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

 

[waross]

Hi VE1BLL;
This post is part of the discussion on simulators versus real world control forces and the generation of the feed back forces related to elevator action.

As I understand the drawing and the related information in the link that you posted, without compensation of some sort, when the stabilizer is moved the elevator would stay at the same angle to the fuselage. The connections between the stabilizer and the elevator linkage called the "ELEVATOR NEUTRAL SHIFT RODS" has the effect of keeping the elevator at the same angle to the stabilizer.
If the elevator PCU were anchored inside the rear of the stabilizer this compensation would probably not be needed.
Boeing chose to anchor the elevator PCU inside the fuselage and compensate for stabilizer movement.


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

 

[waross]

It was almost 50 years ago when I had a ride in the cockpit of a 737.
The first officer grew up next door to me and we went to school together. I rode in the navigators fold down jump seat.

The part of the trip that still stands out in my mind is the spinning trim wheels.
They spun quite fast, faster than they could be turned by hand.
The white stripes created a strobe effect that drew my attention no matter what I was looking at before the wheels started spinning.

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

 

[LionelHutz]

Bill, do you really think you'r the right person to be posting comments about reading links before posting? You continue to refuse to read the Ethiopian initial accident report but rather chose to post conjecture which is starting to give off a rotten stink.

"At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two
momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in
the ANU direction from 2.1 units to 2.3 units."

The FDR recorded both the command and the resulting movement.

As for and elevator feel computer, does anyone actually think this would be a sophisticated electronic device on a plane that went into service in 1968? Feeling the force via a device that takes the mechanical and/or hydraulic pressure required to hold the elevator in position as feedback sounds about right considering the tech available at the time of the 737 development.

 

[waross]

The FDR recorded both the command and the resulting movement.

Thank you for your information Lionel.
I was not trying to put you on the spot.
I was respecting your greater familiarity with the accident report, and rather than make a mistake, I asked you.

As far as reading links, I have read a lot in the link to SatCom Guru's site posted by VE1BLL.
He has a lot of information on the various systems on the 737.

Pioneering airplanes like the Boeing 707 used a mechanical control system to drive the elevator trim tabs.

The Boeing 737 uses Power Control Units (PCU), a hydraulic actuator, to drive the elevator. The indirect control introduces the need for an elevator feel computer, to artificially provide "representative aero loads" onto the pitch control yoke with a variable "neutral shift".

Like power steering but there are many types of power and hydraulic steering.
In an automobile the percentage of the steering force relayed to the steering wheel is a percentage of of the actual force on the steering mechanism, up to a limit.
The limit is often related to the strength of a small spring in the control valve.
In an automobile it is usually possible to steer with great effort in the event that the power steering system fails.
On agricultural and industrial equipment there are at least two other hydraulic systems in use.
In one system the steering wheel is mechanically linked to the steering system but the ratio of actual force to feedback force is so great that it is impossible to steer if the hydraulic system fails.
In another system there is no mechanical connection.
The steering wheel turns a rotary valve that commands steering action.
Feedback from the steering system rotates an internal part of the valve tending to stop the flow of hydraulic fluid.
Imagine a hydraulic cylinder with the control valve mounted on the ram. Pushing on the valve causes the ram to extend which moves the control valve further from your hand, tending to stop the movement of the ram.
This principle has been incorporated into a rotary valve which is used on some heavy equipment.
There is zero force feedback and it is impossible to steer without hydraulic pressure.
I have been reliably told that some new road graders no longer have a steering wheel. They are steered with a joy stick.
The grader operator who told me that missed his steering wheel.
I believe that industrial control computers were available and in use at the time that the 737 was being developed.
Electronic PID controllers were available and in use before the early 70s.

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

 

[debodine]

I noted in an earlier post a passage quoted from the Boeing team that the AOA information was never primary information but supplemental. I understand how supplemental information could be useful to establish a warning for the flight crew to look at something that could be a problem, even without redundancy.

