Lion Air crash: Jakarta Boeing 737 'had prior instrument error'

[KL27x]

What I mean is the plane isn't flying like an arrow. A high AOA would be a situation where the nose is tilted up and the back end of the fuselage passes through a lower point in space than the nose of the fuselage.

When the plane is puling up hard from a dive, it doesn't reach level flight the instant the plane's fuselage is horizontal to the ground. The vector of the plane will still be such that the plane is losing some altitude at this point.

This is what high AOA means, don't it?

[Berni]

At high angle of attack, the plane isn't flying nose first.

What? I think it sums it up pretty nicely. Angle of attack of 0° means the plane is flying perfectly nose first. As in the plane is pointing in the same direction as its air speed velocity.

[KL27x]

^I don't know if Berni is backing me up or piling on. But I fully agree with his statement.

I'm not a pilot and I'm not trying to school anyone. I'm just using common sense. Just because you're not a pilot doesn't mean you have to take words like AOA and treat them like a made-up thing in Star Trek.

[Nusa]

At high angle of attack, the plane isn't flying nose first.

What? I think it sums it up pretty nicely. Angle of attack of 0° means the plane is flying perfectly nose first. As in the plane is pointing in the same direction as its air speed velocity.

No, it doesn't. You're confusing angle of attack with pitch. In a straight and level cruise, you're still going to need a positive angle of attack to counter the force of gravity.

Angle of attack is the difference between the chord of the wing and the actual direction of travel. Note that flaps, when deployed, change the shape of the wing which changes the chord of the wing which changes the angle of attack relative to the pitch of the plane. Similarly, spoiler devices on the wing can spoil the lift, which doesn't actually change the chord, but does change the angle of attack because it affects the direction of flight.

[KL27x]

^Ok. Take everywhere I said AOA and change it to pitch.

So "pitch" is the angle of the plane relative to the vector of the plane's velocity, not an angle in relation to the earth, right? And for simplicity, I'm not worried about wind.

The engines being fixed to the plane, w/e you call it, this is the problem I'm referring to. As the angle of the plane increase upwards in relation to its vector, there is a point where the MAX will be unstable and will quickly go out of whack beyond recovery in a positive feedback loop (at least compared with the original version). This is what seems obvious to me from what we have been told and including what Djacobow has previously explained a few pages back.

[rs20]

Everyone here is being overly specific. Let's start by quoting wikipedia.

In aerodynamics, angle of attack specifies the angle between the chord line of the wing of a fixed-wing aircraft and the vector representing the relative motion between the aircraft and the atmosphere. Since a wing can have twist, a chord line of the whole wing may not be definable, so an alternate reference line is simply defined. Often, the chord line of the root of the wing is chosen as the reference line. Another choice is to use a horizontal line on the fuselage as the reference line (and also as the longitudinal axis).

OK, so:
1. Angle between the longitudinal axis of the aircraft and earth (irrespective of direction of travel): is called Pitch (so KL27x, the answer to your last question is "No, pitch *is* related to the earth")
2. Angle between the longitudinal axis of the aircraft and the velocity of the aircraft through the air: is called AOA (assuming we choice the "another choice" in the Wikipedia definition); and needn't be positive in straight and level flight (especially at high speed)
3. Angle between root chord of the wing and velocity of the aircraft through the air: is also called AOA (assuming the chord line definition is used); and this particular definition I could believe Nusa's claim that this must always be positive in straight and level flight, maybe.

In short, let's stop arguing over an ill-defined term like AOA. A wing designer probably has definition 3 in mind, while a pilot maybe probably has definition 2 in mind.

[Berni]

Ah it is a confusion with pitch. No, pitch and AOA are two different things.

Pitch is the front to back angle of the plane relative to horizontal on earth. It makes up the absolute rotation angle along with Roll (angle left to right relative to horizontal) and Yaw (rotation around the planes vertical axis relative to the earths north pole).

