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

 

[waross]

Hi IRstuff;
Where we disagree is what is/was the primary problem.
Using the five whys approach may take us back from the MCAS system to the engine placement and the resulting unacceptable flight characteristics resulting from the engine placement.
The real cost of that <$5k device is now being counted in the billions and hundreds of lives, and by, the way I understand that the wings were redesigned to withstand the new forces caused by the forward engine placement and the greater mechanical advantage of the longer engine pylons.

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

 

[IRstuff]

I think that the issue will be that anything that eliminates the need for MCAS will be a wholly new plane

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

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[waross]

Why doesn't the A 319 Neo need MCAS?

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

 

[3DDave]

Because the A319, and all Airbus AFAIK, don't allow any feel for airplane controls to the pilots. If the actuators fail, people die. So they build a big pile of redundant actuators into their planes and a big pile of redundant sensors (which hasn't always helped.)

Note that MCAS doesn't operate during autopilot operation because the autopilot won't let the plane get to the high angle of attack. In that way, the fail of the autopilot on the 737 is also due to depending on AoA measurement.

This lack of feel is what let AF447 drop like a stone into the Atlantic and another one rip the vertical stab right off the plane from rudder pedal inputs.

There is a basic difference in philosophy - Airbus believes the plane needs to be protected from the pilots; Boeing believes the pilots should have the most direct inputs. In both cases when the plane has a problem it is possible for inept pilots to become exposed and everyone dies.

 

[RVAmeche]

I thought the reason it didn't need a similar system to MCAS was the plane naturally sat higher, resulting in the larger LEAP engines sitting in the previous engines place; this meant it doesn't have the thrust angle issue that the Max has?

 

[LittleInch]

Because the engines sit in more or less the same location as before. So no major changes to the aerodynamics.

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

 

[3DDave]

Any differences in Airbus aerodynamics is 100% hidden in the same control system that hides all the other airplane performance characteristics from the pilots. The entire Airbus lineup is 100% MCAS 100% of the time. Just with more redundancy.

There is no thrust angle issue (where does this stuff come from?) as the engines have less moment arm from thrust than previous 737 engines did and the engines are still basically parallel to the fuselage. The larger diameter lets the engines produce similar amounts of thrust with less fuel burn; they aren't supposed to produce more thrust.

 

[waross]

It is not the engine thrust, it is the aerodynamic uplift on the physically larger engines at high angles of attack.
Not much of a problem with the engines under the wings.
It is a big problem with the engines out ahead of the wings on the longer pylons. It was the engine location which caused the flight characteristics that MCAS was supposed to remedy.

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

 

[Alistair_Heaton]

The AP will let the plane into high angle of attack, but when the stick shaker triggers it then should dump the whole lot onto the pilots to sort out when the AP cuts out. Well in theory because in these the AP stayed in with the stickshaker going which also needs dealt with.

How to get the AP to command high Alphas is easy you just put it into VS climb mode and set a high rate of climb and don't do anything, eventually the plane will stall and kick the AP out. The altitude you stall at depends on the rate of climb. If you use the level change or ASI then you have protection. You can also get it in ASI mode if the pitot tube is blocked to the active ADC supplying info to the flight director when that happens the ASI turns into an altimeter and the aircraft will pitch nose up to try and maintain a fixed airspeed which will always increase with altitude so the nose just gets driven higher and higher until the stick shaker goes off. Then your dumped with an aircraft with about 40 degs nose up with full power on and full aft trim.


The airbus has a different setup. 737 is a single electrical screw jack. 1 actuator.


Airbus is 3 separate electrical motors then the mechanical trim feeds in from the front (two sets of steel wires one down the left of the aircraft one down the right). This then feeds into twin system hydraulically assisted screw jack one of which is powered by the redundant third backup hydraulic system that's powered when the RAT is out and the other by the one of the main hydraulic feeds. So triple redundancy on the electrical side of things and dual redundancy on the hydraulic, dual on the mechanical linkage. Full range of elevator travel is available to the pilots at any speed or any degree out of trim via the manual trim wheel.

