Boeing 737 Max8 Aircraft Crashes and Investigations [Part 2] 44

[Alistair_Heaton]

This thread is a continuation of:

thread815-445840


****************************
Another 737 max has crashed during departure in Ethiopia.

To note the data in the picture is intally ground 0 then when airborne is GPS altitude above MSL. The airport is extremely high.

The debris is extremely compact and the fuel burned, they reckon it was 400knts plus when it hit the ground.

Here is the radar24 data pulled from there local site.

It's already being discussed if was another AoA issue with the MCAS system for stall protection.

I will let you make your own conclusions.

 

[Alistair_Heaton]

There really isn't much official out there about the system or how it works. There is absolutely zero about it in the pilots training for it.

Apparently the difference training is 2 hours of CBT then go fly the plane according to Boeing.

 

[LittleInch]

I'm just wondering if they ever really software tested it for multiple operations of operating followed by reset followed by... Strange things happen sometimes if the program decides enough is enough and just goes full pitch down.

Rereading the info in the posts above, it looks like the system continues to go nose down if it thinks the previous adjustment hasn't worked and one issue which is impacting the roll out of the mods is whether and how much to limit the max elevator trim.

for Boeing to not release the software update for months after it has become clear it had a large part in the Lion air crash tells you that there is something not right in how this MCAS works under real life situations and bad data.

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

 

[Alistair_Heaton]

Apparently the government shutdown screwed the FAA up somewhat.

 

[VEBill]

And, they ['Black Boxes'] beep.

Traditionally, the smaller (~ 4-inch) Underwater Acoustic Beacons (UABs) attached to the Black Boxes beeped at ultrasonic frequencies. E.g. The very common now-obsolete DK100, or its newer replacement DK120/90, both ping at 37.5 kHz. One may still hear a faint click, if it's still working.

There are now also 8.8 kHz LF UABs, but they're longer (~ 6-inch) and are generally designed to be attached to the fuselage itself. They're more recent, perhaps starting circa 2012 as far as I can tell.

 

[Alistair_Heaton]

Those black boxes are going to be mangled and burnt to a crisp that's why they have to be sent to France.

All the outside of it will be junk and the connectors unusable.

They have to take the solid state memory out of it and remount it to a reading board then download.

 

[LittleInch]

They don't look too bad. a picture just rekeassd by the bea.

https://www.forbes.com/sites/kristy...hat-investigators-will-look-for/#2d4fdf033776

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

 

[LionelHutz]

Sparweb - I already posted it, but I'll do it again for everyone's benefit since what seems to be the wrong info keeps being repeated. It changes at 0.27 degrees per second. The limit is 2.5 degrees of change which would occur over 9.26 seconds.

From the descriptions, the MCAS will apply a trim correction up to 2.5 degrees. It then does it again 5 seconds later if it still believes the plane is close to a stall condition.

The electric trim switch on the stick will control the trim, which means it can over-rides MCAS if the pilot uses it to command trim up. However, the pilot can't hold it or else the plane will stall. So, they have to release it and then the MCAS will start to command trim down again.

From the manual page at the previously linked site, it says for trim, flaps up the stabilizer can change at 0.2 units per second and with flaps not up the stabilizer can change at 0.4 units per second. MCAS is only active with the flaps up. So, what is a unit? Is it possible the MCAS can move the trim faster than the pilots can via the trim switch?

 

[Alistair_Heaton]

The round bit is where the solid state memory lives. The mangled bit is the connectors, pinger, transmitter.

 

[charliealphabravo]

I have a hard time blaming the aircraft design although it seems the MCAS was perhaps a contributor.

I agree. It may well turn out that manufacturing or maintenance problems contributed. And it will be a chilling find if it is determined that there was something simple the pilots could have done.

I wonder what is the maximum speed that you can deploy flaps. Maybe someone knows if this would disabled the automatic trim system.

 

[Alistair_Heaton]

It would disable it.

Flap speeds is 230 knts I think for the 737 for the first stage.


The MCAS system is to add stability to the control pressure. I belive that the control responce is not linear ie we usually have increasing control forces as you get nearer the stall to pitch the nose higher.

Due to the nacelles on the leap engines thay start developing liftat high angles of attack and that means the control forces to increase pitch become less the closer you get to the critical angle of attack which is against crtification rules. So they added in theis automatic trim nose down fuction to increase the stabity in pitch while manual flying. I think its only really meant for the go around situation.


