MCAS ALTITUDE V ATTITUDE ??

[CONSO]

I've started this thread since the other long threads are getting impossible to follow. In previous thread(s) severl have claimed that MCAS has an ALTITUDE input of some sort. Gums and others have not been able to find such a constraint. I think I've found where the confusion started and IMHO its probably a typo !!

OK here is the basic link that may have started

737 MAX - MCAS (http://www.b737.org.uk/mcas.htm?fbclid=IwAR0u1ngnZDrPRPR6oq1sMAs6OvMvL4yBoSiaY1kfsj NCZK6aCSLi61nxBjY)

And if one scrolls down one finds

" Technical Description of Maneuvering Characteristics Augmentation System (MCAS)
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.
The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees



BUT further on is this official document ( pasted part of image since could not copy and paste )


and the term is ATTITUDE ! RUllaWaY Stabilizer
Required by AD 2018-23-51
Disengage autopilot and control airplane pitch attitude with control column
and main electric trim as required.

[CONSO]

Which also IMHO brings me to another related IMHO probable screwup by newstypes

The mention of ' critical' or " overspeed" or max speed in various accounts re Ethopian crash

I suggest the confusion is that 300 or similar knots at less than 1000 AGL for a commercial airliner is definitelty too fast under both rules and common sense, thus for the unwashed, they use critical or overspeed

That meme gets picked up and repeated - and without full context makes ' sense' ?

Whereas 300 knots at any reasonable altitude is a non event and not close to VNE
Just this SLF .00002 cents

[gums]

Salute Conso !!

Yep, although we had a great discussion and got into the technical stuff about MCAS and such about 4 months ago, it may be time to refer newbies here or back to the original thread that arose due to JT610.

Before logging, the ALTITUDE, as well as temperature, is part of the mach value used by MCAS. So in a roundabout way altitude is there for the MCAS magnitude of down trim component ccording to available info.
Tough to analyze something that is a kludge to begin with, and then the inputs to the kludge may have "back doors" the sftwe folks had not thot of. The stability problem was an aerodynamic one that Boeing tried to correct with a "simple software"doofer. But the fix used the aerodynamics of the hozontal stabilizer controlled by a new sftwe module in the FCC, and the fault tree analysis was "faulty"!!!!

I can tellya that having the stick/wheel feel get light as you approach a stall AoA is not good. And then, if you pull back just a tad longer the nose continues to rise all by itself.

Gums sends...

[Vessbot]

BUT further on is this official document ( pasted part of image since could not copy and paste )




There is no "but." The sections before and after say two unrelated things that are not in contradiction. ALTitude is one of the inputs that determines at what AOA the MCAS will make a stab input. ATTitude is what the runaway stab procedure wants you to control with the column and manual trim.

[FCeng84]

There is no "but." The sections before and after say two unrelated things that are not in contradiction. ALTitude is one of the inputs that determines at what AOA the MCAS will make a stab input. ATTitude is what the runaway stab procedure wants you to control with the column and manual trim.

Good idea to pull aside and have a level setting. I concur with everything added to the thread so far. Let me point to one possible source for the confusion about the role of ALTitude in the activation of MCAS. I recall that in the ET accident thread at one point there were several submissions wondering if MCAS could have played a role based on the data suggesting that the airplane did not gain much ALTitude and thus speculating that the flaps would have still been extended. I believe that at least one entry suggested that on climbing out of Addis Ababa it would not be abnormal to be cleaned up to flaps up by the point where the airplane was 1000' above the runway ALTitude. This may have been seen by some as indication that MCAS has logic to prevent activation below 1000'. I am not aware of any direct use of ALTitude in the activation of MCAS. I am checking with a source and will report back once this is confirmed or denied.

[Vessbot]

Yeah, according to something reliable-looking that someone posted in the other thread, the real threshold determiner is Mach. Of course that's a function of temperature, which roughly translates to altitude and airspeed, as the first section quoted by CONSO says. So strictly speaking that's wrong too, but good enough for the purpose of whatever that document is. I was only clearing up the CONSO's perceived contradiction between that and the ATTitude referenced in the second passage.

I also agree that 1000 feet is likely meaningless, and was a result of 1) MCAS activates when the flaps go up, 2) the flaps go up at 1000 feet, and yada yada Chinese Whispers, 3) MCAS activates at 1000 feet. Oops! New wrong fact introduced into the collective consciousness.

[gums]

Salute!

