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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 18th Mar 2019, 20:58
  #2001 (permalink)  
 
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Transport Canada Will Review

Canada re-examining Boeing 737 MAX approval after FAA certification probe

OTTAWA (Reuters) - Transport Canada is re-examining the validation it gave Boeing Coís 737 MAX jets, following reports of a U.S. probe into the aircraftís certification by the Federal Aviation Administration (FAA), Canadian Transport Minister Marc Garneau said on Monday.

Garneau told reporters in Ottawa that Transport Canada might not take any action but he thinks it would be wise to re-examine the validation of the 737 MAX 8 jet, which has been grounded worldwide for safety concerns following the recent crash of an Ethiopian plane of that model, which killed 157 people.

The disaster followed a Lion Air crash in Indonesia in October involving the same model plane.

Canada accepted the FAAís March 2017 certification of the MAX under a deal where such approvals by the United States are accepted by Canada and vice versa.

ďWe may not change anything but weíve decided itís a good idea for us to review the validation of the type certificate that was given for the MAX 8,Ē he said.

Citing people familiar with the inquiry, the Wall Street Journal reported on Sunday that U.S. Department of Transportation officials were scrutinizing the FAA approval of MAX jets and that a grand jury in Washington subpoenaed at least one person involved in developing the MAX.

In addition, the Seattle Times reported that Boeingís safety analysis of a new flight control system on MAX jets, known as MCAS, had several crucial flaws, including understating the power of the system.

Garneau said Transport Canada would do its own certification of a software change being prepared by Boeing within the next few weeks ďeven if itís certified by the FAA.Ē

Reporting By David Ljunggren in Ottawa. Writing by Allison Lampert in Montreal; editing by Jonathan Oatis
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Old 18th Mar 2019, 20:59
  #2002 (permalink)  
 
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The most desirable outcome would be where manufacturers could design new aircraft with state of the art materials, engines and avionics instead of using airframes and certifications from the sixties. The cost of entry is so high this inhibites innovation and safety.

Recently a Canadian design that competed with Boeing was blocked by Boeing because it was a competitor. Airbus stepped in to scoop up the newly certified C series and rebrand it an Airbus A220. Not what Boeing wanted.

If the cost of certification is so high and established manufacturers block new entrants the safety and innovation that has been a hallmark of aviation will suffer.

Flying sixties designs with bandaid fixes will be our future and our grandchildrens
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Old 18th Mar 2019, 21:04
  #2003 (permalink)  
 
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Originally Posted by abdunbar View Post
This question is for FCeng84 who seems to be very specifically knowledgeable regarding the 737Max. Based upon what you are saying about how the elevator works, it does not seem that there would be a situation that would, either through control force, aerodynamic force or a combination thereof, preclude manual trimming of the stabilizer. True?
You are stretching my 737 knowledge! I have never read or heard anything definitive about whether or not horizontal tail control (either electric or manual via the trim wheel) would be stalled out if the elevator were at blowdown and thus generating maximum force on the jack screw. I would hope that someone on this forum with that knowledge would share it with the rest of us. Not wanting anyone to find out the hard way my recommendation is to keep up with the pitch trim so that the column forces don't build up to the point where you become the one to find out in flight.
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Old 18th Mar 2019, 21:04
  #2004 (permalink)  
 
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Well, if questions are permitted from professional engineers, here are two dumb questions:-
1. Looking at the altitude plot in post 1869, the clearance from rising terrain at 02.40 is perilously small, but thereafter there is a brisk climb. Could that rate of climb and altitude gain have taken the aircraft into a stall?
2. Eyewitnesses claimed to have seen items falling from the plane whilst it was still airborne. Could these have been contrails/vortices generated by extreme AoAs?
Thank you in anticipation of patient and polite replies.
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Old 18th Mar 2019, 21:10
  #2005 (permalink)  
 
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Originally Posted by silverstrata View Post


Clearly MCAS is not a runnaway trim condition, otherwise we would be reaching for the cut-out switches every time the speed-trim operated. I think some people here do not realise that the trimmer doing its own thing is operations normal.

