Boeing 737 Max8 Aircraft Crashes and Investigations [Part 4] 28

[Sparweb]

This is the continuation from:

thread815-445840
thread815-450258
thread815-452000

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.

Thank you everyone for your interest! I have learned a lot from the discussion, too.

My personal point of view, since this falls close to (but not exactly within) my discipline, is the same as that expressed by many other aviation authorities: that there were flaws in an on-board system that should have been caught. We can describe the process that "should have happened" in great detail, but the reason the flaws were allowed to persist is unknown. They are probably too complex to reveal by pure reasoning from our position outside of the agencies involved. Rather, an investigation of the process that led to the error inside these agencies will bring new facts to light, and that process is under way, which will make its results public in due time. It may even reveal flaws in the design process that "should have" produced a reliable system. Every failure is an opportunity to learn - which is the mandate of the agencies that examine these accidents.

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

 

[Alistair_Heaton]

I have had a look at Angle of slip sensors.

It appears that to get a real reading using a sensor you need a huge probe out the front into clean air. Certain test flight aircraft have them.

But it seems they use the output from the INS or AHARS, heading and track from GPS and air data computer gives the other stuff, and then derive a angle from that. The slip ball indicator is AHARS derived.

But to be honest its way over the tech knowledge required by pilots of the type on how these things are calculated. Looking at the list of variables outputted by the data recorder it is calculated for my type but how it does it I have no clue.

I suspect that test aircraft are used to calibrate the internal derived data for slip angle and then that's what's used.

To note for commercial aircraft the only time the aircraft is in slip or skid like that is when you are operating single engine and you haven't sorted your rudder trimming out yet and when your doing a cross wind landing. The rest of the time its ball in the centre for coordinated flight. Apart from anything else if you fly with the aircraft skidding all the time you will get ear ache off the cabin crew and increase the likely hood of the pax barfing down the back. And it will increase your fuel burn. I might add the ear ache off the cabin crew is enough that you don't do it unless you want to go hungry and not have a cup of tea while the sun comes up.


To note I have been shown the price list for the MAX and the customer options. To be honest I am a bit scared and won't post it because of all the legal exposure that gives. But the option to give the pilots a AoA indicator on the PFD and additional AoA mismatch warning was utterly disgustingly expensive for a safety addition, for what is only an activation of some code to display a value which is already available and being used for no additional hardware. No accountant would pay that price for something that's not required.

Hell as a Scotsman and pilot I wouldn't pay it either. But then again it was two Scotsmen fighting over a penny that invented copper wire so maybe I am not the best person to do that cost analysis.


Anyway they will all have it now, but I suspect even having it wouldn't have changed the outcome of the two aircraft unfortunately.

 

[Alistair_Heaton]

Just a link from this side of the pond. I can't see that being complete before the end of this calendar year. And I also can't see the other CAA's doing anything until after the EASA review is complete mainly due to cultral face issues if it turns out that it turns up issues which I suspect it will with the elevator power V stab power. If the tail and controls needs redesign and certification we could be talking a year at least.

https://www.bloomberg.com/news/arti...review-of-boeing-max-before-approving-return.

And one from Boeing from just before it was rejected by the FAA. They haven't given the reason yet for the rejection.

https://www.boeing.com/commercial/737max/737-max-software-updates.page

 

[Sparweb]

I have had a look at Angle of slip sensors.

I found an angle of slip indicator for you. Works great in aircraft where the windscreen IS THE Primary flight display.
Think it would work on a 737?

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

 

[waross]

I am interested in any pilots comments on this statement:

There are no pilot actions or procedures during flight which require knowledge of angle of attack.

Link


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

 

[Alistair_Heaton]

Everything is referred to speeds for weight which are given for the envelope range.

This then puts you in the correct AoA.

Military pilots of fast jets fly the AoA for carrier landings which means they don't have to worry about the aircraft weight.

For landing you land at 1.21 of stall speed in commercial aircraft. This is a fixed angle of attack.

Civilian world they for some reason which I can't understand do not want the pilots using AoA.

 

[Sparweb]

Airspeed is normally a very good proxy for AoA. It is measured fairly directly, in conjunction with other aerodynamic parameters, so you can have a good idea of how fast you are going, not just horizontally but vertically and how high you are, all from two simple tubes.

The errors that build up as you increase altitude grow exactly in proportion to the effect altitude has on your angle of attack, therefore, your indicated airspeed still tells you what you need to know.

Because angle of attack can be changed in seconds with the stick, any device to deliver to the pilot AoA info would have to do it continuously, without lag or overshoots. What would a pilot do with the AoA information on a second-by-second basis that can't be done by airspeed?

Bad things happen when pilots fail to second-guess a faulty airspeed indicator. Would the same happen if they had an AoA indicator, which then went bad?

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

 

[waross]

Thanks for the explanation, Alistair and SparWeb.

Another question.
I understand that there are several inputs that already caused automatic trim adjustment.
Is speed trim the correct term?
Is AoA information just an added input to the speed trim system with a new name, or is MCAS an additional stand alone system?



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

 

[Alistair_Heaton]

AoA is one of these things which is progressively getting used more and more. It started just being involved with the stall system but now gets used by various other systems. Eg the FMC to get most economic flight profile settings.

You are correct that you trim for a speed but people don't call it that.

 

[Alistair_Heaton]

I don't have a clue how it's all intergrated. The base control system is easy enough but when you start looking at what boxes are talking to each other and it gets complicated.

 

[3DDave]

There is a Speed Trim System on 737s that automatically changes the trim to match the speed. AoA is also used as a correction for the Pitot-Static system to provide accurate airspeed.