I believe the 777/787 has a similar setup.

AF447 you are completely right it was extremely poor command decision to leave the FO in charge while traversing the ITCZ with the cruise SO flying the aircraft and go to your bunk. Their actions after that again surprising. Basic power pitch equals performance not followed, exercise 5 in basic PPL training. But again they were all first world nation, national airline, 6 year aviation degree, 4 years of training then type rating and into an aircraft selected from the cream of the population certainly not your average intelligence for a pilot or for that matter average amount of training.


AA587 was again bad training but this was more understandable as it was a national issue. In the US the pilots were taught to deal with flight upset by lifting the wing using the rudder pedals. So when you get a wing drop you press the opposite rudder and use the secondary control effect to roll the aircraft instead of using the Ailerons and roll. And you do get force feed back through the rudder on airbus. Not on the stick though because it has autotrim on pitch and roll to keep attitude when you release the stick.

So the FO trying to use the rudder to fight huge roll effects from wake vortex cyclically inputed full rudder deflection. The tail survived I think 6 cycles and the eventual failure stress was calculated at something like 4 times the certififed failure load. Afterwards all OEMs put a warning in the manuals about cyclic inputs and NASA did a load of studies on it including the US stall procedure and reported to the FAA that things needed to be changed. The FAA changed things but both methods are still being taught.

There is a conspiricy theory that the FO couldn't be strong enough to push the rudder pedals to full scale which sort goes against the claims that there is no control feed back forces on the airbus.

I had a FAA trained US passport holder FO rip his hamstring off the bone due to trying to use the rudder to lift a wing with a second generation yaw damper still engaged trying to lift the wing in similar circumstances in Saudi. We couldn't even get him out the cockpit in the air so he was screaming like a banshi for 10 mins until i could single crew it down to the ground and land. And no morphine to boot in Saudi!!!. So i can completely see a adrenaline fueled push of the pedals by a thousands of hours ex military pilot being able to go full defelction if a young lad can rip the muscle off the bone pushing against a rock solid yaw damper controlled pedal. His mistake was not pushing the AP disconnect button twice before pushing on the pedal. After he started screaming I corrected the attitude using roll without touching the rudder pedals. That was in 2012 and this AA587 happend in 2001 and he had been flying for 2 years so this nonsense is still being trained even with NASA and the FAA telling people its the wrong way to operate. He was put on a plane back to the US saying he was going to sue BAe for a bad design because he had carried out his training correctly.

 

[RVAmeche]

The engine was moved slightly forward and higher up…which changed how the jet handled in certain situations. The relocated engines…caused an upward pitching moment. Boeing added a new system “to compensate for some unique aircraft handling characteristics"

I thought it was common knowledge that the 737 engines had to be relocated due to their size; while the Airbus version didn't need to relocate their engines.

 

[davefitz]

I recall that airbus had a crash about 10 yrs ago ( brazil to spain??)due to reliance on a single pitot tube, maybe that tragedy led to the increased redundancy now evidenced in current designs. The lack of reducndnacy on some fighter jets may also be the reason why airfields that host fighter jets need to police the field for debris before allowing the fighters to launch.

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

 

[Alistair_Heaton]

No they had triple redundancy with pitot tubes. That's af447 your on about. All commercial aircraft have triple redundancy of basic flight variables. The only one that doesn't is the TAT probe which isn't mission critical.

Even the dc3 has three pitot tubes.

They iced up after flying into a collosal thunder cloud and there was an issue with one of the heating elements on a pitot tubes.

They then made a complete and utter hash of procedures and killed everyone.

The pitot design was changed but that was it.

Less than 10% technical mostly pilot error. Which to be honest is normal.

 

[waross]

There may have been an attitude at Boeing that may be mirrored here. Once a design decision has been made there is not time to re-evaluate the decision.
To take sales away from the Airbus Boeing needed a plane with the Leap engines.
The first indication that they may have been going off the rails was when flight simulator testing showed the unexpected lift generated by the engines at high angles of attack.

