Ethiopian airliner down in Africa

[Organfreak]

Originally Posted by Chronus

Here is an interesting piece on the STM :


"• Boeing 737NG • Improve field reliability of the Horizontal Stabilizer Trim Motor (STM) • Solder joint fatigue of the memory chip • 6355C0001-01 to 6355C0001-02
""Joint investigation into the STM’s field reliability with Boeing, along with lessons learned from 737MAX qualification, have yielded design improvements to the Control PWA.".

The link is at:

file:///C:/Users/Tony/Downloads/Jay%...A302_FINAL.pdf

Sorry, link is bad. Looks as if it's a local file on your puter.

[Chronus]

Originally Posted by Organfreak

Sorry, link is bad. Looks as if it's a local file on your puter.

Apologies for that, it is a pdf file on www.eaton.eu

Here is the slide :
Boeing 737NG • Improve field reliability of the Horizontal Stabilizer Trim Motor (STM) • Solder joint fatigue of the memory chip • 6355C0001-01 to 6355C0001-02 5 © 2018 Eaton. All Rights Reserved.. Background Part history • Eaton has been the provider of the Stabilizer Trim Motor used on the Boeing 737NG since 1996. • 6355B0001-02/-03:1996 - 2003 • 6355C0001-01: 2003 - 2017 • 6355C0001-02: 2017 - Present • 6355D0001-01: 2016 - Present • Unique to the 737MAX and not interchangeable with 6355B/C configurations 6 © 2018 Eaton. All Rights Reserved.. Background • Stabilizer Trim Motor (STM) exceeds Boeing’s reliability requirements. However, in partnership with Boeing and using Eaton’s continuous improvement process and operator feedback, design improvements have been made to further improve field reliability. • Primary failure mode is solder joint fatigue of the non-volatile memory (NVM) chip due to a combination of uncontrolled environmental conditions causing bending, vibration and thermal expansion stresses. • Eaton implemented a shimming procedure that will ensure the control printed wire assembly (PWA) is held flat, so that all component leads, including the NVM, are free from mechanical bending stress. This change is identified on 6355C0001-01 units as Modification 9 (“Mod 9”). • Boeing has performed an in-depth analysis of the Horizontal Stabilizer Trim System Wiring design to include ships wiring, switches and production breaks and splices. The results of the Boeing Study show that operators may reduce the No Fault Found test results for the STM by checking the airplane wiring and switching components prior to removal. 7 © 2018 Eaton. All Rights Reserved.. Solution • Joint investigation into the STM’s field reliability with Boeing, along with lessons learned from 737MAX qualification, have yielded design improvements to the Control PWA. • Modifications to the NVM chip package which is significantly more robust to vibration and thermal expansion. • Control PWA changes include NVM component replacement, component land pattern optimization, component height spacers, copper balancing of the PWB, reduction of hand soldering operations and addressing potential obsolescence concerns. • An upgrade of a 6355C0001-01 to a 6355C0001-02 will require a new Control PWA (the PWA itself can not be upgraded). • Long term lab testing of the 6355C0001-02 shows that the failure mode of solder joint fatigue of the Non-Volatile Memory (

[Organfreak]

THANK YOU!

[bill fly]

Originally Posted by Albino

Just to confirm, I'm talking about 'Control airplane pitch manually with control column and main electric trim as needed'

This is pretty basic stuff.

Also Memberberry and Derfed,

This illustrates my point that the stab runaway checklist is unsuitable for the false MCAS operation case. Point two in the stab runaway checklist presumed that AP was on and that AP trim was the trouble maker, if runaway stops after disconnecting, so now try thumb trim etc.
You can’t just pick bits out of a drill and say “I’ll use this point now”.
What is needed is a QRH checklist with memory items to cover the specific fault, including possible accompanying distracting indications.
As MCAS logic is in the process of being modified, a checklist which might have covered the two fatal cases may well not be suitable for the new mod.
What these men were faced with was a checklist for a different situation, nevertheless recommended by Boeing - no time to reflect on its merits and no ground study of any other suitable procedure.
We can all sit here and sort out measures at our leisure, which those pilots couldn’t.
A new and case specific drill needs to be made before further ops.

[TryingToLearn]

Originally Posted by Derfred

He did say one strange thing though: if it is correct in translation, he asked the F/O "to trim up with him". This was after the second MCAS. He had only responded to the first MCAS with a relatively short "blip" (2.1 to 2.4 units). The response to the second MCAS (presumably with the F/O "helping") was around 11 seconds and took it from 0.4 to 2.3 units (if only they'd kept going another 11 seconds).

