Boeing 737 Max8 Aircraft Crashes and Investigations [Part 3] 36

[Sparweb]

This is the continuation from:

thread815-445840
thread815-450258

This topic is broken into multiple threads due to the long length to be scrolled, and many images to load, creating long load times for some users and devices. If you are NEW to this discussion, please read the above threads prior to posting, to avoid rehashing old discussions.


Some key references:

Ethiopian CAA preliminary report

Indonesian National Transportation Safety Committee preliminary report

The Boeing 737 Technical Site

No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
STF

 

[LionelHutz]

"I see that you are playing the "Blame the pilots" card."

Not playing any cards. There is plenty of blame to go around. Boeing could have make the system more robust and lkely avoided continually moving the stabilizer nose down by making different decisions. The planes could have been successfully landed if the pilots had made different decisions. More robust pilot training (not just on the MAX but in general) might have helped the pilots cope with the issue and bring the planes back safe.

To be blunt, pilots don't get trained on the technical details of how the various plane systems work. They get trained on how to fly the plane and what to do when certain failure conditions present themselves. Pilots certainly don't get trained on how the software in fly by wire planes is written and how it decides to move the various controls? They don't dive down into the code level and subroutine level. This is why I don't understand the indignation at the MCAS system not being technically explained. Boeing had believed a malfunction was handled as part of the runaway stabilizer procedure the exact same way as all kinds of other failures have procedures.

As for the simulator. What is "high speed"? I just don't see it being a huge screw up if "high speed" is exceeding Vmo. Should the simulator always be accurate even when the pilots fly the plane out of it's flight envelope?

 

[waross]

Apparently a 737 flight simulator can be programmed and some have been programmed to replicate the control forces experienced by the Lion Air pilots.
There was a You Tube video up for awhile demonstrating that the forces on the control surfaces could become greater than could be overcome manually.

Drilling down in the New York Times further we find:

By Jack Nicas, James Glanz and David Gelles

March 25, 2019

During flight simulations recreating the problems with the doomed Lion Air plane, pilots discovered that they had less than 40 seconds to override an automated system on Boeing’s new jets and avert disaster.

The pilots tested a crisis situation similar to what investigators suspect went wrong in the Lion Air crash in Indonesia last fall. In the tests, a single sensor failed, triggering software designed to help prevent a stall.

Once that happened, the pilots had just moments to disengage the system and avoid an unrecoverable nose dive of the Boeing 737 Max, according to two people involved in the testing in recent days. Although the investigations are continuing, the automated system, known as MCAS, is a focus of authorities trying to determine what went wrong in the Lion Air disaster in October and the Ethiopian Airlines crash of the same Boeing model this month.

The software, as originally designed and explained, left little room for error. Those involved in the testing hadn’t fully understood just how powerful the system was until they flew the plane on a 737 Max simulator, according to the two people.

Less than 40 seconds. That does not leave much of the 35 seconds of decision time that was deemed to be reasonable in the investigation into Captain Sullenberger's landing.
Don't forget that one system was forcing the aircraft down and another system was warning that were sinking.
Captain Sullenberger knew that he had hit a flock of birds and lost his engines. He had to make a choice of landing options.
He wasn't getting conflicting information while a system that the pilots had not been properly trained on was trying to crash the plane.
Face it. Boeing sold something that they didn't have and then couldn't deliver on schedule. A lot of airlines trusted Boeing and lost their place in the Airbus delivery schedule.

A question for the aviation experts:
When an aircraft is diving steeply is there any way other than nose up to slow it down? When the aircraft is diving at an extreme angle will shutting down the engines have much effect on the speed?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Alistair_Heaton]

Swept wing jets have spoilers on the wings which are quite good at getting rid of energy at high speed.

On jets shutting an engine down doesn't produce much drag. Turbo props shutting one down without feathering the props gives huge amounts of drag.

Taking the power off when you already have issues with the nose going down usually makes matters worse due pitch power couple. So you have conflicting effects. Overspeed take power off... Power off nose will go down alot and you can't lift the nose.

 

[verymadmac]

Don't know if applicable to the Max but some previous 737 models allow for gear extension as part of emergency descent procedure. Still going cause more pitch down.

