Boeing 737 Max8 Aircraft Crashes and Investigations [Part 2] 44

[Alistair_Heaton]

This thread is a continuation of:

thread815-445840


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Another 737 max has crashed during departure in Ethiopia.

To note the data in the picture is intally ground 0 then when airborne is GPS altitude above MSL. The airport is extremely high.

The debris is extremely compact and the fuel burned, they reckon it was 400knts plus when it hit the ground.

Here is the radar24 data pulled from there local site.

It's already being discussed if was another AoA issue with the MCAS system for stall protection.

I will let you make your own conclusions.

 

[debodine]

I still find myself questioning the use of software to correct a hardware anomaly. I am aware that the military sometimes designs fighter jets that are inherently unstable/unflyable unless the computer controls the aircraft by comparing the flight envelope limits to the pilot's input request, but those pilots have ejection seats when the software becomes unavailable.

Having read several reports about MCAS, I would ask the following questions of whomever signed off on this system.

1. Since the hardware anomaly is always present (the nacelle shape and location does not change, ever) then the instability always exists in the dangerous part of the flight envelope. So what protects the aircraft when MCAS cannot if the aircraft enters that dangerous region of the envelope with MCAS out of the picture due to opening the stab trim cutout switches?

2. Since MCAS only operates in manual flight mode, does that mean it is impossible for the autopilot to command the aircraft into the dangerous part of the envelope?

3. Since MCAS only operates with flaps retracted, does that mean it is impossible to manually fly the aircraft into the dangerous part of the envelope with the flaps extended?

The above questions are why I consider a software fix to a hardware anomaly inadequate when the occupants (crew and passengers) have no escape path (ejection seats) if the computer is not available (stab trim cutout) to protect the aircraft from bad flight control inputs.

 

[waross]

Thank you for taking the time to explain that, Alistair.

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

 

[IRstuff]

Is MCAS really "software," as opposed to "firmware?" In military systems, that's a big difference; firmware is basically hardware and essentially always loaded and running, while software has operating systems, possibility of CPR errors, interrupts, etc. If MCAS is mission critical, then one would think that it'll be firmware and subject to the same reliability analysis as any other of the critical avionics systems.

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[Lou Scannon]

That may be so but it seems like a nuance since as far as I understand we haven't been discussing malfunctions of the MCAS code within its normal parameters.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Alistair_Heaton]

"1. So what protects the aircraft when MCAS cannot if the aircraft enters that dangerous region of the envelope with MCAS out of the picture due to opening the stab trim cutout switches?"

absolutely nothing....

"2. Since MCAS only operates in manual flight mode, does that mean it is impossible for the autopilot to command the aircraft into the dangerous part of the envelope?"

The trim system does not rely on force feedback to know what its doing. And yes it can take the aircraft into all extremes of the flight envelope. When it does and it hits a limit and triggers an alarm be it over speed or underspeed it then gives up and gives control back to the pilots to sort out.

"3. Since MCAS only operates with flaps retracted, does that mean it is impossible to manually fly the aircraft into the dangerous part of the envelope with the flaps extended?"

No it doesn't and there is a standard stall setup which replicates exactly this when you have landing flap and its practised in the sim, but this is where it gets interesting for me flight dynamics wise. And thanks to the test pilot week I understand it these days.


When you deploy flap it has several effects on flight stability. And to note its flaps and slats. Flaps are on the trailing edge and slats are on the leading edge.

It moves the centre of pressure of the wing inboard making the aircraft less stabile in roll.

Its also moves it rearwards increasing stability.

It increases the wing area as well.

Now this alters the pitch moment force diagram. The lift generated by the engines will be constant with alpha. The flaps/slats cause a pitch down moment so they are working opposite to the engine pitch. When you increase the alpha the nose down forces will increase thanks to the flaps. I presume the moment increase from the flaps/slats will over power the opposite effect of the engines so there is always an increase in control force with higher alphas. Now this is the nice version...…

Nasty version is that there is no certification standard for control forces with flaps extended so they decided to just turn it off when they are selected as there are not required to meet any standards on it.

 

[VEBill]

..."software," as opposed to "firmware?"...

The distinction is blurred. Even between "software" and "hardware", the boundaries are fuzzy.

Some policies have had to retreat to "field loadable". Even that distinction is unreliable, as the OEM may only acknowledge that capability when they suddenly decide that they need help in fielding an unexpected update.

 

[waross]

Would deploying mimimum flaps have disabled MCAS and made the aircraft flyable with pilot controlled electric trim still operable?
Would it have been safe to deploy minimum flaps when they got into trouble?

