This article reads well. Unfortunately, it's filled with fundamental mistakes.
> In the old days, when cables connected the pilot’s controls to the flying surfaces, you had to pull up, hard, if the airplane was trimmed to descend. You had to push, hard, if the airplane was trimmed to ascend. With computer oversight there is a loss of natural sense in the controls. In the 737 Max, there is no real “natural feel.”
Whoops. The 737 is one of the few airliners produced today that DOES still directly connect the pilot's controls to flying surfaces. There's literally 12mm wires connecting the yoke to the control surfaces. So, in fact, the 737 does have "natural feel". In fact, that's the whole problem MCAS was designed to solve - how to add force to the yoke when at high AOA in certain speed regions, in order to ensure a linear AOA response. (Certification requires something along the lines of "5 pounds force -> 5 degree AOA, 10 pounds force -> 10 degree AOA, etc") If the 737 was fly-by-wire, this force could be added directly in software. Instead, they added this force by changing the aircraft's trim.
> In a pinch, a human pilot could just look out the windshield to confirm visually and directly that, no, the aircraft is not pitched up dangerously.
No, you can't. There isn't a visual indication for AOA - the author is confusing this with pitch. You can have a 45 degree AOA while still having the nose of the plane pointed at the horizon. (The author mentions using the artificial horizon for this purpose, too) If you could read AOA from sensors that didn't require being exposed to the airflow, planes would use them.
I'm sure there's more, and it greatly reduces my confidence in the article.
Private pilot here. What you say is, strictly speaking, true but that doesn't mean that you can't tell an awful lot about what's going on by looking out the window, and, more importantly, at the other flight instruments. If your attitude, airspeed, and rate of climb are all looking normal, then if the AOA says you're stalling it's almost certainly wrong. And it it says that your AOA is 70 degrees (which the Ethiopian airlines AOA sensor did) then it is definitely wrong.
MCAS was designed to blindly trust a single AOA sensor, which was known to be prone to failures. To call that inexcusable would be quite the understatement.
>If your attitude, airspeed, and rate of climb are all looking normal, then if the AOA says you're stalling it's almost certainly wrong.
Agreed - you can certainly sanity check the AOA data with other data sources. Unfortunately, I'm not sure that will always let you pinpoint AOA as the cause - imagine some nice dirt-loving wasps have built nests in all of your pitot tubes, or they've iced over, and now those values are locked in place. So your IAS and altimeter could both be reading normal despite the fact that you're in a dangerously fast descent at a high angle of attack. (A radar altimeter wouldn't have this issue, of course) Things like this have happened before. Of course, this would require your engines to have failed at approximately the same time as your airspeed and altitude data, but I can imagine failure modes in which that would be the case. (A particularly stupid autopilot attempting to reduce speed but not seeing the speed decrease, repeat in loop)
> And it it says that your AOA is 70 degrees (which the Ethiopian airlines AOA sensor did) then it is definitely wrong.
Or it's accurate and you're about to die unless you fix it. Air France Flight 447 comes to mind. (though 70 degrees is probably past the point of being recoverable)
> MCAS was designed to blindly trust a single AOA sensor, which was known to be prone to failures. To call that inexcusable would be quite the understatement.
> imagine some nice dirt-loving wasps have built nests in all of your pitot tubes
All of them? And I didn't notice any of them during preflight? And none of them were there on the previous flight? And I didn't notice that the airspeed was not alive on the takeoff roll? Not going to happen.
> or they've iced over
Again, pretty freakin' unlikely on takeoff flying out of Addis Ababa. And this is another thing a pilot can rule out by looking out of the window. If you're not in clouds, you're not picking up ice.
Also, there are OAT (outside air temperature) sensors.
> your engines to have failed
Another thing that would be pretty apparent to the pilots.
No matter how you slice it, Boeing screwed the pooch bigly.
> > imagine some nice dirt-loving wasps have built nests in all of your pitot tubes
> All of them? And I didn't notice any of them during preflight? And none of them were there on the previous flight? And I didn't notice that the airspeed was not alive on the takeoff roll? Not going to happen.
You underestimate the abilities of people to fuck things up. Birgenair Flight 301 - one tube blocked by mud dauber wasp, leading to crash. [0] Later that year was the crash of Aeroperú Flight 603 - the static ports were covered with tape, which was never realized by the pilots. [1] (They continued flying as if their altitude and airspeed readings were accurate)
> > or they've iced over
> Again, pretty freakin' unlikely on takeoff flying out of Addis Ababa. And this is another thing a pilot can rule out by looking out of the window. If you're not in clouds, you're not picking up ice.
