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I'm not an aviator by any means, but one thing that has come up in recent conversations with friends is that the MAX8 is inherently unsafe because alone, unpowered, the craft is not 'aerodynamically stable'.

As we understand it, the new engine configuration means that the plane has a tendency to point upwards, requiring software to constantly point the nose down (which also seems kinda sketchy, cuz.. software). I imagine that if there was nothing to run the software, the nose would raise higher and higher until the whole thing was trying to backflip. I understand it wouldn't go totally vertical, but I guess maybe enough to stall or something else just as bad.

Is this true and is this the only aircraft with this kind of 'software or bust' design?

Edit:

My apologies, this question is probably pretty terrible for the majority of the people on this particular stack, but the thought has come up and been validated enough in conversations that I thought to ask folks here to get a better idea and/or be proven wrong (as has happened). I had no idea about phugoid or any of that kind of thing. I only really have "heavier engines cause the craft to pitch upward requiring software to point downward" and "software requires power to run" to base any kind of premise on. I appreciate your patience.

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    $\begingroup$ Unfortunately, your question is based on a flawed premise. See en.wikipedia.org/wiki/Phugoid for a short introduction into the pitch stability of aircraft. $\endgroup$
    – TooTea
    Mar 25, 2019 at 22:18
  • $\begingroup$ Hi, your question has a few ideas which aren't well explained. Maybe addressing them would help answerers. First, are you sure you aren't confusing systems to increase stability in certain scenarios (like the MCAS, a yaw dampener, etc.) with fly-by-wire control of aerodynamically unstable fighter jets? Second, there are a lot of other possible conditions besides unpowered and powered-with-MCAS-engaged that you haven't discussed. Finally, the statement "kinda sketchy, cuz software" as written seems more like a guess than something based on industry experience or a previous incident report. $\endgroup$
    – Cody P
    Mar 25, 2019 at 22:24
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    $\begingroup$ You may also find these questions helpful: aviation.stackexchange.com/questions/1801/… aviation.stackexchange.com/questions/8049/… $\endgroup$
    – Cody P
    Mar 25, 2019 at 22:31
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    $\begingroup$ Yea, I figured I was wrong somewhere, but the premise seemed logical enough to someone who doesn't understand the nuances of aerodynamics or aircraft. I've always held an assumption that aircraft are designed to just fly straight and smooth with only the thrust from the engines and the control surfaces to turn, etc. I was not fully aware that they all have perfectly reasonable flight characteristics that will need to be compensated for. Also, I'm a software developer so thinking about some of the code I've seen used for keeping me in the sky is where that aside was from. $\endgroup$
    – coblr
    Mar 25, 2019 at 22:48
  • $\begingroup$ @codyP, thanks for those super helpful links. I was looking for something along those lines but wasn't pressing the right buttons. $\endgroup$
    – coblr
    Mar 25, 2019 at 22:56

2 Answers 2

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Just to address a misconception

alone, unpowered, the craft is not 'aerodynamically stable'.

I don't believe this is true.

Unpowered

The problem addressed by MCAS is not specifically unpowered flight but flight when the aircraft has a high angle of attack. Because of the need to mount the wider engines further forward to maintain ground clearance, there is a tendency for the engine housing to cause additional aerodynamic forces that rotate the aircraft in the pitch axis - these forces tend to push the nose up. The forces are higher than on prior 737s.

By "unpowered" you might be thinking more of electrical power than engine power. The two are usually related. However any aircraft like the 737 has multiple electric power generators, if the generators on both main engines fail, there is a Auxiliary Power Unit (APU) that can provide electrical power. If both engines and the APU fail, many aircraft have a ram air turbine (RAT) that deploys to produce hydraulic and/or electric power. If both engines, the APU and the RAT fail, there are batteries that provide enough power for minimal necessary controls for a short while, maybe of the order of 30 mins.