This is much like mixing Yaw and Heading. Heading is also relative to the north pole, but its the direction the plane is moving. Wind causes the difference between the two as crosswind essentially makes the plane fly slightly sideways relative to the ground below (This is seen nicely in heavy crosswind landings)

AOA is the angle of the difference between your Pitch Roll Yaw of your plane and the direction of the wind outside (Tho usually you only want AOA to be offset in the Pitch direction). Its essentially what angle the air is hitting the wings. You want a few degrees of AOA for the wings to produce lots of lift, but if the AOA gets too high the wings go into a so called aerodynamic stall where the lift drops sharply, the drag sharply raises and heavy turbulence develops around the wing. This is very bad because no lift means you fall out of the sky and because the wings are now ineffective its very difficult to control the plane to get it pointing forwards again so that the AOA reduces and you get lift again.

[KL27x]

I'm pretty happy with those explanations. I think my previous posts are fairly ok, then.

Regardless if perfectly horizontal flight means the AOA must be positive, that is irrelevant. When pulling up out of a nose dive to avoid crashing the plane into the ocean, one would expect the plane to be put into a high AOA. And this is where the MCAS would kick in to prevent the 737-trained pilot from losing the plane to a stall.

So in the case of having to shut off MCAS because the plane it put into a nose dive with an imminent date with the earth, the pilot would then be putting the 737 MAX into a high AOA and facing the dangerous potential of a stall. Crash into the ocean at 600mph, or fall into the ocean at 200mph. Dead is dead.

[Berni]

The thing is that this MCAS system would take a while to adjust the trim so low that the plane would nose dive into the ground.

The pilots would have seen the big trim wheels moving as MCAS is trimming pitch down. The wheels also make a mechanical noise as they spin on purpose to make them more noticeable. All they would have to do is flick a switch to disable automatic trim, manually trim it back to level and then continue flying as normal.

But the fact that the pilots are not properly trained about the new MCAS and its possible failure modes, likely means that the pilots are suddenly suspecting other failures and reacting according to those, the high stress situation making them too focused on certain things that they would not notice the trim wheel oddity. There are many many things that can go wrong on a plane, troubleshooting a system you don't even know exists on such short notice as you are heading for the ground is not easy.

[Nusa]

New update. This man is probably the best subject-matter-expert reporter available for interpreting the actual facts released from various sources:

[KL27x]

^8:23 for insight over MCAS and the interesting perspective that a third pilot might bring.

There are many many things that can go wrong on a plane, troubleshooting a system you don't even know exists on such short notice as you are heading for the ground is not easy.

Exactly. And even though the problem went on for 10 minutes in the Jakarta case, think how near these "single" spaced out malfunctions would put the plane to death that entire time. In the fully loaded plane, you would lose altitude much more quickly than regaining it. It only took the one double malfunction in rapid succession to end it all. The captain took back control but it was not in time. You know he wasn't taking a nap. He was right there, trying to save everyone's life.

The thing is that this MCAS system would take a while to adjust the trim so low that the plane would nose dive into the ground.

Did you see the altitude graph? I imagine everyone on the plane lost their lunch. These changes don't appear to be gradual. This guy ^ uses the words "absolutely terrifying." "Startle factor." No matter how big the wheel or how loud it clicks, it appears to be quite responsive. See the vid someone posted of the vertical takeoff, how fast it can level out. Just because it is normally steered like the Titanic doesn't mean it can't change vector in a hurry. I imagine the MCAS system would be moving the elevator as fast as possible. The runaway trim which pilots have been trained and done simulation with is never going to move the elevator that much or that fast, lest someone spill their drink. Only 5 degree change needed for full nose dive, per the Seattle Times article, and the MCAS does 2.5 degrees per firing. How much of that elevator range did the test pilot in the stunt takeoff use? Was that even full elevator?

I refuse to believe that 2 out of 2 pilots can get a job flying a hundred million dollar plane with 200 souls on board without being halfway competent.