And the auro-trim systems are always mis-trimming the aircraft (the trim inputs by the speed-trim system are always wrong, and you always have to re-trim manually). So at what point does ‘operations normal’ become ‘operations abnormal’? At what point do you assume that the trimmer has gone awry..??

Silver
I am going to disagree with you. You are technically correct, but the captain on the Lion aircraft that crashed manually corrected for MCAS input 21 or so times. 21 times there was a 10 second trim input by MCAS, followed by 21 times a 10 second ( I assume pilot electric trim and MCAS trim at the same rate but I can be wrong about that) electric trim input by the captain. I know there were 5 second breaks without trim, but I would have to think this is not even close to normal. Of course they had all sorts of other bells and whistles going off, but the captain was able to keep the aircraft in trim, so he must have known there was a trim issue too. I am on brand A, but I have a few hundred hours in the 737 cockpit as JS/non working crew. I have been paying attention, and have never seen STS doing anything close to a 10 second trim run, and it will generally trim ANU after take-off, as the pilot is trying to accelerate, and STS wants to keep the speed constant, whereas MCAS was continuously AND.
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Old 18th Mar 2019, 21:25
  #2006 (permalink)  
 
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Originally Posted by FCeng84 View Post
Got it, thanks for expanding. What I was wanting to know was if you were pointing to an explicit command limit within the control system that would keep from asking more of the elevator actuators than they can deliver vs. a system that asks for everything that is available and takes what it gets when elevator actuators are driven with full available hydraulic pressure. I am familiar with both types of systems. In the case of the 737 the limit is hydro-mechanical in that the elevator actuator control valves are wide open but the elevator hinge moment is more than the actuators can deliver so the surface ends up at a less than full travel position. I am familiar with this being referred to as either "blowdown" or "blowback". Note that if you are flying with high enough dynamic pressure for that to be the limiting factor and you command full elevator with full column and you hold that column and you speed up, the elevator deflection will decrease as dynamic pressure increases. With reference to Star Trek, that's when Scotty says "She's give'n you all there is, there just isn't any more to be had." Glad to provide this answer once I was certain of your question.
The Q limiter was pure mechanical and it limited rudder (DC-9). Same vintage as original B737. I donít know what the MAX or NG has but I believe all modern jet transport aircraft limit fight controls as Q increases as well as Mach.

My point.... and I do have one.... is that the elevator may have increasingly limited authority as the aircraft AS increased.
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Old 18th Mar 2019, 21:39
  #2007 (permalink)  
 
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Originally Posted by dozing4dollars View Post
The USA government shutdown that both political parties participated in was also a factor.

... and hence, the voters like you
I don't remember Indonesians or Ethiopians getting a vote!

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Old 18th Mar 2019, 21:43
  #2008 (permalink)  
 
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Originally Posted by dozing4dollars View Post


The Q limiter was pure mechanical and it limited rudder (DC-9). Same vintage as original B737. I donít know what the MAX or NG has but I believe all modern jet transport aircraft limit fight controls as Q increases as well as Mach.

My point.... and I do have one.... is that the elevator may have increasingly limited authority as the aircraft AS increased.
Let me give two examples:

On 737 the maximum elevator displacement with full column input is either full travel if the elevator actuators are physically capable of pushing the elevators hard enough to get there or where ever the aerodynamic hinge moment balances out against full elevator actuator force capability. In both cases, the control system is trying to move the elevator as far as is physically possible but in high dynamic pressure case it has reached an actuator force limit.

On 787 the maximum elevator displacement with full column input is limited by a control system command limit schedule that is a function of flight condition. (I am simplifying here by ignoring for this discussion any envelope protection functions.) The elevator actuators may be capable of generating more force than required to get the surfaces to the command limits, but they are not allowed to. At low speeds the command limits are such that full elevator travel is achievable. At higher speeds control law limits are chosen to balance between (on the low end) elevator displacement needed to achieve pitch control power to meet maneuver requirements and (on the high end) deflections that would generate higher structural loads than necessary. This scheme allows the elevator limits to be tailored to provide the pitch control power needed, but no more so that the structure can be optimized for minimum weight. This is one of the ways in which FBW control is able to deliver improved performance.

