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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 16th Mar 2019, 10:16
  #1581 (permalink)  
 
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Originally Posted by fdr View Post
§ 25.672 Stability augmentation and automatic and power-operated systems

If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with § 25.671 and the following:

(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.

(b) The design of the stability augmentation system or of any other automatic or power-operated system must permit initial counteraction of failures of the type specified in § 25.671(c) without requiring exceptional pilot skill or strength, by either the deactivation of the system, or a failed portion thereof, or by overriding the failure by movement of the flight controls in the normal sense.

(c) It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system -

(1) The airplane is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved operating limitations that is critical for the type of failure being considered;

(2) The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and airplaneconfigurations) which is described in the Airplane Flight Manual; and

(3) The trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing.

[Amdt. 25-23, [url=https://www.law.cornell.edu/rio/citation/35_FR_5675]35 FR 5675 Apr. 8, 1970]
It all depends whether you believe the AOA vanes and system are part of the MCAS. Its like using an ASI display spec to regulate the pitot/static systems. MCAS worked precisely as it should - it was given the incorrect inputs. Inputs that were also used for ADIRUs and those systems provided a cacophony of warnings - for some reason their logic does not include an AOA disagree and that warning light is an 'optional extra'.

As was stated repeatedly on the AF447 thread AOA should be considered a non-optional display and AOA disagree should also be non-optional. An AOA disagree should result in the disabling of any system would operate dangerously. However, as was stated up thread a lot more logic could be used to identify and isolate the incorrect AOA. Only validated AOA information should be fed to systems or those systems disabled.

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Old 16th Mar 2019, 10:20
  #1582 (permalink)  
 
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Umm, Boeing didn't provide pilots with any information on MCAS until after Lion Air crashed, pilots were not supposed to need to know about it.
I can see what Boeing were trying to do. From their point of view, the pilots are already trained (supposedly) to deal with the situation the MCAS may bring up, so why complicate things?

If the pilots had followed the prescribed procedure for inadvertent/inappropriate/runaway stabiliser trim (a procedure which has existed for decades, across all Boeing models) then these crashes would not have occurred and the pilots would be none the wiser about MCAS.
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Old 16th Mar 2019, 10:37
  #1583 (permalink)  
 
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ET302 Stab jackscrew found , trimmed to dive

The smoking gun implicating MCAS ie. the stabiliser jackscrew has been found.

https://www.ndtv.com/world-news/piec...o-dive-2008100

In the light of physical evidence which is hard to suppress, I think we can now expect some resignations at the FAA, and a hard look at the airplane’s other possible failure modes. I don’t think the 737 Max-8 will get ungrounded as easily as it got certified the first time round.

Edmund
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Old 16th Mar 2019, 10:48
  #1584 (permalink)  
 
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Originally Posted by FCeng84 View Post
MCAS is able to do its job with 2.5 degrees of stabilizer up to Mach 0.4 and less than 1/3rd of that at cruise. That MCAS can insert more stabilizer motion that this design limit is beyond its design requirements. There is not a control power deficiency as there is plenty of elevator to counter the design levels of stabilizer that MCAS needs.
There is plenty of elevator to counter MCAS, but has the pilot enough strength to pull the control column to do so, even when speed increases to 380 kts, given that the artificial feel increases with speed ?

It seems the only chance not to crash, if manual trimming was not possible due to elevator up forces, would have been reducing power, airbrakes out, to cut the speed, then flaps down to kill MCAS and use electric trim again...
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Old 16th Mar 2019, 10:56
  #1585 (permalink)  
 
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Originally Posted by deltafox44 View Post
So... it seems that Boeing confirms this procedure that I had taken for a joke : before trimming, push the stick forward :
Push forward for 2-3 seconds while 2000 [email protected] knots...
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Old 16th Mar 2019, 10:58
  #1586 (permalink)  
 
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Originally Posted by edmundronald View Post
In the light of physical evidence which is hard to suppress, I think we can now expect some resignations at the FAA
You're probably right.

But what's really needed is an in-depth look at the (changed) relationship between the regulators and the industry (by no means confined to the USA).

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Old 16th Mar 2019, 11:03
  #1587 (permalink)  
 
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Originally Posted by DaveReidUK View Post
You're probably right.

But what's really needed is an in-depth look at the (changed) relationship between the regulators and the industry (by no means confined to the USA).
Regulatory Capture.