> Also, there are OAT (outside air temperature) sensors.
Not applicable in this instance, sure. But "are there clouds around" isn't something I'd expect the flight computer to be able to reliably detect, and that's what would have to make the decision here.
> > your engines to have failed
> Another thing that would be pretty apparent to the pilots.
I'd agree that it SHOULD be.
> No matter how you slice it, Boeing screwed the pooch bigly.
Yep. But there's still more complexity here than people seem to want to acknowledge.
> No, you can't. There isn't a visual indication for AOA - the author is confusing this with pitch. You can have a 45 degree AOA while still having the nose of the plane pointed at the horizon. (The author mentions using the artificial horizon for this purpose, too) If you could read AOA from sensors that didn't require being exposed to the airflow, planes would use them.
I think you may have missed the context of that part of the article. The author was talking about diagnosing a possible AoA sensor malfunction.
Changes in AoA should generally correlate with changes in attitude according to the artificial horizon and with changes to what you see looking out the window. So if you have an AoA sensor that claims you are at, say, a 70 degree AoA for several minutes (like the one in the Lion Air crash did), but you can see out the window that the plane alternating between pitching up and down relative to the ground, you've probably got a busted AoA sensor.
I wouldn't say that you can't correctly diagnose the failed AOA sensor in the majority of cases, but the conditionals to disable a safety system are probably going to have to be more rigorous than "AOA changing between extremes without significant change in pitch, airspeed or altitude". Just for starters, imagine you've hit a particularly violent updraft and then a downdraft... Though I'm not sure if such weather conditions would exist anywhere you'd plan to fly an airliner.
(And I'm kind of being "perfect is the enemy of good" here, sorry. Any "AOA sensor is wonky, disable MCAS" methodology would probably have resulted in better outcomes than the one they used. It's just that none of them are really perfect)
> "The solution was to extend the engine up and well in front of the wing. However, doing so also meant that the centerline of the engine’s thrust changed. Now, when the pilots applied power to the engine, the aircraft would have a significant propensity to “pitch up,” or raise its nose."
The centerline of the engine’s thrust moves up, doesn't it (the constraint is the floor, bigger engine requires you to go higher, wings are in the way, so you go higher and forward), so presumably it moves closer to the centre of gravity, giving you a shorter lever, thus full throttle should give you less aircraft nose up momentum.
In reality, the issue is (as far as I understand) that the nacelles generate lift that is further out in front of the centre of gravity, giving you a longer lever, and that's what produces a stronger pitch up (under high AoA) than before.
EDIT to add:
The article mentions that later:
> "And the lift they produce is well ahead of the wing’s center of lift, meaning the nacelles will cause the 737 Max at a high angle of attack to go to a higher angle of attack."
The relevant property is not that the lift of the nacelles is ahead of the centre of lift, but ahead of the centre of gravity.
> In the old days, when cables connected the pilot’s controls to the flying surfaces, you had to pull up, hard, if the airplane was trimmed to descend. You had to push, hard, if the airplane was trimmed to ascend. With computer oversight there is a loss of natural sense in the controls. In the 737 Max, there is no real “natural feel.”
Whoops. The 737 is one of the few airliners produced today that DOES still directly connect the pilot's controls to flying surfaces. There's literally 12mm wires connecting the yoke to the control surfaces. So, in fact, the 737 does have "natural feel". In fact, that's the whole problem MCAS was designed to solve - how to add force to the yoke when at high AOA in certain speed regions, in order to ensure a linear AOA response. (Certification requires something along the lines of "5 pounds force -> 5 degree AOA, 10 pounds force -> 10 degree AOA, etc") If the 737 was fly-by-wire, this force could be added directly in software. Instead, they added this force by changing the aircraft's trim.
> In a pinch, a human pilot could just look out the windshield to confirm visually and directly that, no, the aircraft is not pitched up dangerously.
No, you can't. There isn't a visual indication for AOA - the author is confusing this with pitch. You can have a 45 degree AOA while still having the nose of the plane pointed at the horizon. (The author mentions using the artificial horizon for this purpose, too) If you could read AOA from sensors that didn't require being exposed to the airflow, planes would use them.
I'm sure there's more, and it greatly reduces my confidence in the article.