The story is always a little more complicated than one might at first think.
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Old 18th Mar 2019, 22:20
  #2009 (permalink)  
 
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Originally Posted by Sailvi767 View Post


Bingo, you are reducing the effectiveness of the miss trimmed stab and increasing elevator effectiveness.
Reducing the speed (or not letting it grow) is an obvious remedy but easier to say afterwards. If they have the stick shaker since the take-off, with a normal airspeed, they may think the fault is on airspeed and apply the procedure to keep N1 at a relatively high value that will accelerate the aircraft beyond controlability limits...
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Old 18th Mar 2019, 22:48
  #2010 (permalink)  
 
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Originally Posted by FCeng84 View Post
Let me give two examples:

On 737 the maximum elevator displacement with full column input is either full travel if the elevator actuators are physically capable of pushing the elevators hard enough to get there or where ever the aerodynamic hinge moment balances out against full elevator actuator force capability. In both cases, the control system is trying to move the elevator as far as is physically possible but in high dynamic pressure case it has reached an actuator force limit.

On 787 the maximum elevator displacement with full column input is limited by a control system command limit schedule that is a function of flight condition. (I am simplifying here by ignoring for this discussion any envelope protection functions.) The elevator actuators may be capable of generating more force than required to get the surfaces to the command limits, but they are not allowed to. At low speeds the command limits are such that full elevator travel is achievable. At higher speeds control law limits are chosen to balance between (on the low end) elevator displacement needed to achieve pitch control power to meet maneuver requirements and (on the high end) deflections that would generate higher structural loads than necessary. This scheme allows the elevator limits to be tailored to provide the pitch control power needed, but no more so that the structure can be optimized for minimum weight. This is one of the ways in which FBW control is able to deliver improved performance.

The story is always a little more complicated than one might at first think.
Also, 787 do not need any MCAS

Instead of a 737 MAX Boeing should have built a 787 MIN
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Old 18th Mar 2019, 23:15
  #2011 (permalink)  
 
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Originally Posted by deltafox44 View Post
Reducing the speed (or not letting it grow) is an obvious remedy but easier to say afterwards. If they have the stick shaker since the take-off, with a normal airspeed, they may think the fault is on airspeed and apply the procedure to keep N1 at a relatively high value that will accelerate the aircraft beyond controlability limits...
Keep trimming and if the system continuously trims you nose down as you are accelerating know that is the opposite direction of STS and thus incorrect => shut the stabilizer down. First and foremost keep trimming to keep the column forces from building up. The problem of getting progressively more out of time takes 10s of seconds to build up and will stop if you keep trimming. If MCAS decides it is time to put in 10 seconds of nose down trim and you determine after a couple of seconds that it is doing the wrong thing and add your own nose up trim, MCAS will stop until you have been off of the trim for 5 continuous seconds. If all you do is blip the trim once every 5 seconds MCAS won't do anything even if the AOA it is getting is out of whack high! Eventually it is hoped that the crew will decide that the automatic stabilizer is doing far more harm than good and turn if off via the cutout switches.

Oh - did I mention enough times that you should keep trimming?!

I'm not trying to argue that MCAS as currently design is satisfactory, but with the system knowledge that we had before the end of 2018 I was confident that we had an acceptable workaround until improvements could be brought to the fleet.

If it turns out the the Ethiopian accident was the result of the same issues that led to the Lion Air accident our industry has some major soul searching to do.

MCAS appears to have been designed based on three assumptions that I have suggested before:
1. If a pilot trims column force will be trimmed to or near zero.
2. Having the automatic stabilizer control continuously take the airplane away from trim when otherwise flying a relatively steady condition, pilots will recognize that it is not healthy and would shut it down via the stabilizer cutout switches.
3. An errant AOA signal feeding MCAS would be acceptable because of (1) and (2) above.
Lion Air showed us that for the crew the day before the accident assumptions (1) and (2) above were reasonable so when (3) happened they were able to fly home.
- In fact they elected to go on to their destination with the stick shaker rattling the whole way!
Lion Air further showed us that for the pilot on the accident flight assumption (1) above held. He made more than 20 corrections and was able to maintain his apparent target altitude of 5000 feet reasonably well. He did not get to (2) above, but might have had his partner been able to keep up with (1) after transferring control.
The Lion Air accident clearly showed that neither assumption (1) nor (2) held true for the second pilot and thus control was lost as a result of not meeting assumption (3).