So why did he need the F/O's help to activate main electric trim? It's a thumb switch. Was the stick-shaker that annoying he couldn't activate the thumb switch? Could this mystery have anything to do with the two fatally short "blips" near the end of the flight?

If I turn a thunb switch which should turn a wheel and nothing happens, I release it before I cause further damage. Then I ask another person with another thumb switch if the other one is working... This is the only straight-forward strategy, the last straw...
I already raised the questions if the short blips are short because nothing happened. This woult be in line with the CVR. And maybe the Lion Air PF handed over control because he lost control of the el. trim?

So maybe either:
-> the motor overheatet after x cycles / went into thermal protection
-> Aerodynamic forces blocked the motor
-> The jackscrew got damaged (blocking ANU completely)
Both flights showed such blips before things went wrong completely. Such things will probably never show up in the simulator and you presumably won't try them out on test flights either...

Second mystery: Why were 3 AoA sensors damaged or 'misinterpreted' within 5 months? And why are there so many similarities in both maintainence logs? Is this the second bug they found? Data acquisition?

[silverstrata]

These incidents are simply a continuation of the basic 737 problem - that the design is too old, and would not pass modern certification. So we had...

Single rudder actuators, that had a tendency to turn the aircraft upside down.
Patched.
The same warning horn for config and high altitude, resulting in crews running out of oxy.
Patched.
Speedbrakes that caused elevator vibrations and flight control problems.
Patched.
Wing too close to ground for modern fan engines.
Patched.
Fuel pumps relocated into the center tanks, which overheated and exploded.
Patched (sort of).
Stab-trimmer becomes inoperative with high flight loads.
Forgotten about.
Elevators unable to be separated if jammed.
Forgotten about.
Anti-stall trimmers, which fly the aircraft into the ground.
Awaiting a patch.

But how many band-aids can you stick on a 60 year old design?

Silver

[gums]

Salute!

PEI. And Hans.....

Hans is describing my old VooDoo approach to the infamous “pitchup”. The stick got lighter and if you ignored the wing rock and buffet and then pulled more, then guess what? Duhhhhh. We had a manual limiter when not in AP control stick steering, and it took over 60 pounds to overcome. Then you hit the pusher.

The Viper control laws did not command any control surface opposite pilot input until AoA was above 30 digs or so. Then it removed your pitch and roll authority! It then applied anti-spin rudder. Meanwhile, your stab was fully deflected to get the nose down but could not help - you were in a deep stall. Engage and hold the pitch override switch and rock out of the stall. It was hard to get there, but after the new switch we could get out fairly quickly with 10,000 feet or so below. Maybe sooner if you had practiced, heh heh.

So bottomline, is unlike the MCAS, our FBW did not put in commands except as described above. It limited rates, gee and AoA. No matter how much you yanked and banked. About 35 pounds back stick and 17 pounds for roll.

Later, and we can move to Tech Log

Gums sends...

[MurphyWasRight]

Originally Posted by bill fly

Also Memberberry and Derfed,

This illustrates my point that the stab runaway checklist is unsuitable for the false MCAS operation case. Point two in the stab runaway checklist presumed that AP was on and that AP trim was the trouble maker, if runaway stops after disconnecting, so now try thumb trim etc.
You can’t just pick bits out of a drill and say “I’ll use this point now”.
What is needed is a QRH checklist with memory items to cover the specific fault, including possible accompanying distracting indications.
As MCAS logic is in the process of being modified, a checklist which might have covered the two fatal cases may well not be suitable for the new mod.
What these men were faced with was a checklist for a different situation, nevertheless recommended by Boeing - no time to reflect on its merits and no ground study of any other suitable procedure.
We can all sit here and sort out measures at our leisure, which those pilots couldn’t.
A new and case specific drill needs to be made before further ops.

Precisely.

Reading the emergency AD and revised runaway trim checklist issued after the Lion Air accident it is hard to avoid a conclusion that it was carefully crafted to address the newly revealed MCAS system while preserving the 'party line' that the existing checklist would have worked, hence no blame to Boeing.

Especially for english as a second language crews it would be difficult to 'read between the lawyerse lines' to extract the needed critical information, only some of which was in a 'note' not part of the flow.

Had a new clear and forcefull MCAS misbehave checklist been issued at that time we almost certainly would not be 3500 posts into a discussion of an event that cost 157 lives and billions of dollars.

Plenty of good suggestions here so I will add just one for what should have been in the checklist:

In case of a suspected MCAS issue momentary activation of up then down trim every 3 seconds will effectively disable the MCAS system.
The trim and other issues can be addressed in the normal manner while this sequence is continued.

[spornrad]

Broken Thumbswitch?