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

 

[3DDave]

Yes it would disable MCAS, but it would probably not have saved them. The pilots were so focused on the false stick shaker that they let the plane approach sonic speeds at low levels which would likely have torn any flaps that weren't stowed right off. The main problem is they only countered half the first MCAS increment leaving them with around 30 pounds force to keep the nose up, a force that would increase with increased speed, hence their ineffective attempt to trim again by re-enabling the trim motors without FIRST pushing the nose-trim-up switch.

What would have worked is following the disagree memory checklist to limit the speed and therefore the force required on the elevators and then the trim runaway procedure where they would have used the manual trim switch to return trim to nominal before shutting it off. None of the three crews that encountered this problem did any of that; the first group lucked out by having a pilot fully retrim several times before the pilot in the jump seat suggested shutting off the trim, apparently believing some flaw in the Speed Trim System. That flight went on another hour with the stick shaker hammering away. The second, crash flight, the pilot retrimmed nearly 30 times in response to MCAS without thinking to shut the trim motors off; his turning control to the second in command and not telling him to maintain the stab trim was fatal. The Ethiopians took a short stab at trimming and then just gave up; as if they read about the MCAS problem but not understanding what it meant to establish trim -before- shutting the motors off. They skipped a bunch of steps and got to the end even faster than the others.

I started writing what was wrong with the IEEE article but there's too much. Owning a plane and understanding aerodynamics are separate things as demonstrated by the large number of small plane pilots who slam into the dirt a half mile from the runway while yanking back on the stick trying to stay up longer when the motor failed or the ice built up.

 

[moon161]

Yeah, I thought some of the IEEE article looked off and I took flight dynamics in 1997 or 98.

 

[saikee119]

3DDave,

You seem to suggest there is a way to work around the existing MCAS against a faulty AoA sensor. In that case I don't understand why Boeing need to bother with another update? Just tell the pilot to read the appropriate section of the manual and fly the plane.

If there is something so simple a pilot can get out of the dire situation in 6 to 12 minutes, as experienced by the pilots in the two crashed planes, the aviation industry must be stupid wasting billions on grounding the 737 Max and cutting back its production.

 

[3DDave]

saikee,

It is simple, but it's also obvious that pilot emergency training is poor and it is easier and more certain to update the software than to depend on pilots to do the right thing when it's avoidable. I read both reports and in all three incidents the pilots failed to follow their training. Even with the updated AD from Boeing that said if the pilots ever turned the trim motors off to leave them off, the Ethiopian pilots turned them back on and crashed seconds later directly because of that. Had they continued as the were they would have gained enough altitude to do a series of small pushovers to unload the stabilizer and use the manual crank handles.

You seem almost personally offended, as if you think the pilots were helplessly lashed to a bomb. The first incident showed that was not true and that pilots had all the tools at hand to cope, which they did in just a few minutes. It only became a dire situation on the accident flights when pilots did not delegate (Cockpit resource management) to have one pilot concentrate on guiding the plane while the other concentrated on determining if the alarms are right or not and going through the procedures.

In the first incident the pilots never let trim stay more than one degree from nominal. On the accident flight the pilots let it drift 2 degrees before a transfer of control allowed it to reach minus 5 degree stab trim change. In the case of the Ethiopian flight the pilots let it go almost 3 degrees and failed to monitor airspeed, reaching Vmo* in just 3 minutes and also letting trim reach -5 degrees from nominal by failing the AD procedure and enabling the trim motors.

*Vmo = Velocity Maximum Operating.

 

[Alistair_Heaton]

Its trim units not degrees. These days they have got away from raw numbers because the numbers for the 800 will be different to the max. So the give us flap 1 2 etc and none dimensional units.

The emergency training or lack of it may be correct. But the amount of conflicting information in the cockpit would have been collosal. And there was no singular alarm to tell them that there was a problem with the AoA system which could have led them to the appropriate check list. There was an option for a AoA mismatch indicator but that would have added a 6 figure sum to the airframe price.

This human interaction and interpretation of data in the cockpit is a huge subject. It doesn't sit with engineers because the results are unpredictable and subject to a distribution curve. Experience,age, fatigue and the big one cultural all have an effect.

The fact is two airframes in the space of six months have killed lots of people on a new type which is 18 months old. One which major selling point is that it requires 1 hour on an iPad to fly if you can fly the 737-800 Ng. A type I might add that fly's 10 000's of hours per day using the same pilots without crashing using the bulk of the same parts and systems.

 

[SwinnyGG]

Just tell the pilot to read the appropriate section of the manual and fly the plane.

"just read the iPad for 45 minutes and then go fly the plane" is exactly what 4 dead pilots were told to do.

 

[Sparweb]

Thank you everyone for your enthusiastic contributions!
This topic is being 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.

Please continue the discussion at the new thread:

thread815-452000