[kuzehanka]

This is entirely false. Stop spreading disinformation.

Myth: the 737 MAX 8 is not inherently stable, has relaxed stability, etc. Fact: it's very much inherently stable.

Myth: the 737 MAX 8 is easier to stall than other planes. Fact: no it isn't.

The pitch-up characteristic of the MAX 8 is less strong than of e.g. the 757 and that plane flies just fine.

The actual problem with the MAX 8 is that Boeing added MCAS to allow it to share a type rating with the rest of the 737 family (allowing existing 737 pilots to fly the MAX 8 without additional training), and they fucked up MCAS. There's a number of solutions on the table, including removing rather than fixing MCAS and giving up the 737 type rating.

I am continuously astounded that even on HN people are focusing on news cycle bullshit about inherent instability instead of the actual issues with Boeing/FAA that caused this situation.

Evidence so far suggests that MCAS was originally a non-critical system that was found to be too weak during flight testing, and given significantly more pitch authority. For whatever reasons, this didn't trigger the reclassification of MCAS as a critical system and it all went downhill from there.

Here's a pair of sources slightly more credible than the bullshit news cycle:

https://www.youtube.com/channel/UCphqjYZxxzjNbONVmY-0J7Q

https://www.youtube.com/channel/UCwpHKudUkP5tNgmMdexB3ow

[neuronic]

If a software system has to be designed to make up for aircraft design problems, namely the too large engines that break the limits for ground clearance in the normal 737NG config, then the mentioned issues are not "entirely false" and "disinformation".

Boeing needed to counter the A320neo and they needed to take shortcuts that ended up killing people. I wouldn't be surprised if this were management failure and the actual engineers at Boeing have always been throwing around copious WTFs when building the 737 MAX 8.

"Hey engineers, improve the 737 to be X% more fuel efficient and make sure it doesn't need to be reclassified as new type. Have fun!"

[rbanffy]

> they needed to take shortcuts that ended up killing people.

No. They chose to take shortcuts.

[neuronic]

I stand corrected.

[usrusr]

> I am continuously astounded that even on HN people are focusing on news cycle bullshit about inherent instability instead of the actual issues with Boeing/FAA that caused this situation.

How is it surprising that people are sceptical of claimed native flight characteristics after the way Boeing has handled communications after the crashes? I don't think that there are any independent facts around, even airline pilots who had been flying MAX from day one would not have much (if any) experience at the edges of the flight envelope. One data point we do know is that some people at Boeing were apparently very concerned about MCAS ever being off.

[tjoff]

Skeptical, absolutely! But it doesn't help to make up new facts.

[thefounder]

The engines are too BIG for the airframe! Why people don't get it?

[dingaling]

> The pitch-up characteristic of the MAX 8 is less strong than of e.g. the 757 and that plane flies just fine.

The 757 ( and 737NG etc ) has constant pitch rate. The Max does not, which is the root of the problem.

The actual forces involved aren't pertinent.

[mgsouth]

Ironically, I believe that your information is also inaccurate. MCAS is required in order to meet federal airworthiness requirements. Without it, in certain flight conditions the back-pressure on the control yoke gets less as the yoke is pulled further back. It's like over-steer in a car, and is simply not allowed. Yes, the plane is dynamically stable; if you leave the yoke in one position it won't pitch up even more. However, the forces must be corrected somehow, regardless of type rating concerns.

"The 737 MAX was a bit too easy to pull into a stall when flying with high AoA and making abrupt maneuvers. The larger engines for the MAX hung further forward from the wing, added a destabilizing aerodynamic area ahead of the center of gravity, destabilizing the pitch moment curve at high AoA.

Boeing and the certification authority, FAA, decided added margins was called for. Boeing added a pitch augmentation at high AoA called Maneuvering Characteristics Augmentation System, MCAS.

The aircraft should trim nose down to increase the stick force needed once it passed into the light grey area where the base aircraft had a region of less stability. Before the augmentation, the pilot felt if the aircraft wanted to fly into the stall, it got easier to increase the AoA after 12°AoA. With the augmentation the felt extra force was the same for the first and last part of the curve before the maximum lift was achieved at stall (and stall warning kicked in)." [0]

The manner of the fix (MCAS transparently pushing the nose down) was designed to avoid pilot retraining and thus keep the same type rating.

Edit: The fact that the 737-Max needs a handling tweak is not a failure. Modern planes have all kinds of these tweaks, whether aerodynamic (such as strakes), mechanical (stick shakers) or enabled in software. As the cited article continues: "So far so good. It's common an aircraft’s flight control system has fixes to stability margin changes in different parts of the flight envelope." The problem is that Boeing had a pretty severe collapse of its systems engineering regime.

