Go Back  PPRuNe Forums > Flight Deck Forums > Rumours & News
Reload this Page >

MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

Rumours & News Reporting Points that may affect our jobs or lives as professional pilots. Also, items that may be of interest to professional pilots.

MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

Old 2nd Jul 2019, 06:55
  #921 (permalink)  
 
Join Date: Aug 2007
Location: Alabama
Age: 58
Posts: 366
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by DaveReidUK
ICAO Annex 6 lists mandatory FDR parameters, and is well worth a read. One of those parameters is "Cockpit pitch trim control input position" (my emphasis, not ICAO's).

That's why FDAUs came along in the 1970s.
The FDR trace show only one cockpit pitch trim control input position, it does now show BOTH thumn switches, that might lead to assume is an elaborated signal that is taken at the servo motor controller. Of course the traces might have been elebaorated before been published, however the information published does not validate a theory of the trace representing the thumb switch at the source
FrequentSLF is offline  
Old 2nd Jul 2019, 07:33
  #922 (permalink)  
 
Join Date: Jan 2008
Location: Reading, UK
Posts: 15,784
Received 196 Likes on 90 Posts
Originally Posted by FrequentSLF
however the information published does not validate a theory of the trace representing the thumb switch at the source
Or, more accurately, it neither validates nor invalidates the theory - we simply don't know.

But the fact remains that if it turns out that the FDR isn't showing the thumb switch position, then it contravenes the Annex 6 requirements.

Boeing have been building aeroplanes for long enough that one would hope they know what an FDR is supposed to record.
DaveReidUK is offline  
Old 2nd Jul 2019, 08:47
  #923 (permalink)  
 
Join Date: Dec 2013
Location: Weltschmerz-By-The-Sea, Queensland, Australia
Posts: 1,362
Received 76 Likes on 34 Posts
Servo motors...

The reason that I askd if the stab motor was a servo is that servos behave differently to the normal motors we are all used to. They were developed for cnc machines...they accurately will move about 1/4000th of a revolution per step signal, and then lock their rotor. So they need a command to move x steps in Y direction at whatever speed. That can be a simple analogue instruction; run clockwise at speed number 3 until the limit stops are reached.

But it would make more sense for the FCC to signal a step + direction command. However many steps to get 2.5 units of nose down trim.

The critical feature is again the locked rotor when not turning. If the clutch doesn’t work, or doesn’t work well, a human will not be able to move the rotor from its last commanded position.

I have had four big servo motors in my garage machine shop so am quite familiar with their characteristics.

I am also a 737 pilot, and have had a failure of the stab trim brakes in flight...a soft runaway requiring cranking the wheel. It wasn’t a big deal, but then again we were smack in the middle of the envelope, and the -400 had a both a shorter moment arm and a 10% bigger trim wheel to crank.

Regarding motor overheating: I really doubt it. I have run servos in 45° heat with a big load running constantly for a couple of hours relying only on their passive air cooling. Never had a thermal shutdown.
Australopithecus is online now  
Old 2nd Jul 2019, 08:56
  #924 (permalink)  
 
Join Date: Apr 2019
Location: EDSP
Posts: 334
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by MemberBerry

https://aviaforum.ru/threads/katastr...2/post-2392734
(contains a more detailed schematic for the NG version).
Good find! Thanks!
BDAttitude is offline  
Old 2nd Jul 2019, 08:57
  #925 (permalink)  
 
Join Date: Feb 2015
Location: The woods
Posts: 5
Likes: 0
Received 3 Likes on 2 Posts
Originally Posted by Australopithecus
Servo motors...

The reason that I askd if the stab motor was a servo is that servos behave differently to the normal motors we are all used to. They were developed for cnc machines...they accurately will move about 1/4000th of a revolution per step signal, and then lock their rotor. So they need a command to move x steps in Y direction at whatever speed. That can be a simple analogue instruction; run clockwise at speed number 3 until the limit stops are reached.

But it would make more sense for the FCC to signal a step + direction command. However many steps to get 2.5 units of nose down trim.

The critical feature is again the locked rotor when not turning. If the clutch doesn’t work, or doesn’t work well, a human will not be able to move the rotor from its last commanded position.

