Eden Hill Journal

Comments, dreams, stories, and rantings from a middle-aged native of Maine living on a shoestring and a prayer in the woods of Maine. My portion of the family farm is to be known as Eden Hill Farm just because I want to call it that and because that's the closest thing to the truth that I could come up with. If you enjoy what I write, email me or make a comment. If you enjoy Eden Hill, come visit.

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Location: Maine, United States

Tuesday, March 26, 2019

MAX Time

I need some way to store and sort out all the information I've been coming across online concerning the 737 MAX issues that have the airplanes grounded. Using this blog seems like as good a way as any. This way anybody can have access to whatever I have found. I will need to update whenever I find new material or new information becomes available so I'm sure this entry in my blog will grow to enormous proportions. I intend to discuss ambiguities that seem to exist, different viewpoints that various sources are presenting based on different understandings of how the systems involved are designed and intended to work. There is a near vacuum of technical information from Boeing itself so there is a lot of room for misunderstandings and confusion. Even now after two disastrous crashes leading to the grounding of all 737 MAX-8 and MAX-9 airplanes, pilots still don't have the full picture. In my mind from what I've seen so far, pilots have not been well informed about the problem.

There have been two crashes. The first crash happened at about 6:30 in the morning on October 29, 2018 minutes after the airplane had taken off from Jakarta, Indonesia. All on board lost their lives as the airplane dove steeply into the ocean.
The second crash happened minutes after takeoff from Addis Ababa, Ethiopia on March 10, 2019. Again all onboard lost their lives as the airplane dove at a steep angle into the ground.


This article sums up the situation. From IEEE Spectrum dated April 18, 2019:
How the Boeing 737 Max Disaster Looks to a Software Developer
One of the comments on this article adds a bit to the article:
Very good analysis, but fatally incomplete. One really essential reason those planes crashed was that each time the MCAS triggered, it acted like it was the first time. If it added 1 degree of trim last time, it adds a second this time, a third next time, up to the five degrees that runs the trim all the way to the stops. A second reason is that, under the design still on file at the FAA, it could only add a maximum of 0.8 degrees (each time). This was raised to 2.4 degrees after testing, so only two hits could, in principle, put you almost to the stops. A third was that the only way to override the MCAS was to turn off power to the motor that worked the trim. But above 400 knots, the strength needed to dial back the trim with the hand crank was more than actual live pilots have, especially if it is taking all their strength to pull back on the yoke. A fourth was that, with two flight control computers, the pilot could (partly) turn off a misbehaving one, but there is no way to turn on the other one. You have to land first, to switch over, even though the other is doing all the work to be ready to fly the plane. A fifth was that it ignored that pilots were desperately pulling back on the yoke, which could have been a clue that it was doing the wrong thing. A sixth was that, besides comparing redundant sensors, it could have compared what the other flight computer thought it should be doing.

Here from SKTbrary is a good basic summary of what was known on April 4, 2019 following the second 737 MAX crash which happened March 10, 2019 in Ethiopia.

The real culprit is explained in this April 4 article from Air Current.

A New York Times video published here on YouTube, March 14, 2019, makes a good introduction for the problem the world is currently facing with this airplane. How Boeing Sold Its 737 Max | NYT News

The 737 MAX has software running on its flight computers that goes by the name of MCAS.

This November 13, 2018 updated November 17 article in The Air Current is a good introduction to the 737 MAX problem.

Here is an article from Leeham News dated November 14, 2018 giving a basic understanding of why MCAS is necessary in the MAX: Boeing’s automatic trim for the 737 MAX was not disclosed to the Pilots

For the technically inclined here is a rather complex explanation from Satcom Guru of the 737NG and to some degree the 737 MAX stabilizer trim systems. Fair warning: As with much of the technical information I am discovering, this discussion is seriously complicated by a failure to distinguish between the NG (New Generation model prior to the MAX) and the MAX model which is the most recent 737 model. There were apparently significant design changes between those two models in the computerized control systems with regards to the systems discussed in this article.

B737 NG Flight controls slide show for the previous 737 (NG) version - Horizontal stabilizer controls begin with slide 44 with elevator and stall information in preceding slides

Some seem to say this is the only reference made to MCAS prior to the Lion Air crash. Brazilian authorities conducted a study of the new MAX version of the 737 and found some differences and reported on them. MCAS appears on page 18 basically as not needing special attention.

The Second Problem
This article from Satcom Guru dated April 2, 2019 discusses the second issue related to the second crash, the Ethiopia crash in March 2019. MCAS had driven pitch down trim beyond the point where it could be cranked back to normal while the pilots were holding the airplane's nose up. They did not use the "roller coaster" or "yo-yo" maneuver to recover. Trim Cutout with Severe Out-of-Trim Stabilizer can be difficult to recover

Criminal Investigation

Speaking of Satcom Guru...
Boeing probe reaches Kirkland local after deadly crashes Peter Lemme, Satcom Guru, media consultant, served federal grand jury subpoena to testify as an expert witness

Pilot Checklist Recall Study

Here's an interesting tidbit of information from an FAA archive concerning a study done by a student to evaluate how familiar pilots were with emergency checklists. Note Figure 3 on page 23 and the introductory text for Figure 3. Four of the sixteen pilots studied tried using the electric trim switches instead of adhering to the runaway stabilizer trim protocol. After the Lion Air crash that became a part of the official protocol but, I have noticed in the forums,remains unnoticed by most to this very day..

MCAS - Maneuvering Characteristics Augmentation System

Schematic Diagram

There is a schematic diagram here posted by PixelFlight on or about April 8, 2109 showing signal flow and various switches related to MCAS and STAB TRIM in the 737 MAX.

What Is MCAS?