 

[3DDave]

waross - that 40 seconds includes failing to notice that holding pitch has changed the control yoke force from a pound or two to (estimating from pprune comments as I haven't seen what the actual force was) nearly thirty pounds. It's not like the fire on the Concorde or the hole in the space shuttle wing where some structural effect is happening that cannot be felt by the pilots or a sudden explosion of an engine disk or loss of an engine entirely, it's an increasing tug which both crews on the Indonesian planes countered successfully with electric trim for a reasonably long time until, in the accident flight, the copilot failed to understand that just stopping the trim wheel from turning by a short click on the button wasn't what the primary pilot had done the nearly 30 times before.

If you were on a mountain road with a tire losing air and it kept tugging the steering towards the cliff, but you had an inflation button to push and knew from experience that low pressure is what the tugging was from, you'd have a similar experience. As long as you pushed the button the steering would not pull to the side. Let the tire go flat enough and off the cliff you go. It may be that a system is installed that automatically deflates the tires for better traction, but a sensor has failed, and that's what's happening, or the tire has a nail that sometimes hits just right to let air out - doesn't matter which, it deflates, tugs at the steering and the driver either pushes the button or muscles up and holds the car straight until they can't hold against the pull anymore - which is what the Ethiopians effectively did. In addition, the Ethiopians kept the throttle to the floor, making the pull worse.

And yes - a tire system on a car or MCAS on the airplane is not acting the way it should and should be changed, however in either of these cases it isn't catastrophic until human inaction lets it move from an annoyance to a disaster.

Sullenberger's problem is significantly different. The MCAS accident planes were fully functional. They had all surfaces working and thrust available, the two items required for powered flight. Sullenberger had half that completely gone and had only enough energy to make it back to an airport if there were no problems, such as a headwind. To do that calculation in a few seconds is much different than feeling the yoke tug away from where you want it. Setting trim is the primary control on a plane and should be as second nature as breathing. Just like the car tugging, there's no decision to make. Either muscle it or push a button that is just under the left thumb.

That's what is so vexing about both the crashes. Even if they just kept up what should have been a petty slap-fight between MCAS starting trim down** and the pilot immediately stopping it with a bump of trim up, they should both have had hours to call in to their chief pilot for suggestions. Instead the pilots acted as if this was their first day handling trim on their own. It's very possible it was.

**MCAS takes almost 10 seconds for a full increment; at any time the pilot can push a trim button which instantly stops the trim increment; holding the trim button allows the pilot to set the stab trim anywhere he likes. He can exactly counter the fraction of increment, exceed (move it so it requires a yoke push to hold the pitch), or use less (so it requires a pull to hold the nose pitch). As long as the pilot continues to adjust pitch and for 5 seconds after any trim change, MCAS does nothing. The amount of time to disaster is, I expect, the amount of time MCAS takes if completely unchallenged at the speed the plane will reach at near 100% throttle to either tear off it's own control surface or for the pilots to run out of elevator authority with full aft yoke.

 

[LionelHutz]

You do know that Captain Sullenberger made the best decisions he could and successfully put the plane down in a manner that saved all on board. It certainly seemed that his main focus was figuring out how to get the plane down in 1 piece right from the start and he didn't feel he could get back to a runway so he took the only other path available. Sure, simulations showed the plane should have made it back, but would anyone making that claim want to try it in real life when knowing that coming up short puts you down into buildings in a fiery crash? Overall, it was a hell of a flying job.

Not holding the trim up button long enough to counter the stick forces - not so much.
Not reducing power so the plane doesn't exceed Vmo - not so much.

If you want to keep making such comparisons, then a simulation should be setup not to duplicate what happened but rather show a successful recovery simply by putting the spoiler back to a trimmed position before using the cutout switches. Then, it could be easily proven that the plane could have been landed just like they "proved" that Sullenberger could have got his plane back to a runway.

You do realize that while the Ethiopian plane was flying at a reasonable speed that MCAS moved the stabilizer to within a few tenths of a percent to full down (over 1/2 a degree further than the movement that caused the crash) and yet the pilots had little difficulty recovering from that? You also realize it was more than 40 seconds after they shut off the electric stabilizer control before they got further into trouble by allowing the plane speed to get above Vmo?