737MAX was permitted to continue flying after the data from the Lion Air accident revealed the facts outlined above, through so many PPRuNe pages, and across so many other information outlets including FAA and Boeing communications because an assumption was made that with increased awareness pilots would be able to live up to assumptions (1), (2), and (3) above. I would like to think that most of the pilots flying 737MAX airplanes in February were prepared to live up to these assumptions. If the recently decoded FDR from the Ethiopian accident shows that to be a repeat of the Lion Air event this latest assumption will clearly not have been valid for at least one 737MAX crew.

While the MCAS software update developed after the Lion Air accident that is almost ready to go to the fleet will likely remove reliance on the three MCAS design assumptions listed above and thus would have greatly improved the likelihood of a safe outcome for the Ethiopian event we are left with a huge elephant in the room. After making the planned update we still must address the following:
A. How many other key points in the 737MAX safety story are based on pilot response assumptions that may not be valid?
B. How about other airplane models? Are they deemed safe based on faulty assumptions regarding pilot action?
- For instance, how may current 737 crews (all models) would not respond quickly enough to a classic stabilizer runaway that was not arrested by column cutout (i.e., pulling the column far enough)? I know this is covered in simulator sessions for 737 pilots, but is that enough?
C. Moving forward with the current status and future of commercial aviation have we gotten to the point where basic flying skills and system awareness are so low that we are at risk throughout the whole industry?
D. Can current and future pilot reaction short falls be addressed through training? If so, what kind, how much, and how often?
E. How will we know that we have achieved a sufficient industry wide level of safety?

Hoping to see FDR data from the Ethiopian accident soon. I sure hope someone from the PPRuNe community will find a way to get ahold of it and share it here.

Last edited by FCeng84; 19th Mar 2019 at 20:48. Reason: typos
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Old 18th Mar 2019, 23:40
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FCeng84,

Given your familiarity with 737 systems I wanted to address a question to you:

As far as I understand, FCC-enabled MCAS receives single channel sensory data from the ADIRU, mainly AoA (so as to trigger MCAS in the first place) and speed (so as to regulate just how much trim input is required based on Mach number schedule).

My question is about the total control column forces with both SMYD stall protection features ANDhaving MCAS in its maximum setting. Allow me to explain further:

1. MCAS: assuming an AoA sensor failure where the data is relayed as equal or higher than the activation threshold of MCAS, then MCAS kicks in. Assuming a low speed readout by MCAS, it is then set to its maximum throughput (10 seconds of trim and 2.5 degrees nose down, since it works on a speed schedule based on Mach).
2. SMYD/EFS: As I understand, the SMYD computer enables the EFS (Elevator Feel Shift) and STS (Speed Trim System) to do the following during low airspeed conditions - EFS on one hand doubles Hyd A pressure to control column in its AFT axis, and STS trims the stab Nose Down (though respecting the column cutout switches, something MCAS ignores altogether).

The conclusion is the STS trimming you down (so as long as you don't pull aft), the EFS doubling aft forces, and MCAS also trimming at up to 2.5 degs nose down within 10 seconds (if uninterrupted). Essentially, you can end up in full Nose Down stab trim with doubled elevator control forces, thus severely affecting your authority.

Do you agree with my reasoning that SMYD/EFS interaction with MCAS (when low speed is relayed) can be additive in terms of struggling to keep the nose up and having limited authority?
Makes you wonder just how much attention Boeing engineering gave to both these systems' interaction at the design stage.