*You can also see in this video the location of the AOA sensor is on the sides of the plane near the nose. Thus, the AOA, as measured by these vanes, is the angle of the plane vs the air it is passing through. It is only measuring the angle of the "wings" by virtue that the wings are attached to the fuselage. So I don't get the hubbub over my usage of AOA. I didn't think modern commerical jets rotate their wings to adjust AOA. They may have some control surfaces to change the curvature of the top of the wing to change the amount of lift. And there are flaps. But the wings are pretty fairly well fixed in place, otherwise.

Tooki: the pilot in this video states what you did, as to the reason for MCAS. That it is there because the position of the engine produces substantial nose up force. But we know by virtue of how it works, this is not correct IMO. It kicks in based on AOA sensor. It only kicks in after the nose has gone up. It is not there to make the plane handle like a 737. With the MAX, the pilot can get the nose up by increasing thrust. This isn't counteracted by the elevator under normal AOA. It's only when the AOA gets extreme that it kicks in. So as long as the AOA is not nearing the danger zone, yeah, the nose goes up under hard acceleration. No magic prevents this from happening. The magic pixies kick in if this goes too far out of the normal range. IOW, the MAX can't necessarily pull up with greater force than the NG. The elevator can do that just fine; the engines need not do this at all. What this means is the MAX loses control.... if not before, at least more suddenly... than the NG does.

[GeorgeOfTheJungle]

Why did none of the pilots at least grab and stop the trim wheel? I still don't get it. The damn thing was spinning under their noses. They knew what that wheel spinning meant, didn't they? Three minutes is a long time, it's not a sec, yet they couldn't connect the dots. Poor guys. It's so sad.

[Brumby]

Three minutes is a long time...

In a confusing, high stress, life and death situation, 3 minutes will pass very, very quickly.

[Berni]

The thing is that this MCAS system would take a while to adjust the trim so low that the plane would nose dive into the ground.

Did you see the altitude graph? I imagine everyone on the plane lost their lunch. These changes don't appear to be gradual. No matter how big the wheel or how loud it clicks, it appears to be quite responsive. See the vid someone posted of the vertical takeoff, how fast it can level out. Just because it is normally steered like the Titanic so no one spills their drink doesn't mean it can't change vector in a hurry. I imagine the MCAS system would be moving the elevator as fast as possible. Only 5 degree change needed for full nose dive, per the Seattle Times article. How much of that did the test pilot in the stunt takeoff use? Was that even full elevator?

Well yes you can certainly make negative G or pull over 2G in a big airliner if you try.

But the quick maneuvers are done using the control surfaces moved by powerful hydraulic pistons, these are the ones tied to the pilots control column and can certainly move very fast. But things like trim are different, here the entire wiglet on the tail moves. This is moved via a motor turning a worm gear, all this gearing down gives it plenty of oomph while turning very slowly, as in normal use it is never needed to turn quickly. This is what MCAS is moving via trim.

The problem is that the entire wiglet is much bigger than the small flap part at the end of it. This small flap is what is moved quickly and responsively by hydraulics. If the whole wiglet is tilted up a little bit then you can just tilt down the flap to counteract it, but once the wiglet tilts more it can produce more force than the little flap can at full deflection. At this point no matter what the pilot does on the control stick he can't get the nose back up (Well apart from rolling the plane on its back, but then you have a uncontrolled pitch up and its certainly not something you are supposed to do in an airliner, especially if you ask the passengers).

The two fighting forces of the wiglet and its flap also produce more drag, slowing the plane down, making it even harder to keep altitude.

Keep in mind that the pilot can keep it perfectly level while the trim is being turned down by simply pulling back on his controls more and more. But eventually he hits the end of his control range and that's when the plane starts to plummet. (Tho if the plane was warning him of a stall he might not pull up since that's the opposite of what you are supposed to do if approaching a real stall)

[TerraHertz]

https://www.rt.com/news/454275-indonesia-crash-third-pilot/
Off-duty pilot reportedly saved Boeing 737 MAX from crashing day before disaster
Extract:

An extra pilot, who hitched a ride on a Lion Air Boeing 737 MAX a day before its crash in October, saved it during an emergency strikingly similar to the one that proved fatal, Bloomberg reports.

