For the aviation fans

[mazambaan]

What incident was this?

 

As I recall from Air Crash Investigation snippets it was on a flight from China, auto pilot malfunctioned and on switching it off, main flight computer starting throwing up errors then pitched the nose down, 4 degrees, then another data misinterpretation sent it another 6 degrees; passengers and non-flight crew not strapped in suffered some quite serious negative g injuries and it took a few attempts before the pilot could get some control back, then it happened again. Three man crew got it to an airfield but it may still have crash landed - no loss of life.

 

Airbus worked with the Australians to dig out the problem in features with acronyms I've forgotten.  But this just recall; I could easily have some stuff wrong. I will dig if needed.

[kosmonooit]

They are saying that moving the engines forward on the 737 max series can cause the nose to pitch up under power/certain circumstances - but how so? 

 

Moving weight forward would move the COG forward, causing a tendency to pitch down surely under static conditions.

 

So what causes the aircraft to pitch up here? I am stinking maybe its the point of thrust from the engine causes a moment of force up relative to the COG that needs to be compensated hence the MCAS?

[patham]

They are saying that moving the engines forward on the 737 max series can cause the nose to pitch up under power/certain circumstances - but how so? 

 

Moving weight forward would move the COG forward, causing a tendency to pitch down surely under static conditions.

 

So what causes the aircraft to pitch up here? I am stinking maybe its the point of thrust from the engine causes a moment of force up relative to the COG that needs to be compensated hence the MCAS?

 

I will have to hunt for the links to the online info I paraphrase below. The engines are now way ahead of the CoG and Centre of Lift lines. I gather that the engine nacelles themselves are very poor wings, i.e. they generate their own lift.This is pretty nominal at normal flight angles, but as the Angle of Attack increases they produce more lift. As they are ahead of the CoG, CoL line; that extra lift tries to pull the nose of the plane even higher. That increases the Angle of Attack, which means they produce more lift, which increases AoA...you can see where this is going, its a negative feedback loop. Eventually the plane will be too nose high, the normal wing would have stalled and it's all gone horribly wrong. 

 

The MCAS is meant to pick up the warning signs of this situation and automatically intervene to bring the nose down.

[patham]

To celebrate post # 747...

 

BA-Landor-unveil-2-e1552292913478-916x515.jpg

 

 

[edit]  - seeing as LWB missed a golden opportunity 

Posts 757, 767, 777 and 787 coming up soon now that you have set the precedent. But well spotted !

[Capricorn]

apparently, the discovery at the crash site that convinced the FAA to reverse their world-beating decision, and that caused Boeing to discovery an abundance of caution, was that the jack-screw was found to be set in the nose-down position.

 

Is the jackscrew seems to actuate the horizontal tail, which pitches the nose up or down. Perhaps it's rearward location is what caused it to remain as tangible evidence despite the catastrophic nature of the crash.

[flymango]

[Caution may disturb some folks]

 

 

 

 

 

Also have a horror story regarding a crash in Angola from high altitude. When they eventually found the crash site it was just a hole in the ground... my boss at the time also went as part of the accident investigation and he used the term mincemeat

 

RIP Bennie

[kosmonooit]

apparently, the discovery at the crash site that convinced the FAA to reverse their world-beating decision, and that caused Boeing to discovery an abundance of caution, was that the jack-screw was found to be set in the nose-down position.

 

Is the jackscrew seems to actuate the horizontal tail, which pitches the nose up or down. Perhaps it's rearward location is what caused it to remain as tangible evidence despite the catastrophic nature of the crash.

 

 

that screw was the trim on the elevator / horizontal stabiliser

 

 

The trim tabs are the small little wings at the tail  end of the control surfaces - normally say when climbing out one trims so that you don't have to keep holding the stick back, it becomes a second instinct to always trim for your intended path of flight.

 

The MCAS apparently uses the trim tabs for control as well, but that can easily be overridden with switches and apparently these pilots were trained in this system and were made aware of the issues post Lion Air disaster. So the plot thickens.

 

 

 

Edited March 16, 2019 by kosmonooit

[kosmonooit]

I will have to hunt for the links to the online info I paraphrase below. The engines are now way ahead of the CoG and Centre of Lift lines. I gather that the engine nacelles themselves are very poor wings, i.e. they generate their own lift.This is pretty nominal at normal flight angles, but as the Angle of Attack increases they produce more lift. As they are ahead of the CoG, CoL line; that extra lift tries to pull the nose of the plane even higher. That increases the Angle of Attack, which means they produce more lift, which increases AoA...you can see where this is going, its a negative feedback loop. Eventually the plane will be too nose high, the normal wing would have stalled and it's all gone horribly wrong. 

 

The MCAS is meant to pick up the warning signs of this situation and automatically intervene to bring the nose down.

 

 

Interesting - you are saying the lift for the nacelles are the culprit, would be interested to read about that, I'll do some trolling of the interwebs later - it has to be out there.

[patham]

Interesting - you are saying the lift for the nacelles are the culprit, would be interested to read about that, I'll do some trolling of the interwebs later - it has to be out there.