MCAS is characterized as an SAS, stability augmentation system and shares commonality in many respects with the STS, Speed Trim System, from earlier 737 NG models. MCAS is used when flaps are retracted whereas STS seems to be for conditions where flaps are extended (during takeoff and approach conditions).
MCAS can engage only when flaps are retracted. STS can engage only when flaps are extended.
MCAS is not disabled by control column cutout switchers. STS is disabled by control column cutout switches.
MCAS before the 2019 software update could drive the stabilizer to full pitch down. STS could do the same thing.
MCAS and STS are both overridden by yoke mounted electric manual trim switches.

Rumor of MCAS Authority Limit

Here's a good one. This is from a discussion on PPRuNe (Professional Pilots Rumors Network). The comment is by FCeng84 dated November 24, 2018 07:00. The commenter points out that MCAS has authority to issue a 2.5 pitch down command but will not issue another one unless the pilot uses manual electric trim or the autopilot has been engaged and disengaged. I did read or hear that the updated MCAS software would be limited to one command but was it originally limited to one? Maybe so.

Angle of Attack (Alpha) Sensors

From Boeing: 737 MAX Updates

YouTube video from a foreign source (showing the AoA unit but not in English): KNKT Temukan Persoalan Baru Lion Air JT-610

Horizontal Stabilizer Pitch Trim Theory

A good source of information on Facebook seems to be B737Theory.
Here's a good basic description of the 737 horizontal stabilizer trim setup and what all the numbers mean: Stabilizer Trim green band

This 737 blog entry has pictures and information about the pitch trim system in earlier models of the 737 and includes a picture of the jackscrew

737 NG Pitch Trim Block Diagram

Slide 48 of this instructional slide show for the 737 NG version shows a diagram of how pitch trim cutout switches are connected.

This November 7, 2018 article in Leeham News gives diagrams of the system as it was set up in the 737 NG. Don't confuse these diagrams as being MAX diagrams but even so, many of the ideas are the same for the MAX. Some are not.

Stabilizer Pitch Trim Jackscrew

There was a comment dated 24th Nov 2018, 18:33 at this PPRuNe web address that shows a drawing of the jackscrew including the cable drum. Indonesian aircraft missing off Jakarta

Jackscrew in action: Boeing 737NG horizontal stabilizer jackscrew cycle

Ball screw actuators, 737 has 25,000 pounds "limit load" Horizontal Stabilizer Trim Actuators

STS - Speed Trim System

One of the automatic systems used by the 737 that involves horizontal stabilizer pitch trim is Speed Trim. Here is a generalized 737 speed trim summary. Speed Trim System (STS)

An article in Aviation Stack Exchange offers insight into the difference between STS and MCAS.

YouTube video of speed trim on 737 - 300

SAS - Stability Augmentation System

Stability augmentation systems "SAS can automatically stabilize the aircraft in one or more axes." From Wikipedia.

Pitch Trim Jackscrew Jamming under Force

This April 2, 2019 article in Satcom Guru explains why the horizontal stabilizer may become jammed under certain opposing forces applied by the elevators: Trim Cutout with Severe Out-of-Trim Stabilizer can be difficult to recover

Great Graphics - False Information

Here's an informative article including some great graphics from Reuters Graphics. The picture at the top of this article is of a 737 MAX in flight steeply turning.
  • Take note of the steeply turning attitude of the 737 MAX in the initial background photo
  • Scroll down to the black and white picture of the cockpit of the 737 MAX and then place your mouse pointer over that picture and use the highly unusual scroll feature built into that picture.
  • As text scrolls up through the picture various flight controls become outlined in red
  • Note the text for the "Stabilizer Trim Control" switches on the two yokes
  • The claim made that MCAS can override these switches is false according to all information I have found to date including data from the Lion Air flight data recorder
  • The switches override any current MCAS commands but don't (or at least didn't before the April 2019 software fix - after the fix the operation is unclear) prevent MCAS from then issuing another pitch down trim command. 

The March 2019 Boeing Fix

From the same Leeham News website, here is a description of Boeing's software fix for MCAS.
From this article I learn:
  • that there is a difference in the Angle of Attack sensor outputs left and right when the airplane is in a sideslip
  • that MCAS will have limiting parameters for Angle of Attack sensors disagreeing
  • that the original design was bad engineering
  • that a commercial 737 pilot would normally never fly a 737 in such a way as to "experience an MCAS augmentation"
From News in Flight March 28, 2019
Features added on the updated MCAS software…

Pilots Unaware of MCAS

This March 15, 2019 Washington Post article highlights the issue that pilots were not informed about MCAS: How the FAA allows jetmakers to ‘self certify’ that planes meet U.S. safety requirements


These articles summarize the MCAS problem:
737 MCAS - Failure is an Option - from Satcom Guru November 15, 2018

A somewhat more judgmental evaluation with input from other sources is here on The Greanville Post: Ethiopian Airline Crash – Boeing Advice To 737 MAX Pilots Was Flawed

Lion Air Flight 610, October 29, 2018, Jakarta Indonesia


Quick Reference List

Official Documents
Preliminary Aircraft Accident Report - Flight 610 - Released late November 2018

Introductory Information

Lion Air was the airline operating this airplane.
Here are some references concerning that crash. The first is something I just found from the Insurance Journal dated November 8, 2019 and it gives preliminary insight into the causes of the crash.

This next link from Leeham News and Analysis dated March 22, 2019 is a discussion of a novel theory about what finally ended Flight 610. Although I see no particular reason to either believe or disbelieve the writer's theory, I find the article enormously informative and worth reading. Figure 2 is a chart made from data on the airplane's flight data recorder (the black box). Note the blue vertical line labeled B on the right side of this chart. That vertical line was added to highlight the point in time where the pilot handed over the controls to his copilot and down they all went.