I agree with the above that trimming a plane should be second nature to a pilot.

 

[HamburgerHelper]

I always question how much of a decision it was. You have three possible outcomes: making it to an airport, crashing it in the middle of a the city, or putting it into the river which likely will mean everyone on the plane gets killed but no one additionally on the ground. As soon as it became a question if the plane could even making it to an airport, landing or crashing it into the river becomes the moral choice.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[VEBill]

Very good summary.

Vestigial design issue clouds 737 Max crash investigations - The Air Current (2019-Apr-04)

 

[3DDave]

Right off the bat the article misstates what the pilots were supposed to do, critically leaving out step 1: Trim the aircraft with the trim switches. Then shut off the motors. Then use manual trim.

"the crew's initial by-the-book efforts" is a lie, as the article is in response to the released preliminary report that clearly shows the crew didn't follow anything in the FOM or the AD.

 

[LittleInch]

3D dave - I disagree. The pilots did follow the AD, but crucially both the AD and their action didn't specifically state that they should initially trim the plane back to the same angle it was before the MCAS. From the report IIRC, they only trimmed it back to about half of what it was before MCAS intervened, before they flipped the cut outs. This left them with an increasing load on the stick as the plane increased in speed.

They then appear to have tried to trim it manually and failed due to the load on the jackscrew so turned on the electric trim again but got a severe jolt when they tried to trim the aircraft as it was going rather fast at that point. When MCAS deployed again they were apparently into negative G and were probably off the seat for a few seconds. Then it was too late to do anything.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[VEBill]

Required by AD 2018-23-51
Runaway Stabilizer
Disengage autopilot and control airplane pitch attitude with control column
and main electric trim as required. If relaxing the column causes the trim to
move, set stabilizer trim switches to CUTOUT. If runaway continues, hold
the stabilizer trim wheel against rotation and trim the airplane manually.
Note: The 737-8/-9 uses a Flight Control Computer command of pitch
trim to improve longitudinal handling characteristics. In the event of
erroneous Angle of Attack (AOA) input, the pitch trim system can trim
the stabilizer nose down in increments lasting up to 10 seconds.
In the event an uncommanded nose down stabilizer trim is experienced
on the 737-8/-9, in conjunction with one or more of the indications or
effects listed below, do the existing AFM Runaway Stabilizer
procedure above, ensuring that the STAB TRIM CUTOUT switches
are set to CUTOUT and stay in the CUTOUT position for the
remainder of the flight.
An erroneous AOA input can cause some or all of the following
indications and effects:
• Continuous or intermittent stick shaker on the affected side only.
• Minimum speed bar (red and black) on the affected side only.
• Increasing nose down control forces.
• IAS DISAGREE alert.
• ALT DISAGREE alert.
• AOA DISAGREE alert (if the option is installed).
• FEEL DIFF PRESS light.
• Autopilot may disengage.
• Inability to engage autopilot.
Initially, higher control forces may be needed to overcome any
stabilizer nose down trim already applied. Electric stabilizer trim can be
used to neutralize control column pitch forces before moving the STAB
TRIM CUTOUT switches to CUTOUT. Manual stabilizer trim can be
used before and after the STAB TRIM CUTOUT switches are moved
to CUTOUT.

 

[VEBill]

Really emphasizes turning off the Electric Trim, which I understand includes both MCAS and trim switches.

It does offer "can be used", which is not really aligned with some opinions.

 

[waross]

3DDave, LionelHutz;
Thank you for taking the time to compose your courteous responses.
Thank you for allowing me to play devils advocate.
I am sure that there are a few people who are silently sharing my bias and attitude.

You will be happy to know that I have just deleted a lengthy post.

As far as we Know, 7 pilots have been involved in a MCAS runaway trim.
Of the 7 pilots, only one, the pilot deadheading on the flight that survived, could handle the emergency.
The score:
Pilots 1, MCAS 6.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

I have a few questions for the those familiar with the 737.