Partnerships with regulators.
Steak dinners, self regulation.
Soft corruption abounds.

Revolving doors between the regulator and industry. Just look at Aii Barhrami

As Charlie Munger says, Show me the incentive, I show you the outcome.
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Old 16th Mar 2019, 11:11
  #1588 (permalink)  
 
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Originally Posted by deltafox44 View Post
So... it seems that Boeing confirms this procedure that I had taken for a joke : before trimming, push the stick forward :
That looks like they are admitting the jackscrew can be prone to binding right at the time when you can least afford it to, when it has an extreme load on it in the wrong direction.
Is it possible once the loads were too great the jackscrew locked up and they never had time/realisation to unload it enough to free it?
Non pilot here, studying for ppl though and very interested in electromechanical engineering.
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Old 16th Mar 2019, 11:11
  #1589 (permalink)  
 
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Old 16th Mar 2019, 11:12
  #1590 (permalink)  
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Originally Posted by GarageYears View Post


So that puts the ball in the court of Ethiopian - I assume that’s your point? Because that’s my take.

- GY
I was trying not to draw conclusions, merely present the dialogue of the interview as best as I could recall it. (Apologies if I have misrepresented anything the pilot said)
But to clear up one point where I said "Channel 4 News (UK) has just screened an interview with a "Senior Ethiopian Pilot"."
I think the meaning was Ethiopian the nationality, or else the word Airlines would have been used, for the audience of this programme.

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Old 16th Mar 2019, 11:40
  #1591 (permalink)  
 
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(pax). In #1633 the two aoa traces track each other with what looks like a fixed offset but, at the end, the top trace moves down to match the other. Why may that be?
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Old 16th Mar 2019, 12:04
  #1592 (permalink)  
 
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Originally Posted by Mr Optimistic View Post
(pax). In #1633 the two aoa traces track each other with what looks like a fixed offset but, at the end, the top trace moves down to match the other. Why may that be?
higher drag due to increased speed finally moved the AoA vane in the correct position?
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Old 16th Mar 2019, 12:07
  #1593 (permalink)  
 
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Originally Posted by fdr View Post
§ 25.672 Stability augmentation and automatic and power-operated systems

If the functioning of stability augmentation or other automatic or power-operated systems is necessary to show compliance with the flight characteristics requirements of this part, such systems must comply with § 25.671 and the following:

(a) A warning which is clearly distinguishable to the pilot under expected flight conditions without requiring his attention must be provided for any failure in the stability augmentation system or in any other automatic or power-operated system which could result in an unsafe condition if the pilot were not aware of the failure. Warning systems must not activate the control systems.

(b) The design of the stability augmentation system or of any other automatic or power-operated system must permit initial counteraction of failures of the type specified in § 25.671(c) without requiring exceptional pilot skill or strength, by either the deactivation of the system, or a failed portion thereof, or by overriding the failure by movement of the flight controls in the normal sense.

(c) It must be shown that after any single failure of the stability augmentation system or any other automatic or power-operated system -

(1) The airplane is safely controllable when the failure or malfunction occurs at any speed or altitude within the approved operating limitations that is critical for the type of failure being considered;

(2) The controllability and maneuverability requirements of this part are met within a practical operational flight envelope (for example, speed, altitude, normal acceleration, and airplaneconfigurations) which is described in the Airplane Flight Manual; and

(3) The trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing.

[Amdt. 25-23, [url=https://www.law.cornell.edu/rio/citation/35_FR_5675]35 FR 5675 Apr. 8, 1970]
A chapter dear to my heart and one that came immediately to mind after the first crash. My initial assumption was that the fleet would be grounded shortly after the first crash given the suspicion that the aircraft did not comply with the section quoted above - effectively removing a major strand of its certification.

A great deal of flight testing is devoted to assuring the requirements above are met. What surprises me is that the system does not appear to be designed to meet these requirements at the outset.
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Old 16th Mar 2019, 12:12
  #1594 (permalink)  
 
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Before the days of MCAS

And now for something completely different.
I believe this may have been mentioned before but it is an interesting read.
It occured in 2007 a refers to a Boeing 737-3Q8 landing at Bournemouth and at a height of 2,500'.
The full report can be downloaded at
https://www.gov.uk/aaib-reports/aar-...september-2007
A brief summary includes:-

The Boeing 737-300 was on approach to Bournemouth Airport following a routine passenger flight from Faro, Portugal. Early in the ILS approach the auto-throttle disengaged with the thrust levers in the idle thrust position. The disengagement was neither commanded nor recognised by the crew and the thrust levers remained at idle throughout the approach.
Because the aircraft was fully configured for landing, the air speed decayed rapidly to a
value below that appropriate for the approach. The commander took control and initiated a go-around. During the go-around the aircraft pitched up excessively; flight crew attempts to reduce the aircraft’s pitch were largely ineffective. The aircraft reached a maximum pitch of 44º nose-up and the indicated airspeed reduced to 82 kt.

