Boeing 737 Max8 Aircraft Crashes and Investigations [Part 5] 19

[Sparweb]

This is the continuation from:

thread815-445840
thread815-450258
thread815-452000
thread815-454283

This topic is broken into multiple threads due to the length to be scrolled, and 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.

Some key references:
Ethiopian CAA preliminary report

Indonesian National Transportation Safety Committee preliminary report

A Boeing 737 Technical Site

Washington Post: When Will Boeing 737 Max Fly Again and More Questions

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

 

[Sparweb]

The LEAP Pratt& Whitney engines on the A220

I don't see how this is related.
If you're concerned about a P&W engine problem, please start a new thread to discuss it.

http://www.sparweb.ca

 

[Alistair_Heaton]

sorry I got the wrong engine type. I was just very interested in it as I have been flying my bum off thanks to ours getting sorted.


Anyway to get the thread back on course in this link the accident report and regulator review JATR is discussed. Some interesting stuff and I suspect a lawyers dream brief.

https://www.satcom.guru/2019/10/flawed-assumptions-pave-path-to-disaster.html

 

[waross]

Maybe I missed it.
Looking through the Satcom Guru's article I didn't see any mention of the inability of the crew to trim with the hand-wheels.
Has there been any word on a response to the excess manual trim forces?

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

 

[Alistair_Heaton]

No, that's a bit of a elephant in the basement issue.

There is no regulation that requires to be able to manually trim across the whole envelope.

After that you get into what level of risk it is not being able to. And that's effected by the failure modes and next level down abilities.

Currently with the max its single failure = dump it on the pilots and CATASTROPHIC if they are outside the force window. How they are going to lower that risk level down to MAJOR is Boeings problem.

There are plenty of other jets of the same era 1960 which will have been the same in regards of not having full trimming over the envelope. But they had additional failure layers eg the classic 737 had two stab screwjacks, one of the pilot electric trim and one for the auto pilot and it had a cut out switch on the control column with full aft movement. So by the time they got to manual trim they were 3 levels down. The NG you could kill the AP input and leave the pilot electric trim turned on. And none of them had funny aerodynamic effects going on depending on which part of the envelope you were in.

Like everything else about what's happening the information releases are driven by financial regulations and everyone is keeping extremely tight on what's said.

Nothing is expected to be said until end of November when the FAA test flights will either go ahead or not as a lot of us expect. If they do then its EASA test flights next before Christmas.

From the stuff I have seen Boeing is determined to try and get away with only a software fix and a 2 hour ipad training session instead of an hour on the ipad. We shall see if the regulators accept that. If its physical system changes then we will still be discussing this same day next year

 

[LionelHutz]

How do two separate independent screwjacks move the same stabilizer???

 

[Alistair_Heaton]

Don't have a clue you would need to have access to the maint manual.

I presume it would be some form of differential gearing driving the actuator shaft.

The motor's are only a small part of the unit there is a great big gear box on the bottom of it on the classic. The NG is much smaller

I think this is a diagram of the NG screw jack.

to add I am not 100% sure about the top pic is 737 but anyway its a old school one can't find anything on line. The NG screw jacks are a 1 man lift, the 737-500 its 3 blokes a forklift a loads of swearing from what I have seen.


Found this which is a more generic diagram.

 

[moon161]

So, I'll ask the naive question, why not self locking threads?

 

[Sparweb]

Gave the Satcom Guru a read myself tonight.
The clever guy - a bit smug - but most of the latest article concentrates on a detailed analysis of the JATR report and really hits home, too.

The part about the trim wheel effectiveness vs. cranking forces is definitely missing, WARoss, but it is very closely related to the other point that SG really does make: the 737 Max has no column trim cut-out switches. I admit that I hadn't caught on to this problem until reading it in detail just now, and what a difference that would have made. Imagine a system that allows a pilot to respond to "something wrong" before even being conscious of it. Those cut-out switches have been on the 737's all along, until the Max. They let the pilot stop the trim runaway by doing what he's been trained to do ever since first sitting in an airplane: pulling on the control column when the nose is low, and pushing when it's high.

Why were they omitted???

MCAS wouldn't have driven the stab so far out of whack if there had been a simple set of limit switches to prevent its continued operation while the pilots were already hauling on the controls.

http://www.sparweb.ca

 

[Tomfh]

Why were they omitted???

Isn't that the point of MCAS - to manhandle the pilots?

 

[LittleInch]

Because MCAS is designed to work at high angles of attack caused by the pilots pulling the stick back.

If indeed Boeing are trying to limit changes as described I can't see it getting past the non FAA regulators.

I hope this issue has a big impact on looking much more at assumptions on pilot input (3 seconds to react) and the cumulative impact on the pilots of alarms following a failure of a single sensor.

Plus this 1 hour training I believe simply came to be too big a burden. You only have to look at the cockpit pictures of the max vs NG and it's a different plane.