I don't know about the 737, but the 30-year-old A320 family of fly-by-wire aircraft has a mechanical backup mode where the pilot can control the aircraft using purely mechanical means and non-electrical instruments. In 30 years I think the closest any aircraft has come to this was a 2005 A319 incident where the cockpit went dark and I believe the pilot flew for a while without any computerised aids, they quickly restored power and continued to their destination. I know of no reason to think the 737 is more dependent on computers for stable flight.

Even without any electrical power, an aircraft like the 737 ought to be no less "aerodynamically stable" than the earliest 737.

Aerodynamically stable

It is normal for aircraft trim to be adjusted during various phases of a flight. The aircraft is still considered 'aerodynamically stable'. When commencing an unpowered glide, the pilots would likely adjust trim. This applies to almost all aircraft.

It is normal that an aircraft in flight, without manual control inputs and without automated inputs from an autopilot etc, can exhibit undesirable flight characteristics such as a phugoid cycle. An aircraft with this sort of behaviour is normally regarded as aerodynamically stable.

I have not read anything to suggest the 737 NG or MAX series are inherently unstable. Even in an unpowered glide.

So far as I know, the only aircraft that are deliberately made inherently unstable are high-agility fighter aircraft.

Maneuvering Characteristics

MCAS stands for Maneuvering Charactersitics Augmentation System. From what I've read, a large part of the motivation for it was to make the aircraft controls feel the same as an older generation 737 - hence the name. Boeing felt that a pilot getting out of an older 737 could get into a 737-MAX and use & experience the same forces on the control yoke as before when climbing out after take off etc.

Related:

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  • $\begingroup$ It’s not quite true that the increased engine power requires MCAS. It’s the different engine geometry that requires it. Due to the engine being further forward, at high angles of attack the nacelle acts like “an extra wing”, providing lift and with it a moment pitching the nose up. This is what MCAS counters (regardless of thrust), not the thrust-pitch-coupling. $\endgroup$
    – Cpt Reynolds
    Mar 26, 2019 at 11:09
  • $\begingroup$ @CptReynolds: Ah I misremembered, I'll correct my answer. Thanks. $\endgroup$
    – RedGrittyBrick
    Mar 26, 2019 at 11:18
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No the software was used to fix a specific characteristic that was problematic if left to natural behaviour in a specific configuration and flight regime. If was a cheaper fix then the alternative, significant redesign with material implications for the physical aircraft.

But in itself this is not such a big deal because all sorts of airliners you are flying in today are totally dependent on software, not just partially dependent like the MAX. So if you want to freak out over software, ride in any of the newer fly-by-wire Airbuses, or the 787.

What seems to have happened here is a typical case of a failure to cover every possible eventuality in how this particular software based solution was applied. We live in an imperfect world, still. The alternative is to stay home and watch TV.

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  • $\begingroup$ I see, thanks for your answer. This was basically what I was looking for but don't know any pilots or aircraft engineers to ask. $\endgroup$
    – coblr
    Mar 25, 2019 at 22:38
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    $\begingroup$ It should be added that (unlike the Airbuses or the 787), the Max is perfectly capable of being flown without the MCAS software in question, as long as the crew stay vigilant enough to fly normal speeds. On the Airbuses or 787, more software is required for them to fly nicely, but then again the level of redundancy and fault tolerance is much higher on their software than on that particular 737 Max MCAS function everyone is keenly aware of now. $\endgroup$
    – Cpt Reynolds
    Mar 25, 2019 at 22:51
  • $\begingroup$ It should be noted that aircraft produced by Airbus can be controlled by pilots even when both engines fail and when the auxiliary power unit fails. They can also be flown in what is known as "direct law" with software safeguards, such as stall-prevention, disabled. $\endgroup$
    – RedGrittyBrick
    Mar 26, 2019 at 9:32
  • $\begingroup$ @RedGrittyBrick True. You still need the computers to run to get direct law, though. On the 737, you don’t need any computers AFAIK. $\endgroup$
    – Cpt Reynolds
    Mar 26, 2019 at 11:10
  • $\begingroup$ @CptReynolds: Sure but there is also mechanical backup - aviation.stackexchange.com/q/20963/1289 $\endgroup$
    – RedGrittyBrick
    Mar 26, 2019 at 11:32

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