Boeing 737 Max8 Aircraft Crashes and Investigations [Part 3] 36

[Sparweb]

This is the continuation from:

thread815-445840
thread815-450258

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.


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

 

[saikee119]

I have been trying to find out why the Indonesian 737 Max survived in the flight JT043 the day earlier but not the later flight JT610. I superimposed some lines over the MCAS events in doomed JT610.

It seems the Indonesian pilots were attempting to fly the plane at 5 pitch trim units a value the same plane flew successfully the day earlier to its destination to about 25,000ft.

After the flap was retracted for the second time there were 25 cycles the JT610 pilots used the power-assisted trim, which I believe was from the switch at the column yoke, to cancel the nose down actions from MCAS. The plane was successfully flying an average 5 pitch trim units except for the last four MCAS nose down commands which drove the plane into the Java sea.

The JT610 pilots fought about 7.5 minutes with MCAS and failed. They could not over-trim the plane to get out the difficulties as the plane would gain altitude when it was supposed on their way back to the airport.

The day earlier the JT043 pilots on the same plane fought with MCAS for about 6 minutes too but survived. They did so by throwing the two Stab Trim Cutout switches and completed the rest of the journey by flying the plane manually. In fact after the MCAS was disabled the pilots only manually trimmed the plane just once for the whole journey.

One important factor for the survival could be the JT043 pilots made the decision to fly to destination instead of aborting the flight by returning to the airport. The 737 Max on that day was able to climb out of the difficulties and continue the journey after halting MCAS.

JT610 and the later Ethiopian ET302 pilots both considered the flight control problem to be unsafe to continue with the journey, sought and got permission to return to the airport, stopped gaining any more altitude and suddenly MCAS took control to pitch the plane to the point of no return.

The altitude the two doomed 737 Max were flying did not have enough headroom to survive the known MCAS malfunction with a faulty AoA sensor. JT610 was finished by the last four MCAS activation and gone in 50 seconds while it was flying a pitch trim averaging 5 units at 5000ft. ET302 was knocked off in just 24 seconds by just one MCAS nose dive command while flying steadily at 2 pitch trim units at 7000 ft.

It seems any MCAS update has to be safe for the 737 Max to do emergency return to the airport at low altitude ready for descend/landing. It is a frightening that MCAS could suddenly destroy the two 737 Max in less than a minute.

 

[RVAmeche]

Given the legitimate need for MCAS due to the engine location I agree it'd be a major backpedal to "just remove it".

It seems like the some of the bigger issues were the lack of redundancy on the AoA sensors, MCAS' apparent inability to ignore or react to bad data, and the lack of pilot awareness/training on MCAS.

 

[BrianPetersen]

Bear in mind that "just remove it" and "switch it off" (manually via the existing stab trim cutout switches) and proposed MCAS automatic switch-off due to any future fault-detection scheme, all result in an airplane that is in the air without functioning MCAS. If the plane does not have acceptable flight characteristics in that situation ... hmmm.

 

[3DDave]

saikee -

The crash of JT610 was due failing to trim following the hand-off off of the trim task from PIC to the SIC. This hand-off was to allow the PIC time to look through a manual. The SIC apparently noted that tapping the switch stopped the trim motors, but did not understand that the altered trim was not restored to its previous state. This is why the manual inputs are so short and completely ineffective at the end.

What's not clear is why the PIC and SIC did not turn off the trim motors. They hit the trim button nearly 30 times. They should have known the plane was flyable with the trim wheels.

The one pilot input to manual electric trim on the survival flight was on the ground, weight on wheels.

 

[waross]

First rule of problem solving:
Identify the problem!
Second rule of problem solving:
Make sure that you have identified the problem.
Third rule of problem solving:
Don't address the symptoms, fix the problem.
The problem is an inherently unstable aircraft.
MCAS is not fixing the problem, it is covering up a symptom.
Maybe it is time for Boeing to bite the bullet and stop pushing an old old design to greater performance.
The problem is not with MCAS. The problem is a flawed design change that should not have been approved in the first place.


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

 

[3DDave]

The 737 is not and has never been unstable; there is no normal envelope area where there is positive feedback that causes divergent flight. This repetition of this untruth is not helpful. As for covering things up, that's the primary design philosophy of Airbus in order to make all their aircraft appear to have similar handling characteristics and completely isolating their pilots from the feel of the actual controls.

For an expert write-up see

http://avherald.com/h?article=4c534c4a/0045&opt=0

In particular "... the first and foremost being the startle effect, in particular with a permanent stick shaker activation that does not stop despite lowering the nose in an instinctive reaction. The added stress of the continuous noise and rattle must have contributed to further confusion - and this scenario has never been trained for, no pilot has been prepared for such a scenario yet, in which a faulty AoA value could cause a permanent stick shaker. Therefore it appears likely to me, that the focus of the crew was to keep the aircraft flying and clear of terrain while trying to get rid of that noise and stress."

