technology can reduce the length of checklists.

The silent completion of some checklists is not uncommon. Other checklists that contain safety critical items generally require verbalisation of the action and a verbal response.

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#2214534 10-Apr-2019 17:24

As far as I’m aware, both the after take off and the after landing checklists on Airbuses are performed silently.
Ones that check flight critical configurations, such as the before take off and the landing checklists are challenge and response.
I have long wondered why Boeing hasn’t developed a single aisle airliner by scaling down the 787 and all they have learned from that.

“We’ve arranged a society based on science and technology, in which nobody understands anything about science technology. Carl Sagan 1996

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#2215083 11-Apr-2019 15:26

Dingbatt:
I have long wondered why Boeing hasn’t developed a single aisle airliner by scaling down the 787 and all they have learned from that.

Time and money, from what I can gather.

They didn´t want to have re-certifications for the plane or the pilots, and they didn´t want to use the money to develop it. And they didn´t want to loose the money on being late to market.

Its all about the $.

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#2215105 11-Apr-2019 16:05

Much of Boeing's free cash flow for the last several years has been spent on buying back their own shares.
This has elevated the share price and - purely coincidentally, you understand - thereby sharply increased the value of share options owned by senior executives. I understand that a significant proportion of some top-level managers' 'performance incentive remuneration' is also directly linked to share price.

A lot of the remaining money has been spent on debt reduction and dividends.
Relatively little (by Boeing historical standards) has been invested in commercial R&D that would produce the New Small Airplane B737-replacement.

Short-term financial objectives have been favoured ahead of longer-term product strategy.

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#2215404 12-Apr-2019 05:21

Sidestep:
The trim motor (pdf here) drives the jack-screw through a reduction gearbox, on the side of which is the pulley for the manual (cabled) trim wheel. A torque clutch between the motor and gearbox allows the manual trim pulley to have priority in the system.

It does indeed operate at two speeds – faster with flaps down (when low speed requires faster trim movements) slower (1/3 that rate) with flaps retracted (higher airspeed assumed - smaller adjustments required)
(Edit: controlled by a speed change relay in the STM)

The slower speed is the one MCAS operates at – as MCAS only operates in manual, flaps up flight.
MCAS in operation moves the horizontal stabiliser trim .27° per second, up to 2.5° and 9.26 seconds per activation.
(Edit: and repeats after 5 seconds if activation conditions remain)

The yoke mounted electric trim switches (theoretically) move the trim at the same speed.

In comparison (I read on PPRuNe forum that) one turn of the manual wheel equals about 0.07 trim units, so requires 15 rotations per unit of trim.
So MCAS trims down at 0.27 units per second, or 3.5 rotations of the manual wheel per second.

That's a lot of manual winding to reverse every 9 second MCAS activation.

In addition, compared to earlier iterations Boeing had reduced the trim wheel size on the NG & MAX (to clear the new display panels) reducing it's mechanical advantage.

A big Question will be exactly how the trim motor has been wired. Assuming it is a DC motor, its direction of rotation would be changed simply by changing the polarity of the power applied to the motor. But a major issue is that the 2 pilots both have their own trim switches. Then there is MCAS, and also the speed trim and other systems that also control trim. So the system has to be able to handle a situation where the motor is commandeered to both lower and raise trim at the same time.

The circuit is probably based on the H bridge circuit. A poorly designed implementation of the H bridge circuit would cause a direct short circuit and blow the fuses. A simple way of avoiding this is to use relays with multiple sets of contacts, so that a short cannot happen. But depending on the design, trying to command that the motor be both raised and lowered at the same time, will either mean that the motor wont do anything, or whichever command is first will be carried out.

And then there is the control circuits as part of the H bridge circuit. Have all of the possible input sources been given the same priority? (easiest circuit to implement) Or do some of the sources have the ability to override others? And how exactly has MCAS been designed to operate the trim motor? It was said earlier that MCAS adjusted the trim in 2.5deg steps. Is this implemented by switching on the motor, and just waiting until the position sensor reports that trim has been lowered by 2.5deg Or is there also a timeout that would cancel the “lower trim” command. If the trim hasn't been lowered after a certain time?

If there is no timeout, and assuming that the system has been designed to simply “do nothing” if both raise and lower commands are attempted. Then even if the pilots hold the “raise trim” buttons, they would be unable to override MCAS using the buttons on their control yokes.

Anyone got a circuit diagram for the 737 max trim control motor?