O.K., Vessbot. Show us the source code that uses altitude to determine the AoA and not the Mach value/gain for the application of trim value, rate and such.[/SPOILER]
Our post crossed, and I am glad FC clarified my original understanding of the activation criteria. No hard feelings. None, nada, no-way dot com.

The available documents show AoA as the main driver once flaps up, and Mach determines the gains that the FCC uses to move the stab. FCeng84 has explained this before, as has Bjorn. Being the all around good guy that I am, I can stand to be corrected if I misinterpreted our documentation.. Heh heh.

The problem is aerodynamic, and then a kludge sfwe fix by Boeing that is poorly understood , even by pilots flying the plane, will now open the door for billions of $$$$ that Boeing will have to payout. Hell,if I was flying that plane and having over a hundred folks depending upon me to visit Aunt Clara, I might file a suit myself.

Gums bitches.....

[ASRAAMTOO]

I'm sure we all remember the graph showing angle of attack against lift with the stall occurring at about 15 degrees. Fairly true for a straight winged light aircraft and lowish speeds and maybe not too far out for a 737 at 2000'. The same graph for a (slightly) swept wing aircraft flying at altitude above 0.7M would look completely different.

So presumably MCAS has a set of stored parameters as to what is an acceptable AOA in both regimes. It might allow say 14 degrees at 2000' but only 6 degrees at FL 390. Hence the need for inputs other than AOA.

[CONSO]

feng said I am not aware of any direct use of ALTitude in the activation of MCAS. I am checking with a source and will report back once this is confirmed or denied.https://www.pprune.org/images/statusicon/user_online.gif https://www.pprune.org/images/buttons/report.gif (https://www.pprune.org/report.php?p=10419944)

Granted I did a poor job in my comparison/confusion between altitude and attitude. And gums rightfully explained the difference - BUT that leaves myself and perhaps a few other much more qualified to guess just what sensors are involved in MCAS.

It ** seems** to me that the so called official descriptions and documents released so far leave MUCH to ' can you spot the problem within the next few minutes or die ' complex

MY confusion comes from the **apparent** assumption *** that IF the amount of stab trim is a function of speed and altitude- that both speed and altitude should/must have some sort of inout to when AOA triggers AND hows much movement per tweak results.
IF in general the trim movement is a function of speed - then at low speeds them trim/second would logically be larger . OK
Since speed is usually lower at low altitudes - re takeoff- in the simplified world-- then a logical question is how is altitude measured ? AGL or the standard baro altitude set at airport So what happens when airport alt is say 7000 above sea level - how is this compensated for re how much trim movement per second/ tweak?

While one can assume the whiz kids who programmed this kluge did take such issues into account- the Ethopian accident makes one wonder.- especially when one knows that mach number ( actual speed ) varies with actual temperature NOT altitude per standard tables. Which suggests that somehow a local ( to airplane ) temperature or at least at takeoff must somehow be fed into MCAS to prevent errors in stab movement.
Which then drives into the REAL effects of airspeed, altitude, temperature, airport altitude, inputs to MCAS and how derived/measured.

Seems to me one could legitimately question just how/where/when the above inputs are accommodated in a reliable fashion re MCAS activation. ? Dont expect to get answers here- but hopefully the code sausage making will be fully questioned.
Of course none of the above applies to the microsquish simulator crowd- and maybe not even the real simulator inputs ???

For the above - other than raise what I believe are real questions that need real answers by the experts on MCAS- I'll consider it my .0003 worth :8

[Vessbot]

Salute!

O.K., Vessbot. Show us the source code that uses altitude to determine the AoA and not the Mach value/gain for the application of trim value, rate and such.

The available documents show AoA as the main driver once flaps up, and Mach determines the gains that the FCC uses to move the stab. FCeng84 has explained this before, as has Bjorn.

You misunderstood my post. The threshold is determined by Mach, which is a function of airspeed and temperature. This can also be said to be airspeed and altitude. As altitude does not strictly relate to temperature this is not strictly correct, but it's a good enough approximation for a general description for a lay reader, which is why it's written on B737.org.uk; and I'm clearing up CONSO's confusion between that use of "ALTitude" and his perceived contradiction between that and pitch ATTitude which is referenced in a completely different document that he quoted.

[gums]

Salute Vess !