The action of it isn;t just one cycle and then stop at 2.5 degrees it pauses and the system rests then does it again and repeats until the AoA reduces or the autopilot gets engaged.

 

[Alistair_Heaton]

Here is the mechanical side of the system for those interested

 

[BI1]

The 737 was designed in the 1960's. What have you still got running that was designed in the 60s? My toothbrush? Not even. The 757 and 767 were developed decades later than the 737 and both of them have been retired. 737 Max? More like a 737MF (MaxFrankenstein). I suspect that design modifications to the 50+ year old airframe have finally caused inherently unstable flight within certain areas of the performance envelope required to conduct normal operational manuevers. Now some obviously low IQ, AI software was patched in as a vain attempt to keep the aircraft from entering attitudes that have been found to provoke the resultant situational aerodynamic instabilities and it may think it is smarter than the PIC. And that may be right, given the lack of speed and clarity of the "work arounds" issued to pilots by Boeing itself, which may or may not work. Whao! When Boeing (or anybody) starts thinking that hastily written AI software (did it even make ver 1 yet?) is the best possible method to correct bad aerodynamics, I'm bailing out literally! What the heck are they thinking... all this is some FlightSim game? Take your eyes off the turn and bank indicator and look out the window once in awhile.

My humble advice is don't fly a Max until the airframe is substantially corrected. I doubt that will ever happen, given the certification costs and time schedules involved in that. Leave unstable flight envelopes to bees and helicopters.

BTW, waiting for more data before grounding the MaxF was not my idea of a risk adverse industry and ... who trusts any branch of USA.gov to make a responsible decision these days? 50 other countries being first shows how much evidence there is for that.

 

[BrianPetersen]

https://www.seattletimes.com/busine...-max-system-implicated-in-the-lion-air-crash/

So only one of the two available AoA sensors was used in the MCAS?

The control strategy is not AI; it's the opposite ... overly simplistic.

 

[saikee119]

The Seattle Times report posted by BrianPetersen is chilling.

Am I the only one to interpret

(1) The certification authority FAA has partially or substantially delegated the safety assessment authority to the manufacturer Boeing.
(2) A design change of 400% on the limit of the tail rotation angle, from 0.25 to 2.5 degree, was implemented "after" the 737 Max8 was certified its airworthiness.
(3) The design of the MCAS can allows unlimited self-activation, possibly at the maximum permitted tail rotation, over the pilot's intervention.
(4) Both FAA and Boeing were aware of the first three issues and did nothing even before the second plane crash occurred in Ethiopia.

I can understand in (1) this is possibly a common practice in view of Boeing's expertise and FAA's lack of resources and (2) The subsequent flight tests could demand a large increase of the tail rotation to make the 737 Max8 work. However if the rumour that the recovered jackscrew, which pushes the horizontal stabilizer up and down to rotate the tail in Alistait_Heaton's supplied diagram, were true in the Ethiopian plane then the MCAS could have played a major role in bringing down both planes?

 

[Alistair_Heaton]

Yes both screw jacks have been found and both were at maximum travel to pitch the nose down.

 

[waross]

A few years ago, I was working at a large heavy oil plant. This was a rebuild following a fire and the plant was down to 50% production.
Gross revenue was down over $10,000,000 a day.
In the wake of several serious safety violations by one contractor, the contractor was removed from site and another contractor brought in.
Note that there were no deaths, injuries or equipment damage. It was a case of workers being directed to do unsafe work.
The crew was allowed to remain and was directed by the incoming contractor.
Basically, all management personal were removed.
Crew foremen, who relay instructions but do not effectively have decision making authority were allowed to stay.
All management with decision making authority were removed from site.
I wonder if there will be a similar top down purge at the FAA.
That would send a strong message for a generation or so.
Does my post sound a little harsh?
187 deaths due to pressure to meet deadlines.
That's harsh.
Four months later another 157 deaths.
That's a little more than harsh.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[MDEAus]

I saw an air crash investigation of the TAM Fokker 100 that crashed in Brasil in 1996, something struck me as similar. A safety system that pilots were not trained on functioning in an unexpected way during take off. On the Fokker 100 it was a safety to reduce power to an engine if the thrust reversers deployed during flight, pilots at TAM weren't aware of that function so they fought the reduction in power as there was no warning of thrust reversers being deployed. On the 737max the MCAS system is meant to keep the aircraft in the flight envelop yet the pilots were not aware of a system and the way it functioned (granted every max pilot should have after the lion air accident).

