Hacker News Reader

Comment by bambax | Hacker News Reader

It's now fairly obvious MCAS was at the root of those two crashes. But, did it save lives in other circumstances?

Meaning, was there ever a case where the nose went up so much, because of the bigger motors and their different location on the wings, that the activation of MCAS prevented a stall? I'm surprised no one seems to talk about this.

It's also surprising the MCAS was implemented at all, instead of an alarm. Yes, the point of MCAS is to make appear the MAX is the same plane as earlier models (when in fact it's not), but if pilots need to learn how to deal with MCAS malfunction, isn't it the same as learning to deal with unexpected stalling during take-off? The whole reasoning sounds kind of circular.

View on HN

You’re missing the point of why MCAS was implemented in the first place - for type certification & to avoid retraining.

Essentially it was an emulation layer to make what is effectively a new plane behave like an old plane. And now 300+ people are dead.

BA dropped the ball - single sensor, and business shortcut to improve margins.

They are liable for the deaths. The real question now is what is a human life worth to a corporation? (i.e. payout).

Next Steps: + BA should be heavily fined. +scrutinize ties and lobbying with FAA. + Fire head of FAA, and teams involved in approving MCAS.

You nailed it. I'd bet my last dollar that there were engineers at Boeing who objected (possibly not loudly enough) to the idea.

That is what it looks like. In some sense, you are not “contrlling” a 737MAX, you are flying a virtual machine emulating the 737 previous gen, whose host is a new generation.

Quite the feat, Boeing.

Calling it an emulation layer is more than slightly disingenuous. MCAS just sits and watches until a specific set of circumstances occur. Only then does it try to trim the nose down. Did they implement it well, no. Is that an "emulation layer", no.

They were going to lose customers to Airbus, who had a longer range plane. The solution was to retrofit larger engines on the 737. But they were too large and collided with the ground. They moved the engines to the front of the wing for ground clearance. This made the plane unstable and tend to tilt upwards, so they came up with MCAS to work around the instability.

The plane is not airworthy.

> It's also surprising the MCAS was implemented at all, instead of an alarm.

Regulations are that "(1) The stick force vs. g curve must have a positive slope at any speed up to and including VFC/MFC; and......(a lot of other text)......During the approach to the stall, the longitudinal control pull force should increase continuously as speed is reduced from the trimmed speed to the onset of stall warning."

The Max's aerodynamics are such that as you approach a stall angle, at a certain point the force inverts and the aircraft "pitches nose up" reducing force against the stick close to the stall angle.

An alarm thus fails to meet the stick force requirement. The intention behind the MCAS is to push force against the stick throughout the approach to the stall angle to meet the requirement.

(Which obviously created a whole host of failure modes that everyone seems to have entirely missed).

> The Max's aerodynamics are such that as you approach a stall angle, at a certain point the force inverts and the aircraft "pitches nose up" reducing force against the stick close to the stall angle.

Which anybody who claims that MAX is safe without MCAS is ignoring: without properly working MCAS the plane effectively starts to misbehave exactly when it shouldn't (when it's harder to save it).

Unfortunately, with improperly working MCAS, the way it was designed (non redundant single sensor which is blindly trusted by MCAS and which is in effect stronger than the pilots fighting against it) there were already two crashes.

>Which anybody who claims that MAX is safe without MCAS is ignoring: without properly working MCAS the plane effectively starts to misbehave exactly when it shouldn't (when it's harder to save it).

MCAS is there to prevent stalls at high thrust and AoA. It's also disabled when flaps are down. Meaning that it's turned off for take offs and landings where you'd most expect high thrust and AoA as well as the worst times to stall. It's hard to square that with MCAS being necessary for safe operation.

As far as I know, MCAS has never activated in service due to actual high AoA. However, even the 737 classic could have "interesting" pitch up characteristics:

"The Boeing 737-300 was on approach to Bournemouth Airport following a routine passenger flight from Faro, Portugal. Early in the ILS approach the auto-throttle disengaged with the thrust levers in the idle thrust position. The disengagement was neither commanded nor recognised by the crew and the thrust levers remained at idle throughout the approach. Because the aircraft was fully configured for landing, the air speed decayed rapidly to a value below that appropriate for the approach. The commander took control and initiated a go-around. During the go-around the aircraft pitched up excessively; flight crew attempts to reduce the aircraft’s pitch were largely ineffective. The aircraft reached a maximum pitch of 44º nose-up and the indicated airspeed reduced to 82 kt. The flight crew, however, were able to recover control of the aircraft and complete a subsequent approach and landing at Bournemouth without further incident."

https://www.gov.uk/aaib-reports/aar-3-2009-boeing-737-3q8-g-...

If that had happened in a Max, without MCAS, would it have ended badly? We'll never know, but it seems possible.

MCAS wouldn't have been active since the flaps were down.