Originally Posted by Derfred

...He did say one strange thing though: if it is correct in translation, he asked the F/O "to trim up with him". This was after the second MCAS. He had only responded to the first MCAS with a relatively short "blip" (2.1 to 2.4 units). The response to the second MCAS (presumably with the F/O "helping") was around 11 seconds and took it from 0.4 to 2.3 units (if only they'd kept going another 11 seconds).
So why did he need the F/O's help to activate main electric trim? It's a thumb switch. Was the stick-shaker that annoying he couldn't activate the thumb switch? Could this mystery have anything to do with the two fatally short "blips" near the end of the flight?

Before the MCAS input the PF trims up twice with the thumb switch for appr. 3 s each. This results in appr 1 U uptrim stab movement, twice.
At 5:40:15 after the first MCAS ND he trims again up for appr. 3 s. The stab movement is barely visible now on the chart. Then, unusual:
At 05:40:27, the Captain advised the First-Officer to trim up with him.
Why? Is his switch not working correctly?
The trim up (together or) by the FO is the only long one in the entire FDR recording. It shows again a stab movement at appr 1 U / 3 s. It is followed by STAB TRIM CUTOUT.
Is it possible that the PF had a broken trimswitch?

[wheelsright]

Originally Posted by Derfred

That’s not entirely true, as soon as MCAS starts trimming you can stop it with a reverse blip. You don’t need to wait for it to punish you for 9 seconds.

How do you know that?

[MurphyWasRight]

Originally Posted by Takwis

I think my first line of defense would be to leave the flaps at 1...or get them there. Completely agree on the instructions after the Lion Air crash.

As I said plenty of ideas on what should be in checklist and order of actions etc. I am sure that Boeing engineers/pilots would be able to provide the best one if given the mandate.

An interesting outcome of the inevitable legal discovery actions will be the internal Boeing discussions leading to the AD and revised checklist. How much pressure to "keep it in type" and similar.

[RickNRoll]

Originally Posted by Capn Bloggs

So we're test pilots too now? No mention anywhere by Boeing of keeping the speed down, these guys were probably totally focussed on the MCAS issue, not your bog-standard UAS/80%. Ask yourself: when did you last pull off a handful of thrust at 1000ft above ground, straight after takeoff with the stick shaker going? I would suggest that some pilots don't even touch the thrust levers after takeoff normally, let alone have it in their muscle memory to do it when all hell has just broken loose.


Any professional pilot would already have a very good visual concept of what happened here. Absolutely no need to watch someone dying, thanks.

that's the thing. What testing was done by Boeing to ensure this vital sequence of actions was debugged? The stick shaker was also present on other flights but was ignored by the AD. MCAS runaway was just the endgame of a series of events. That would have taken more time to fully test and document. It would have also been more complex, perhaps necessitating the grounding of the plane so that pilots could have more training. There was no time in that cockpit for the data of debate that are going on in the web.

[deltafox44]

Originally Posted by Albino

You need to go and read step 2 again. It wasn't completed, at any point after the MCAS AND.

What would disconnecting the autothrottle have achieved? Well you could set thrust to something more sensible and fly the aircraft.

You mean page 2 of the memo that gave specific operating instructions to crew? It even included a note (obvious to most with any common sense!) to put the aircraft in trim, then cut out the stabs.

Not so. Step 2 is "Autopilot (if engaged) disengage". A/P was no longer engaged. So forget Step 2. Even if you don't, it says "Control pitch with column and trim". The pitch has been controled, the trim was back at the limit of green band though a significative column force remained.

Never is said to use electric trim to full trim before cutout, nor any warning that manuel trim might be impossible at higher mistrim/speeds

[deltafox44]

Originally Posted by GordonR_Cape

The takeoff ground speed, engine power, and pitch attitude in the climb, would probably all have been higher, to compensate for the lower air density (about 80% of sea level pressure). Not a pilot, but I can't think of any other differences, once they were airborne.

Less oxygen coming to the brain also. May be negligible, especially as they were used to this altitude, not higher than cabin altitude in flight.

[jimjim1]

Originally Posted by wheelsright

"as soon as MCAS starts trimming you can stop it with a reverse blip."

How do you know that?

I can't recall exactly but I have seen quite authoritative looking documents on t'interwebby that state it.

You can look for yourself at the LionAir preliminary report and notice the frequent truncated AND automatic trim pulses coincident with manual ANU trim pulses beginning. This was during the period when the captain was maintaining good trim on average for several minutes.

http://www.flightradar24.com/blog/wp...ary-Report.pdf

Edited to add:-
https://theaircurrent.com/aviation-s...ion-air-crash/

Has an image of what appears to be a Boeing document that includes the text "The nose down stabilizer trim movement can be stopped and reversed by the use of the electric stabilizer trim switches ... but may restart ...". See the Orange marked section below.