The new airliner plunged into the Java Sea killing all 189 people on board apparently due to a malfunction in an anti-stall system which pushed the nose of the aircraft down.

A day before the crash the same aircraft experienced a similar problem but was saved by an off-duty pilot who realized what was happening and instructed the crew on how to stop the system from affecting the flight, the agency said.

I wonder if the MCAS software interprets 'rapidly declining altitude' as 'must be still stalled, try more pitch down' ?
Also, with all the spoken warnings to pilots in modern cockpits, you'd think someone would add some spoken messages from MCAS, to let pilots know why the 'safety system' was killing them.
Though given the attitude inherent in implementing something like MCAS (rather than just warning the pilots), such a message would probably be something unhelpful like "I hate you, die die dieeeee!"


This is why I refuse to own a car with computerized engine and drive systems. Well, ONE reason why.

[KL27x]

Berni, I wonder where you are getting this information that is not in the news? Are you interested in aviation or do you have personal experience?

Google "737 winglet" and it says this is the little vertical thing at the tips of the wings. The elevators appear to me as one piece on either side of the tail. As far as I know, the elevator is the most powerful control surface for rapidly changing the attitude/AOA of the plane under normal flight. And the same elevator used by MCAS or by autopilot trim is also used by the pilots for manual control.

GeorgeoftheJungle, in my understanding, everyone who is talking about the trim wheel slowly clicking away until the snoozing pilot realizes his plane is facing the ground is not on the right planet. This kind of incident might have surprised the pilots the first time, but not the 22nd time. If the wheel makes a clicking sound, maybe watch a video of a Marlin fisher and the sound the reel makes when the fish is pulling away at 50 knots. When this happened, the pilots didn't need to hear the clicks to suspect something was wrong. Their seats falling out from under them and their lunch pushing into their esophagus provided all the notice they needed. If that didn't work, I'm sure the 200 screams would have woke the dead. This is not a gradual trim adjustment or course change. It's a last ditch massive response for when the S has already HTF, and all the drinks have already been spilled.

[ogden]

Google "737 winglet" and it says this is the little vertical thing at the tips of the wings.

I think he did mean horizontal stabilizers. Indeed it would be good that we do not re-invent naming of airplane parts & systems
https://theaircurrent.com/aviation-safety/what-is-the-boeing-737-max-maneuvering-characteristics-augmentation-system-mcas-jt610/

[KL27x]

There are control surfaces on the wings for banking. There are flaps on the wings for increasing lift and drag during takeoff and landing. There are sliding surfaces on the top rear of the wing for changing the amount of lift.

Don't let the words "trim" or "stabilizer" fool you. The elevators are the most powerful control for changing the plane's vector. If you want to do a loop de loop or a rapid nose dive, you use the elevators.

[Berni]

Yes the technical term for that part of the tail is the elevator but i wanted to stick to generic terms. So winglet as in "a small wing shaped thing" rather than those little upwards curving things on the end of wings, but yeah those are indeed called winglets (Unintended name clash).

I do have some interest in aviation and work at a company that makes avionics, but i am not a pilot, so don't take my words as fact.

Was mainly trying to explain why trim does not cause the plane to suddenly nose dive out of the blue within a second, but rather its a slow gradual thing that makes the plane more and more nose heavy as time goes on until at some point it becomes impossible to keep it up by normal control column input.

[Nusa]

There are control surfaces on the wings for banking. There are flaps on the wings for increasing lift and drag during takeoff and landing. There are sliding surfaces on the top rear of the wing for changing the amount of lift.

Don't let the words "trim" or "stabilizer" fool you. The elevators are the most powerful control for changing the plane's vector. If you want to do a loop de loop or a rapid nose dive, you use the elevators.