A flight simulator does not have to simulate flight outside of the normal flight envelope

Boeing's flight simulator was good enough to show up this problem.
Well, now a problem has been identified and we need a solution.
Maybe that was a bad decision to put the engines in that position.
Maybe yes, maybe no.
But neither Boeing nor some posters here will allow that decision to be re-evaluated.
How many here have seen a an attempt to fix a bad design with a kludge. Sometimes the kludge results in more problems. Eventually kludge upon kludge results in a solution that sort of works, but not as well as it could.
Going back to the original design decision and changing that may result in much better performance all around.
The problem arises when someone refuses to allow the original design to be considered.
Attempts to explore possible alternatives here are quickly shot down.
A suggestion that longer landing gear would have allowed the engines to be placed under the wings was shot down.

If the plane was higher off of the ground there would be difficulty loading the luggage

That may have been an issue 40 or 50 years ago, but most luggage is now loaded with a conveyor and the MAX is competing with a plane that is higher off of the ground and that has the leap engines fitted under the wings.

Longer landing gear would have to be re-certified and the wings may have to be strengthened.

There are reports that the wings were strengthened to withstand the added force and torque generated by the forward engine placement.
I hope that these changes were certified.

Longer landing gear could not be stowed in the existing wheel wells.

But when a potential tail drag issue was identified on the Max 10, Boeing tried harder and developed an extending landing gear for the max 10 that gave them the needed clearance but could still be stowed.

But the Max 10 landing gear only extends on rotation.

Try harder. Boeing tried harder once, maybe they can do it again.

Changes to the landing gear would have to be certified.

I hope that the extending landing gear on the Max 10 was certified.
It would be nice to have a discussion in regards to possible alternative solutions without every suggestion being shot down because it is not MCAS solution.

My suggestion that the existing Max aircraft be fitted with the original engines in the original position was not intended as a final solution.

changing engines and engine location may lead to more problems and issues.

A year or so ago I doubt that this position would go unchallenged.
Now, given the mistakes, bad judgement, oversights, forgetfulness and response to the Max issues, the thought is gaining in credibility.
The suggestion to change the engines was intended to be a stopgap measure to get seats in the air and to limit losses.
Compare the cost of unavailable flights and penalties and refunds or the cost of using charters to fly those flights with the cost of doing business at last years fuel cost.
Since making this suggestion I understand that the worst crisis will be over when the present flight schedule ends. Subsequent schedules will factor in the shortage of aircraft.
This may still turn out to be the best financial solution if things don't go well with the international authorities.
When the Max finally flies again, these planes could be scheduled for rotation back to Boeing for the upgrade.
Eventually when all the existing and future Airbus orders have been filled a couple of years from now, the operators of the older engines will be at a disadvantage.
At present there may not be much disadvantage as much as the operators with Leap will be reaping windfall profits.
It has been reported that ticket prices will be high in the near future due to a shortage of seats.
All flights will be doing well financially. A shortage of seats means high prices and full aircraft.
The Leap operators will be doing VERY well.



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

 

[IRstuff]

Changing the engines won't help the MAX, since it won't be any better than the existing models, which already have the same engines. The high demand for MAX basically shows that the buyers want the LEAP engines.

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[3DDave]

Increasing the gear a few inches is not the same as increasing the gear several feet.
The existing 737 ground equipment works with the new 737s, not Airbus
The existing 737 pilot training works with the new 737s, not Airbus
The existing 737 maintenance management works with the new 737s, not Airbus
What production line is tooled up to make the old engines in quantity?
Making a move to Airbus would wipe out airlines like Southwest that has specialized in 737s.

Every solution that isn't MCAS is building a new plane and scrapping the 737 cockpit, increasing the ground, maintenance, pilot training, and other costs. Sure, the aluminum can be recycled as scrap and the seat rails and seats stored for the next generation, but I don't see Boeing as reconfiguring the cockpit with large trim wheels so a 1 percentile pilot can crank the stab at Mach .6 from more than 2 units out of trim.

