> You and other keeps parroting this line, I don't believe it to be true.
You are free to believe anything you want to believe. Of course if you where involved in the 737 Max development and you have inside information to the contrary then I'll accept that.
It's the explanation which best tallies with the official description of the system, the circumstances surrounding its introduction and the certification requirements.
Many aircraft have systems implemented to prevent stalls (see stick pusher); and systems to prevent undamped yawing movements becoming uncontrollable (see yaw damper). And to prevent mach tuck (see mach trim system); and to prevent changes in pitch trim due to speed changes (see Speed trim system).
1. The position and size of the upgraded engines for the 737 MAX caused the plane to tend to pitch upwards, which could cause a stall.
2. Boeing was concerned designed MCAS to automatically push the plane's nose down to prevent stalls
Every single piece of reporting I've seen on the matter refers to MCAS as an anti-stall device.
Pilots of 737 who have talked to reporters refer to it as an anti-stall device.
I have a hard time believing that this information hasn't been fact checked to hell and back yet.
> Every single piece of reporting I've seen on the matter refers to MCAS as an anti-stall device.
Assuming you are a developer, have you ever seen some reporting of something technical in the news? Does it not make you cringe?
> Pilots of 737 who have talked to reporters refer to it as an anti-stall device.
Even Boeing themselves do; so who can blame them. The reason that the certification item exists is to prevent certification of aircraft which demonstrate increasingly lighter control forces as the aircraft approaches a stall. The reason being that it makes it easier for an inattentive pilot to accidentally fly the aircraft into a stall. So if you want to shorten that to anti-stall then I'm fine with that.
What I don't really like is the retoric about how these aircraft would fall out of the sky without MCAS "controlling" the plane. It isn't a closed loop control system implementing PID control to account for some crazy instability in the aircraft.
> Every single piece of reporting I've seen on the matter refers to MCAS as an anti-stall device.
"Anti-stall" has become a catchphrase wrt the 737 MAX. I wish everyone would stop using it, but the horse has left the barn it seems.
MCAS was cooked up to maintain the handling characteristics required by the FAA certification specifications.
Other commenters in other threads have explained why consistent response curves are vital in operating an aircraft, so I won't take on the why of the regulation other than to say it's well founded and some smart systems/human factors engineers have elucidated on this in earlier threads. Otherwise, wouldn't it have been easiest of all for Boeing to beg "let us have the airplane respond this way; all planes are different, right?" And even if the FAA allowed that, it'd force a new type rating, I suspect; the very thing they so badly wanted to avoid.
The standard (see below) requires stick resistance to increase as the critical (stall) angle of attack is approached. The MAX violated that requirement due to the aerodynamics of the new engine cowls. (So: why didn't they mess with the cowls? I suspect that wasn't possible without impacting efficiency, which is a key selling point of the MAX.)
Back to the spec: as noted, it exists to provide a consistent response curve to the pilot compliant with the regulation.
As borne out by these accidents, MCAS is Boeing's (quick & dirty) implementation of "artificial feel" to meet the spec. Big, transport planes with hydraulic flight controls have had artificial feel for many years to provide a consistent and properly scaled input response (not too heavy, not too light, etc.)
As far as the FAA specification in question, the critical section is in CFR 14 §25 Subpart B—Flight ¹, notably the sections on Controllability & Maneuverability and Stability. In these sections 'stick force' appears sixteen times; 'stick force curve' appears six times.
It's "stick force curve" that MCAS was created to tweak. There's no mention of "reduce the critical angle" or "change the stall onset speed" or anything about the aircraft performance. It's about how the plane handles in a particular part of the flight envelope².
> Every single piece of reporting I've seen on the matter refers to MCAS as an anti-stall device.
Because it was printed repeatedly doesn't make it true (see, Gell-Mann amnesia effect³).
²–I'm surprised that MCAS reacts fast enough to count as artifical feel. Electric trim doesn't move all that fast, AFAIK. Still an open question in my mind.
The 737 Max does fly differently, the MCAS and AoA checks to offset the engine "pitch up" due to more powerful engines was to get around a regulation/certification [1].
> The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.
The regulation that they had to make to fit within the 14CFR §25.203(a) regulation/rule was first tried by physical designs but ultimately settled on a sensor/software solution that constantly polls every 9 seconds to check the nose pitch/attitude and adjust, if it goes to0 far MCAS is triggered and pulls the nose down. Since there was a faulty sensor, this caused the nose dive catastrophic failures.
The 14CFR §25.203(a) regulation (a) is [2]:
>It must be possible to produce and to correct roll and yaw by unreversed use of the aileron and rudder controls, up to the time the airplane is stalled. No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls.