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#2215405 12-Apr-2019 05:29

PolicyGuy:

Much of Boeing's free cash flow for the last several years has been spent on buying back their own shares.
This has elevated the share price and - purely coincidentally, you understand - thereby sharply increased the value of share options owned by senior executives. I understand that a significant proportion of some top-level managers' 'performance incentive remuneration' is also directly linked to share price.

A lot of the remaining money has been spent on debt reduction and dividends.

I don't know if faulting Boeing for that particular issue's fair.
Market conditions that support debt repayment & share buyback (low interest rates, the recent US Tax overhauls, wads of cash) aren't Boeing's doing.

Many companies (from Buffet's Berkshire Hathaway - nearly US$1 billion, Apple at $100 billion!, to Cisco - $25-billion, and Wells Fargo - $22 billion) are buying back their own shares, so it's probably more a market wide response to regulatory, tax and market conditions, rather than a particular scheme to enrich Boeing executives...

Recent Insider Trading stats don't show executives engaging in pump-and-dump activities, and as a BA shareholder myself, what floats director's boats floats my share of the boat too.

We'd all like to see share valuations based on business fundamentals. Boeing's still a huge, diverse – and profitable - company with it's fingers in many pies (including some, in their Defense, Space & Security operations, that might pose an 'ethical investing' dilemma).

What matters to me when I invest in a company (apart from the ethicality of their business) is that I have a basic understanding of what they do – and would use their products myself. I'm a Boeing fanboi - I admire and fly on their aircraft, ogle their shiny spaceships and use data transmitted through their satellites.

I also don't see where they've cut back massively on research.
The airline industry's one where the major players leapfrog each other in development, production and sale of new, more efficient aircraft.

Boeing's continued production of (an updated) tried and true aircraft while r&d'ing other entirely new products such as a mid-market airplane (NMA) to be a widebody 757 (or even Airbus A321 NEO) replacement with over 200 seats and range of 5,000 nm was, at the time, seen as a good business decision.

Their manufacturing move - from in-house aircraft manufacture to head of a global supply chain - shifted major systems production, and that value, away from Boeing to suppliers (who now collect something like 70% of profits generated through the chain) and leaves Boeing Aircraft as a design, marketing (and final assembly) business. Those suppliers now have skin in the game too (eg. Eaton who make the - initially suspect - trim motor system).

While the initial reporting of these crashes as purely a design fault took me aback, a bit of research has shown that the fault doesn't lie exclusively with Boeing's aircraft, and doesn't appear to be a systemic fault in their design process. So I'll remain a believer and keep buying their shares on the dips..

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#2215406 12-Apr-2019 05:54

Aredwood:

Anyone got a circuit diagram for the 737 max trim control motor?

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#2215553 12-Apr-2019 09:52

Aredwood:

A big Question will be exactly how the trim motor has been wired. Assuming it is a DC motor, its direction of rotation would be changed simply by changing the polarity of the power applied to the motor. But a major issue is that the 2 pilots both have their own trim switches. Then there is MCAS, and also the speed trim and other systems that also control trim. So the system has to be able to handle a situation where the motor is commandeered to both lower and raise trim at the same time.

The circuit is probably based on the H bridge circuit. A poorly designed implementation of the H bridge circuit would cause a direct short circuit and blow the fuses. A simple way of avoiding this is to use relays with multiple sets of contacts, so that a short cannot happen. But depending on the design, trying to command that the motor be both raised and lowered at the same time, will either mean that the motor wont do anything, or whichever command is first will be carried out.

And then there is the control circuits as part of the H bridge circuit. Have all of the possible input sources been given the same priority? (easiest circuit to implement) Or do some of the sources have the ability to override others? And how exactly has MCAS been designed to operate the trim motor? It was said earlier that MCAS adjusted the trim in 2.5deg steps. Is this implemented by switching on the motor, and just waiting until the position sensor reports that trim has been lowered by 2.5deg Or is there also a timeout that would cancel the “lower trim” command. If the trim hasn't been lowered after a certain time?

If there is no timeout, and assuming that the system has been designed to simply “do nothing” if both raise and lower commands are attempted. Then even if the pilots hold the “raise trim” buttons, they would be unable to override MCAS using the buttons on their control yokes.

Anyone got a circuit diagram for the 737 max trim control motor?

The basic components of trim system have been in use for many many years, so it's reasonable to assume Boeing have the control circuit design well sorted out.

I cannot tell from the diagrams how its achieved but it is usual for one trim switch to have priority to guard against a null output where one pilot trims up and the other trims down. That's certainly been my experience. In some situations there is also a separate "override" switch to override the switch that has priority.

The control wheel switches override the MCAS.

As I said in an other post, I believe MCAS is being used as a scapegoat. In my opinion and that I've read of others there is nothing substantially wrong with the MCAS system.