 

[Alistair_Heaton]

"Speed Trim System (STS)
An electrical stabilizer trim input automatically controls certain aircraft attitude conditions when undergoing large thrust changes in the lower speed region, such as takeoff and go-around. These conditions require high thrust settings and are especially present with a low weight aircraft and a relatively aft center of gravity where the aircraft wants to “nose up”. The STS supports the crew during these conditions when manually controlling the aircraft without the use of an autopilot by an opposite stabilizer trim, commanding a nose down force by use of the autopilot trim.
STS activates:
– Between 100 KIAS and Mach 0.60 (fading to zero after M 0.68)
– 10 secs after takeoff
– 5 secs after releasing trim switch(es)
– N1 >60%
– Autopilot disengaged
– Trim required
A short manual trim selection overrides the speed trim and will inhibit it for around 10 seconds, just in case that the speed trim inadvertently provides an incorrect input.
The SPEED TRIM FAIL light Illuminates amber:
– accompanied by the FLT CONT annunciator and MASTER CAUTION with a failure of the STS
– after RECALL is pushed together with the FLT CONT annunciator and MASTER CAUTION but extinguishes when Master Caution System is reset with a single FCC channel failure.
The stabilizer speed trim itself (not to be confused with STS) depends on flap position as it is most needed with the flaps extended around the takeoff and landing phase.
When the flaps are up, the low speed trim moves the stabilizer at 0.2 ups (units per second) and when the flaps are extended the high speed trim moves the stabilizer at 0.4 ups.
This stabilizer speed trim is available during autopilot operation, than stabilizer trim speed changes when the flaps are up to 0.09 ups and when the flaps extended to high speed of 0.27 ups."


Seems there is another system for dealing with flight characteristics.

Is that the system you mean or is there another one?

The list of what AoA is used for these days is huge and it sneaks in as 3DDave says to do corrections on multiple systems. It its also involved in the rudder systems eg yaw damper.

Quite how the 737 deals its inputs and failure modes I really don't know. With electrical failures its normal for systems to progressively be shed and as the heaters are reasonably high load one side of the Pitot static system is shed, I can only presume one of the AoA vanes also goes but don't know. It could be both of them. But if this then automatically then kills the AoA usage I don't know. If the electric trim has already been shed as well then it won't matter.

On my type we loose what we call the low speed cue which is the stall protection which does give you an idea of AoA but not a real value quite soon in the proceedings. Now if this is due to a safety assesment that its more likely to cause issues than solve them or the sensor is off line I have no clue. The only bit of advice in the QRH when you get it is "maintain a safe airspeed for the conditions" and its a de brief point if you don't work you way through to that checklist for that obvious advice in the sim.

And another punch in the guts for 737 operators if they haven't got enough to deal with just now trying to plug holes timetable.

https://www.cnbc.com/2019/06/02/boe...parts-that-may-be-susceptible-to-failure.html

 

[davefitz]

Sparwweb,

My own personal opinion ( based only on the publically reported summaries) is that the reason for the failure to catch the basic design error is neither complex nor complicated, but instead it is too simple to accept publically. The instant problem was just the nexus of 2 common faults in our system of government regulation and corporate management.

In nearly all large companies the weight given to sales success decisions simply outweighs all other considerations, as measured by the corporate internal poltical power given to sales VPs and their commercial decisions ( together with incentives such as bonuses etc) are totally focused on sales success. Likewise, the evolution of our government regulation mechanism has, in all instances without exception, caused the regulators to be "owned" by the regulated industry.This instant spate of fatal accidents will not change either of these systemic defects , and the current effort is merely focused on finding words and narratives to help cover these systemic issues that are integral to our way of life.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[charliealphabravo]

I agree. These incidents are looking less like engineering failures and more like corporate management failures.

I'm sure engineers raised red flags along the way but what are they in the path of an ill conceived corporate agreement to make red lines with green ink.

 

[IRstuff]

It's almost always both; management certainly didn't force MCAS to act on stupid data; that was an engineering fubar.

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

https://www.youtube.com/watch?v=BKorP55Aqvg

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[3DDave]

Based on personal interaction with several Boeing Systems Engineers, I know where I would start digging.

 

[Alistair_Heaton]

Is there likely to be a commission type thing aka what they had after the shuttle failure of the solid booster o rings in the cold?

Just been reading the book by the engineer that spoke out about the whole thing then took the lead on the redesign.

 

[Sparweb]

No.
Boeing is not a public agency like NASA.
Several state legislatures would burn down before that kind of inquiry happened.

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

 

[IRstuff]

I used to be a Boeing systems engineer; what are you suggesting?

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

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[Alistair_Heaton]

I wasn't thinking Boeing would be the focus of the commission. They were only playing the system.

The system would be the focus of the commission.

They need to do something. If the US system is deemed none compliant by the rest of of the worlds CAA's then it becomes a very difficult market for any aviation product from the USA.


Sparweb just seen the perfect angle of slip indicator The first form of HUD ever and works a treat. Failure mode the bit of string comes off.... Effect on flight zero.... Fix another bit of string.....

Unfortunately the boundary layer on the windscreen on powered twin aircraft means it won't work. A few times I have had feathers trapped in the wipers to see the effect.

Oh that's another completely useless system on all commercial aircraft.... the wipers. Highly expensive to maintain, create loads of noise in the cockpit, utterly useless for doing the job they were fitted for. Thankfully centre line lighting does work, as does taxing by brail.

Takes me back to the day.... I came to gliding after doing power. Gliding makes you aware of the rudder pedals even more than flying tail draggers. Can't wait until my kid is old enough to be able to go gliding...