"The implementation for the 737 MAX had two problems, however:

- The fault checking of the triggering AoA signal was not rigorous enough. This problem has been discussed a lot. No need to add anything.

- The judgment the pilots would identify a problem with the augmentation as a trim runaway and shut the trim off was wrong. Why the pilots didn’t see MCAS rouge actions as a trim runaway is poorly understood."

(The article was published in February. Since then lots of information has come to light about how MCAS determinedly fought correction, and the huge mental and physical loads imposed on the pilots.)

Edit 2: FAA regulation mandating increasing elevator forces for all transport aircraft: FAR §25.253 High-speed characteristics, (a) Speed increase and recovery characteristics, (3):

With the airplane trimmed at any speed up to VMO/MMO [maximum operating airspeed], there must be no reversal of the response to control input about any axis at any speed up to VDF/MDF [maximum airspeed demonstrated in testing]. Any tendency to pitch, roll, or yaw must be mild and readily controllable, using normal piloting techniques. When the airplane is trimmed at VMO/MMO, the slope of the elevator control force versus speed curve need not be stable at speeds greater than VFC/MFC [maximum control airspeed], but there must be a push force at all speeds up to VDF/MDF and there must be no sudden or excessive reduction of elevator control force as VDF/MDF is reached. [1]

[0] https://leehamnews.com/2019/02/08/bjorns-corner-pitch-stabil...

[1] https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se1...

[kuzehanka]

"MCAS is required in order to meet federal airworthiness requirements"

Cite this. Specifically this. The rest of your comment agrees with mine without this being true, and I have not seen any evidence of this being true.

[mgsouth]

Hmm. OK, better wording would be "Handling mitigation such as MCAS is required in order...". I've seen several news articles, such as the one cited, which reported that the yoke forces decreased near stall AoA in certain flight regimes. Here's one from the NYT: [0] Originally MCAS was implemented for rather extreme maneuvers, and required both the AoA sensor and a G-force sensor to agree. Later, it was discovered that low-speed stalls also had yoke-force problems, and the control authority was increased, and the G-force requirement dropped.

[0] https://www.nytimes.com/2019/06/01/business/boeing-737-max-c...

[kuzehanka]

And as stated in countless well cited articles, this was done in order to maintain the 737 type rating and make the MAX 8 handle like other 737. Not to meet regulations that were otherwise unmet.

[nobodyandproud]

I don’t think this is correct.

Your reference suggests that this tweak is to match the expectations of pilots certified to fly 737s, which handle in a certain way.

[mgsouth]

See edit 2, citing regulation which must be met by all transport aircraft. I think also §25.255 Out-of-trim characteristics, (b) (1) The stick force vs. g curve must have a positive slope at any speed up to and including VFC/MFC

[nobodyandproud]

I don’t see how the FAAs assessment that the 737 MAX was easier to put into a stall for greater that 12 degrees AoA is the same as a “reversal of controls” or a “sudden or excessive reduction of elevator control” stipulated in 25.253

What am I missing?

I am not an aviation expert in any sense.

[mgsouth]

"Reversal of control force", not "reversal of controls". There's also section 25.175, which stipulates "The stick force curve must have a stable slope at speeds"... and gives speed ranges for climb, cruise, approach, and landing. For example:

§25.175 Demonstration of static longitudinal stability.

Static longitudinal stability must be shown as follows:

(a) Climb. The stick force curve must have a stable slope at speeds between 85 and 115 percent of the speed at which the airplane—

(1) Is trimmed, with—

(i) Wing flaps retracted;

(ii) Landing gear retracted;

(iii) Maximum takeoff weight; and

(iv) 75 percent of maximum continuous power for reciprocating engines or the maximum power or thrust selected by the applicant as an operating limitation for use during climb for turbine engines; and

(2) Is trimmed at the speed for best rate-of-climb except that the speed need not be less than 1.3 VSR

These are transports, not fighters. The basic idea is that you want it to be harder work to make a sharper maneuver, and want the aircraft to naturally level out. If the force required decreases for a steeper pitch/roll/yaw, then the plane will naturally want to intensify the maneuver. It's like a car with oversteer--let go of the steering wheel and it will make a sharper turn. As the article cited said, the FAA and Boeing's test pilots weren't happy with the yoke forces in certain situations.

[nobodyandproud]

Thanks for clearing that up.

Now is it unusual for sensors and computer controls to help meet the airworthiness requirements?

I’m guessing it’s definitely not preferable.

I also think everyone is agreed that Boeing (the company and decision makers) really fucked up here, but it seems like a chain of bad decisions at every stage has played a part in this disaster.