No misunderstanding , I really DID misunderstand! and I am very reasonably knowledgeable about all the thermodynamic relations.
My position is simple, and FC eng and others can refute:
The primary activation for the MCAS nose down trim is AoA
- MCAS inhibited with A/p engaged ( maybe)
- MCAS inhibited if flaps are not up ( demonstrated on two or more flights, .one fatal)
- MCAS can be defeated with a manual trim switch on the stick/yoke ( at least two flights in Indonesia, one fatal)
- The amount of trim rate applied by MCAS, and possibly the total amount has a mach function as has been explained by FCeng and other sources. [deleted using CONSO's explanation and review of books, heh heh] But mach is not what triggers MCAS nose down trim, it tailors it

That's my story, and I am sticking to it.

Gums sends...

[Vessbot]

Salute Vess !

No misunderstanding, and I am very knowledgeable about all the thermodynamic relations.
My position is simple, and FC eng and others can refute:
The primary activation for the MCAS nose down trim is AoA
- MCAS inhibited with A/p engaged ( maybe)
- MCAS inhibited if flaps are not up ( demonstrated on two or more flights.one fatal)
- MCAS can be defeated with a manual trim switch on the stick/yoke ( at least two flights in Indonesia, one fatal)
- The amount of trim rate applied by MCAS, and possible the total amount has a mach function as has been explained by FCeng and other sources. And mach has temperature and air density and less factors. But mcah and its contributing components are not what trigger MCAS nose down trim.

That's my story, and I am sticking to it.

Gums sends...

I didn't say you misunderstood MCAS or thermodynamics, but rather that you misunderstood my post. This is spurred by your challenging me to produce proof that MCAS "uses altitude to determine the AoA and not the Mach," while that's not a position that I take.

As I clarified in my last post, my position is that the AOA activation threshold is determined by Mach, and my invocation of altitude is not to say that MCAS uses it, but to relate CONSO's quote of "altitude," from the beginning of this thread, to the real Mach-based usage.

[Dairyground]

Since speed is usually lower at low altitudes - re takeoff- in the simplified world-- then a logical question is how is altitude measured ? AGL or the standard baro altitude set at airport So what happens when airport alt is say 7000 above sea level - how is this compensated for re how much trim movement per second/ tweak?

While one can assume the whiz kids who programmed this kluge did take such issues into account- the Ethopian accident makes one wonder.- especially when one knows that mach number ( actual speed ) varies with actual temperature NOT altitude per standard tables. Which suggests that somehow a local ( to airplane ) temperature or at least at takeoff must somehow be fed into MCAS to prevent errors in stab movement.
Which then drives into the REAL effects of airspeed, altitude, temperature, airport altitude, inputs to MCAS and how derived/measured.

My recollection from hydrodynamics courses taken as part of a maths degree almost 60 years ago is that mach number is airspeed relative to the local speed of sound. The speed of sound depends on temperature and pressure (assuming constant air composition), so MCAS activation and operation should depend on air pressure, temperature and airspeed. It should be independent of altimeter setting and height above the surface (unless it is considered useful to involve the terrain clearance system in the picture). Perhaps if MCAS was overridden by TCAS we would not be having this discussion.

[CONSO]

The speed of sound depends on temperature and pressure . . .

Nope - speed of sound in AIR in a 500 psi tank is only dependant on temperature. In the extreme ( vacuum ) with no air molecules speed of sound is of course zero. But we are not talking the extreme here but within the normal range of flight- and as such its strictly temperature

basic calculation speed of sound (m/s) = 331.5 + 0.60 T(°C) note only a relation to temperature - not ' pressure' or altitude withing the normal range. Check any table for speed of sound vs altitude and note that it is the temperature standard at altitude which controls

Which gets back to my oversimplified question - what other inputs control MCAS -- and how measured or compared to what standard versus actual ??

[Vessbot]

Which gets back to my oversimplified question - what other inputs control MCAS -- and how measured or compared to what standard versus actual ??
This is total supposition, but I would bet that it's fed Mach from the ADC, where it's already calculated and fed to probably a dozen other systems.

[CONSO]

This is total supposition, but I would bet that it's fed Mach from the ADC, where it's already calculated and fed to probably a dozen other systems.

IMHO probably right - more sensors to impact calculations - but at least some sort of visible aural warnings if mismatch gets us to ???

Hmmm- Nowadays, ADCs are digital. The transducers called Analogue to Digital Interface Units are present near the static and pitot source. The readings are converted to digital signals before the ADC receives the information. The Total Air Temperature has always been an electrical reading as it uses a temperature sensitive resistor which gives out a Voltage.

OK - so at a high temperature near takeoff , the mach calculation would give a LOWER mach number meaning a greater stab deflection ? And if ANY other sensor ( static v dynamic , temperature ) than AOA was incorrect on the same computer as AOA - the old standby match the curve game goes bonkers and ....