The primary problem in these crashes is crews unable to deal with the stick shaker, a symptom of an AoA system failure. Thus leading the Ethiopian Air crew to allow an overspeed condition which produced higher than usual control forces, especially to the trim wheels

 

[Lou Scannon]

3DDave, waross, it seems you guys are arguing from extremes. We're engineers, let's argue from the middle. Some most likely rational yet apparently misguided people at Boeing got onto a slippery slope of meeting a specification that required no re-certification or major re-training for the 737Max, and let that cloud their judgement regarding how to compensate for the undesirable aerodynamic effects of the engine mounting configuration.
It seems that some regrettable decisions were taken.
It remains to be seen whether the 737Max can yet be tailored to meet the specification, no re-certification or re-training as a core requirement, or whether these requirements get thrown overboard, along with possibly the whole project.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Alistair_Heaton]

I must admit the unable to cancel stickshaker has me a bit confused. Unless it's another grandfather difference between current certification standards.

I though there had to be a way to cancel the stick shaker. Its normally in the spurious stick shaker qrh checklist. For the three types I have flown you just press the stall caution light when it's flashing and it kills it until you do a full power kill reset on the ground. The stick push which is the next level up of protect and you pull through it requires a reset by a technician in the hydraulics bay.

Oh and for some of our procedures you are meant to pitch until the stick shaker triggers and hold it there. Windshear being the main one.

Windshear the MCAS wouldn't be functioning because the flaps would be out.

 

[Alistair_Heaton]

What phase of flight did the previous event occur? Dealing with something at fl350 is completely different to at 1000 ft.

 

[3DDave]

The flight that survived started with symptoms at takeoff just like the flight the next day. It appears they engaged Autopilot a minute after takeoff and that Autopilot was cancelled a minute after that. The FO tried again and it was cancelled almost immediately.

The accident flight seems to have delayed retracting flaps and gained more altitude before the first MCAS input; they continued to gain altitude as the PF continued to counter the MCAS input.

 

[waross]

Part of the underlying issue is the large size of the leap engines. They moved the engines forward and up to get the ground clearance they needed.
Why, I asked, did they not just extend the landing gear and put the engines under the wings.
Apparently there are a lot of reasons that they can't extend the landing gear.
It wouldn't fit in the existing wheel wells.-
This would void type approvals.-
And maybe a few other reasons.
Then they had another problem with the Max10.
Possible tail strike on takeoff.
What did they do?
Well they found a way to lengthen the landing gear and still fit it into the existing wheel wells.

https://www.flightglobal.com/news/a...etails-737-max-10-landing-gear-design-451546/

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

 

[3DDave]

The other reason is the 737 can operate by ground crews handing luggage into the fuselage. Longer gear would mean that it can no longer be usable at many of the airports it's used at now. Only the Max 10 the main gear lengthens only during landing and takeoff to avoid tailstrike on the lengthened fuselage.

 

[Alistair_Heaton]

I suspect also if it was higher off the ground they would have to fit wing slides for the overwing emergency exits.

The 800 they select max flaps as part of the evacuation checklist before killing the engines. Then the punters using the overwing have to slide down the flaps.

A requirement for wing slides would be a collosal hit on dry operating weight and service maintenance. I think they have to come out every two years and weigh 100kg but could be wrong. So 200 kg and 100k $ every two years. For every extra 1 ton of load on a 737 classic it's an extra 50kg of fuel per hour.

 

[LittleInch]

It looks like the landing legs basically extend another 9.5 inches only and then retract when the gear is stowed. That might be enough for the tail strike issue but for the engines?? The static height of the plane looks to be the same as before.

Also adds a level of complexity as the wheels need to retract that extra distance using some linkages. more weight and potential to go wrong which needs to be designed out.

Also from the video it looked like the wheels just extend their travel as the load lightens as the plane accelerates but on landing have the same height once the entire plane weight is on the wheels which would be no use for the engines. I guess this way they don't have to change anything height related ref escape slides etc.

I must admit I've always thought it a bit "sketchy" to have to basically slide off the wing onto tarmac or whatever from those mid wing exits via the flaps which the pilot has hopefully remembered to lower in the middle of dealing with whatever crisis has happened at the same time



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

 

[Alistair_Heaton]

It could be one of these grandfather deviations that's the only reason they get away with it.

To be honest the holds on the 737 are lower than the hold on the q400 and some places they load the q400 without a baggage belt. I just avoid being present when they are putting 100kg pallet of dangerous goods on.

 

[saikee119]

Why are we obsessed with the height of the landing gear or any part of the bad design that works?

I thought we are looking at the faults of the design like MCAS that most likely caused the plane crashes.

It is the engineer's role to make things work. Just like one can put an engine in the front, middle or back of a vehicle. None of the position is ideal but one live with its characteristics and arrangement.

 

[RVAmeche]

Saikee,

If the 737 sat higher off the ground normally (through taller landing gear for example), they likely wouldn't have needed to change the location of the engines to a location that necessitated MCAS. The Airbus version sits higher naturally, so they didn't have to move the engine placement (their LEAP engines are even larger).

 

[saikee119]

RVAmeche,

I understand what you said but is the landing gear the root cause of crashes or is it a major contributory factor?

Unless the landing gear has a major design fault contributing to the two crashes I doubt if the investigator could recommend a change in its design. A bad design is not always a faulty design.

Has it become obvious that MCAS, assuming it were the root cause, will never work no matter how much engineering is thrown at it?

 

[RVAmeche]

Saikee,

Like LittleInch said, the landing gear itself didn't cause the accidents but its design, combined with the new engines, forced a new engine placement that was inherently unstable. Therefore they developed MCAS to counteract the engine placement. Unfortunately it seems the MCAS system was poorly implemented and even more poorly explained to pilots because, as Boeing said, "they should never see its operation".