Sydney Airport Message Board - Ethiopian B38M Crash

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- - Ethiopian B38M Crash (http://www.yssyforum.net/board/showthread.php?t=11999)

On the midday news, ET crew reported followed SOP (from Boeing) for MCAS issue.

As I understand it, MCAS was limited to 2.5 degrees of horizontal stabiliser deflection, which in theory could be overcome by manual nose-up input from the pilots. However, Boeing's SOP, which recommended deactivating the MCAS, did not explain that such a procedure just reset it rather than disabled it. If the same faulty AOA input recurred MCAS would then deflect the stabiliser up to 2.5 degrees. But here's the fatal flaw: it had no "memory" of previous deflections after each reset, and it had no provision for checking the current deflection, so each activation of MCAS ADDED ANOTHER 2.5 DEGREES of nose-down!!

That explains why both Lion Air and EA appeared to porpoise - manually regaining climb after the first deactivation but increasingly unable to do so as the jackscrew added each incremental 2.5 degrees of MCAS-commanded nose-down to its existing setting until the horizontal stabiliser was beyond the pilots' ability to counter with elevator nose-up commands. What is shocking is both the flaw in the software and Boeing's deliberate decision to withhold full information about MCAS so that they could market MAX as a no-conversion option for existing B738 pilots. As is now apparent, no B738 simulator incorporated MCAS and no pilot was aware how to fully disable MCAS to avoid it continually re-activating, assuming they would even appreciate the need to do so given the limits of the SOP Boeing had provided.

If my theory is confirmed by the investigation team Boeing's civil liability may be the least of its problems ... :(

EA preliminary report just released

http://www.ecaa.gov.et/documents/204...8-d7af1ee17f3e

The key finding from the FDR, is that this occurred AFTER the electric stabiliser trim cutout switches had been pulled:

Quote:

At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down. The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.

So automatic nose down trim commands were issued by MCAS and honoured despite the stabiliser trim cutout switches having been pulled. Boeing's only suggestion in that situation is to manually grab the trim wheel to stop it from rotating. Obviously if it's already at 40 degrees nose down grabbing the trim wheel is not going to help much.

Quote:

Originally Posted by Philip Argy (Post 107549)

http://www.ecaa.gov.et/documents/204...8-d7af1ee17f3e

40 degrees nose down grabbing the trim wheel is not going to help much.

Didn't they turn the trim switches back on?

Happy to stand corrected, but the way I understand it was that they were unable to exert enough force on the manual trim wheel (due to the airspeed) and reactivated the trim cutout switches.

Quote:

At 05:40:35, the First-Officer called out "stab trim cut-out" two times. Captain agreed and First-Officer confirmed stab trim cut-out.

At 05:40:41, approximately five seconds after the end of the ANU stabilizer motion, a third instance of AND automatic trim command occurred without any corresponding motion of the stabilizer, which is consistent with the stabilizer trim cutout switches were in the "cutout" position

So that indicates with the switches in the cutout position, MCAS continues to operate but cannot move the stabiliser as the cutout switches de-energise the motor?

Quote:

At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working.

I assume the FO is confirming the manual electric trim is not functioning, consistent with the switches being in cutout, so tries the manual trim wheel without success (Due to aerodynamic forces of the aircraft being above VMO??)

Then;

Quote:

At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,400 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units.

At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40Â° nose down. The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.

So at some point between 05:41:54 and 05:43:11 the trim cutout switches must have been reactivated for the manual electric trim inputs to move the stab in the ANU direction - which explains why MCAS suddenly had the ability to command AND movements again?

Interesting comment on the investigations to date:

Quote:

Examining both accidents separately provides valuable insights—it’s easy to understand how these unrelated airlines and crew may have responded in similar ways—but the overall conclusion in our previous article, “Boeing’s Grounding: Catastrophic Crashes, and Questions About Boeing’s Liability And 737 MAX Aircraft Viability,” still stands—the major contributing factor to these accidents was pilot error.

After a more comprehensive analysis of each of the two accidents, especially Lion Air Flight 610, we are persuaded more than ever that the case for pilot error—as well as inadequate training—are the dominant contributing factors in both accidents, not the only ones but the most serious factors.

https://seekingalpha.com/instablog/3...se-pilot-error

Pilot error claim objectively unfair

The fact is that Boeing promoted the MAX as requiring little to no conversion training from a 737-800 - an hour on an iPad was one quote I saw. In that context any inability of crew to cope with differences with which they were not familiar, and which had never been incorporated into any simulator, cannot be the principal cause of either event. Pilot error based on informed hindsight has no place here.

Having said that, yes, I have speculated more than I should have about the role of MCAS, but the inferences are there to be drawn.

Looks like Boeing have decided to put the accidents back on the pilots:

Boeing CEO Points at pilot error

Primitive inputs need more redundancy and resilience

However sophisticated the avionics, the old adage of garbage in garbage out applies. Primitive physical sources like pitot tubes and AofA sensors need redundancy and resilience before they are taken as trusted sources for sophisticated avionics. Taking a single sensor as a trusted source was objectively poor risk management in the implementation of MCAS.

There have been too many examples of erroneous data from pitot tubes and AofA sensors causing adverse avionics responses. This needs much more focus going forward IMHO.

Blaming pilots is hardly helpful - human error needs to be anticipated in system design. It's no use if the cure is worse than the complaint. Pilots know what to do when the stick shakes. MCAS is not necessarily the best solution to the MAX propensity to stall!

:(

As I was saying ...

How dare they blame the pilots!

Now this: https://airlinerwatch.com/boeing-nev...sources-claim/

My case rests ...