[fdr]

per conso's req:


§ 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 (https://www.law.cornell.edu/cfr/text/14/25.671) 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 failuresof the type specified in § 25.671(c) (https://www.law.cornell.edu/cfr/text/14/25.671#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 (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=1&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:94:25.672) 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 airplane (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=2&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:94:25.672)configurations) which is described in the Airplane (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=3&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:94:25.672) 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, 35 FR 5675 (https://www.law.cornell.edu/rio/citation/35_FR_5675) Apr. 8, 1970]

[CONSO]

per conso's req:


§ 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 (https://www.law.cornell.edu/cfr/text/14/25.671) 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 failuresof the type specified in § 25.671(c) (https://www.law.cornell.edu/cfr/text/14/25.671#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 (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=1&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:94:25.672) 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 airplane (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=2&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:94:25.672)configurations) which is described in the Airplane (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=3&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:94:25.672) 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, 35 FR 5675 Apr. 8, 1970]

And adding ( my emphasis added )§ 25.671 General.
(a) Each control and control system must operate with the ease, smoothness, and positiveness appropriate to its function

.(b) Each element of each flight control system must be designed, or distinctively and permanently marked, to minimize the probability of incorrect assembly that could result in the malfunctioning of the system.

(c) The [URL="https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=1&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:94:25.671"]airplane (https://www.law.cornell.edu/rio/citation/35_FR_5675) must be shown by analysis, tests, or both, to be capable of continued safe flight and landing after any of the following failures or jamming in the flight control system and surfaces (including trim, lift, drag, and feel systems), within the normal flight envelope, without requiring exceptional piloting skill or strength. Probable malfunctions must have only minor effects on control system operation and must be capable of being readily counteracted by the pilot

.(1) Any single failure, excluding jamming (for example, disconnection or failure of mechanical elements, or structural failure of hydraulic components, such as actuators, control spool housing, and valves).

(2) Any combination of failures not shown to be extremely improbable, excluding jamming (for example, dual electrical or hydraulic system failures, or any single failure in combination with any probable hydraulic or electrical failure).

(3) Any jam in a control position normally encountered during takeoff, climb, cruise, normal turns, descent, and landing unless the jam is shown to be extremely improbable, or can be alleviated. A runaway of a flight control to an adverse position and jam must be accounted for if such runaway and subsequent jamming is not extremely improbable

.(d) The airplane (https://www.law.cornell.edu/definitions/index.php?width=840&height=800&iframe=true&def_id=ee9803083700896cd85aff74cb4f95ea&term_occur=2&term_src=Title:14:Chapter:I:Subchapter:C:Part:25:Subpart:D:S ubjgrp:94:25.671) must be designed so that it is controllable if all engines fail. Compliance with this requirement may be shown by analysis where that method has been shown to be reliable.[Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25-23, [URL="https://www.law.cornell.edu/rio/citation/35_FR_5674"]35 FR 5674 (https://www.law.cornell.edu/rio/citation/29_FR_18291), Apr. 8, 1970]

[gums]

Salute!

I have corrected the misunderstanding(s) that existed a few post back ( Vess and Gums). Not surprising when dealing with the not so simple implementation and explanation of MCAS from various sources.

@FDR thanks for the references. I had to look them up myself back in November because I couldn't believe the plane got certified with a crappy pitch momentum plot at high alpha.

It still amazes me that many folks posting lately think MCAS is a "stall prevention" or "stall warning" feature. Oh well, being the old instructor pilot, I explained how it works to my wife using a steering wheel analogy where the wheel moved all by itself as she tried to turn harder near the stops. "Oh yeah, I get it"

Thanks for putting up with my rants, but this whole thing bugs me. If we prevent just one accident then it is worth it in spades, huh?

Gums sends...

[CONSO]

CURRENT - 833 PM SEATTLE TIME
BA plans to provide software fix within about 10 days and mandate installation. Then after a test flight- plane can be returned to service ! meanwhile the FAA and Boeing are going rounds as to type and kind and level of ' training ' on the ' new' software/system/ effects shall be required. !