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

 

[Sparweb]

to manhandle the pilots?

I know you meant it in humour but I realize that you're partly right. If the cut-outs were there, then the pilots would be exposed to the non-linear stick-force trends at high angles of attack and high g maneuvers. So eliminating the switches (a safety feature AND a fault tolerance mechanism built into many system safety assumptions) were eliminated to prevent the bad aerodynamic behaviour from becoming obvious to anyone. Oh man this rabbit hole gets worse the farther down I go.

http://www.sparweb.ca

 

[Alistair_Heaton]

Remember mcas triggers both ends of the envelope.

Originally it was linked to AoA and g sensor. And this was deemed ok by test pilots grudgingly. Then they had issues with another area of the envelope and the g sensor was removed and its power and duration was changed. But it was never sent back to the full test pilot approval system and FAA wasn't informed.

 

[LittleInch]

And this is the issue in a nutshell. It's the same aircraft with the same controls and pilot actions - except when it isn't.

The inability of Boeing to square the circle sufficiently is why we have planes parked up all over the world.

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

 

[Tomfh]

I know you meant it in humour

Not really. It was to keep the pilots away from the forbidden part of the envelope, wasn’t it? to avoid all the hassles of recertifying the plane and retraining the crews?

If the pilots know how to easily just turn it off it defeats that original purpose doesn’t it?

 

[waross]

Or; It was to make an unstable plane appear to be stable.
It was meant to comply with the requirement that the force on the controls must be linear.
It can not have been a stall prevention/intervention system as that would have required more rebates, approvals and training. (A problem with the sarcasm font is that even if you use it, no-one recognizes it.)
We started with management overriding good engineering.
Will we end with politics overriding good engineering?



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

 

[Alistair_Heaton]

Just a few updates.

737 max Boeing failed a paperwork audit which will put back the start of certification testing of the fix by the regulators both SIM and aircraft.


737 NG pickel forks the failure rate is over 20% now and includes aircraft below 22k cycles. Plus also it doesn't seem to be linked to winglets. Not enough jigs or parts in the world for a quick fix.

787 O2 has some issue now with the pax emergency 02 system the explosive charge that opens the valve apparently has a 25% failure rate. Both Boeing and FAA know about it and have done SFA about it.

The altitude 787's fly at if you depressurize you have 5s of useful consusness without O2. And by the time you get down to breathable ALTs half the folk in the back will have the bends and brain damage with no 02. Not counting the ones that stroke out anyway.

 

[IRstuff]

I don't think you can get the bends without external pressure. Your last item is not unique to 787s, all planes flying at altitude have that issue; that's why cabins are pressurized and why pilots their own air supplies. Many people have died without cabin air pressure; see

https://en.m.wikipedia.org/wiki/1999_South_Dakota_Learjet_crash

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

http://www.eng-tips.com/forumlist.cfm

 

[LittleInch]

Difference between cabin pressure and pressure at 38,000 ft is only about 60 kPa, so equivalent to 6metres below sea level. If you sat at 6 m underwater for a few hours is instant de-pressurisation to surface enough for the bends?

And 5 seconds is rather quick - the google consensus seems to be 15 to 20 seconds before you loose it and then 30 secs before hypoxia kicks in - but sure - it all happens rather fast and no way near enough time for the pilots to get down to 10,000 ft or so - assuming they've managed to connect to the O2 fast enough.

If you had a total decompression at 38 to 40,000 ft I don't know what the max descent rate is but prob 4-5,000 ft/min so 6 minutes. They always show it much more dramatically in the movies, but reality seems to be different.

Anyway a side issue - failing this audit doesn't sound good.



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

 

[Alistair_Heaton]

You can get it easy enough that's why astronauts have to breath pure O2 for a few hours before doing an eva. The suits are much lower pressure than inside. It's to do with the partial pressure of nitrogen, pressure gradient and tissue half life for off gassing.

In the front we have a full face mask with mixed and 100% demand and forced feed.

Most places in the world you can head straight down to 10k and we can do 7k per min. There are areas where the MSA is greater than 10k so you have to level off higher and then step down. Think Everest.

Oh 787 will start at about 380 to cruise then end up at 420 or 430 after a few hours.

Our books say 5s useful high level brain function. And after doing the chamber training it's about right. There are plenty of YouTube vids showing it.

Normally most aircraft pressurise to 8k.

https://www.nasa.gov/audience/foreducators/mathandscience/research/Prob_PreventDecomSick_detail.html

 

[zeusfaber]

I'll buy the 5s useful time (I remember getting stuck after fitting the third piece into my 12-piece kiddies jigsaw puzzle in the chamber at Boscombe (and that was coming from breathing 100% on demand with a couple of mm of safety pressure to boot).

Altogether less comfortable with the idea that the oxygen concentration in one of those diluter masks we get in the back is going to provide much useful protection against DCI.

A.