Electric trim versus manual trim with the trim wheels:
Apparently electric trim may be over-ridden by holding the trim wheel.
Does this imply that the manual trim wheels are more powerful than the electric trim?

This leads to another question.
When the forces on the stabilizer are too great to allow trimming with the hand wheels, are the forces also too great for electric trim to overcome?

And this leads to another question.
As I understand it, in the later stages of the event the electric trim was turned back on but that the switch was not held long enough to correct the trim. As I understand, the switch was activated for several short times.
Does the flight log record commands to the jack screw or actual jack screw movement?
I can visualize the co-pilot attempting to use electric trim when manual trims was not possible.
When using manual electric trim the hand-wheels spin.
If the hand-wheels did not move when electric trim was tried, it may be reasonable to use several short applications in an attempt to jar the electric trim loose.
Comments?

More a comment than a question;
I have trouble understanding the instructions. Manual operation? Is this manual with the thumb switches or manual with the hand-wheels or both? Is anyone else confused by this or is it just me?

Recovery from a stabilizer runaway. Given enough altitude, is a runaway trim always recoverable, or is there a combination of speed and dive angle from which it is not possible to recover?



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VEBill]

FAA AD states, "If runaway continues, hold the stabilizer trim wheel against rotation and trim the airplane manually."

The above extract implies that the human can overpower the electric motor.

This leads to another question. When the forces on the stabilizer are too great to allow trimming with the hand wheels, are the forces also too great for electric trim to overcome?

Bill, that's a very intriguing question.

I wonder if there's a simple explanation?

 

[waross]

A comment on design;
Now it seems as if the Max 8 simulator has been improperly programmed.
The 737 NG simulator may be programmed to replicate the control forces resulting from a runaway trim down condition but apparently the Max simulator does not replicate the extreme control forces experienced with a runaway trim.

Apparently the wings of the Max series have been strengthened to withstand the greater torque exerted on the wings by the uplift of the engines and the greater MA of the longer engine mounting pylons.
Apparently the tail has also been strengthened.

This may have serious implications related to the flawed simulator.
Simulators are programmed with flight data supplied by the manufacturer.
Was faulty data supplied to the simulator manufacturer?
This may call into question the forces calculated during the re-design of the wings and tail.
Are there any reports on the source of the flaw in the simulator?



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[SwinnyGG]

The assumption that the simulator can accurately simulate every possible flight state, including every possible control force state, etc is a false premise. That's not a goal of flight simulation.

 

[VEBill]

"...false premise..."

But in this case:

Boeing statement[/color]]"Boeing has made corrections to the 737 MAX simulator software and has provided additional information to device operators to ensure that the simulator experience is representative across different flight conditions."

Straight from Boeing (reportedly this is an exact quote).

 

[SwinnyGG]

'representative' does not mean 'an exact, perfect replication'.

So we're clear, the point I'm making here is that an inability to perfectly replicate extremely high control forces during certain coffin-corner areas of the flight envelope, especially those which the designers of the airframe would not anticipate end users encountering possibly ever, does not indicate that the simulator is 'flawed' as others have stated.

I'm not saying that the simulator is perfect- I'm just saying that the simulator, is a simulator.

The other questions raised in the last few posts (trim actuator authority at extreme AoA, flight performance data potentially used to build software models, etc) are valid/interesting/worth thinking about and discussing. This bit about control forces in the simulator is jumping at shadows.

 

[SwinnyGG]

This leads to another question. When the forces on the stabilizer are too great to allow trimming with the hand wheels, are the forces also too great for electric trim to overcome?

I believe (but do not know with certainty) that the answer to this question is 'no'.

I found a drawing at one point in this discussion that showed a detail of the trim jack screw and trim wheel mechanical layout. I'm attempting to re-find it and if I do I'll certainly post it here.

I'm pretty sure that turning the trim wheel manually in either direction, or holding it against torque supplied by the drive motor, causes the drive motor and jack screw to decouple.

If anyone else knows better please correct this or link to the drawing I'm talking about.

If I'm remembering incorrectly and the actuator doens't have enough authority to overcome the forces generated in these situations, that's obviously REALLY bad for Boeing.