Boeing 737 Max8 Aircraft Crashes and Investigations [Part 7] 16

[Sparweb]

This post is the continuation from this series of previous threads:

thread815-445840
thread815-450258
thread815-452000
thread815-454283
thread815-457125
thread815-461989

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 (Link is now broken. See PDF download below, 3 MB)

Indonesian National Transportation Safety Committee preliminary report

A Boeing 737 Technical Site

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

BBC: Boeing to temporarily halt 737 Max production in January

www.sparweb.ca

 

[waross]

Thanks Alistair.
And a few other people who didn't ask thank you also.
From your explanation we can also understand how/why it is acceptable to fly with a check valve locked down.
[link ]Bill[/url]
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Sully made a “force off airport landing”. The expression seemed to convey its meaning. The landing will be when/where it is with few options. Up to the crew to make the best of it. The closer to the ground the plane is when it becomes a glider the fewer the options; go around probably isn’t on the option list.

 

[waross]

Then there was the Gimli Glider.
They landed on a drag-strip that used to be an airstrip.

[link ]Bill[/url]
--------------------
"Why not the best?"
Jimmy Carter

 

[Alistair_Heaton]

A forced landing is having to land anywhere on an unprepared area and you have to do it like it or not. Aircraft like gliders do it all the time. Powered aircraft less so and usually due to a mechanical issue. Sometimes its a crash and the plane needs fixed and some of the time the aircraft gets on the ground no damage. You also have a full fatal crash with everything involved never flying again.

Big commercial aircraft such as in the discussion it rarely happens and its even rarer that no damage is done.

IF the HP compressor valve is locked down shut it means that no high pressure air can get into the LP compressor. As such you will be fine in normal flight but at some points the LP won't be enough to control the temp in the aircraft, so it might get hot as hell on the ground before takeoff and on the descent. They will have ram air to help out with pressurisation. And more than likely a restriction that they can't fly in icing conditions which would need the high pressure valve open to drive that system while descending.

What I think has happened in this case is several aircraft have had to shut engines down which is a reportable event to all regulators. They have then strip the engine and discovered the corroded valve. Then then went and had a look at the other engine and the valve was in a similar out of limit state. There are enough parked aircraft about it was relatively simple to say get the cowls open on that one over there and lets have a look. It wasn't pretty either and a couple more to make sure and no point looking at others if they are all screwed. Maybe they got some in Alaska inspected and some in the south and they were all the same. Spoke to Boeing who said err they are all the same across all 737 types, Quite rightly the FAA puts out this emergency AD to get them fixed world wide ASAP before planes go flying. I suspect the reason why the English is different to normal is because it bypassed several stages which output of the FAA normally goes through to maintain image. No committee's involved in this one, straight to the director of the FAA who then goes on the phone to the Boss of Boeing to let him know before its public. Some technical type has written it, It will be proof read by another tech type then released. Zero media guru's involved just stop the aircraft flying before inspection. I wouldn't be to harsh on the wording. It pretty clear that if planes have been in storage then these valves have to be inspected before flight. And it gives a method to get them to a hanger to get fixed.

The English might be a bit dodgy but the emergency AD has got the required info out quickly and prevented any fatality's. I suspect it not the last one on various aircraft types which have been in storage.

 

[Alistair_Heaton]

Found out today that the check valve is a Boeing only fitted part which is common across the 737 fleet.

Any other type with the same engines is not effected. Apparently they had a look at pretty much every type that was in storage Apparently its not a big issue to check them or to swap out the check valve If you can get hold of the spare part. Which is normally not an issue.

 

[Alistair_Heaton]

Rumour has it that the MAX doesn't not comply with rags for stability and control forces. And requires a "special" for it to fly again with out the flight control computer system and sensors going FBW regs.

As things stand MCAS doesn't comply with the failure analysis numbers matrix. You have to go to the next level of redundancy before you get the required failure per million flight hours. To get that then smart probes have to go in and 3 flight control computers i am told.

The FAA is more than likely willing to give a dispensation, the other regulators are not keen. Nobody can go to the USA anyway due to covid and its unlikely that infection rates are going to drop to allow unrestricted travel to do flight tests for quit a few months.

The USA market alone is not enough to sustain the model if the FAA decided to go it alone. So I doubt Boeing is keen about just getting it flying again without it being world wide.

There is also some issue with the test aircraft being one of the small models. Most of them will be longer and heavier. And if the short one is twitchy they want to know what the longer ones will do before pax get on them.

The complete lack of "noise" after the flight tests by anyone gives a good indication about the state of play.