You are, of course, correct. I've not heard anything about whether lift from the nacelles on the Max could be more of an issue than on older 737s in such circumstances. Perhaps it's not an issue at low airspeeds? Would be interesting to know.

> It's now fairly obvious MCAS was at the root of those two crashes. But, did it save lives in other circumstances?

Let's say Boeing ships a new airliner, where in 1/1,000 flights, the wings fall off for no reason.

Would you be asking the question of: "But did the wings save lives in other circumstances?"

Of bloody course, they did. Doesn't mean that they should have ever shipped an aircraft, where the wings fall off for no reason!

MCAS was a hack around a redesign of the plane. If the plane would have crashed at an even higher rate, without that hack, then the conclusion is not: "WOW, MCAS saves lives."

The conclusion is: "That airplane should never have been shipped, because it is a deathtrap."

If it would have crashed at a lower rate, without MCAS, then no, MCAS was not a net life-saver.

Boeing chose to redesign the place, making it more dangerous to operate. To mitigate the danger, they added MCAS, which was also dangerous to operate. It doesn't matter whether or not a MAX with MCAS, or a MAX without MCAS is more dangerous. What matters is that a MAX with MCAS is dangerous. Nobody put a gun to their head, and made them redesign their plane.

> Nobody put a gun to their head, and made them redesign their plane.

The execs at Boeing probably felt the A320 was a gun to their head. The A320 was set to eat their lunch, and they made a decision that ultimately has resulted in 300+ deaths. Was it a malicious decision? I give them enough credit to say that they honestly believed they solved the problem with MCAS. Was it a malicious decision to lie about pilots not needing to re-train? I'm leaning towards yes.

I'm not trying to defend Boeing, I'm trying to understand if MCAS is actually needed in real life.

Reading the other comments it seems the answer is yes.

That said, it's unclear that a Max with MCAS is dangerous: the danger comes from pilots not being aware of it / trained for it. Which, again, doesn't disculp Boeing in any way.

>did it save lives in other circumstances?

It's a bandaid fix for an airframe issue. So yeah it probably saved lives, but ones which shouldn't have been at risk in the first place.

The whole thing wasn't kosher - re-using an old airframe to save costs even though its not really compatible with new engines to rush something to market with reduced certification requirements. The bandaid coming off shouldn't be where the focus should be

Culturally move fast & break things shouldn't be how aircraft go to market

The problem with dealing with a departure stall is they’re extremely deadly, happen faster than a person can react to, and are in general unrecoverable all because of the lack of altitude. If it was the case that you could easily deal with a departure stall in the 737-MAX then they would have seen no need to implement MCAS.

Even if you could recover without MCAS Boeing still wouldn’t have released a version without it as the recovery procedure have needed new training.

Let’s not forget MCAS only exists to protect Boeing’s profits by avoiding a new type certification.

And if Boeing had originally went with a new type certification, the plane they'd build wouldn't need MCAS. The MAX can't exist without something like MCAS, can it?

MCAS only operates with flaps up, though, so it's actually irrelevant during most departure stalls at low altitude.

Are you claiming that the fact that without MCAS, the plane being fundamentally dangerous is a good thing? I am not sure that "MCAS saved lives because that plane is begging to crash otherwise" a big selling point for Boeing.

i didn't take it as them claiming anything. it seemed them just asking a question out of curiosity.

"It's now fairly obvious MCAS was at the root of those two crashes. But, did it save lives in other circumstances?"

Very possible. The question is, should you use a plane with an unstable design in civilian air transport? AFAIK, a stealth bomber can't fly without the help of a computer to keep in in the air. But for a civilian aircraft this should be a very conscious decision and nothing that is hidden under band aid.

If you don't want to fly in a aircraft which requires a computer to fly then avoid anything Airbus and almost all other modern passenger jets. The 737 is literally as close to that ideal as you'll find in service today. If an EMP goes off your are still in trouble though, the engines have FADEC, so they'll probably shut down.

Well, fly by wire (airbus) and impossible to keep the plane in air without a computer (Stealth bomber) are not the same thing.

MCAS is an example of where ugly hacks take lives, not save them. If the plane was properly engineered hundreds of people would be alive, it's not a choice between MCAS and a dangerous plane.

Simply put, commercial imperatives overtook solid engineering practices.

Here's one example on why they might have thought it was necessary: https://en.wikipedia.org/wiki/Air_France_Flight_447

I guess you can't tell. You could see how often it operated but not know whether the pilots would have saved it. That said it's pretty unusual for professional pilots in an airliner to get into a stall situation.

The only time it has been utilized that in the manner it was intended that I'm aware of is when it was exercised during it's test flights.

The desired mode of operation for a plane is to keep flying, without crashing. MCAS did not save anybody. By that logic steering wheels are also saving billions of people because without a steering your car would veer off the road.

Reply on news.ycombinator.com