And yet I would ask Boeing, why did they design to allow supplemental information to assume priority command of a critical flight control? Not only was there no redundancy of the sensor input, but there was obviously no "reasonableness check" of the incoming information! The AOA input value spiking back and forth to unreasonable values in an unreasonably short time should have triggered an internal filter to say, NO WAY AM I USING THIS DATA.

Note that my question is simply rhetorical and there is likely no one satisfactory answer. This forum has provided many potential answers such as profit motive, regulatory oversight failure, budget constraints, ego, hubris, etc. I recognize the true answer probably encompasses most or all of the potential answers presented on this forum.

 

[SwinnyGG]

Like power steering but there are many types of power and hydraulic steering.
[long paragraph about hydraulics]

I'm not trying to be rude here- but what you're saying just isn't correct.

All 737s have manual elevator control capability. It's in the manual. You can read all about it whenever you want.

If what you're saying was accurate, a failure of A and B hydraulics would result in zero capability to operate the elevator.

That is not what happens. During a complete failure of the hydraulics, elevator control is maintained (albeit with very high control forces) in a manual reversion mode. Period.

 

[LionelHutz]

Bill - what does steering on a wheeled vehicle have to do with a plane? There's really no point to any of that.

Just to make my point about this "computer" clear - I would bet against it being electronic. Trying to say that it's an electronic system because you read the word "computer" and then further implying that Boeing stupidly screwed up the simulator because it should have been easy to transition an electronic algorithm to simulator land is silly conjecture when you have no clue how the whole elevator system actually works.

That's why I didn't comment on how it's possible that the trim wheel could over-ride the electric motor even though the electric motor can provide more turning force to the jack screw. I don't know why and it's pointless to post random conjecture to try and explain it.

 

[waross]

All 737s have manual elevator control capability. It's in the manual. You can read all about it whenever you want.

I'd like to. Can you post a link?

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

 

[waross]

Lionel:

The elevator PCU acts as a booster to forces on the cables- it does not terminate them. It's exactly like power steering in your car. You can't draw conclusions from the simplified schematic.

Responding to this comparison.

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

 

[SwinnyGG]

See below.

Manual Reversion

With both hydraulic systems A and B inoperative, the ailerons and elevator are controlled manually. A noticeable dead band will be observed in both of these controls. High control forces are required for turns and the control wheel must be forcibly returned to the aileron neutral position.Both electric and manual trim are still functional. Do not over trim. The airplane should be trimmed slightly nose up and a light forward pressure held on the control column to minimize the effects of the elevator dead band.The rudder is powered by the standby hydraulic system. Caution must be exercised to not over-control the rudder.

 

[LionelHutz]

Bill - doesn't really matter, I still don't see the whole point of your blurb about power steering or the grader without a steering wheel since none of it has anything to do with how a 737 works. Besides, I think you're reading that statement way, way out of context.

 

[Alistair_Heaton]

The PCU is nothing special to be honest and does work on the same principles as power steering but has a couple of other inputs which are electronically calculated ie the yaw damper to stop dutch roll.

I can't find a diagram of the elevator system but here is the rudder PCU set up.

The elevator per say is not trimmed its the tail plane/horizontal stabiliser which is moved for trimming via the screw jack. This then sets the base point and the elevator allows input from there.

A common misconception is that trimming sets a control force for an attitude its actually trimming for a speed at which the control forces are zero at a given power setting. It doesn't set an attitude.

Because the PCU gives through direct control as well the amount of additional power it can give is linked to the piston size and hydraulic system pressure which is 205 bar.

The only additional inputs for the elevators is the mach trim and feel unit. The mach trim compensates for mach tuck and lifts the nose. At the levels that the incident occurred I wouldn't expect it to have got involved as they wouldn't have got over mach 0.6 even at 400knts. The feel unit is electronic controlled these days and it will increase the resistance to pilot input as the airspeed increases. This is the diagram for that system for the NG. Its also hydraulically powered. It would have got involved as the speed increased opposing the input of the pilots. But the base setting is the AoA of the tail plane/horizontal stab.