Apart from holding the control column fully forward, the flight crew made
no other pitch control actions throughout the 44º nose-up excursion until the
aircraft had stalled and the nose had dropped towards the horizon. At this
stage the thrust was reduced to go-around thrust. This thrust reduction allowed
sufficient control authority to recover the aircraft.
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Old 16th Mar 2019, 12:15
  #1595 (permalink)  
 
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I will freely admit to being a bit of a dinosaur, having started my flying career on an aircraft with no computer assistance, via aircraft that did to finish on another with minimal assistance. Along the way I learnt a healthy distrust of software being written to help my job - most of the time it worked, but sometimes it didn't, with the most common statement seeming to be 'what is it doing now?' or words to that effect.

Now we have a software input which has possibly, maybe even probably caused two major accidents on a newly introduced virtually computer driven ac. Everyone seems to be looking at just the MCAS algorithms, but has anything else in the entire flight control system been added/removed which could give rise to other maybe unexpected but related happenings in the cockpit?

Software engineers have 'improved' incrementally many other non-aviation systems in the last few years - TSB and other banking outages spring to mind, as does Facebook the other day. Who's to say some apparently minor code change has not given rise to something else happening when MCAS trips in.

I ask as an 'interested observer', no longer an occupant of the panoramic window seat.
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Old 16th Mar 2019, 12:40
  #1596 (permalink)  
 
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Originally Posted by Compton3fox View Post
MCAS is not supposed to operate if the AP is engaged. I would want to look at these events further to be sure they are not related or that they were a result of crew error. If not, there maybe a further complication to the current issue or another issue lurking that we don't know about.
In both cases when the crew took over manually the nose down stopped. It MCAS had been involved then the aircraft would still have had nose down trim in manual control. These cases were activation of an autopilot that for whatever reason was set for a lower target altitude, that is a human factors issue in its own right and needs to be assessed.
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Old 16th Mar 2019, 13:28
  #1597 (permalink)  
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Originally Posted by XN593 View Post
And now for something completely different.
I believe this may have been mentioned before but it is an interesting read.
It occured in 2007 a refers to a Boeing 737-3Q8 landing at Bournemouth and at a height of 2,500'.
The full report can be downloaded at
https://www.gov.uk/aaib-reports/aar-...september-2007
A brief summary includes:-

The Boeing 737-300 was on approach to Bournemouth Airport following a routine passenger flight from Faro, Portugal. Early in the ILS approach the auto-throttle disengaged with the thrust levers in the idle thrust position. The disengagement was neither commanded nor recognised by the crew and the thrust levers remained at idle throughout the approach.
Because the aircraft was fully configured for landing, the air speed decayed rapidly to a
value below that appropriate for the approach. The commander took control and initiated a go-around. During the go-around the aircraft pitched up excessively; flight crew attempts to reduce the aircraft’s pitch were largely ineffective. The aircraft reached a maximum pitch of 44º nose-up and the indicated airspeed reduced to 82 kt.

Apart from holding the control column fully forward, the flight crew made
no other pitch control actions throughout the 44º nose-up excursion until the
aircraft had stalled and the nose had dropped towards the horizon. At this
stage the thrust was reduced to go-around thrust. This thrust reduction allowed
sufficient control authority to recover the aircraft.
Are you saying then that the recovery of a MAX pitching up the same as seen at Bournemouth could then be recovered in the same way without MCAS features added on?
In which case why have MCAS at all?
So as long as pilots know that at slow speeds with a high thrust setting applied will lead to a high AoA - thus avoidance of this scenario surely is what pilots are trained
for?
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Old 16th Mar 2019, 13:37
  #1598 (permalink)  
 
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Originally Posted by GlobalNav View Post
Agreed. This failure mode, whatever it turns out to be, has demonstrated catastrophic consequences, possibly in two cases The system safety of the type design requires improvement. Training is not an acceptable alternative to safe design. I would be curious to know how a software "enhancement" answers the mail, and I wonder what the design assurance level of the software is.
Design Assurance Level assures the software is working as designed, whatever level was used, I strongly suspect that the MCAS software is working exactly as designed.

