Comment by mannykannot | Hacker News Reader

Comment by mannykannot | Hacker News Reader

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This question -- if it is a safety feature, why is it acceptable that it can it be disabled -- comes up a lot, so I think we should recognize that, in general, there is an answer, though whether it was the right answer for MCAS goes a step beyond.

Whenever you have something that would usually improve safety, but which presents a risk if it fails, then the rational response is to ask whether it demonstrably improves safety overall. This calculus depends on how much it improves safety when working, how much harm it does when failing, and how likely it is to fail. These considerations can be modified by limitations on operations both when things are working and when they fail, as is the case of twin-engined aircraft use on trans-oceanic flights.

If, in addition, this thing can be disabled, the principle is the same, but there are more cases: what are the chances it be disabled even though working, or (as in this case) not be disabled when failing.

Disabling MCAS does not itself put an airplane in a dangerous situation, it merely increases the risk somewhat. The part of the analysis that seems to have been flawed is that covering the risk introduced when it fails but is left engaged.

MCAS isn’t needed to fly the 737 MAX, you can perform a perfectly fine flight without it — the autopilot directly trims, pilots could fly via trimming themselves using the control on the stick, and there’s a manual wheel for emergencies when the jackscrew motor fails (or is disabled to override MCAS).

Rather, MCAS is needed to fly the 737 MAX like other 737 type aircraft.

MCAS has the capacity to override pilot control via continued trimming; MCAS is not a safety requirement, but can trigger a critical failure; MCAS was not built to critical failure specs; pilots were not trained on MCAS; MCAS is required to maintain the type rating, because otherwise the plane would handle differently during a stall. (This is the only purpose — tipping the nose to fake handling characteristics during a stall.)

It sounds like a reckless design, made out of political rather than safety considerations.

That's a bit of a misunderstanding. MCAS is needed to satisfy the certification requirements for an aircraft under part 25. It only makes the 737 Max more like the 737NG in that it also makes it more like any other certified aircraft.

To put it another way, the alternative to MCAS was not more training for the pilots. It was the 737 Max not passing certification.

This is still a bit unclear to me. It's possible that it was originally intended to provide equivalent handling characteristics to the NG, but after flight testing, the magnitude of the MCAS correction increased by over 4X. It's possible that based on the predicted performance, the MAX would have met FAR 25 but not had equivalent handling to the NG, but after flight testing, MCAS took on a more important role in making the MAX certifiable at all.

Do you have a source?

My information suggests that while all modern aircrafts require trim control for stabilization, particularly near stall, the way that MCAS operates isn’t required. Rather, pilots would simply have to fly slightly differently to control the 737 MAX compared to the 737 NG. However, that difference in stall prevention would require the 737 MAX to be certified fresh, as opposed to the same type as the 737 NG.

In short, that MCAS trimming is only required if you’re not going to train pilots to trim correctly on the new airframe, because you’re trying to assert type compatibility.

I admit it's not as authoritative as I'd like but this is the best source I've found:

> MCAS is a longitudinal stability enhancement. It is not for stall prevention or to make the MAX handle like the NG; it was introduced to counteract the non-linear lift of the LEAP-1B engine nacelles and give a steady increase in stick force as AoA increases. The LEAP engines are both larger and relocated slightly up and forward from the previous NG CFM56-7 engines to accommodate their larger fan diameter. This new location and size of the nacelle cause the vortex flow off the nacelle body to produce lift at high AoA; as the nacelle is ahead of the CofG this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to further increase the back pressure on the yoke and send the aircraft closer towards the stall. This non-linear/reducing stick force is not allowable under FAR §25.173 "Static longitudinal stability". MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching stall.

http://www.b737.org.uk/mcas.htm

However, on Boeing's own website they give the 'makes it fly just like the NG' explanation. On balance of probabilities I think that's unlikely to be the engineering justification.

My reasoning being I don't believe that there is any formal requirement for an aircraft to exhibit the same handling behaviours to be counted on the same type rating. For example the 757 and the 767 shared a common type rating. The 757 was a pilots favourite precisely because it was sporty in comparison to the 767.

This is a good source: https://www.satcom.guru/2019/03/regulations-around-augmentat...

FAR 25 has a number of specific requirements around pitch stability, specifically, FAR 25.255(b)(1) and FAR 25.203(a), and some others. FAR 25.203(a) says "No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall"

My understanding is that on the MAX, without MCAS, once you've pitched up beyond some AoA beyond 12 degrees or so, you can let go of the yoke, and the plane will continue to pitch up further until it stalls. That does not comply with the regs, and so you have MCAS which dials in some nose down trim in this situation to counteract the aircraft's natural tendency to pitch up further.

If the chart presented in the article below [1] is fairly accurate and not merely representational, I think that the 737 MAX, without MCAS, does not actually become statically unstable in pitch at least until after the stall, but the response was still unacceptable, and the reason MCAS was needed for certification.

There is a complication here in that the stick forces are generated by an elevator Feel and Centering Unit, which is fed by a dedicated pitot tube and the stabilizer position.

Pitch instability after the stall was part of the rear-engined jet deep stall problem, and the reason for stick pushers [2].

[1] https://leehamnews.com/2019/02/15/bjorns-corner-pitch-stabil...

[2] https://leehamnews.com/2018/12/07/bjorns-corner-pitch-stabil... Figure 3.

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