The area of the stab is 32.78 sq meters and the area of the elevators is 6.55 sqr meters. You can't really give the elevators more power or increase the size because the loads will go through the roof and there will be a huge weight penalty for beefing everything up to take the loads.

I can only comment on my own type which has a fixed stab but if the feel system goes off line the autopilot is then off line and your limited to 200 knts and you have to be extremely carefull you don't bend the tail. And it feels extremely wierd while your hand flying in fact its easier to fly it via the trim instead of direct control inputs which is usually extremly bad practise. Personally I use a thumb and index finger touch on the control and then adjust with trim. 12 mile finals and stabilised fully configured and trimmed by 2000ft. The last TRE that gave me that exercise said I know your flying by trim I can see it on the board but I can't argue with the results so we will leave that debrief point at that.

 

[Alistair_Heaton]

"FAA head says Boeing 737 MAX will return to flight, but timetable is uncertain
May 22, 2019 at 5:00 pm Updated May 22, 2019 at 8:09 pm
Dominic Gates By Dominic Gates
Seattle Times aerospace reporter


FORT WORTH, TEXAS — The head of the Federal Aviation Administration (FAA) on Wednesday insisted there’s no fixed schedule for lifting the order that has grounded Boeing’s 737 MAX since March 13.

“It takes as long as it takes,” said Acting FAA Administrator Dan Elwell. “The 737 MAX will fly again when we have gone through all of the necessary analysis to determine that it is safe to do so.

“If it takes a year to find everything we need to give us confidence to lift the order, then so be it,” he added. “I’m not tied to a timeline.”

Though the plane could still potentially return to service in the U.S. as early as August, that fast-track schedule privately suggested last month by both Boeing and the FAA may have been delayed by technical hitches and by public unease.

Ahead of Thursday’s meeting of top officials from civil aviation authorities around the globe to discuss what’s needed before the MAX can fly again, Elwell said Boeing has still not submitted its final proposed software fix for the flight-control system that erroneously activated on Lion Air and Ethiopian Airlines 737 MAXs and led to two deadly crashes — despite Boeing’s announcement last week that the software fix was “completed.”

An FAA spokesman on the sidelines of the meeting said the safety agency has a clear idea of the main elements of Boeing’s fix and knows the steps that need to be done to certify and validate Boeing’s work. How long those steps will take remains fuzzy.

Daniel Elwell, the acting administrator of the Federal Aviation Administration. Daniel Elwell, the acting administrator of the Federal Aviation Administration.

Elwell said Boeing had earlier committed to deliver its software fix on March 26, but at the last minute decided it needed to make adjustments after an independent internal Boeing review found problems that needed to be addressed.

Fifty-seven delegates representing civil aviation authorities in 33 countries have gathered for the meeting Thursday at the FAA’s southwest regional office in Fort Worth — including officials from Indonesia and Ethiopia, the two countries leading the investigations into the two fatal MAX accidents that killed 346 people.

“We will be sharing with them the safety analysis that will form the basis for our return-to-service decision,” Elwell said.

At his side during the press briefing was Ali Bahrami, the FAA’s head of aviation safety, who called this “a very extensive review” of Boeing’s software fix.

Elwell was asked how this safety analysis will be done differently than the FAA’s safety analysis of the original flight-control system, an analysis that, as the Seattle Times reported in March, was largely written by Boeing engineers and missed several crucial flaws that are now being fixed in the software update.

He responded only by saying that during the MAX’s initial certification the FAA followed the same proven procedures that have been used for 50 years.

The FAA has confirmed that among other issues, the original safety assessment, as detailed in the March Seattle Times story, did not take into account an increase in the authority of the problematic flight control system, called MCAS (Maneuvering Characteristics Augmentation System).

Boeing found during flight tests that for certain flight conditions, it needed to allow MCAS 2.5 units of movement instead of 0.6, quadrupling its power to move the jet’s tail and push the its nose down. Many FAA technical staff who worked on the MAX were never made aware of this change, and the system safety assessment done during original certification was never updated to include it.

An FAA spokesman said that the higher authority was designed only for “high angle of attack, low-speed turns,” and that “the change to MCAS didn’t trigger an additional safety assessment because it did not affect the most critical phase of flight, considered to be higher cruise speeds.”