[deltafox44]

Originally Posted by Ian W

99.9% of PF would trim back to in trim if MCAS did operate incorrectly giving significant uncommanded nose down trim and after 2 or at worst 3 trim back would switch off Stab Trim.

99.9%, really ?

The pilots of the 2 last Lion Air flights did it. The Copilot in Lion Air crash and the Captain in ET302 did not. That's only 50% !

By the way, when speaking that an "average pilot" should be enough to succeed avoiding a crash, it implies that the crash probability is very near 50%, since by definition 49,99% of the pilots are higher than average and 49,99% under average

[Airbubba]

Here's an explanation of the EASA position on the observation that the yoke trim switches on the MAX don't work throughout the entire speed envelope (with some highlighted text):

Explanatory Note to TCDS IM.A.120 – Boeing 737 Issue 10

EQUIVALENT SAFETY FINDING:
B-05/MAX: Longitudinal trim at Vmo
APPLICABILITY: Boeing B737-7/-8/-9 REQUIREMENTS: CS 25.161(a), CS 25.161(c)(3), CS 25.1301(a) and CS 25.1309(a) ADVISORY MATERIAL: N/A

STATEMENT OF ISSUE

The aisle stand trim switches can be used to trim the airplane throughout the flight envelope and fully complies with the reference regulation. Simulation has demonstrated that the thumb switch trim does not have enough authority to completely trim the aircraft longitudinally in certain corners of the flight envelope, e.g. gear up/flaps up, aft center of gravity, near Vmo/Mmo corner, and gear down/flaps up, at speeds above 230 kts. In those cases, longitudinal trim is achieved by using the manual stabilizer trim wheel to position the stabilizer. The trim wheel can be used to trim the airplane throughout the entire flight envelope. In addition, the autopilot has the authority to trim the airplane in these conditions. The reference regulation and policy do not specify the method of trim, nor do they state that when multiple pilot trim control paths exist that they must each independently be able to trim the airplane throughout the flight envelope. Boeing did not initially consider this to be a compliance issue because trim could always be achieved, even during the conditions where use of the aisle stand trim switch was required. Subsequent to flight testing, the FAA-TAD expressed concern with compliance to the reference regulation based on an interpretation of the intent behind “trim”. The main issue being that longitudinal trim cannot be achieved throughout the flight envelope using thumb switch trim only.

EASA POSITION

Boeing set the thumb switch limits in order to increase the level of safety for out-of-trim dive characteristics (CS 25.255(a)(1)). The resulting thumb switch limits require an alternative trim method to meet CS 25.161 trim requirements in certain corners of the operational envelope.

The need to use the trim wheel is considered unusual, as it is only required for manual flight in those corners of the envelope.

The increased safety provided by the Boeing design limits on the thumb switches (for out-of-trim dive characteristics) provides a compensating factor for the inability to use the thumb switches throughout the entire flight envelope. Furthermore, the additional crew procedures and training material will clearly explain to pilots the situations where use of the trim wheel may be needed due to lack of trim authority with the wheel mounted switches.

The trim systems on the 737Max provide an appropriate level of safety relative to longitudinal trim capability.

Page 15 of 114

From: https://www.easa.europa.eu/sites/def...20ISS%2010.pdf

[AerocatS2A]

Originally Posted by deltafox44

Not so. Step 2 is "Autopilot (if engaged) disengage". A/P was no longer engaged. So forget Step 2. Even if you don't, it says "Control pitch with column and trim". The pitch has been controled, the trim was back at the limit of green band though a significative column force remained.

Never is said to use electric trim to full trim before cutout, nor any warning that manuel trim might be impossible at higher mistrim/speeds

The AD does in fact say to trim the forces out before using the cutout. https://theaircurrent.com/wp-content...AX-AD-1107.pdf

[locblue]

Why didn’t Boeing incorporate an MCAS cutoff switch or circuit breaker? Rather than cut out an otherwise perfectly functioning stab trim, just isolate MCAS.

[deltafox44]

Originally Posted by Just the fax maam

Seems to me that once the stab due to MCAS or runaway trim reaches a certain point it is unrecoverable without the old roller coaster manouvre few have been trained on or practiced. And possibly not just the Max.

Please let's all forget the roller coaster theory. At 340 kt releasing aft column pressure would mean negative g's. So if you want to have the time to make the numbers of turns of the trim wheel you need to get trimmed again, you have the choice between an inverted loop or 180° roll before releasing pressure

At the end of the flight they got -2g with still both pilots applying full aft force !