Jakarta Weather

Here is information about the weather conditions in Jakarta the day of the crash.
This shows overcast skies and 79 degrees for the overnight low. It looks like winds were calm overnight but picked up by noon to a northwest wind between 7 and 8 miles per hour. So the only factor here that might be important is that skies were overcast. This might have meant the airplane was in low clouds during its flight and that means the pilots couldn't orient themselves by looking out the windows. The insurance article that I link to here seems to contradict me on this point. Other sources contradict me here too. The common story is that it was daylight with clear skies.
Sunrise that day was 5:28 AM so the flight was in daylight with the early morning sun in front of the airplane. Even if pilots could see the sun through the clouds, there's no way they could estimate their pitch and roll and yaw this way. They needed to see the ground to get that kind of information.

MCAS - Pilots Not Informed

Here is an early (November 16) report in Forbes that brings out one of the main points concerning this crash, namely that pilots were not prepared or even informed about what is being attributed as the cause of this crash, MCAS. I found a discussion referencing this article in an airliners.net forum.

Preliminary Accident Report for Lion Air Flight 610

A considerable amount of the information I have studied concerning this Lion Air Flight 610 crash comes from the Preliminary Aircraft Accident Report which is a PDF file that you can save to your computer. This report contains much information that has helped me understand what went on in that airplane that morning.

An article in DefenseWorld.net dated November 28, 2018 titled Installation Record of AOA Sensor not in Lion Air Crash Investigation Report: Boeing discusses maintenance issues on that airplane on previous flights. This article also brings attention to FAA Emergency Airworthiness Directive (AD) Number 2018-23-51 and Boeing Flight Crew Operations Manual Bulletin TBC-19. Both of these official documents were published on November 6, 2018 just one week after this Lion Air Flight 610 crash.

Here is a fairly well researched and well written article in The Seattle Times dated November 28, 2018 that I find helpful. In this article I discovered:
  • that the problem ensued "the moment they retracted the wing flaps at about 3,000 feet"
  • that the angle of attack problem existed undetected while the airplane was still on the ground before takeoff
  • that maintenance records indicate that the angle of attack problem was recorded on the flight from Bali to Jakarta the evening before
  • that the angle of attack sending unit was replaced in Bali before the Bali to Jakarta flight where the pilots used the cutout switches and continued their flight on to Jakarta (That's the flight where it was announced in March of 2019 that there was a guest pilot on board who solved the MCAS problem by suggesting that the pilots use the cutout switches)
  • That maintenance records for more than one flight before Bali indicate altitude and airspeed problems. [no mention of angle of attack before Bali]
  • That it's inconceivable that a pilot would have let this situation go on for as long as it did and not realized they needed to use the cutout switches
  • that the pilot , after 21 times using his yoke STAB TRIM buttons to counter the MCAS malfunction, handed over the controls of the airplane to the copilot (first officer) who then did not respond to the next two MCAS pitch down commands. Actually it appears from the data that the first officer did respond but only by making small adjustments. He did not use the STAB TRIM the way the pilot had been using it and he allowed MCAS to pitch the nose of the plane down into a dive.
  • That when the pilot recognized the emergency he took the controls back and pulled back hard on the control column but by then it was too late
  • that after the Lion Air crash and the Boeing and FAA response "every 737 pilot in the world" now knows about MCAS
A key factor apparent in stories related to this fatal Jakarta flight is that maintenance reports indicate that in prior recent flights of this airplane including the flight that landed just six and a half hours prior to the crash the airplane had maintenance issues related to faulty airspeed and altitude indications as well as a persistent problems with the elevator feel system, a seemingly unrelated system which relies on a pair of air pressure sensors mounted externally in the airstream on the tail of the airplane. Airspeed and altitude sensors are mounted near the nose of the airplane.

Another aspect of this story is the reaction of pilots after the Lion Air crash. It was only when information about the cause of this crash started coming out that pilots ever even heard the term MCAS. I found a Seattle Times article from November 16, 2018 covering these concerns.
Some things I discover here...
  • that there was a lot of disagreement and I'd think confusion about MCAS way back then [and still is]
  • that MCAS can be triggered in a steep bank or when speeds are so low the wings are about to stall [remind you, MCAS cannot be triggered when any flaps are in use]
  • that not only flight crews but maintenance technicians as well didn't have sufficient knowledge of the MCAS system
  • that some pilots and at least one pilot union didn't feel the need for concern about MCAS after the Lion Air crash
  • that in normal flying pilots should never see MCAS operating [meaning either that it does its thing all the time and the pilot never notices - or - meaning the airplane in normal flying will never be placed in an attitude extreme enough to trigger MCAS... big difference there between those two interpretations and your guess is as good as mine which meaning it should have]

John Q steps in...

John Q is my somewhat opinionated but outspoken objective observer, my fly-on-the-wall character invented very recently here in my blog. There is one huge question that John Q keeps asking me to answer for him and that is this:
Since it was well known to the various maintenance people that this airplane had needed maintenance in its last few flights and seeing as how the airplane was actually saved on the flight just prior by a guest pilot riding in the cockpit's jump seat, and since the maintenance people in Jakarta were dealing with this same dangerous repeat condition, then why - considering the nature of the problem - why didn't maintenance in Jakarta have a maintenance technician sitting in that jump seat just in case their shop hadn't repaired the problem? It makes absolutely no sense in John Q's mind for there not to be a maintenance guy onboard sitting right there with the pilots observing the airplane and giving advice.

The Maintenance Technician Question

I just came across a comment posted here by JayinKitsap after the Jakarta crash back in October that states this:
"A concern I have is this plane was serviced for this problem at least 2 times in the prior 4 flights and a tech was sent on this flight."

Although I long since lost the reference link I am certain that I read within the past two weeks that there was a maintenance technician on Lion Air flight 610 at the time of the crash. I'll be looking for that tidbit to show up again and either be confirmed or debunked.

I reference here a source that states, "One of the crew was a technician and three were flight attendants in training, per the airline."

