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

 

[RVAmeche]

3DDave,

Sounds like we're splitting hairs. If Boeing didn't have the "closeness" that they have, the FAA presumably would've more thoroughly reviewed the design and raised questions. Even if that's not the case, why didn't Boeing's internal safety reviews show anything? Or did they not consider it because "the pilots should never know it's operating"?

 

[waross]

I understand that, Keith.
As understand it, MCAS was not intended to correct a high AoA.
MCAS was intended to linearize the control "feel" at High AoA.
There may big difference in the amount of trim required.
I understand that the 737 has flown for over 50 years without MCAS.
My point is that MCAS was implemented to counteract the extra lift from the engines at high AoA, not to correct a full stall condition.
There are other systems to warn of an impending stall.

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

 

[TenPenny]

"I understand that the 737 has flown for over 50 years without MCAS."

I think the big point is that the 737 that's been flying for 50 years is, essentially, a different aircraft.

 

[charliealphabravo]

The fact that the trim system (horizontal stabilizer) can exert greater tail authority than the actual primary control surface (elevator), under any circumstance, seems telling. I'm sure it tells an expert more than it tells me.

 

[Alistair_Heaton]

Its normal that the trim can do this

There are huge changes in trim required when the flaps move from lift flap to drag flap. And there are automatically trim changes occur to counter this effect. But they are standard amounts so if you select flap 35 it will automatically trim forward by say 5 units. If you then select flap 15 it will trim back by 5 units and the time it takes is about the same as it takes the flaps to travel.

So if you were in trim before selecting then you will be in trim afterwards as well.

 

[waross]

I think the big point is that the 737 that's been flying for 50 years is, essentially, a different aircraft.

Yes, a different aircraft flying under 50 year old certification.
Designed to management criteria that may have been unrealistic.
I wonder what the turnover rate of the better designers is at Boeing?
I don't think that the joint review panel will be quite as compliant as the FAA.

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

 

[LionelHutz]

charliealphabravo - I wouldn't say under any circumstance. At one point in the Ethiopian flight the stabilizer went full down yet the pilots maintained reasonably level flight until they trimmed the stabilizer back up.

There are lots of "ifs" relating to the Ethiopian flight along with the corresponding question of why. For examples:
#1. If the pilot had trimmed the stabilizer to the proper angle before using the cutout switches then they wouldn't have been struggling with the controls and might have been able to put their focus elsewhere.
#2. If the pilots had not let the plane speed increase to too high a speed then the struggle to hold the yoke wouldn't have increased to the point they felt the only option was to enable the trim again and take a chance on it working correctly.
#3. If the pilot had not let the plane speed increase to too high a speed then they might have been successful at re-enabling the stabilizer and trimming the plane.

As with most plane accidents, there are a number of thing that go wrong before the crash finally happens. Blaming the crash solely on MCAS is being short sighted. The MCAS system mis-operating is serious, but ultimately the mishandling of the MCAS failure is what caused the plane to crash. Of course, this doesn't mean the crash is only the fault of the pilots. Boeing's needs to build a plane that makes it possible for the pilots to successfully troubleshoot failures of the various systems so they can do whatever is possible to keep the plane safe. The various regulatory bodies need to have rules and requirements in place so the pilots are required to get enough training.

 

[VEBill]

If the pilots had not let the plane speed increase to too high a speed...

A lesson from US Airways Flight 1549 is that it's not reasonable to assume that pilots can always react instantly. Famously, the 1549 crew were granted an allowance of 35s analysis and reaction time. Thus it was concluded that landing in the Hudson was the optimum decision.

This MCAS incident may be somewhat similar in that regard, i.e. expectations of reaction timing. Ultimately, system designers need to account for a range of human responses.

 

[Sparweb]

...system designers need to account for a range of human responses.

Well, actually, they do. I can think of some abnormal & emergency situations where the minimum pilot reaction time is published directly in the advisories that designers are supposed to refer to. Fires and cabin decompressions quickly come to mind.
Delay in pilot reaction time is expected and planned for in so many scenarios, that it would be a significant error to omit this consideration from the MCAS trim system design.

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

 

[LionelHutz]

This MCAS incident may be somewhat similar in that regard, i.e. expectations of reaction timing. Ultimately, system designers need to account for a range of human responses.

I'm not sure what the first part of your post has to do with what I wrote or the part you quoted. The plane certainly didn't accelerate to above Vo speeds instantaneously. As for this, so you agree with what I wrote later in my post but worded differently?