The above is a paraphrase of the news release by a few ' sources'

https://www.thestreet.com/investing/stocks/boeing-shares-jump-amid-reports-of-737-max-software-fix-14897876

[QUOTE]Boeing Co. (BA (https://www.thestreet.com/quote/BA.html)
- Get Report (https://secure2.thestreet.com/cap/prm.do?OID=039042&ticker=BA)) shares rose Friday following a report that that planemaker is set to roll out a software upgrade to address safety issues linked to the grounded 737 MAX series.
France's AFP news agency said the rollout, which will come in the next ten days, would focus on the MAX's automatic stall-prevention system that was linked to the Lion Air crash in Indonesia, but has not yet been proven to have caused Sunday's Ethiopian Airlines ET 302 disaster. Reuters reported Friday, however, that the timeline for the software fix "has not changed" and will be installed "in the coming weeks".
Boeing shares were marked 1.52% higher Friday to close at $378.99 each.

[QUOTE]

I'm simply reporting- I'm sure within a few hours or less, more details will be available-

IMHO- while FAA may allow this in the U.S- I doubt anyone else will . but I'm just a poor SLF retired mechanical injun ear from BA and not an aeronautical whiz bang at all- Not even a microsoft simulator player .

[fdr]

getting each pilot into a simulator and making sure they can recognise a trim fault, can hear the stall warning over their ipods, and can find the cutout switches and stop themselves from vertical parking would appear to not be asking too much at this point. If the crews are turning up to the aircraft and cannot ascertain what various messages mean on the PDF or ND, then there is a hint that all is not well with the preparatory training packages. The training doesn't need Level D, it needs guided instruction to ensure the guys have some confidence in their own operation.

[CONSO]

getting each pilot into a simulator and making sure they can recognise a trim fault, can hear the stall warning over their ipods, and can find the cutout switches and stop themselves from vertical parking would appear to not be asking too much at this point. If the crews are turning up to the aircraft and cannot ascertain what various messages mean on the PDF or ND, then there is a hint that all is not well with the preparatory training packages. The training doesn't need Level D, it needs guided instruction to ensure the guys have some confidence in their own operation.

The quality of that report might well be summed up in the following quote from the article listed
would focus on the MAX's automatic stall-prevention system that was linked to the Lion Air crash in Indonesia

juxst wunnerful !

Thanks by the way for your post here from the other thread - perhaps some will try to see if the current ' system ' kludge would or did meet the requirements ? i dont think so !

[GordonR_Cape]

CURRENT - 833 PM SEATTLE TIME
BA plans to provide software fix within about 10 days and mandate installation. Then after a test flight- plane can be returned to service ! meanwhile the FAA and Boeing are going rounds as to type and kind and level of ' training ' on the ' new' software/system/ effects shall be required. !

The above is a paraphrase of the news release by a few ' sources'

https://www.thestreet.com/investing/stocks/boeing-shares-jump-amid-reports-of-737-max-software-fix-14897876

I'm simply reporting- I'm sure within a few hours or less, more details will be available-

IMHO- while FAA may allow this in the U.S- I doubt anyone else will . but I'm just a poor SLF retired mechanical injun ear from BA and not an aeronautical whiz bang at all- Not even a microsoft simulator player .

A BBC report suggests that the FAA expects the B737 MAX to be grounded until at least May: https://www.bbc.com/news/business-47578555

As investigators work to determine the cause of the tragedies, the US regulator said the aircraft would be grounded until at least May.

Boeing has halted 737 Max deliveries and some airlines say they will demand compensation.

Some customers have signaled they could back away from orders.

But analysts say the long-term impact on the firm will depend the outcome of the investigation.

[CONSO]

maybe worth taking a close look at

https://www.flightradar24.com/blog/wp-content/uploads/2018/10/2018-035-PK-LQP-Preliminary-Report.pdf


especially pages 16 to 18 re maintenace log

page 23 to 26 plots
and additional info on pages 28 and on

now add in the possibility of a switch between FCC and ' active' AOA sensor -

Sort of like my brakes grabbed and pulled me to the right - OK we checked the front wheel brakes- PUT IN NEW PADS and all OK - but problem really was with right rear brake - so when you hit the ice . .

Which is the long way of saying DID MAINTENACE CHECK BOTH AOA SENSORS ??

[wiedehopf]

Which is the long way of saying DID MAINTENACE CHECK BOTH AOA SENSORS ??