However, at a crucial moment during the doomed Ethiopian flight, MCAS moved the tail 2.5 units and pushed the nose drastically down, even though the aircraft was moving at higher than normal speed.

The politics of getting international consensus as well as restoring the confidence of the traveling public may prove lengthy.

Elwell conceded that there may be a crisis of public confidence in Boeing’s jet “right now,” but said that the longstanding and proven processes of his agency that have produced the unrivaled U.S. aviation safety record over the past two decades will restore that lost confidence over time.

“I’m not worried about the future of public confidence, because I’m not worried about the future of aviation safety.” Elwell said.

He described the purpose of the international meeting as information sharing, so that a consensus may emerge.

“The idea is that we are working globally from the same sheet of music,” Elwell said. “So that when we are ready to lift the prohibition (on the MAX flying), there’s absolutely no question in any country’s mind why we are doing it and how we got there.”

He said the FAA will “explain our understanding of the risks that need addressed, the steps we propose to address those risks and how we’ll propose to bring the 737 MAX back into service.”

Though the U.S. safety agency typically takes the lead in the aviation world, consensus could be elusive after regulators in other countries decided to move first in grounding the plane and then learned of flaws in the FAA’s certification processes.

Countries such as China have indicated that they want to study the safety of the new system for a longer period and do their own assessment of it rather than just following the FAA.

It’s unclear if the FAA would move forward without key players on its side, including the European Union Aviation Safety Agency (EASA) and the Canadian civil aviation authority. But Elwell seemed to suggest that the US will take the lead and move first.

Because the U.S. is where the MAX was designed and issued its initial certification, which was then separately validated by regulators around the world, the ungrounding of the aircraft must follow a similar pattern, Elwell said. When its analysis is complete, the U.S. will certify the fix and lift the grounding, then other countries must validate the FAA’s work and make their own decisions, he said."

On a separate note this is what's going round in the pilot circles in Europe.

https://www.eurocockpit.be/news/boeings-max-return-service-we-need-answers-and-transparency

 

[itsmoked]

Ouch.

We could make a fortune on canvas plane covers...

Keith Cress
kcress -

http://www.flaminsystems.com

 

[3DDave]

Meanwhile, no airline has grounded all their pilots or all their mechanics until it's proved that they are all up to the task of operating with or diagnosing any other sensor problem. Those holes are going to remain lined up forever, even when exposed by a hole from the airplane maker.

 

[Alistair_Heaton]

The airline I work for has fired both pilots and mechanics for not making the grade in the last 6 months.

And 2 first officers have failed command upgrade. And it will be a year before they can start the process again which takes 18 months.

When I joined there was 4 of us doing the type rating with the airline and 3 of us passed the skills test. The guy that failed is still flying to my knowledge but not with us.

Sim checks in Europe are done with a three year cycle covering all the systems on a type in both winter and summer environments. Along with the mandatory single engine work and low viz requalification and the latest thing RNP approaches. These checks occur every 6 months so in the three years that's 48 hours I will have spent in the sim over 12 sessions and every session will include 1-2 known sensor issues, again over the three years all of them will be covered. At least once a year I will have to do a single approach, hand flown, raw data on emergency battery power which means 1 screen out of 5 working and standby artificial horizon, to go around and then landing with 200ft cloud base and 550meters vis. The first officers doesn't have to do that mainly because all there instrument screens are dead and anyway and they are usually too broken after pumping the gear down.

But I agree the holes will always be open. But the idea that airlines don't fire incompetent pilots or technicians is false. But most pax choose their airline by ticket price and trust the regulator to look after the safety. When the regulator doesn't do their job the whole system falls down and holes merge into one huge orifice complete with vortex.

To be honest most pilots would be more than happy to go and prove to who ever you like that they are competent to fly the MAX before strapping into one with paying pax down the back. Just the airline accountants don't want to pay for it. Boeing has fiddled things so they just have to do 45 mins on an Ipad and the FAA has certified that that's all required, and the rest of the aviation authorities have agreed and let them do it.