I have had four big servo motors in my garage machine shop so am quite familiar with their characteristics.

I am also a 737 pilot, and have had a failure of the stab trim brakes in flight...a soft runaway requiring cranking the wheel. It wasn’t a big deal, but then again we were smack in the middle of the envelope, and the -400 had a both a shorter moment arm and a 10% bigger trim wheel to crank.

Regarding motor overheating: I really doubt it. I have run servos in 45° heat with a big load running constantly for a couple of hours relying only on their passive air cooling. Never had a thermal shutdown.
Good info OZ,
Dont forget though, that the workshop motors are designed for continuous running with appropriate cooling.
Stab motors are designed for intermittent use with cooling appropriate to that task.
As Yoko says, earlier 737 types (which had a different motor(s)) could suffer from stab motor overheat, so it is not unknown.
Air speed is also possibly relevant depending on cooling air source - and OAT.
bill fly is offline  
Old 2nd Jul 2019, 09:06
  #926 (permalink)  
 
Join Date: Apr 2019
Location: EDSP
Posts: 334
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by Australopithecus
Servo motors...

The reason that I askd if the stab motor was a servo is that servos behave differently to the normal motors we are all used to. They were developed for cnc machines...they accurately will move about 1/4000th of a revolution per step signal, and then lock their rotor. So they need a command to move x steps in Y direction at whatever speed. That can be a simple analogue instruction; run clockwise at speed number 3 until the limit stops are reached.

But it would make more sense for the FCC to signal a step + direction command. However many steps to get 2.5 units of nose down trim.

The critical feature is again the locked rotor when not turning. If the clutch doesn’t work, or doesn’t work well, a human will not be able to move the rotor from its last commanded position.

I have had four big servo motors in my garage machine shop so am quite familiar with their characteristics.

I am also a 737 pilot, and have had a failure of the stab trim brakes in flight...a soft runaway requiring cranking the wheel. It wasn’t a big deal, but then again we were smack in the middle of the envelope, and the -400 had a both a shorter moment arm and a 10% bigger trim wheel to crank.

Regarding motor overheating: I really doubt it. I have run servos in 45° heat with a big load running constantly for a couple of hours relying only on their passive air cooling. Never had a thermal shutdown.
From Eaton:
Eaton’s new Model 6355C Stabilizer Trim Motor features: • Brushless three phase motor design • Low loss power bridge with IGBT switches • Processor based motor commutation and velocity control • Dual current limit (torque) control circuits • Power up built-in test • Continuous fault monitoring • Fault storage (non-volatile memory) • RS-232 test/maintenance interface • Investment cast housing • Two stage spur gear train • Modular, bottom up assembly — two electronic sub-assemblies, motor, housing with gear train
BDAttitude is offline  
Old 2nd Jul 2019, 09:25
  #927 (permalink)  
 
Join Date: Nov 2002
Location: Northern Europe
Age: 45
Posts: 152
Received 0 Likes on 0 Posts
Originally Posted by BDAttitude
From Eaton:

Eaton’s new Model 6355C Stabilizer Trim Motor features: • Brushless three phase motor design • Low loss power bridge with IGBT switches • Processor based motor commutation and velocity control • Dual current limit (torque) control circuits • Power up built-in test • Continuous fault monitoring • Fault storage (non-volatile memory) • RS-232 test/maintenance interface • Investment cast housing • Two stage spur gear train • Modular, bottom up assembly — two electronic sub-assemblies, motor, housing with gear train
Current-limiters are great. That will stop the motor from any
overexertion during high loads.
It may of course slow the motor down, or even stop it completly...
The Bartender is offline  
Old 2nd Jul 2019, 09:42
  #928 (permalink)  
 
Join Date: Jul 2013
Location: Norway
Age: 57
Posts: 140
Likes: 0
Received 0 Likes on 0 Posts
A brushless three phase motor could be a "normal" induction motor. Or it could be a permanent magnet motor. And it can have some position feedback added to it. And even though it's not specifically mentioned it can very well have termistors or temperature sensors embedded in the windings for overheat protection.
SteinarN is offline  
Old 2nd Jul 2019, 09:51
  #929 (permalink)  
 
Join Date: Mar 2006
Location: Vance, Belgium
Age: 61
Posts: 262
Likes: 0
Received 3 Likes on 2 Posts
Originally Posted by DaveReidUK
Can you point to the entry on the CVR extract (i.e. the timestamp) where you believe that happened ?
Here it is:
http://www.ecaa.gov.et/documents/204...+,(ET-AVJ).pdf
Originally Posted by Aircraft Accident Investigation Preliminary Report - ET-AVJ
At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer 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.
...
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.
So it is a bit more than 1 minute after the cutout switches had been actioned.