Actually when looking at that maintenance writeup it looks like the AoA sensor was intermittent before maintenance in Denpasar. (But not necessarily wrong)
(The Captains AoA sensor was replaced as the captains airdata was intermittent)

After the maintenance in Denpasar the MCAS problem started showing up which suggests the problem originating with that maintenance action.
(Also in the FDR traces it is very clear it is that freshly replaced AoA sensor which is the problem on the last two flights of the aircraft)

[CONSO]

AND NOW

https://www.inc.com/chris-matyszczyk/sully-sullenberger-just-made-chilling-statements-about-boeing-737-max-crash-other-pilots-disagree.html

[CONSO]

(Also in the FDR traces it is very clear it is that freshly replaced AoA sensor which is the problem on the last two flights of the aircraft)


But since the ' active' AOA sensor changes from left to right, depending n power down and or WOW, and the default is ?? what sensor was checked/replaced ? Not clear from log

[wiedehopf]

But since the ' active' AOA sensor changes from left to right, depending n power down and or WOW, and the default is ?? what sensor was checked/replaced ? Not clear from log

The FDR traces for the sensor data are separate, nothing switches.
Also the captains air data are always from the captains AoA sensor, those were intermittent.

Only thing that switches between flights is the FCC which is used for MCAS or something.
In this case after the maintenance in Denpasar, the left FCC (connected to captains AoA) was used for MCAS.
Then the following flight it was again the left FCC since the aircraft was powered down over night.

So yes it is quite clear which AoA sensor was replaced and on the following two flight provided wrong data.

[CONSO]

FWIW

https://www.youtube.com/embed/AgkmJ1U2M_Q


starts with a running jackscrew in ?? airplane
its about 2 min in before he identifies himself - and then starts to show a typical control diagram

And also this FWIW

https://www.youtube.com/watch?v=TlinocVHpzk

But must go 16 min in to get to MCAS

[CONSO]

RE install of AOA sensor from previous thread in december

https://www.pprune.org/tech-log/616624-maintenance-lapse-identified-initial-problem-leading-lion-air-crash.html?highlight=SENSOR#post10344284


BUT that would also infer that the same error happened twice - wrong clocking or ??? Especially after the first earlier report ? About the swiss cheese lining up ?????

[wiedehopf]

BUT that would also infer that the same error happened twice - wrong clocking or ??? Especially after the first earlier report ? About the swiss cheese lining up ?????

No after Denpasar-Jakarta they basically reset the computers and cleaned a couple of plugs.
(Not sure if flushing means flushing computers or flushing the actual pitot static system)

But they didn't touch the AoA sensor.

So they basically just didn't change anything as no one looked at the FDR or QAR.
For the IAS disagree they checked the pitot static system instead of checking the real source of the problem, the AoA sensor.
Of course the error description not mentioning the stick shaker can be considered lacking description of the problem.

The Lion Air incident basically shows lack of training and corner cutting everywhere.
That is one of the reasons Boeing wasn't as much under scrutiny i believe, a good part of the focus was on Lion Air.

[CONSO]

And about what Boeing thinks the AOA sensor does ( dated approx year 2000 )

Aero 12 - Angle of Attack (http://www.boeing.com/commercial/aeromagazine/aero_12/attack_story.html)

he U.S. National Transportation Safety Board (NTSB) has recommended visual indication of AOA in commercial airplanes. This indication may take the form of a dedicated AOA indicator or other implementation, such as the PLI. A dedicated AOA indicator shown on the primary flight display (PFD) recently has been developed in cooperation with airline customers. The new indicator is offered as an option on the 737-600/-700/-800/-900, 767-400, and 777 at this time. During the development of the new indicator, discussions with airlines, the NTSB, and U.S. Federal Aviation Administration (FAA) pilots and engineers provided a unique opportunity to examine potential uses of AOA and the many existing uses that have evolved in recent decades along with advances in display and indication technology.

...The indicator developed shows body AOA in degrees and is not normalized, which is related to the second objective above, that the indicator be useful when pitot or static data, and therefore Mach calculations, are unreliable because of blockage or a fault in the system. The pointer of a normalized indicator in this condition would behave erratically, making the indicator unusable. With the nonnormalized design, the position of the needle is a function only of sensed AOA. The red tick mark for stall warning may behave erratically in a pitot or static failure state, as may stick shaker, PLI, and speed tape amber and red bands. However, the AOA needle and digits will remain stable, and the indicator itself still will be useful as a backup for unreliable airspeed, provided the AOA vanes are undamaged.

I think the whole document is worth rreading now !!