 

[waross]

For operators faced with a shortage of seats, there may now be more pressing issues than "helping Max".
The operators facing equipment shortages will be looking at alternatives to the Max, if only for the short term.
They want Leap engines but they can't have Leap engines right now.
Airbus has a pretty long waiting list and we don't know how long it will take to get the Max approved.
If the wait for the Max turns out to be much longer than anticipated I am sure that some operators will be running the numbers on an engine swap.
Putting last generation engines on the Max airframe leaves the option open for a later upgrade.



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

 

[3DDave]

The problem extends to the NG as well and maybe earlier planes. That's what is taking so long. For a similar trim runaway, managed as poorly as Ethiopian Airlines did, the NG will also crash. Eliminate MCAS as a potential accident initiator and the Max is no different than the NG. There just haven't been any trim runaways for pilots to face or, if they have happened, they happened to crews who dealt with them correctly.

Again - where do those extra engines materialize from?

 

[LionelHutz]

Its impossible to move the manual trim wheel at elevator deflections under half deflection and full forward trim cannot be overpowered by full aft elevator. Stick forces increase by 50lbs per 2.5degrees out of trim.

Well, it can be (at least mostly) if you don't overspeed the plane....

Which brings up another question. The stick feedback appears to be hydraulic so why not reduce that force?

The 737 being an old fly by cable design doesn't have electrical systems redundancy because the cables are supposed to be the redundancy. It doesn't appear to have any redundancy for the instrumentation side to side so a single AOA sensor failure throws off a whole bunch of stuff on that side.


Bill - Doesn't Max 10 gear just lengthen on rotation? I'm thinking it was a very specific design to keep the forces in the gear equal to the old gear design. Otherwise, why didn't they just make new gear that was extended all the time while deployed and only retracted when stowed?

I doubt the old 737NG engines are available in production quantities anymore.
The Max planes are being ordered because of the fuel savings.
Airbus has way more than 2 years of neo orders. There is probably about 8-10 years worth of orders.

You also got one paraphrase partly wrong. The 737 was originally designed to sit lower to help with loading luggage and passengers, which became an issue when fitting bigger engines.

Lots of planes use fly by wire controls. So, why does the proper fix have to be a kludge?

 

[waross]

Good points Lionel

Which brings up another question. The stick feedback appears to be hydraulic so why not reduce that force?

I can make a suggestion;
Anecdote alert. A few years ago I had occasion to move an older truck, one of the first models with power steering.
Things went well until I made a sharp turn into a driveway at low speed.
The early versions of power steering added assist to basically manual steering. It took about twice as many turns of the steering wheel as I had expected.
Over the years the steering ratio has been reduced. You no longer need a lot of mechanical advantage because hydraulics are doing the heavy lifting.
Anecdote off.
Had the artificially generated control forces been dialed down incrementally over a span of years it may have been acceptable.
But I don't think that it would be a good idea to abruptly change the control force feel from similar aircraft.
Imagine a pilot moving from one plane to a similar plane but the control force for a given result is only half of what he is used to.
I don't see any technical problem to reducing the stick force feel, but there may be insurmountable problems with usability.

Doesn't Max 10 gear just lengthen on rotation?

That is my understanding.
I was probably unclear with my comments.
Boeing tried harder and came up with the extending gear to avoid tail scrape.
I am sure that there were problems to overcome.
My thought was by trying harder again, an extending gear may be developed that will both give the needed clearance and still fit in the available space.
I'm out of time just now.
More later.



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

 

[IRstuff]

I meant helping MAX in the sense that if someone has to choose between a rebuilt/redesigned MAX vs. Airbus, particularly if the MAX won't provide the promised fuel savings, then the MAX is essentially dead as a product; everyone that needs seats now, or within the next couple of years, will be more than willing to get an Airbus, if the delivery is not substantially longer.

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