Luc
Luc Lion is online now  
Old 2nd Jul 2019, 09:59
  #930 (permalink)  
 
Join Date: Apr 2019
Location: EDSP
Posts: 334
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by SteinarN
A brushless three phase motor could be a "normal" induction motor. Or it could be a permanent magnet motor. And it can have some position feedback added to it. And even though it's not specifically mentioned it can very well have termistors or temperature sensors embedded in the windings for overheat protection.
It says:
• Low loss power bridge with IGBT switches • Processor based motor commutation and velocity control
And I guess that says it all. Not having thermal sensing on IGBT bridges would be very unusual.
BDAttitude is offline  
Old 2nd Jul 2019, 10:32
  #931 (permalink)  
 
Join Date: Jul 2013
Location: Norway
Age: 57
Posts: 140
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by BDAttitude
It says:

And I guess that says it all. Not having thermal sensing on IGBT bridges would be very unusual.
I didnt say anything about thermal protection of the IGBT, I was only adding that there could well be that there was thermal sensing of the windings in the motor. But ofc, that is pure specualtion and has no significance for the accident under discussion in this thread.
SteinarN is offline  
Old 2nd Jul 2019, 10:36
  #932 (permalink)  
 
Join Date: Feb 2013
Location: 60 north
Age: 59
Posts: 17
Likes: 0
Received 0 Likes on 0 Posts
Dear All
Been a busy month June 2019.
Will never forget it, hell!
But enough about my problems.
As some of You have observed I am a simple chap.
And here is a simple observation:
MCAS is the most retarded system I have ever encountered!
It now is clear to me that Boeing has lost the plot , curtsy of FAA.
I am afraid the treason by Boeing can not be forgiven, as it is systemic!
The MCAS should be scraped and a aerodynamic improvement applied.
Or its Coke Cane Time!
Flight safety on a basic aerodynamic level is not a political thing.
Looking forward to fly the MAX 2.0.
Regards
Cpt B

Last edited by BluSdUp; 2nd Jul 2019 at 10:37. Reason: d
BluSdUp is offline  
Old 2nd Jul 2019, 11:55
  #933 (permalink)  
 
Join Date: Dec 2018
Location: 8th floor
Posts: 0
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by FrequentSLF
The FDR trace show only one cockpit pitch trim control input position, it does now show BOTH thumn switches, that might lead to assume is an elaborated signal that is taken at the servo motor controller. Of course the traces might have been elebaorated before been published, however the information published does not validate a theory of the trace representing the thumb switch at the source
Well, the trim schematic shows the thumb switch outputs for "up" and "down" connected together, and there are direct connections that go to the DFDAU (Digital Flight Data Acquisition Unit). This strongly suggests that the FDR traces are based on the actual position of the switches. I took a section of the schematic and colored in green and turquoise the paths of the signal from thumb switches to the DFDAU:

MemberBerry is offline  
Old 2nd Jul 2019, 12:26
  #934 (permalink)  
 
Join Date: Jan 2010
Location: UK
Posts: 180
Received 0 Likes on 0 Posts
Originally Posted by yoko1
3. Trimming forces. Speaking as someone who has actually used the manual trim wheel on the Boeing, I can assure you that the trim resistance increases in both directions. It is not like rolling a boulder uphill where it is very difficult in one direction and easy in the other. The increased resistance is due to binding forces at the jackscrew thread/nut interface. Not sure if it is strictly frictional force or some type of deformation or misalignment. Keep in mind that while the aerodynamic forces are acting roughly parallel to the jackscrew body, those forces are mostly perpendicular at the thread/nut interface. There will be some directional bias, but that is a relatively small component. BTW, this is one of the reasons why a jackscrew mechanism is used in the first place - to minimize those directional forces and make it easier for the motor. However, one point needs to be emphasized again: You completely avoid these issues by not letting the stab get out of trim in the first place. There is no, none, not one iota of evidence that the Main Electric Trim did not move the stab when it was used. And yes, by not being attentive to such matters, it is absolutely possible to place the aircraft in a state where things don't work as they should.

