In what world is scrapping the airframes due to a (serious) software fault the best and most sensible solution? Do you believe there could be undiagnosed problems with the wings, fuselage, tail, hydraulics, electrics, fueling system, gear, etc.?
the airplane did not have stable flight characteristics because of its physical design.
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.
Fun fact. The original 737 actually had two yaw dampeners because they expected it to have diverging yaw stability. Just by chance the airframe is actually positively stable so can operate safely with no yaw dampeners so one was removed.
That's the devilish thing, I think. Making the plane unstable then patching things up with MCAS, or something else, isn't a fault in any straightforward way. But it is taking a risk, and the source of risk isn't simply the risk that eg. MCAS will fail on any given flight, it's also the higher-order risk that eg. the MCAS system will have a faulty design. (A bit like rewriting code in C for performance, or keeping on generally unloved code to satisfy a paying customer, presumably.) I assume that makes the ethical and safety decisions a lot more murky for the engineers than having a specific fault to point to, especially in an organisation afflicted with go fever.
I believe the airplane is aerodynamically stable even at high alpha. What it fails in certification is the requirement to have continually increasing pitch feedback forces. I believe the pitch feedback forces are still in the stable region, just too low. This is not a flying wing or intentionally unstable airplane (as the F-16).
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.
>> [...] In most of the flight envelope the aircraft is stable.
>> 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 haven’t looked at the fars in quite a few years, but i’m pretty sure there would be stuff in there that references stuff like pitch stability (which is how i’d define the “hardware problem”), is that not the case? i’m afk atm.
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.
I'm not so sure that it is good engineering to be inherently stable during normal takeoff and landing operations.
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.
A common fallacy I'm afraid - modern fighter planes are unstable for improved supersonic performance - reduced drag. So relaxed stability may benefit civil aircraft in terms of fuel efficiency. There is an argument that inherently unstable aircraft makes the manoeuvrability worse or harder. What many forget is that issue with manoeuvrability is actually at the end of the manoeuvre. The handling qualities aim is to point the nose in a new direction and an unstable aircraft makes the design challenge harder to stop the aircraft at the position required.
You are confusing stability, which is a specific aerodynamic term, with two examples of catastrophic outcomes. Reread the original post I replied to, none of the first three sentences are true: unstable flight characteristics, instability is a hardware problem, software routine only exists to paper over the hardware problem.
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.
I'm one of the chief repeaters who has Harper on the stability issue and the control stick force curve; I usually Eve up dropping a post or two about it in each MAX thread.
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.
Even the 737 Max variants are statically stable in pitch, at least up to the stall. The issue that MCAS was intended to address was the handling characteristics, again in the flight regime prior to the stall. I do not know whether the airplane could have been certified, as a separate type, without some sort of augmentation, but it seems that Boeing did not think it could be certified under the common type rating that covers prior variants.
Maybe we should just scrap all airplanes due to the inherient risk of strapping jet engines on metal tubes and forcing them 10's of thousands of feet into the air.
I think a better alternative would be to deny it a shared type certification with the prior 737 and requiring recertification (and pilot retraining), thus removing the incentive to use shortcuts.