[CONSO]

Seems to this SLF that a simple damped pendulum free to swing in the fore and aft direction ( eg a plumb bob ) and properly oriented to maybe 3 degrees body pitch with simple visual readout would make a simple ' AOA' sensor backup and could be part of a simple "inertial" standby system NOT dependant on anything else - sort of a needle , ball, ( and non alcohol ) system

[DaveReidUK]

Seems to this SLF that a simple damped pendulum free to swing in the fore and aft direction ( eg a plumb bob ) and properly oriented to maybe 3 degrees body pitch with simple visual readout would make a simple ' AOA' sensor backup and could be part of a simple "inertial" standby system NOT dependant on anything else - sort of a needle , ball, ( and non alcohol ) system

(post deleted as there's no point in leaving it now that the mods have deleted all the subsequent discussion)

[7p3i7lot]

Apart from all this tech discussion (which is great) there are some other factors to consider. Namely politics.
No one at the FAA is gonna lift the grounding unless it is absolutely, positively, guaranted fixed so that any monkey could agree the problem is corrected. That includes the President of the US who made (so far as I know) first ever presidential grounding of an aircraft series. I don't see any bureaucrat in either the government or Boeing putting their reputation and company survival on the line until all possible questions have been satisfactorily answered. Oh and don't forget the lawsuits and congressional investigations which are just getting started.

[LNAV VNAV -]

Any one can explain why an erroneous AoA input can cause an IAS disagreement? Aren’t the inputs to the airspeed indicators just the static and pitot pressures?


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1123x1498/dde46f37_7ef6_425d_b459_b76325503a57_e04741ac23a5b8ef336287b 9af7f7e5baaeac76d.jpeg

[Ian W]

Any one can explain why an erroneous AoA input can cause an IAS disagreement? Aren’t the inputs to the airspeed indicators just the static and pitot pressures?


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1123x1498/dde46f37_7ef6_425d_b459_b76325503a57_e04741ac23a5b8ef336287b 9af7f7e5baaeac76d.jpeg


No the ASI is driven by the ADIRUs and they correct the inputs from the pitot and static ports using the AoA inputs. It is the ADIRU getting AoA disagree that alert to the airspeed being unreliable, I would have thought that the first check on unreliable airspeed in the Max would be set stab trim switches to CUTOUT that means manual trim but MCAS is now out of the loop.

I am surprised that after the Lion Air crash and the AD that any pilot on the Max would be unaware of the MCAS issue and have briefed accordingly before any flight about stab trim cutout if trimming became difficult - it is part of the standard runaway trip checklist. The fact that another crew had crashed and could have survived if they had switched stab trim to cutout should have concentrated minds.

[futurama]

Any one can explain why an erroneous AoA input can cause an IAS disagreement? Aren’t the inputs to the airspeed indicators just the static and pitot pressures?

There are a couple of aspects to this I believe:

The AOA is used as part of Static Source Error Correction (SSEC) calculations, basically to compensate the fact that the pitot tube isn't usually aligned exactly with the airstream.
Since there is a relationship between AOA and IAS, the AOA can be used as a "cross check" to detect pitot/static problems (blocked ports, etc.) . So a malfunctioning AOA could erroneously show up as an IAS issue.

[safetypee]

LNAV VNAV,

Much as above except I understood that the computation is in the ADC; thus AoA is used in the ADC for pressure error correction; if each AoA feeds it ‘own side’ ADC then the ADCs speed computations differ and alert the crew, similarly for Alt Disagree.

Also, if AoA is in error, so too the computation for low speed awareness (AoA to speed relationship) on the speed scale; this may / may not contribute to the ‘IAS disagree’ alert.

Ian W, the ADCs could be responsible for the AoA disagree (or fly guidance comp); however the alert is an optional installation, normally associated with the optional AoA dial display on EFIS. Which in this instance would be meaningless because there is no way to determine which AoA value is correct, whereas for IAS and ALT there is a third system to crosscheck.

Thus although the Lion maintenance electronic log flagged AoA amongst other problems, the crew’s report concentrated on the indications of unreliable airspeed and stick shake.

[LNAV VNAV -]

Thank you Ian. Do you by any chance have any reference to any manual that shows that the ADRs correct the pitot and static inputs using AoA data? I used to fly the NG and I don’t remember reading anything to this effect and there is nothing in the A320 manuals as far as I know. Unless this is just a Max thing.

[Mark in CA]

Stumbled upon this site. Posting here in case it may be of use to anyone.

The Boeing 737 MAX (http://www.b737.org.uk/737max.htm)

[FCeng84]

Good idea to pull aside and have a level setting. I concur with everything added to the thread so far. Let me point to one possible source for the confusion about the role of ALTitude in the activation of MCAS. I recall that in the ET accident thread at one point there were several submissions wondering if MCAS could have played a role based on the data suggesting that the airplane did not gain much ALTitude and thus speculating that the flaps would have still been extended. I believe that at least one entry suggested that on climbing out of Addis Ababa it would not be abnormal to be cleaned up to flaps up by the point where the airplane was 1000' above the runway ALTitude. This may have been seen by some as indication that MCAS has logic to prevent activation below 1000'. I am not aware of any direct use of ALTitude in the activation of MCAS. I am checking with a source and will report back once this is confirmed or denied.