Another mention comes in this discussion in pprune (Professional Pilots Rumor Network) in a comment by AO283 dated November 25, 2018 15:03. Speaking of the identification of bodies recovered he mentions one mechanic identified.
Further on in that same pprune thread there is mention of an AMP (Aviation Maintenance Professional?) onboard and speculation that the crew might have been troubleshooting the airplane's previously known flight control problems. See the post by threemiles, November 26 , 2018 14:29 and the following comment.
Another mention clearly suggesting a maintenance technician, "teckie", on the fatal flight, same forum, offered by CYTN November 9, 2018, 09:52.
From the same source, comment by SLFstu dated November 16, 2018 14:23 speaks of what a technician might have been doing on the airplane during the fatal flight and references a third quarter 2014 article in a Boeing publication beginning on page 05. The article deals with advanced maintenance data access in late NG and MAX airplanes, even suggesting maintenance teams onboard during flight.


Ethiopian Airlines Flight ET-302 March 10, 2019

Google this phrase: "aerodynamically relieve the airloads"

Quick Reference List

Official Documents
Introductory Information

From  from SKTbrary is a good basic summary 

From Air Current - jackscrew limitations  - "mistrim"April 4 article from Air Current

Aviation Herald

Aviation Herald beginning March 10, 2019 Crash: Ethiopian B38M near Bishoftu on Mar 10th 2019, impacted terrain after departure - updated frequently and with informative comments

A continuation of the March 10 article published April 16, 2019: Crash: Ethiopian B38M near Bishoftu on Mar 10th 2019, impacted terrain after departure Note: Comments from the prior article appear after this article as well.

Leeham News

ET302 used the Cut-Out switches to stop MCAS Leeham News - April 3, 2019 prior to release of official preliminary report: 
Bjorn’s Corner: ET302 crash report, the first analysis - Leeham News April 5, 2019

Professional Pilots Rumor Network

From an online pprune pilot's forum dated November 26, 2018 at 21:33 - MickG0105 discusses the 737 training concerning trimming manually from full nose down trim. He says it would take 50 rotations of the manual trim wheel to get from zero to five on the trim scale and adds a quote that it might take both pilots and even might require the need to "aerodynamically relieve the airloads"
That is confirmed here for the 737-300 in this comment dated March 2, 2008.


April 5, 2019

Manual Trim Not Working

Since the release of the Flight 302 preliminary accident report and even just prior to that release it has become increasingly evident that the pilots were somewhat familiar with the 737 MAX MCAS problem and were familiar with the runaway stabilizer trim procedure to use in the event of another MCAS malfunction similar to the Indonesian Lion Air problem. Yet what they encountered was that they could not relieve the forces on the rear stabilizer using manual trim.
I have observed by studying the available data that neither pilot on Flight 302 seemed to make the connection between using electric manual trim - STAB TRIM on the yokes - and disabling bad MCAS commands. In other words they didn't seem to discover that the yoke switches could be used to overcome MCAS. They therefore disabled both MCAS and the yoke STAB TRIM controls using the STAB TRIM CUTOUT switches and then unsuccessfully attempted manual trim using the trim wheels in the center console. Very recent speculation is saying that the forces acting on the stabilizer and therefore on the stabilizer jack screw would make this attempted manual trim difficult or even impossible without doing certain special and somewhat radical maneuvering, pilots pushing the yoke forward instead of holding it back with all their might.

I'll start here dated today, April 5, 2019: Bjorn’s Corner: ET302 crash report, the first analysis

This blog discusses the technicalities involved.

Another discussion in a pilots' forum is here... and here...

Here is the same situation from a less-informed perspective dated April 3, 2019:
What this article lacks is a good understanding of how the yoke switches override MCAS commands. Up until about the time of this article that was the consensus view among most media reports and online chatter even though the Lion Air data clearly proves otherwise.

The problem seems to be that there is a fundamental misunderstanding of the Boeing and FAA advice issued last November. It is gradually becoming evident that the yoke switches override MCAS and should be used to trim the airplane before cutting out electric trim entirely. Most observers - and I'd guess most pilots - overlooked that advice up until just the past few days.


Limitations for Electric Manual Trim (yoke STAB TRIM switches)

Read page 15 of this official document which has, as far as I can tell, no date but clearly refers to the 737 MAX.
For me it is unclear just exactly what this information implies. I have read and seen diagrams that show limitations on the trim range of the yoke mounted electric manual trim switches referred to here. However, and I may provide a reference to this in the future if need be, I have read reassurances that those yoke mounted trim switches can be used when the stabilizer trim is beyond the range of those switches. In other words, even though the pilots cannot use those switches to set stabilizer trim to its extremes, the switches can be used effectively to recover when some other systems sets trim beyond the range established for those switches. You can't use the switches to make trim get out that far but if it is already out there you can use those switches to recover.
This official document seems to add confusion rather than to add clarity. The question in the Ethiopian Flight 302 discussion is was there some reason the pilots didn't recover from the erroneous MCAS trim settings using the yoke mounted electric manual trim. I tend to think the reason is that in the stress of the moment they chose not to, not that there was some mechanical or electrical or computer limitation that prevented them from doing it. After all, the data shows that these switches were used one time to successfully return stabilizer trim from its full nose down jackscrew position. That's proof enough for me.

This document was revealed by Reuters March 29, 2019.