Is there a readout somewhere of what angle trim settings are at during flight available to the pilot and FO? The manual trim would be spinning which should be a key to the pilots decision but if they think they've countered it with the electric trim switch but haven't fully countered it (1.5 degrees up every time MCAS commands degrees nose down), what in the cabin (apart from looking out the window) would shout that the trim has reached maximum nose down trim (even before the aircraft is pointing nose down)?

Would an augmentation system meant to make it feel like flying the NG really need to access the full stroke of trim? Is there even position monitoring of the trim ram that is fed into the FCCs?

 

[LittleInch]

Saikee,

I tend to agree with you.

The key issues coming out seem to be the significant change in max angle from 0.6 to 2.5 degrees and the clear lack of understanding that this system refuses to die after effectively being cancelled and also keeps increasing the trim until it can't increase it anymore. For it to work "in the background" and not advertise itself in any form as a light, warning buzzer or similar is clearly key also in terms of pilots not understanding what is going on with probably a host of other alarms and errors popping up all over the panel.

The desire to both get a certification and also make the system appear identical to earlier versions is the thought process which goes wrong as well as only using a single sensor and not recycling the decision when the parameters change implies a lack of frozen design or poor Management of change procedures.

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

 

[Alistair_Heaton]

There is usually a trim indicator which needs set for a CoG position prior to departure. If it s not set within a certain range a config warning sounds when the aircraft thinks your about to go flying. If you get it wrong it may mean that you can't lift the nose off the deck if its trimmed to far forward or if its to far rearward the plane will start to pitch nose up before you get to rotation speed which may cause a tail scrape.

If the aircraft thinks its stalling there are multiple alarms both tactile,visual and audio, The stick has a vibrator on it which shakes it which creates quite a loud noise, there will be main caution light flashing and also a indicator which side of the aircraft is about to stall. There are three modes normal flight with nothing going. incipient stall with caution lights and stick shaker. Then full stall which is the master caution and a stick push which shoves the nose down in types which can develop into deep stall. There are links to the rad alt with cut offs of the stick push to prevent the nose getting shoved down in the last x amount of feet before landing which varies with type.

The stall system going off is extremely attention getting. The flight deck will be extremely alive with multiple warnings going off, it would be pretty easy in my opinion to miss the trim wheel moving un-commanded.

 

[debodine]

Alistair_Heaton is on target, from my perspective. On the ground with the pilot reading a manual or using CBT for familiarization, it is easy to understand, "If the trim appears to be arguing with the intent of the pilot, use the STAB TRIM CUTOUT switches." But takeoff is already well known among pilots as a high workload phase of flight with plenty of factors capable of killing you. As such, taking the wrong action during takeoff can be as bad or worse than doing nothing. So as Alistair indicated, recognition is vital, and recognition during a very busy flight phase is difficult. Also as Alistair noted, the trim wheel and indicators are well down out of the line of sight unless the pilot takes their eyes off of the outside horizon or the control instruments, whichever is being used to establish their situational awareness.

Using the Lion Air preliminary report from the authorities, the Lion Air accident aircraft had the same activation of the MCAS during the flight just the day before. While maintenance cleared the issue by following the manual, apparently the true fault was not corrected. The accident flight crew would have had the aircraft log available for review, but there is no way to tell if the wording of the log communicated to the accident crew the true circumstances, and most importantly, the actions taken by the previous crew which resolved the trim runaway and allowed for continued safe flight. It is unlikely that the two flight crews had the opportunity to speak directly to each other.

So for a recap (admittedly abridged) and an application to the Ethiopian accident:

1. STAB TRIM CUTOUT switches to stop any type of runaway trim have been available on the flight deck of commercial aircraft for decades. Trim runaway would be defined approximately as, "The trim is moving when it should not be moving, or is moving in a direction it should not be moving." It does not matter if the MCAS is moving it, or some other system failure is the root cause. At that point a typical pilot would not care about the root cause, they just want to stop the runaway. Thus, STAB TRIM CUTOUT switches exist. Pilots likely have an "official" definition that is better than mine of trim runaway - but I think I am reasonably close.

2. IF (and this is the big IF) the Lion Air accident crew and the Ethiopian accident aircrew recognized trim runaway then they should have already been trained, as aircrews have for decades, to use the STAB TRIM CUTOUT switches.

With all of the above being said, unless they were unaware of the Lion Air crash with trim runaway as the preliminary target of the investigation, it is difficult to understand why the Ethiopian flight crew did not actuate the STAB TRIM CUTOUT switches.