 

I found the one source here:

 

https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/

[Robbie Stewart]

I found the one source here:

 

https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/

This is a rather shocking ommission by Boeing, not informing pilots about the fact that (MCAS induced) trim will not be negated by pulling on the yoke as would logically be applied in the NG. Never mind that they also neglected to inform the pilots about MCAS and instead insisted that the flight characteristics of the MAX and NG are identical, when they clearly are not.

[Capricorn]

This is a rather shocking ommission by Boeing, not informing pilots about the fact that (MCAS induced) trim will not be negated by pulling on the yoke as would logically be applied in the NG. Never mind that they also neglected to inform the pilots about MCAS and instead insisted that the flight characteristics of the MAX and NG are identical, when they clearly are not.

 

the automatic pitch control introduced via the MCAS to counter the additional pitching generated by the new nacelle design, and the initial lack of awareness therefore by the pilots poses two potentially mutually exclusive, but likely co-dependent possibilities,  as cause for the crashes:

• the MCAS solution creates it's own instabilities. The fact that boeing is busy with an FAA mandated change to the control system software hints very strongly at this. Also Boeing will be including input from the second AOA sensor to counter problems induced by a single faulty AOA sensor. This feels like a bizarre lack of redundancy of a critical input.
• the pilots are still not hunkydory with MCAS and its behaviour, leading to outdated recovery actions when the doodle hits the fan.  tThe statement in the article that "both planes fly the same as long as no faults appear",and the fact that the plane was advertised as compatible with flight training for NG versions leads me to believe there's possible delays in ensuring MAX pilots receive proper training regarding the behaviour of MCAS and how they need to respond in event of a major issue. Clearly, highly experienced pilots at the helm did not avert the disasters, and might actually be the cause of it: they did what came instinctively. Repeated training is the basis for instinct.

Edited March 16, 2019 by Capricorn

[kosmonooit]

I found the one source here:

 

https://leehamnews.com/2018/11/14/boeings-automatic-trim-for-the-737-max-was-not-disclosed-to-the-pilots/

 

 

That is someone's blog, they need to provide references to reinforce their claims instead of stating it as fact.

 

I am not saying that what they are saying is not true but we need verifiable facts here.

 

In one sentence this blog states:

 

"All objects on an aircraft placed ahead of the Center of Gravity (..) will contribute to destabilize the aircraft in pitch."

 

So that is weight fwd of the COG that will tend to weigh the nose DOWN or even perhaps induce some oscillations, as I suggested earlier unless this is compensated some how with more mass aft of the CoG, I find it hard to believe the designers would not have compensated for that.

 

"But if the pilot for whatever reason manoeuvres the aircraft hard, generating an angle of attack close to the stall angle of around 14°, the previously neutral engine nacelle generates lift. A lift which is felt by the aircraft as a pitch up moment (as its ahead of the CG line), now stronger than on the 737NG."

 

But what has been said here earlier is that the nacelles push the nose up, this blog is saying that the lift from the nacelles only kick in at high AoA - this is what we need a reference to source for.

Edited March 16, 2019 by kosmonooit

[kosmonooit]

its not trim tabs its the AoA of whole horizontal stabiliser that gets adjusted by that trim system,as the blancolirio explains

 

 

[SwissVan]

More on the Stab trim "jack screw" 

 

[patham]

That is someone's blog, they need to provide references to reinforce their claims instead of stating it as fact.

 

I am not saying that what they are saying is not true but we need verifiable facts here.

 

In one sentence this blog states:

 

"All objects on an aircraft placed ahead of the Center of Gravity (..) will contribute to destabilize the aircraft in pitch."

 

So that is weight fwd of the COG that will tend to weigh the nose DOWN or even perhaps induce some oscillations, as I suggested earlier unless this is compensated some how with more mass aft of the CoG, I find it hard to believe the designers would not have compensated for that.

 

"But if the pilot for whatever reason manoeuvres the aircraft hard, generating an angle of attack close to the stall angle of around 14°, the previously neutral engine nacelle generates lift. A lift which is felt by the aircraft as a pitch up moment (as its ahead of the CG line), now stronger than on the 737NG."

 

But what has been said here earlier is that the nacelles push the nose up, this blog is saying that the lift from the nacelles only kick in at high AoA - this is what we need a reference to source for.

 

I hear you, discussions that are fact based will be far more productive for everyone. But to be 100% verified facts, the only people who help in that are Boeing as they are the only ones with the intellectual property of the design calculation package. And technically, they would still be unverified until some other qualified outfit can review them.

 

I suppose we could also consider it as factual if the FAA or similar investigator releases their final reports into the crashes if they have been supported in their investigations by Boeing.

 

I do note this piece has linked below has a short little video (with not much detail) attributed to Boeing, but I have a feeling that everyone will be forced to join the dots in their own little way for now.

 

https://www.nytimes.com/2019/02/03/world/asia/lion-air-plane-crash-pilots.html

[mazambaan]

Gleaning from Avcom; seems to be a puzzle as to why MCAS was not switched off as soon as automated nose down was initiated.  Also that some instability was noted with the aircraft autopilot engaged and on disengaging, MCAS "took over" and drove to the crash site.