https://en.wikipedia.org/wiki/Yaw_damper
Aircraft design is a giant bag of compromises between desirable characteristics, most which are in conflict with each other.
This is a gross oversimplification of the problem. In most of the flight envelope the aircraft is stable.
At high alpha the aircraft has pitch problems.
There are myriad ways to address this, and MCAS was one (bad) choice of many available to Boeing.
That would put you at odds with Boeing's test pilots.
The issue is not that the airplane shows negative or neutral aerodynamic pitch stability, it's that it does not exhibit an increasing stick force gradient, as required by certification rules.
https://airfactsjournal.com/2019/03/can-boeing-trust-pilots/
>> At high alpha the aircraft has pitch problems.
> I believe the airplane is aerodynamically stable even at high alpha. [...] This is not a flying wing or intentionally unstable airplane (as the F-16).
In some ways that's likely worse news than an airplane that's inherently unstable in general, no? A corner case, and one that evidently isn't actually all that uncommon. If you're building something like an F-16 you know that you absolutely have to make the fly-by-wire correct and robust, and the ground crews similarly know that if anything affects its performance the plane isn't fit to fly.
FAR 25 applies to all transport category aircraft. The section on stability (§§ 25.171 - 25.181). In exactly what manner is the airplane not stable, with or without MCAS?
But a few weeks later, Mr. Wilson and his co-pilot began noticing that something was off, according to a person with direct knowledge of the flights. The Max wasn’t handling well when nearing stalls at low speeds.
i still maintain my macro point, either way.
making the airframe on a pax airliner aerodynamically stable during normal takeoff and landing operations seems like basic “good engineering” to me.
Modern fighter planes are inherently unstable because this is required for better maneuverability. Passenger planes can certainly benefit from that too.
Consider gusty crosswinds during a landing. With an inherently unstable aircraft, there is greater capability to compensate. You can have a computer stabilize the plane, preventing the tail or wing tips from striking the ground. When wake turbulence threatens to flip the plane or when a microburst threatens to pound the aircraft into the ground, a fast response is possible. Stability would deaden the performance of the needed response.
The extreme example is probably wings that are low-mounted anhedral and forward-swept, with the bending controlled by rapidly actuated aerodynamic surfaces near the tips.
In exactly what manner is it not obvious this is not an acceptable design outcome?
The problem results from an edge case or it would be happening a lot more often. That it's an edge case doesn't mean it isn't serious or shouldn't be fixed or that it's not a design flaw. But it is not a stability problem, it's the wrong word to use.
It misdirects the conversation from where it should be. The airplane aerodymics are the distraction. The central problem is when perturbed, this feature becomes a saboteur, 2.5 degrees of deflection in 10 seconds is asinine at Vmo. A human pilot acting on all the same information the flight computer has available, would be considered a maniac to correct for a clearly bogus angle of attack value with 40 degrees of nose down. It's that insane. And Boeing knew about the possibility, classified it as hazardous, and yet somehow no further exploration of what would happen upon arrival at such a hazardous event (MCAS upset) by any team at Boeing or 3rd parties or the FAA. It's mindboggling.
Meanwhile some people prefer distractions from those issues by using the wrong terms: it was designed badly, and the whole plane should be scrapped. With the above systemic problems at Boeing and FAA, who knows what kind of airplane they'd design to replace it and what sorts of problems it would or could have.
The whole impetus of the 737 MAX was a race against time to compete. If they had faced a much longer time frame for a whole new model, the pressure to cut corners is even higher. The opportunities to make mistakes are even higher.
You are right on target, but I do wish to point out the aerodynamics are still a problem, and a problem that has caused a great deal of grief in aviation history.
Take a trip down memory lane, and give the D.P. Davies Interview from the Royal Aeronautics Society a listen. Specifically, the one revolving around the 727 certification.
There seems to be two schools of thought to aircraft design. One is the test pilot's wet dream: simple layout, well behaved, neutral stability, or minimal bad behavior up to the corners of the flight envelope, then easily discernable, and recoverable stall behavior.
The other school is the realm of the Engineer. The Tricky Sick school if you will. Apply enough computer and piloting aid to the properly shaped brick, and it can be flown like a 737! Or Airbuses version of "let the plane fly itself, just tell it where to go."
Even as far back as the certification of the 727, test pilot's saw the shift away from the meutrally stable machine that "just flew" to an ever increasing complex mishmash of complex systems working in the background to male unstable airframes fly like naturally (neutrally) stable ones. Which is all fine and good until something goes wrong, and those systems fail, leaving a pilot in uncharted waters.
The control stick force stuff is not a distraction, just another link in the chain of normalization of deviance that resulted in a departure from "building an airworthy frame" to figuring out how to mask the "unairworthyness" of a frame sufficiently so as to get it by the regulators.
That's not to say it can't be done, but one approach is definitely inherently riskier than the other, and requires increased levels of communication among everybody involved.
Point being: this has been built up to since as far back as the 60's. See the 727 certification in above mentioned interview, the many difficulties that the MD-11 ran into with it's LSAS, and note the similar less than stellar results that emerged from trying to optimize for fuel efficiency at the cost of having to implement increasingly complex control system hacks to maintain parity with regulations/previous airframes.
Interview:
https://www.aerosociety.com/news/audio-the-d-p-davies-interv...
that’s a hardware problem.
MCAS only exists because of that hardware problem.
the fact that boeing also did not train or tell pilots about MCAS, in order to make the airplane more financially appealing by retaining the 737 type rating, is a separate (also bad) issue.