Last edited by jagema; 18th Mar 2019 at 23:52.
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Old 18th Mar 2019, 23:52
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Originally Posted by yanrair View Post
Hi guys
A few points being missed here I think so far. Now some of these are of course just opinion but here goes.....
1 the 737 can fly with everything turned off. Itís a mechanical plane. That is itís great virtue. So to crash one you have to forget that itís actually a real ordinary plane and you are meant to be in total control! Always.
2 that being so, if STAB moves unexpectedly, turn off STAB SWITCHES as generations of pilots have done ,and fly straight and level. Why not take control of the plane? You can.
3 MACS only works with autopilot out at high AoA slow speed
4 the fact MACS malfunctions or maybe is not weíll designed which we donít know yet, is not the cause of the crash. although it would be desirable if it worked in a sensible manner. Eg two AoA inputs, doesnít repeatedly incrementally trim nose down and a limit on the number of trim wheel turns etc etc as postulated here.
5 this nose down trim goes back to the 707 which had a stick pusher- itís not really a new idea at all
6 the cause here? Looks to me like weíre forgetting the basics and the accumulated knowledge of 50 years of operating the 737.
as someone said, the MAX is like a cart (or perhaps a 707)with a couple of computers and glass screens, but underneath itís a very simple uncomplicated plane. Fly it 6/60 and it will fly. 6 deg nose up. 60% power. Like any other plane.
7 the stabilizer trim switches on the yoke-the ones you use all the time, cannot apparently override (apply automatic brake) against MACS. Now- Thatís something very different from the previous models and if true an important issue. But not the cause.
Thats my ten pence worth for today
i would fly a Max in a heartbeat. Provided the pilots
Non of which is particularly relevant to good basic aerodynamics and safe logical system design and some points of which even contradict themselves !

nose down trim by a stick pusher a very different thing than motoring the whole stab 'in the background' without so much as a 'by your leave' or whisper to the pilot !!!
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Old 19th Mar 2019, 00:00
  #2014 (permalink)  
 
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Originally Posted by FCeng84 View Post
You are stretching my 737 knowledge! I have never read or heard anything definitive about whether or not horizontal tail control (either electric or manual via the trim wheel) would be stalled out if the elevator were at blowdown and thus generating maximum force on the jack screw. I would hope that someone on this forum with that knowledge would share it with the rest of us. Not wanting anyone to find out the hard way my recommendation is to keep up with the pitch trim so that the column forces don't build up to the point where you become the one to find out in flight.
As a long-time 737 driver I'll just chime-in a few points. Regarding the trim wheel ability to move the stab at high speed from full deflection: I seriously doubt that there would ever be too much force on the elevator that the trim wheel could not move it. There is a lot of leverage in the jack screw, and the turns of the trim wheel make very minute changes to the angle of the stab.

But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)

My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.

Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"

(1) Just after takeoff there is a lot going on with trim, power, configuration changes, and as noted above, the darn speed trim is always moving that trim wheel in seemingly random directions to the point that experienced NG pilots would treat its movement as background noise and normal ops. Movement of the trim wheel in awkward amounts and directions would not immediately trigger a memory item response of disconnecting the servos. No way.

(2) The pilots could very reasonably not have noticed the stab trim movement. Movement of the stab trim on the 737 is indicated by very loud clacking as the wheel rotates. On the -200 it was almost shockingly loud. On the NG, much less so. HOWEVER, the 737 cockpit is NOISY. It's one reason I am happy to not be flying it any more. The ergonomics are ridiculous. Especially at high speeds at low altitudes. With the wind noise, they may not have heard the trim wheel moving. The only other way to know it was moving would be yoke feel and to actually look at the trim setting on the center pedestal, which requires looking down and away from the windows and the instruments in a 'leans'-inducing head move. On the 717, for example, Ms. Douglas chimes in with an audible "Stablizer Motion" warning. There is no such indication on the 737.