April 4, 2019

Preliminary Report Released

The Ministry of Transport for the Federal Democratic Republic of Ethiopia issued its preliminary accident report for Flight 302. Another copy of the report is here in a PDF file. The report is similar to the Lion Air report in format and includes a chart on page 26 showing pertinent flight control data plotted on a timeline from takeoff until the crash.
Times given are in UTC, Coordinated Universal Time.
Here are some of my early observations:
  • Unlike the Lion Air crash, the left Angle of Attack indication was normal until somewhere between 50 feet and 200 feet above ground level. The airplane was climbing out from takeoff, briefly pitched up sharply, and at its steepest pitch the left Angle of Attack indicator went haywire with a Flight Data Recorder reading of +75 degrees. This triggered the pilot's "stick shaker" which remained on for almost all of the remainder of the flight.
  • So the problem started somewhere between 50 feet and 200 feet of altitude above ground level.
  • Unlike in the Lion Air crash, the pilot of this Ethiopian flight engaged autopilot. It didn't stay engaged but did stay on for about 30 seconds
  • During the time the autopilot was engaged, the pilots raised flaps.
  • About 4 seconds after the autopilot disengaged and just as flaps were approaching full retraction the first MCAS command began and lasted long enough to move Stab Trim pitch down from 4.6 to 2.1. MCAS pitch down commands are set to adjust Stab Trim down by 2.5 units (degrees) for each 9 to 10 second command interval so this was a full, uninterrupted MCAS pitch down command.
  • The second MCAS pitch down command was delayed slightly by a short yoke-mounted electric manual trim command and was cut short by the pilots' use of those same yoke-mounted switches.
  • During the 10 seconds in which the pilots used the yoke-mounted Stab Trim switches, the pilots agreed to use the STAB TRIM CUTOUT procedure.
  • The third MCAS command was initiated about 5 seconds after the pilots released the yoke Stab Trim buttons. It was interrupted and cut short by unknown factors.
  • There is no indication that "Pitch Trim" changed at all during this third MCAS cycle. This seems to suggest that the pilots physically restrained the STAB TRIM wheels for the duration of this third MCAS cycle. The report reached a different conclusion about that fact but the data seems to contradict their conclusion.
  • From 05:40:52 until 05:43:09 there were no more recorded instances of either electric manual pitch trim or MCAS pitch trim commands.
  • During that time there is no significant change in the horizontal stabilizer "Pitch Trim" data. This suggests that pilots were unable to use the manual trim wheels to adjust pitch trim. This necessitated the pilots to hold significant pitch up elevator using the control column.
  • Then at 05:43:09 electric pitch trim appears to have reactivated with brief indications first of seemingly ineffective pilot commanded (yoke-mounted Stab Trim switches) electric manual trim up and then MCAS automatic trim down. This final MCAS command lasted only half the normal cycle but sent the stabilizer to very close to its maximum pitch down position leading to an out-of-control downward dive into the ground.
  • It seems likely to me that this 4th and final MCAS pitch down command was cut short not by the yoke mounted Stab Trim switches and not by the jack screw reaching its limit but rather by the pilots setting the STAB TRIM CUTOUT switches back to CUTOUT for the second and final time.
From this data it seems to me that:
  • The pilots never came to understand that the yoke-mounted Stab Trim switches could override the faulty MCAS commands. They only one time actually did that but it seems to be by coincidence.
  • They never used electric manual trim to neutralize control column forces.
  • They were unable or unwilling to use the manual pitch trim wheels except possibly to arrest the third MCAS pitch down command.
  • After MCAS had set pitch trim below 2.5 the pilots never returned trim above that figure even though they were operating in the more normal 5.0 range before the MCAS malfunction began.

Boeing's Response April 4, 2019

Boeing issued a response to the release of the Ethiopian preliminary accident report.
A video version of this statement is posted here by Boeing.

My Version of Events - April 6, 2019

I just made a comment in one of the comment forums that went like this:
Is it possible…
Bjorn, I wonder if I might suggest a few alternative interpretations of the flight data. I have noticed a few things that don’t add up if I see them the way you do or even the way the preliminary report does.
In the Ethiopian Flight 302 data there are four instances of MCAS inputting pitch down commands. In only one instance, the first of the four, does the MCAS command reach its designed 9 plus seconds.
The second MCAS command lasts only about 6 seconds at which time MCAS was overridden by the pilot’s electric manual trim (yoke switch) pitch up command.
The third MCAS command, if you look closely, doesn’t reach the full 9 plus seconds either. Something interrupted it as well. But what? There is no pilot-initiated electric manual trim (yoke switches) command at this point in time. So what cut short this third MCAS command?
The report seems to conclude that about the time of that third MCAS command initiating, the pilots switched off the STAB TRIM CUTOUT switches, yet the only evidence they cite is the discussion to do so which took place 6 to 8 seconds before this third MCAS command initiated.
Further complicating the data is the fact that there was no observed change in stabilizer position during that third MCAS command interval.
I have not seen any suggestion that it may have been that the first officer actually physically held the trim wheel to prevent it from turning during this third MCAS command cycle yet I have watched videos that clearly demonstrate how this can be done. This would explain why the MCAS command shows up in the data but the stabilizer did not move. If that was the case, then it is reasonable to conclude that this third MCAS command functioned only until one of the pilots flipped the STAB TRIM CUTOUT switches to CUTOUT. First they decided to do the runaway trim procedure. They waited with hands on the trim wheel to verify that MCAS was still active. Then one of them held the trim wheel to keep it from further pitching down, and about 9 seconds later flipped the cutouts, just slightly abbreviating that MCAS cycle.
The fourth MCAS command lasts only about 4 seconds before it is interrupted, but interrupted by what? My guess is that once again they hit the cutout switches during the MCAS command cycle and cut that cycle short.
That seems to me like a more accurate interpretation of the available data. Everything fits this way, no hanging mysteries. This is all physically possible but it is also highly probable. To me, what this would indicate is that these two pilots worked as a team. They had talked this all over before and understood their version of how to proceed. They were probably familiar with the Lion Air cycle of events. The one thing that never dawned on them is the one thing I have found it almost impossible to point out to anybody and that is that electric manual trim overrides MCAS and should be the first thing used to recover from MCAS. Recover, then disable stab trim, not the other way around.
You see the same thing happening during the fourth MCAS command which lasted only about 4 seconds before it was overridden by something. Since there is no electric manual trim (yoke switches) input corresponding to the end of this fourth MCAS command that would have overridden MCAS, it suggests to me that for the second time, MCAS was cut short by the use of the STAB TRIM CUTOUT switches.