In my opinion, the design (of MCAS) is broken, but I am not sure the blame sits with whoever designed MCAS but rather with whoever designed the handling of the plane so that MCAS was required (and possibly then failed to spot the issue until very late in development). I suspect whoever designed MCAS was backed into a (coffin) corner constrained by schedule and to use only what hardware was already on the aircraft.
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Old 16th Mar 2019, 13:55
  #1599 (permalink)  
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Originally Posted by rog747 View Post
Are you saying then that the recovery of a MAX pitching up the same as seen at Bournemouth could then be recovered in the same way without MCAS features added on?
In which case why have MCAS at all?
So as long as pilots know that at slow speeds with a high thrust setting applied will lead to a high AoA - thus avoidance of this scenario surely is what pilots are trained
for?
No, it's been explained before but MCAS is required as stick force requirement reduces at high alpha on the MAX and that is a certification failure. Nothing to do with the pitch power couple.
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Old 16th Mar 2019, 14:07
  #1600 (permalink)  
 
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Originally Posted by DaveReidUK View Post
Thanks for that.
Can you clarify - are you using the value for hPa/ft at SL, or the value at 8000' ?
Hi Dave. It depends.
My answer to your first question was:
Originally Posted by Luc Lion View Post
Originally Posted by DaveReidUK View Post
Let's put it another way - what pressure difference at Bole's elevation (i.e. QFE) would you expect to result from a difference at SL between 1029 mb and ISA?
∆p = (1029 – 1013.25) hPa * (1 - 0.0019812(°K/ft) * 7657 ft/288.15 °K)^5.2561
∆p = 11.85 hPa
So, physically, your question is equivalent to "If a translation is applied to the ISA pressure model so that 0 ft is moved from 1013.25 hPa to 1029 hPa, by how much pressure is translated the point at altitude 7657 ft ? As the whole model is translated everywhere by the amount of feet, 420 ft, it means applying the pressure lapse rate that exists at 7657 ft to the height difference that matches 1029 hPa - 1013.25 hPa = 15.75 hPa at sea level pressure lapse rate.
As the pressure lapse rate at 7657 ft is about 75% of the lapse rate at sea level, I found 15.75 hPa * 0.75 = 11.85 hPa.
So it is pressure lapse rate at 7657 ft for your first question.

Your second question was more straight forward:
Originally Posted by Luc Lion View Post
Originally Posted by DaveReidUK View Post
Let's try a thought experiment: sitting on the runway at Bole in a helicopter on the day in question, if you set the QNH (1029) your altimeter should read the field elevation (7,625'). Now adjust the subscale to 1013.2 - how much lower will the altimeter read, and why ? How much must you climb in order that the altimeter once more reads 7,625' ?
∆h = 1/(0.0375 hPa/ft) * (1029 – 1013.25) hPa
∆h = 420 ft
Again, we know that the whole ISA model is translated everywhere by the amount of feet when the reference isobaric surface is moved.
You move the 0 ft surface from 1013.15 to 1029, so basically these are atmospheric layers very close to the 1013.25 reference surface where the pressure lapse rate is 0.0375 hPa per feet.
And 0.0375 hPa/ft * 15.75 hPa = 420 ft
So it is pressure lapse rate at seal level for your second question.

There is a common misconception, even in the pilot community, about the altitude difference versus pressure difference (aka pressure lapse rate) when adjusting the value in the Kollsman window.
It is well known that pressure lapse rate of the real atmosphere is decreasing, but it is less known that the altitude/pressure ratio for each hPa tuned in the Kollsman window also decrease with decreasing pressure.
It follows an exponential curve with (almost) the same coefficient as the pressure/altitude relation of the ISA model.
When one changes the altimeter setting with a value very close to 1013.25 hPa, each 3.75 hPa moves the altitude by 100 ft.
When one changes the altimeter setting with a value very close to 800 hPa (setting only possible with SAE AS8002A altimeters) only 3.03 hPa is enough to move the altitude by 100 ft.

Last edited by Luc Lion; 16th Mar 2019 at 14:33.
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