(3) The fact that they were at high power and high speed tells me the stick shaker was activated. With that massive vibrator between your legs, alternating blue sky and brown out the window, your eye balls bouncing up and down in their sockets as the plane lurches up and down in positive and negative G's, it would have been a miracle if one of the pilots calmly reached down, flicked off the stab servo cutout switches, folded out the trim handle, and started grinding the wheel in the direction of normalcy. These pilots said over the radio that they had "unreliable airspeed". So they did not even know which instruments to rest their eyes on for reliable info. Their eyes were all over the cockpit looking for reliable info, the plane is all over the place like a wild boar in a blanket not behaving in any rational way. And the flying pilot may have been using the tiny standby IFDS for airspeed and attitude. Ouch.

Finally, runaway stab trim is a very, very rare occurence up until now. We trained it about once every other year in the sim because it is so rare. And when we did it was obvious. The nose was getting steadily heavier or steadily lighter with continuous movement of the trim wheel. That is a VERY different scenario than what these pilots faced.

We also trained for jammed stabilizer, the remedy for which is overcoming it with force. The information they were faced with could very reasonably have been interpreted that way, too.

An URGENT AD from the FAA/ Boeing after Lion Air would have helped get it back to the front of the pilot's minds for sure. Extra training by the airline or an urgent pilot memo would have helped. Maybe one was issued, we don't know yet.

A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
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Old 19th Mar 2019, 00:03
  #2015 (permalink)  
 
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deltafox44
Instead of a 737 MAX Boeing should have built a 787 MIN
I love logic!!!!
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Old 19th Mar 2019, 00:14
  #2016 (permalink)  
 
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How many turns of the wheel are required to change trim by one unit?
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Old 19th Mar 2019, 00:38
  #2017 (permalink)  
 
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Why not allow MCAS ‘if activated’, to operate one cycle with aural alert, no repeat for 60 seconds?
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Old 19th Mar 2019, 00:51
  #2018 (permalink)  
 
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Originally Posted by Aloha_KSA View Post
As a long-time 737 driver I'll just chime-in a few points.

[Long-ish, totally on-point observations, clearly based upon long experience, here.]

A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.
This, to me, a once-upon-a time mech and long-time engineer, is the "truthiest" post of this long thread. From the moment, in the Lion Air thread that we began to understand the existence and potential reality of MCAS, my image of being on the flight deck of one of these accident aircraft has been eerily like Aloha_KSA's depiction.

I understand why some pilots want to believe that they would have recognized what was happening, hit the pedestal cutout switches and trimmed the aircraft manually, but everything I know about human factors in sudden and new emergency situations says, "No, you very probably would not." This whole, frankly unforgivable, design and certification mess is a formula that was destined to end up with HAL killing everyone.


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Old 19th Mar 2019, 00:53
  #2019 (permalink)  
 
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Originally Posted by deltafox44 View Post
Also, 787 do not need any MCAS

Instead of a 737 MAX Boeing should have built a 787 MIN
737 power structure in descending order: SWA-Boeing-FAA.
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Old 19th Mar 2019, 01:04
  #2020 (permalink)  
 
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Originally Posted by OldnGrounded View Post
This, to me, a once-upon-a time mech and long-time engineer, is the "truthiest" post of this long thread. From the moment, in the Lion Air thread that we began to understand the existence and potential reality of MCAS, my image of being on the flight deck of one of these accident aircraft has been eerily like Aloha_KSA's depiction.

I understand why some pilots want to believe that they would have recognized what was happening, hit the pedestal cutout switches and trimmed the aircraft manually, but everything I know about human factors in sudden and new emergency situations says, "No, you very probably would not." This whole, frankly unforgivable, design and certification mess is a formula that was destined to end up with HAL killing everyone.
The problem with the above is that at least in the Lion Air accident it did not happen that way. The MCAS commanded 2.5 units of nosedown trim which was countered by the CA retrimming the aircraft. 5 seconds later MCAS again commanded 2.5 units nose down trim and again it was electrically countered by the Captain. This cycle repeated itself 21 times. Thatís a lot of time to diagnose the problem. MCAS does not drive the stab full nose down and cause a instant plunge into the ground. It moves in 2.5 degree increments and electrical trim is available to counter it. Not that difficult to retrim electrically and then disable the trim motors. Your now in trim and have manual trim available.
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