John Q's View

It would seem to John Q that had the pilots realized they could use the electric manual Stab Trim switches on their yokes to adjust stabilizer trim to its normal range, they probably then could have neutralized the elevator and control column forces, set STAB TRIM switches to CUTOUT, and then flown the plane to a safe landing. The one piece of information they didn't seem to have was that their yoke stab trim switches had priority and could have gotten them out of danger if only they had used them to neutralize control column forces before using the STAB TRIM CUTOUT switches.


Two mysteries stand out for me.

First there is no indication that the pilots were able to bring "Pitch Trim" above approximately 2.4 once it had dropped below that level. Did the jack screw mechanically malfunction and somehow prevent the stabilizer from recovering above 2.4? Unfortunately after reengaging electric trim, neither pilot used electric manual trim (yoke switches) for a long enough period of time to judge one way or the other if there was a mechanical problem with electric trim up. Clearly electric trim down still worked because MCAS used it to deliver its 4th and final blow.

Second, one paragraph in this preliminary report seems confusing:
"Control column actuated stabilizer trim cutout switches stop operation of the main electric and autopilot trim when the control column movement opposes trim direction. When the STAB TRIM override switch is positioned to OVRD, electric trim can be used regardless of control column position."
No other documentation that has caught my eye has referred to that switch but research does show there is this additional switch in the center console of the 737. I've come across pictures of it so it is real and it is not the STAB TRIM CUTOUT pair of switches. It was a feature in earlier 737 models.
Possibly that this statement was placed in the preliminary report by mistake. Either that or it was placed there deliberately to suggest to readers that the investigation team was basically clueless. Studying the report I'm inclined to think that might be the case. Several of their observations seem to me to be misinformed.

April 4, 2019 - Prior to the release of the preliminary investigation report

As of this update the preliminary report on the crash of flight 302 has not been released. Nevertheless, many news outlets and even Ethiopian officials are indicating that the pilots of this fatal flight followed Boeing and FAA directives that came out after the lion Air crash. I will not speculate until I am able to see the data from this crash just as I have seen the data from the Lion Air crash.

Another pre-accident report perspective that finally discusses the point I have been trying to make in online forums is this article dated April 3, 2019 in Leeham News and Analysis.


March 2019

There is one source of information that I have found especially informative concerning the crash in Ethiopia this month (March 2019) and that source is an article in The Aviation Herald:
This article has been updated several times and there are informative comments in the comments section. Information from this crash has been slow in coming and until it does come it is only speculation that this airplane crashed from the same causes as the Lion Air airplane back in October. There seemed to be enough evidence though for officials around the world to conclude the likelihood and take action to ground all Boeing 737 MAX-8 and MAX-9 airplanes.

Update 3/26/19

The New York Times has an article out today about the Ethiopian flight 302 but it is behind a subscription wall. Yesterday The Guardian had an article that is the closest I have seen yet linking the cause of the Lion Air crash and the cause of the Ethiopian crash, that being the MCAS system which has been in the news since the Lion Air crash back in October. Recently I have begun to suspect that MCAS is not an actual physical system. In other words it doesn't add any weight to the airplane because it is software. It is a program running I would imagine on the airplane's primary flight control computers. There is what is called SMYD on the 737 MAX. I know this because that system was tested and even failed a maintenance check on October 27, two days before the Lion Air crash. Maintenance records also show a test of this system on October 26 where it appeared to be "normal". This information is in the preliminary accident report. More on this yaw damper system later.
This Satcom Guru reference indicates that The 737MAX does not use a separate SMYD "box", rather the functions have migrated to other "boxes".
If MCAS is a computer program it seems possible that it is part of the SMYD (Stall Management and Yaw Damper) system but it seems just as likely to me that the SMYD is also just a computer program. If that is the case then on what computers do these systems (computer programs) run? I'll be looking for an answer to that question but I would imagine these programs run on the two FCC Flight Control Computers. I could easily be wrong about that.
This same Satcom Guru reference does add some more information about where MCAS exists.


The 737 MAX Pitch Control

Pitch Control

I have found a ten part discussion of pitch control following the Lion Air crash:

Flight Control Laws

The term "control law" came up in my research so I went looking for more information and came up with this explanation which describes first the Airbus system and then the Boeing approach to the issue:
This discussion apparently predates the 737 MAX design which may vary from the 777 design that is described here.


MCAS is the name of a system used in the Boeing 737 MAX airplane only. This system, from what I can figure since the information needs to be gathered like pieces to a puzzle, but as near as I can figure MCAS is a function programmed into the two identical flight control computers.

"737 MAX Speed Trim System, including the MCAS function"
My reference for that quote comes under the heading "Training" at this Boeing webpage
I use that same resource for this next claim from the Key Definitions section
  • that MCAS is what is called a control law- "a set of software that performs flight control function or task"
  • That MCAS is part of the speed trim system - "a system that uses multiple components to provide additional speed or pitch stability when needed"
This next website is full of information about the 737 family up to and in many cases including the MAX version. Here's their take on MCAS.
Most of that information I tend to agree with but it seems noteworthy that there are several pieces of what I would think would be vital information that just are not being brought up in the media.
  • That in the event of an MCAS malfunction, electronic stabilizer trim can be used [and should be used] to neutralize control column pitch forces BEFORE  moving STAB TRIM cutout switches to CUTOUT. This is a life saving tip originating in Boeing's documentation dated November 6, 2018 that is not being noticed even by many pilots who are making informative videos or comments in the various forums.
  • That MCAS is a function of the Flight Control Computers(s) - I would presume a software function, not a hardware addition
  • The real reason why MCAS was installed in the MAX
  • What is meant by Steeply Turning, a criteria for MCAS to activate - addressed only briefly here
  • That in the event of a malfunctioning MCAS the pilots can always override it each time it kicks in just by using the STAB TRIM switches on the yokes
  • Not only that but the flaps have to be up in order for MCAS to activate. Put flaps down and MCAS deactivates
  • That when all is going as designed MCAS is programmed such that after it has altered STAB TRIM and the angle of attack has been reduced enough, MCAS then sends pitch up commands to return the airplane to its trim before MCAS became activated. In other words it doesn't leave the pitch trim in a dangerous nose down configuration. It corrects itself back to whatever was normal STAB TRIM. That fact, if it is an actual fact, is huge. In the Lion Air crash MCAS never corrected itself because the angle of attack information it was being fed never returned to a safe range.
It's all there on that one web page.