Got definitive answer on MCAS activation and altitude. Only inhibit is when within a few feet of the ground. Takeoff with Flaps Up would allow for MCAS activation as soon as combination of autopilot not engaged and sensed AOA above MCAS activation threshold occurs. Similarly, if takeoff involves some flaps out, MCAS is enabled as soon as flaps are fully retracted with no required minimum altitude.

[gums]

Salute FCeng !

Great to have most of our understnding confirmed.

Assume the "few feet of the ground" criteria might even be ground effect that influences AoA/downwash, or #2 radar altimiter.
@JT PLZ let this one stay! Several of the inputs here by Dave, CONSO and Gums were deleted and they included real world examples that the lay person could visualize concerning AoA, as well the fallacy of using pendulums or plumbobs as AoA or other indicators. Many on the main threads are still confusing attitide they reference to siting in a living room chair with aerodynamic pitch and AoA and .....

Gums sends...

[infrequentflyer789]

I am surprised that after the Lion Air crash and the AD that any pilot on the Max would be unaware of the MCAS issue and have briefed accordingly before any flight about stab trim cutout if trimming became difficult - it is part of the standard runaway trip checklist. The fact that another crew had crashed and could have survived if they had switched stab trim to cutout should have concentrated minds.

ATC comms from both accident flights, and reports from the flight that survived, indicate that the unreliable airspeed was the first focus - I don't know if that is right or wrong or if the prioritization should have been different, but that is what all three crews appear to have done.

According to the report the crew that survived MCAS ran 3 NNCs - IAS disagree, ALT disagree, and then Stab Trim Runaway. From the FDR traces they had around four minutes of fighting MCAS, after flaps up, before they turned it off.

Ethiopian was in the ground after six minutes of flight.

[kiwi grey]

I would have thought that the first check on unreliable airspeed in the Max would be set stab trim switches to CUTOUT that means manual trim but MCAS is now out of the loop.


But that would put the aircraft into a known unsafe condition.
The MCAS was created solely because the B737MAX is sufficiently unstable without it (under some circumstances) to not be certifiable, and the stab trim switches are required to be on for MCAS to work.
Boeing have their "gentleman's parts" in a vice on this one

[futurama]

ATC comms from both accident flights, and reports from the flight that survived, indicate that the unreliable airspeed was the first focus
Actually the "unreliable airspeed" report has never been confirmed for the Ethiopian flight. There was a rumor that a crew on another flight heard ET302 reporting "unreliable airspeed" to ATC, but no one can seem to locate this other flight. Leaks from those who have listened to the ATC tapes (e.g., via Reuters, etc.) state that the pilot reported a "flight control" problem, but not "unreliable airspeed" specifically.

From AvHerald: "..., that there had been no PAN PAN or MAYDAY call, there was also no change in transponder code away from the assigned code 2000, there had been no indication of unreliable airspeed by the flight crew, there had been no distress in the voices or abnormal background noises during transmissions from the aircraft that would have alerted air traffic control to assume a serious problem on board of the aircraft."

[Ian W]

But that would put the aircraft into a known unsafe condition.
The MCAS was created solely because the B737MAX is sufficiently unstable without it (under some circumstances) to not be certifiable, and the stab trim switches are required to be on for MCAS to work.
Boeing have their "gentleman's parts" in a vice on this one

My understanding is not that the Max is unstable without MCAS but that the pull force on the control column is not linear close to stall AoA. This is why MCAS does not operate with AP engaged. So a regulatory 'protection' was put there for linear control forces. Or as in the case of the Max pilots with little feel for manual flying could to pull through into a stall as the column pull force got lighter. This is probably more likely in a high speed stall such as in a steep turn.

[gary f]

Right on, whether it's communication breakdown or crew members not knowing where to look up important data.

I wouldn't discount the pressure for on time performance from the airlines and flight crews racing to get to the plane from a previous flight and getting into a hurry up situation.

[Vilters]

Back to the original question : Altitude versus Attitude.
It does not matter; when sensors:systems feed wrong information, the outcome is predictable.

About the pilots and pressure on them;
Most pilots are trained using the autopilot and protection systems. But => Most are "scared as hell" to turn protection systems OFF.