Since the change had to do with "No abnormal nose-up pitching may occur" in that rule, then the plane definitely flies differently as the cause of the MCAS and sensor flow is to constantly check for a nose pitch up that it corrects, if too far, enable MCAS to bring the nose down. This plane flies differently definitely and a bit opposite in that other planes will pitch down, it pitches up due to engine power of the larger more fuel efficient LEAP engines.
Ultimately the root of the problem was cost cutting which led to retro-fitting the 737NG to the 737 Max 7-10 to fit within cost/training/testing/regulations and in the end that was the problem that caused the end result of a software/sensor single point of failure which has squandered trust in Boeing engineering, management and safety as well as the 737 brand. Most people don't know the 737 Max is essentially an entire new plane. How the FAA let Boeing get away with this will be a focus as time goes on.
As a software engineer, the 737 Max looks like a legacy system hack that they tried to version 2 with it rather than make a new plane and incur all the training/testing/certifications of a new plane.
Boeing other planes don't have these issues, the 767 is has only had a couple issues beyond terrorism and those were related to pilots and fuel [3]. Two crashes of two new Boeing 737 Maxs within months of release and service is not a good trust enabling product.
I'm unconvinced that a software routine (MCAS) that can be (indirectly) disabled, and pilots are instructed to disable in the case of MCAS upset, is an acceptable work around for any portion of FAR 25 certification.
I think it's more likely that MCAS was intended to avoid a new type certificate for the MAX, thereby triggering FAR 61.31(a) requiring that the pilot be type rated.
Either way we have a problem here, as I see it. The aircraft flight manual and emergency AD 2018-23-51 specifically recommend a procedure that instantly makes the airplane not airworthy, or renders the pilot flying an airplane with behavior he's not really type rated for. This seems highly problematic, and I'm not able to account for it.
As far as I understand, the older 737 models also exhibit a nose-up pitching behavior in high-thrust/low-speed situations, to the point that stall recovery procedure explicitly specifies that the nose has to come down first, before thrust is increased. And maximum thrust may not be used, only max continuous thrust (or something similar) because otherwise the pitch moment from the engines may overwhelm elevator control.
Good question, apparently on the 737 Max this is more pronounced with the larger/forward LEAP engines.
This was enough of a problem that it constantly has to be checked and MCAS has to trigger if it is off by too much.
Most planes have a nose down pitch over time but the 737 is smaller/lighter so maybe the engines have always affected flying.
With the 737 Max and new LEAP engines, here it was enough for Boeing to make a system for it, they didn't make it just for extra, it was a cost cutting measure already.
> stall recovery procedure explicitly specifies that the nose has to come down first, before thrust is increased
Pulling the nose down is probably standard on any stall recovery because the nose up exacerbates conditions for a stall.
The problem wasn't really with the nose pitch up, but the nose pitch up created the need for nose up detection/monitoring and an MCAS in case of issues.
The major problem is that MCAS relies on a single sensor and a single point of failure, which can result in the MCAS constantly pitching the nose down if it is bad data or a broken/incorrect AoA sensor.
The root cause of this major problem is the cost cutting retro-fitting that Boeing did and their reliance on a single point of failure that can trigger a catastrophic nose dive.
This is one of those cases that the fix to keep the 737 flying the same as previous versions, without pilot knowledge of the MCAS initially, caused more damage than just the pitch up might do.
I don't think you understand the market the A32x and the 737 are in. They are planes flown by carriers who want to keep costs low. The MAX was designed the way it was because the airlines wanted more of the same with minimal retraining and support retooling. Boeing had originally proposed and may still pursue a carbon-fiber redesign of the 737 incorporating more of the lessons learned from the 787, but for now the airlines wanted and got a progressive improvement on the 737.
The MCAS system was designed as a response to FAA requirements and directives based on a concern about performance at the edge of the flight envelope. Since as we see it now (the public does not have enough information to conclude on how well the design process went except in the result) it was designed poorly and improvements are being made.
Once those improvements are made I expect the MAX to be, at that point a safer aircraft than the 737 NG. Its unfortunate that so many people had to die before the issue was identified, maybe congress should provide more funding for aviation safety enforcement and research at the FAA, NTSB and NASA.
You and other keeps parroting this line, I don't believe it to be true. MCAS is an anti-stall device, show me once source that says otherwise.
> Not because the aircraft is impossible to fly without it.
Not impossible, but possibly also not safe. The system was implemented to prevent stalls in the MAX 8 planes due to it's flight characteristics.