Another web page with MCAS information with familiarization with the 737 NG New Generation prior to the MAX.
737 FCC Pitch Axis Augmentation - Command Integrity Mandate for Dual Channel, Fail-Safe

In that article there are mentions of MCAS as being a function of the FCC flight computer, the pilot's one. Where the NG version had two separate Stall Management Yaw Dampers, the new MAX uses other systems, I'm looking for a connection between the elevator feel system and MCAS, perhaps as an input, and perhaps the speed control system which outputs to the stabilizer trim.

Autopilot Question
One thought just dawned on me. These two STAB TRIM CUTOUT switches. Let's say you're the pilot flying along on autopilot and you reach down to that center console and you flip those two switches down to CUTOUT, then what comes next. Does the autopilot kick out? If those switches are in CUTOUT then is it even possible to fly the airplane on autopilot. If not, then how could a pilot justify continuing the flight? One did exactly that and the second, the fatal Lion Air one seemed to be contemplating it. I'll need to find an answer to that question. Maybe autopilot can still have control of STAB TRIM bypassing those cutouts. I've seen at least one of those same two switches on the previous models of the 737 labeled autopilot if that tells you anything.


Zeke Chimes In

John Q has been theorizing that none of the discussions about this MCAS problem with the 737 MAX is based on good solid common sense. Then he came across this comment in an airliners.net forum posted by zeke four months ago:
I beg to differ, let me explain my reasoning.
With normal trim in manual flight the trim switches not only command the direction they physically complete the circuit to supply power to the activate the stabiliser, when the switches move the pilot is sending power to the trim.
Pilots who fly the 737NG would know the speed trim function provides a nose down trim incremental command as the airplane slows into a stall when being manually flown (the autopilot provides nose up trim in the same situation), MCAS also provides nose down trim.
When a pilot in manual flight uses the trim switches it pauses the MCAS nose down trim for 5 seconds. Whereas the normal trim in manual flight also has the speed trim which is commanded by the autopilot even when the autopilot is disengage. Both speed trim and MCAS are autopilot functions however they are different.
Speed trim is stopped by the column cutout switch that trips if the column is moved significantly in opposite direction to the trim. The pilot pulling back with nose trim down would stop the nose down trim command with back pressure on the control column, it will physically open the electrical circuit removing power from the trim. That is the normal behaviour pilots on the 737NG expect.
However when MCAS is activated it ignores the control column cutout switches (the normal behaviour on the 737NG) and keeps trimming nose down.
Unlike the 737NG to interrupt the MCAS, the pilot either presses the trim switches on the control column which will interrupt MCAS for 5 seconds, or they have to do the counterintuitive thing and turn off the autopilot trim switch when flying manually.
This is fairly detailed systems knowledge which has little to do with basic airmanship.
Until this comment I had not come across any information suggesting that the earlier NG version of the 737 used automatic pitch trim (horizontal stabilizer) pitch down commands when approaching a stall with autopilot off. That's what zeke seems to be saying when he says, "Pilots who fly the 737NG would know the speed trim function provides a nose down trim incremental command as the airplane slows into a stall when being manually flown". He's saying that pulling the yoke back disengages that stall avoidance function on the 737NG version of the airplane. He goes on to say that in the MAX version, pulling the yoke back does not disengage the function but using the trim switches on the yoke does. He refers to turning off the trim switches as "counterintuitive" and John Q fully agrees on that point. So far, though, zeke is the only human who seems to be aware of that fact. John Q is fictional.

MCAS and Steep Turns

I just found this article in FlightGlobal which has an informative graphic loosely summarizing MCAS, what triggers it and what overcomes it. This is only the second source of information indicating that the MCAS system is triggered in steep turns, the first being here. Other than these two graphics there is no talk whatsoever that I have come across concerning the steep turn aspect of MCAS other than here on this webpage:
737 MAX - MCAS
"MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching stall."

I take it back. The notion of steep banks being related to MCAS has been brought up although I have yet to find a clear understanding of just how does a steep turn affect MCAS, how does MCAS determine it, and is this in conjunction with a high angle of attack? Questions still unanswered.

MCAS AND/OR Logic Mystery

None of the sources I have found to date explains the "Steeply Turning" aspect of MCAS. Clearly the idea of the airplane being steeply turned triggering MCAS pitch down commands exists. The idea exists. But does the airplane have to be steeply turning before MCAS can be triggered? That's the mystery. Nobody is saying.
  • It could be that high angle of attack AND steeply turning when detected together are the only conditions under which MCAS is triggered.
  • It could be that high angle of attack, steeply turning or not, triggers MCAS.
  • It could be that steeply turning regardless of angle of attack triggers MCAS.
Which is it? Who knows. Nobody's talking about it and I can't find the answer. Everybody's talking as though all it takes is a high angle of attack but if that is true, then why does the steeply turning factor even appear in the documentation?