BTW the pilots in Europe are pushing that until the manual trim can overcome the aerodynamic loads to be able to retrim from full aft or forward limits via the manual trim wheel without having to do the roller coaster yo-yo procedure which needs 8000ft to pull off. Then the plane will remain uncertified and banned from EU airspace.

 

[btrueblood]

Alistair,

"BTW the pilots in Europe are pushing that until the manual trim can overcome the aerodynamic loads to be able to retrim from full aft or forward limits via the manual trim wheel without having to do the roller coaster yo-yo procedure which needs 8000ft to pull off. Then the plane will remain uncertified and banned from EU airspace."

But wasn't that procedure also called for under circumstances with the original through NX types of 737? Are they advocating banning all 737s? Is this really just Airbus pushing for banning their competitors product?

 

[3DDave]

Too bad these pilots weren't fired before it became obvious they didn't know how to manage an intermittent trim failure based on the yoke-force alone.

Too bad the Ethiopian pilots failed to prove it. Even the one who had just finished training. Would have taken, what, 3 minutes? I'd say that's ample evidence.

 

[Alistair_Heaton]

I don't have a clue to be honest.

As none A or B pilot I tend to stay away from the pair of them and the willy waving.

Airbus isn't involved and to be honest have been saying absolutely nothing on the subject that I have seen. They have 7000 orders for the neo and are targeting 63 airframes a month which is the next 9-10 years production taken care of, so what happens to the 737 really doesn't make the slightest bit of difference to them apart from any fall out from regulation changes. I suppose you could argue that the cut in 737 production has allowed them a more constant engine supply which has been an issue with the leap for a while. 10 years is enough to do a clean sheet next type.


What I am presuming the pilot group means is that if there is a system fitted that has the ability and capability of driving the trim to either limit then the pilot should be able to recover from it without performing aerobatics. . As the other variants don't have anything that will have this effect then they are ok. If you look at the MOR's on issues, trim runaway is single digit occurrences every year across all types flown in europe. Bird strikes on the other hand we are murdering thousands of them a month. Engine shut downs are in the 10-20 a month figure. trim runaway is way way outside the 1 in million event status.

Its coming from the scandi pilots, not the central europe ones. Which isn't surprising as you taxi past 3 max's every time you land on 04l at Helsinki and 8 of them every time you go past the 747 hotel in Arlanda I don't know where they are parking them in Oslo. And you don't mess with the scandi unions they took SAS down for 10 days on strike recently and basically wiped out all profits for over a year by doing so.


The only other major single Isle Boeing user in Europe is Ryanair who had their last NG delivered last month. And have delayed their max's until next summer. Between them and norwiegen they have under 250 on order. Its flydubai, lion air, Vietjet and spice jet you really need to worry about. Between those four you have 16% of the orders (800ish). I have heard nothing from the Ryanair pilots on the subject. They are more worried about Brexit than what aircraft they are going to fly next year and if the tax man is going to come knocking at thier door. And to be honest they are that disorganised as a group they would never get a united face for making a statement, they will get a choice of fly it or get another job.

To be honest anything that screws Ryanair up will gather more support across europe from pilots than anything to do with the country of origin of the aircraft type or the OEM. It might be seen as an anti boeing and anti US stance by the european pilots but its really not. There would be parties all over europe if they announced that they were going weld a couple of leaps onto a 757 airframe. There would be zero ground clearance issues if they did that!!!

 

[Alistair_Heaton]

You do know that Ethiopian pilots are trained and signed off in the US by FAA Boeing examiners for their type ratings?

Its also US check pilots that do the testing after that.

Its one of the few African counties that have stuck with the FAA.

There is nothing in the full 737 NG type rating syllabus issued by Boeing that covers the subject.

And to be honest if you did fire all the pilots in the world that would have crashed in similar circumstances.... aviation would over night become extremely safe basically because the sky would be virtually empty.

If you want to sell product to these countries your going to have to design it so that the locals can fly it and won't kill everyone if a system fails.