 

[VEBill]

The plane certainly didn't accelerate to above Vo speeds instantaneously.

The word choice "instantaneously" makes this a 'straw man argument'; because I explicitly referred to allowable pilot analysis and reaction times on the order of 35 seconds.

[I'd used the word 'instantly', so I'm guilty of the same thing.]

Also, "too high a speed" may be lower than Vo. The 737 runaway trim simulator video (linked elsewhere; hopefully everyone has seen it) demonstrated how the trim wheel forces can be extreme, and I won't repeat all the complicating factors here.

"... agree with what I wrote later in my post...?"

Yes.

Except that we know who downplayed the differences between the 737 variants, and thus instigated the lack of training.

The speed versus time, given a reasonable crew reaction time, and the resulting forces on the trim wheel, it all seems like a recipe for disaster.

So my point was just that "If the pilots had not let the plane speed increase to too high a speed..." may be somewhat related to the reasonable expectations of reaction timing.

Which is clearly a valid point, and I thought worth mentioning.

 

[VEBill]

Delay in pilot reaction time is expected and planned for in so many scenarios, that it would be a significant error to omit this consideration from the MCAS trim system design.

My post was in the context of the quotation that I had included at the top.

If the pilots had not let the plane speed increase to too high a speed...

Clearly this -^ is directly related to reaction time.

 

[VEBill]

Here's the 737 Runaway Trim simulation video mentioned above (it's not a 737 Max, but is still quite informative). I believe that this video was previously linked elsewhere in these threads.

It seems to be essential viewing, in terms of context.

Acknowledge in advance all the other contradictory factors that I'm not going to bother listing here.

The link below is cued-up at 10m05s. Rewind if you would like to see the introduction.

https://youtu.be/aoNOVlxJmow?t=605

 

[Alistair_Heaton]

Swept wing jets are very slippery. You can easily go from stall speed to busting Vmo inside 35 seconds with the power off never mind power on.

If you think the plane goes from 0 to 135 knts inside 2000 meters in 10 seconds on take off. Going the rest of the way to Vmo doesn't take long straight and level. Add in potential energy conversion descending and it's under 10 seconds.

 

[Eufalconimorph]

Reuters is reporting that the AOA disagree option was supposed to be enabled on all aircraft by default, but wasn't due to a bug. Boeing knew of this and chose not to disclose the issue for 13 months.

So if I'm reading correctly the pilots would have expected a warning indicator on the MFD, would not have seen one, and could have concluded that there was no issue with the AOA sensors. Of course they weren't told about MCAS either, so this probably wouldn't have saved anyone but it certainly wouldn't have hurt!

 

[IRstuff]

interesting article about DERs and ARs:

https://www.seattletimes.com/busine...-in-race-to-certify-planes-including-737-max/

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[3DDave]

Had they paid attention to the instruments they had, all three sets of pilots would have executed the unreliable/disagree airspeed protocol. None did.

Concern over adding another warning is a red herring to draw blame to things that would have made zero difference.

The Ethiopian pilots knew about the MCAS, but failed to follow the AD as issued and failed to trim the plane before shutting off the trim system.

Over at pprune it's now about 50/50 on whether the pilots are to be trained and accept responsibility for operating a plane that is otherwise controllable. With the crash of the Russian plane I think the scales are tipped farther.

 

[Alistair_Heaton]

Smacks of O riñgs and space shuttles and cold weather.

 

[VEBill]

Some decision making history, this info from Boeing.

 

[LionelHutz]

Approximately 2 minutes from stick shaker until Vo is reached. Then, about 2 minutes more until the pilots decide their only available course of action is chancing the activation of the electric trim again. 4 minutes total goes well beyond needing to give the pilots 35 seconds to make a decision about the plane speed.

Do I need to point out that it should be common knowledge among pilots that stick forces increase with airspeed? This fact isn't something new that was introduced with the MAX. A plane accelerating (or decelerating) when the pilot doesn't pay enough attention to maintaining speed isn't new to the MAX either. So, not controlling the plane speed can't be blamed on the lack of MAX training.

Also, "too high a speed" may be lower than Vo.

Which is EXACTLY WHY I wrote too high a speed instead of Vo in the initial post you felt necessary to take issue with.


Apparently it's a touchy subject to point out the pilots contributions to the accident.