Slip in a Turn

It is insightful at this point to study a topic that is well described on this web page:
Why Skids Are More Dangerous Than Slips

John Q sees a lot of similarity between what this article describes and what MCAS seems to be designed to detect and automatically counter.
Here's a perfect example of a large Boeing-built airplane doing exactly this at an airshow and diving right into the ground at the 1994 Fairchild AFB airshow.
1994 Fairchild Air Force Base B-52 Plane Crash

Ironically enough, MCAS is not programmed to act this way if the airplane has flaps extended, in other words any time the airplane in normal flight is flying low such as during takeoffs and landing approaches, MCAS is inactive. MCAS would only save a 737 MAX in a situation like this Fairchild AFB case if the pilot was not using any flaps such as when an airplane pulls up sharply and goes into a banked turn after a high-speed pass at an airshow.

Who is in control?

This same b737.org.uk website, as I pointed out above, refers to MCAS as a function of the FCC, the Flight Control Computer. In other words it is a program that runs on the Flight Control Computer that uses inputs to the Flight Control Computer and sends outputs to at least one channel that this computer controls already, Pitch Trim, or probably more accurately the STM. STM is the stabilizer trim  motor. Depending on various conditions, flaps up or down, autopilot on or off, this trim motor operates at different rates of trim, different degrees per second. MCAS drives it at .27 degrees per second which isn't the fastest it can go but three times faster than it is driven by the autopilot in normal flight when flaps are up (retracted).
There is a puzzling sentence on this page in a picture that contains the designator 27-41 January 2017. It reads thus, "Only the F/Os column cutout switch module is affected because it is the only module that interfaces with the FCCs." The column is the post that the yoke is mounted on and it moves fore and aft to control the airplane's elevator. F/O refers to First Officer, in other words the copilot. The cutout switch module here is described in this same picture in the section titled "Column Cutout Switches and Column Input" and refers to switches that prevent STAB TRIM commands opposing the direction the pilots hold on the elevator. I interpret this to mean, as an example, that if the pilot is pulling back on the yoke asking the elevator to pitch the nose up, his STAB TRIM switch will not let him pitch the stabilizer trim for nose down. But he can still use STAB TRIM to pitch the nose up. The only time the opposite is allowed is when MCAS is triggered. Then pulling or pushing the "column" for elevator control does not disengage the opposing MCAS trim commands. These cutout switches are not to be confused with the STAB TRIM cutout toggle switches on the console.

Oh and STM is the abbreviation for Stabilizer Trim Motor. There are a bunch of abbreviations listed on pages iv and v in the preliminary Lion Air crash report but STM isn't one of them.
That's my take on it from what I've gathered so far.

Flight Control Computers (FCCs)

It remains a mystery to me how the airplane decides which of the two Flight Control Computers (FCCs) is actually in control of the airplane at any given time. I have read that they switch themselves from one flight to the next but it doesn't make sense for the pilot and first officer to not have the ability to switch from one to the other in flight. There must be a way to do that. I mean what if the pilot realizes his instruments aren't reading accurately. Can't he then switch the airplane's displays and systems to the other Flight Control Computer if he feels that the other one is behaving more reliably?

Information about the MAX Flight Control Computer software is here:
Flight Control Computer (FCC)

From this source comes this bit of information:
"Since MCAS is an FCC function, the AoA source for MCAS is that of the FCC in use; ie FCC 1 uses the Captains AoA probe and FCC 2 uses the F/Os AoA probe. When the 737 is powered up the FCC used is FCC 1 for that flight, this changes for each subsequent flight until the aircraft is powered down. Therefore the AOA sensor that is used for MCAS changes with each flight post power-up."
I have no idea how reliable that information is.

Angle of Attack

Central to the discussions about MCAS and the Jakarta Lion Air crash are the angle of attack signals which originate from the angle of attack sensors located outside the airplane, one on the pilot's side close to the pilot's position and one in the corresponding location on the copilot's side. Information that I cannot confirm indicates that, "FCC 1 uses the Captains AoA probe and FCC 2 uses the F/Os AoA probe" (http://www.b737.org.uk/mcas.htm#aoa). Again, FCC means Flight Control Computer.

This same source discusses the Angle of Attack Disagree Alert which was an option offered by Boeing before the grounding but as I understand will become a standard function after the software fix that restores the 737 MAX airworthiness. Here is a curious statement, again from this same source:
"Crucially, there is an AoA Disagree Alert which displays the word "AOA DISAGREE" in amber at the bottom left of the PFD if the AoA vanes disagree by more than 10 degrees for more than 10 continuous seconds."
Think about that. This seems to indicate that if the angle of attack sensor disagree by less than ten degrees, the condition can persist continuously without triggering the "AoA Disagree Alert". Even a disagreement for under ten seconds of ten degrees or more won't trigger the alert.
I'm thinking that angle of attack readings can differ from one side of the airplane to the other within what might be considered the normal maneuvering envelope for this airplane. That's what that sentence means to me.
But what conditions might cause that variation in angle of attack sensor readings in the first place? I'm going to go out on a limb here and say it's when the airplane is slipping or skidding.
This article which I addressed above under "Slip in a Turn" has graphics which illustrate air movements over the airplane in slip and skid conditions. The article even discusses how in such conditions the angle of attack for the wing on one side of the airplane can differ from the angle of attack for the opposite wing in these same flight conditions.
So perhaps the angle of attack indicators actually reflect these differences when the airplane is in a slip or skid. That would explain the deviation allowed by the "AoA Disagree Alert" detection.
OK, so good so far but that brings up these questions:
  • Is it possible for the airplane to enter a slip or a skid without the pilots being aware it is doing it?
  • What systems or what malfunctions might cause it to do such a thing?
My guess is it has to do with the yaw damper system, either by itself (faulty inputs perhaps) or in conjunction with something else such as excessive drag on one side of the airplane dragging it into a yawed flight attitude or perhaps something that causes the engines to have unequal thrust like perhaps a sticking reverse thrust panel. This is speculation but I'd be looking for clues indicating such a condition in both of these 737 MAX crashes.

[More to come as time allows. This is an unedited version so far as of 3/26/19]


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