The problems were with the angle of attack sensors used to drive the MCAS system. The same sensors are used to drive the stick shaker system, which as also operating when it shouldn't have been due to the sensor fault. In one accident there had been maintenance carried out which involved the angle of attack sensors which may have created a fault with the sensors and in the second accident there has been speculation a bird strike damaged a sensor.

There are bigger issues around training and the understanding of the trim system and associated components and knowing what to do in the event of a failure of the various systems involved.

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#2215624 12-Apr-2019 11:16

MCAS shouldn't exist, It's solving a problem that shouldn't have been allowed to exist, It also shouldn't be used with a dual AOA system given that when you have an AOA error it turns into a murder machine. Here are some other pilots takes on it:

"thats not at all what I was getting at. i am going to need to unpack my understanding of the full background to explain:

1. facing the prospect of losing 737 sales to the A320neo (up-engined A320), boeing had a problem: they could not mount larger diameter engines to a 737 due to its low wing. the 737 is a 1960s airframe originally designed for narrow turbojets, and boeing had already pushed engine diameter about as far as it could go in the 1980s when converting it to turbofans.

so, boeing initially planned to take the hit and design an all new airframe. 737 customers told boeing they’d just go buy airbuses instead.

that led to

2. boeing came up with a flawed plan to push the envelope of what kind of modifications can be made to an airframe and its flight handling characteristics without making it unsafe. they wanted to sell airlines and the faa on the idea that, for type certification and training purposes, the 737max was just another 737. also it had to be a rush job.

but,

3. boeing knew that moving the engine mount point way forward and up (to get enough ground clearance) was going to produce a strong nose-up torque, doing bad things to handling and stall risk at high AoA, making the plane potentially very dangerous.

so,

4. they invented MCAS. it’s intended to automate the nose down control input required to recover the airplane from flight regimes that aren’t dangerous on other 737 models. they claimed that MCAS would only trigger in extreme circumstances, so pilots didn’t need much training for it

and then,

5. by running so much of the type certification themselves they managed to fool themselves into thinking mcas was safe and working as planned

so,

6. pilots round the world started flying the 737max without adequate training on mcas, its potential failure modes, emergency procedures, how to disable, etc. boeing claimed it was just another 737 after all, and the faa rubberstamped it, and they’re both trustworthy right? boeing even went so far as to sell a mcas related warning indicator as an extra cost option so how serious could it be?

and last but not least,

7. they weren’t trustworthy. turns out there are some pretty awful design flaws in the mcas system

so, far from believing the pilots were at fault or invoking racist stereotypes, i think that boeing and the faa have blood on their hands. the pilots were in effect poorly trained on the type, but not due to any failing on their part. sorry for my op compacting all that into a post without nuance adequate to distinguish it from “lol third world pilots amirite???”. i believe these things are all true:

1. they weren’t trained adequately
2. it was in no way their fault, or related to where in the world they were from
3. they did the best they could given the resources they had
4. the primary cause of the accidents was bad automation, the role of training here is only that better training would’ve given them a better chance of surviving the murderous robot"

and some good info on the manual trim wheels:

the MCAS system can command the stabilizer to trim down by 2.5 degrees every 15 seconds. (10 seconds to trim down, then a 5 second wait before trying again).

if MCAS is faulty, you have to turn off the electric trim motors because otherwise MCAS will keep pushing the trim down like that every 15 seconds continuously.

the stabilizer can rotate for trim through a range of 17.1 degrees.

when the motors are off, trim is effected through the manual wheels, which are the black and white wheels on the outside here:

in manual mode, it takes 250 full rotations of the wheel to move from full up to full down trim (17.1 degrees). that is, one rotation of the wheel corresponds to 0.069 (nice) degrees of rotation.

so, to manually recover from MCAS' trim command you would have to spin the wheel 40 times to match the nose-down commands being generated by the computer

do you think you could do that? how about with one hand, while trying to hold the yoke back with the other? how about when the wheel is actively fighting you, because you're trying to push a 15-meter-wide airfoil against a 300mph slipstream? like, as a human factors question, you don't even really need a professional ergonomist to tell you that it's an unreasonable demand, right?"

and finally on documentation:

"Saying “MCAS is poorly documented” is a gross understatement. Parts of it are entirely undocumented and for other parts, the documents lie. Boeing said MCAS can command maximum downward trim of 0.6°. It turns out the real figure is 2.5°."

Most problems are the result of previous solutions...

All comment's I make are my own personal opinion and do not in any way, shape or form reflect the views of current or former employers unless specifically stated

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