If that were true, we wouldn't be in this situation. In fact, the elevators are the second-most powerful control for that purpose, having less surface area than the horizontal stabilizer itself. Part of the reason the stabilizer jackscrew has that much travel is that it's intended to serve as a backup system for a jammed or inoperative elevator. In normal operation, the two systems work together.

[GeorgeOfTheJungle]

GeorgeoftheJungle, in my understanding, everyone who is talking about the trim wheel slowly clicking away until the snoozing pilot realizes his plane is facing the ground is not on the right planet. This kind of incident might have surprised the pilots the first time, but not the 22nd time. If the wheel makes a clicking sound, maybe watch a video of a Marlin fisher and the sound the reel makes when the fish is pulling away at 50 knots. When this happened, the pilots didn't need to hear the clicks to suspect something was wrong. Their seats falling out from under them and their lunch pushing into their esophagus provided all the notice they needed. If that didn't work, I'm sure the 200 screams would have woke the dead. This is not a gradual trim adjustment or course change. It's a last ditch massive response for when the S has already HTF, and all the drinks have already been spilled.

You're seeing the plane's trying to kill you, every time you release the nose up button in the yoke the trim wheel next to you starts spinning again in reverse, for gods sake at least grab the damn wheel to stop it going nose down again. Why not?

[ogden]

Yes the technical term for that part of the tail is the elevator but i wanted to stick to generic terms.

In this discussion we better use terms that do not lead to confusion

[Nusa]

^ Indeed. Those ARE the generic terms. If people don't know them, they better learn them to talk in this thread.

[tooki]

What?!? The whole issue with the engines is that the MAX has too much lift, not too little. MCAS’s job is to nudge the nose back down, not up.

The reason for this is because the MAX will easily enter an unrecoverable stall if you are at a high angle of attack, which would not happen with the previous versions. You would be using a high angle of attack when pulling up from an unexpected dive at low altitude. And you would have to deactivate MCAS to do this. The pilot would be manually flying a plane that handles very differently and stalls much easier than the plane he was certified on. The new plane was slipped under the same certification because of this MCAS system.

Yup, I already knew and fully understood this. Hence why I was like "wtf" at your comment.

You focused on the wrong aspect of the word "lift".  It was not the magnitude of the lift force that was intended in the cited statement, but the aspect of "lift and handle" (perhaps better stated as just "handle") "like a regular 737".

The point being that when thrust is added in the MAX, it has a (significantly) greater upward turn moment than that of earlier models of 737.  This will cause more pitch up than a pilot used to the older models would expect ... ie. it does not handle "like a regular 737".

I understand this. But I didn't "focus on the wrong aspect", actually: at the moment when I actually clicked "Quote", it only said "lift", not "lift and handle" — that was edited in later. (I didn't notice it sneak into the quoted text.)

[KL27x]

Yup, I already knew and fully understood this. Hence why I was like "wtf" at your comment.

This is clear as mud. Not the transparent kind.

I get wordy and confusing, at times. But this pretty much sums up what I meant:

The MAX can't necessarily pull up in a tighter radius or G force than the original 737. Yeah, the moment of inertia from the engine will push the nose up more than the original. But the elevator can do that just fine as much as you would ever need; the engines need not do this at all. It seems evident to me that MCAS is needed because the MAX is not as controllable at this high AOA. Even after you cut the engine back, the aerodynamic lift and drag of the larger, higher, farther forward engines in this high AOA presentation make the plane want to nose up (more).

So I sorta disagree with Brumby and the popular explanation of this in the news. Yeah, the plane wants to nose up during engine acceleration, but MCAS is not going to kick in until there's near stall conditions. It is expected that 100% of pilots will adapt to the way the plane flies and handles without ever activating MCAS. Else Boeing would be even dumber and more brazen than anyone here has even thought to hand out planes they expect pilots to inadvertently stall. No. MCAS is there for the unlikely event a pilot needs to intentionally utilize an extremely high AOA.