 

[LittleInch]

Well if that's only half correct it does look like the chickens have come home to roost, i.e. all those things they fudged around because of the bigger engines and short undercarriage (MCAS etc) and the fact that no one now trusts the FAA to do the right thing.

I've said before I thought this half way house between Boeing design and FBW could cause more trouble than the initial fault.

And all these new safeguards to stop MCAS operating (disagreement between sensors or FCCs, only operate once (per flight?)) means that it may not operate when it should and create a different hazard and is then itself a failure to operate correctly?

So they're damned if it goes off when it shouldn't and damned it is doesn't go off when it should.



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

 

[RVAmeche]

Given the current state of affairs, I'd be surprised if international travel from/to the US opens up before 2021.

Maybe Boeing will followthrough with their threats and spin off the commercial and just let it tank.

 

[davefitz]

There should be written a "lessons learned" chapter in some MBa textbook outlining how mindless devotion to profits and bonuses combined with a "responsive regulator" ( read corrupt) leads to self destruction of a company.

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

 

[Alistair_Heaton]

o they're damned if it goes off when it shouldn't and damned it is doesn't go off when it should.

That's a pretty good summary I think.

 

[3DDave]

The "hazard" if MCAS doesn't function is that, near stall, the pilots aren't given quite as much linearity as they haul back on the control wheel to produce that stall. Since the accident pilots were unable to detect or understand what it means to have more than 50 and up to 90 pounds of force on the control wheel I'd say that particular requirement is no longer useful and should be dropped entirely in favor of simply maintaining a positive stability margin into stall so any pilot not actively trying to crash the plane can just let go and the plane settle itself down.

 

[waross]

So they're damned if it goes off when it shouldn't and damned it is doesn't go off when it should.

I agree.
I have been concerned with that aspect for some time now.


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

 

[waross]

Maybe a little worse than that, Dave.


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

 

[moon161]

is no longer useful and should be dropped entirely in favor of simply maintaining a positive stability margin into stall so any pilot not actively trying to crash the plane can just let go and the plane settle itself down.

I'm afraid they can't do that Dave (Sorry, I had to work that in). That sort of redesign would qualify as a horse of a different color, and mean certification as such, or rebuild as an NG without the leap engine which doesn't meet the design economy case, as discussed quite a ways back. I mean they could but, maybe not a viable product any more if they did.

 

[3DDave]

There is no redesign required. The MAX maintains positive pitch stability to stall.

 

[waross]

The MAX maintains positive pitch stability to stall. ????

pilots were unable to detect or understand what it means to have more than 50 and up to 90 pounds of force on the control wheel

What that meant and what was not covered in any training was that they were trying to fly a broken airplane with a very active and agressive death wish.


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

 

[3DDave]

Taking replies out of context again? The stability is what happens when there is no MCAS.

Yeah - and they never learned that the trim button is used, as the first crew did, to offset wheel pitch forces.

How is that day-one-in-the-cockpit training overlooked?

Let's examine the Ethiopian crash report for details on the enhanced training they had after the details of the Lion Air crash were released.

I'll wait while responders look that up, but I won't hold my breath.

 

[Alistair_Heaton]

The control force decreases the higher to alpha critical they get. If it increased there wouldn't be a problem.

This reg about the control forces as alpha increases is one of the original and was enforce when the original 737 rolled out the hanger.

Basically you can't have the control forces decrease as the alpha increases. They used to test it with spring force gauges hooked to the yoke and the test pilot pulled back through them and read out he readings and it was plotted while decelerating at 1 knt per second in level flight when they got to Vref they stopped trimming then took it to stall. How they do it these days I have no clue.

This is different to stability which is the aircraft returning to the trimmed condition after the controls are released. At lower alpha it will do it quicker on the 737 max at high alpha it will still do it but it will take longer. But it will always return. If it was unstable it would never get back to the trimmed condition.

 

[3DDave]

Alistair - the slope of the increase of force is not supposed to decrease. The force in the MAX doesn't decrease, it just doesn't increase as fast. There is no force reversal. This is Aero 101.

 

[Alistair_Heaton]

For increase of alpha after as certain point requires less force input than the previous increase. If it was the same there would be no problem if it increased everyone would be happy. It doesn't it decreases which is the reason why MCAS was invented to artificially knock out the trim to keep the force the pilots feel the same or increase.

It definitely decreases because of the lift off the engines forward of the point of pressure creating a moment adding to the moment of the tail plane. This reduces the amount of elevator required to increase pitch. This decrease is not allowed under regulations. IF it was allowed they would have never have put MCAS in, in the first place.