4. There is a lot of reference of what are perceived as anomalies in the trim system in the final moments of both accidents. Personally, that's not what I see, however I'm not going to claim I have a better microscope than anyone else here. That being said, I must again point out that in those final moments, the aircraft had exceeded the certified flight envelope. Engineering safety factors aside, there is not any expressed or implied guarantee that any system will work as it is supposed to once you exceed the limits of that system. The lesson here is that you keep that aircraft within its design envelope - just like the crew of the penultimate Lion Air 610 flight did.
Yet again it must be pointed out the only reason the crew of the penultimate Lion Air 610 managed to keep the aircraft within its design envelope was because they were lucky enough to have a jumpseater who not only observed the operation of the trim wheel but also persuaded the crew to disable automatic trim.

Frankly the lesson here is that Boeing should never, ever, ever again ship an aircraft with functionality that can programmatically take the aircraft out of its design envelope due to an easily foreseeable fault. Boeing made that mistake all by itself and should cease trying to muddy the waters by pointing the finger at overwhelmed crew trying to deal with multiple alarms caused by that fault.
SamYeager is offline  
Old 2nd Jul 2019, 12:47
  #935 (permalink)  
 
Join Date: Jan 2008
Location: Reading, UK
Posts: 15,784
Received 196 Likes on 90 Posts
Originally Posted by Luc Lion
Here it is:

At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and FirstOfficer 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.
...
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.
So it is a bit more than 1 minute after the cutout switches had been actioned.

Luc
You are assuming that "The First-Officer has replied that the trim was not working" means that he attempted unsuccessfully to use the electric trim switches.

It could equally have been him just confirming that (as you rightly say) just over a minute previously both pilots had agreed to disable the stab trim (05:40:35), given that it was followed immediately by asking "if he could try it manually". In other words, the electric trim switches obviously weren't going to work, and there wouldn't have been any point in trying them.
DaveReidUK is offline  
Old 2nd Jul 2019, 13:36
  #936 (permalink)  
 
Join Date: Jul 2002
Location: Ireland
Posts: 596
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by DaveReidUK
You are assuming that "The First-Officer has replied that the trim was not working" means that he attempted unsuccessfully to use the electric trim switches.
Given that the reply came about 8 seconds after asking if he could try, it is highly likely (though not 100% certain) the the FO was attempting to operate the trim switch during those 8 seconds.
Speed of Sound is offline  
Old 2nd Jul 2019, 13:41
  #937 (permalink)  
 
Join Date: Jan 2012
Location: Welsh Marches
Posts: 77
Received 0 Likes on 0 Posts
Originally Posted by MemberBerry
Well, the trim schematic shows the thumb switch outputs for "up" and "down" connected together, and there are direct connections that go to the DFDAU (Digital Flight Data Acquisition Unit). This strongly suggests that the FDR traces are based on the actual position of the switches. I took a section of the schematic and colored in green and turquoise the paths of the signal from thumb switches to the DFDAU:

Sorry, a dumb question but electric circuits are a black art to me. I've seen this, or a similar diagram before, and am aware of all the comments that the two switches now peform an identical function. If that is the case, what is the function of the circuit 8 - 9 in the S272 Stab Trim Cutout Switch?

Thanks

Alchad
Alchad is offline  
Old 2nd Jul 2019, 14:24
  #938 (permalink)  
 
Join Date: Jan 2008
Location: Reading, UK
Posts: 15,784
Received 196 Likes on 90 Posts
Originally Posted by Speed of Sound
Given that the reply came about 8 seconds after asking if he could try, it is highly likely (though not 100% certain) the the FO was attempting to operate the trim switch during those 8 seconds.
That's not how it reads.

According to the transcript "The First-Officer has replied that the trim was not working and asked if he could try it manually", followed (8 seconds later) by "the First-Officer replied that it is not working".

That would suggest that what he was doing during those 8 seconds (having already acknowledged that electric trim was inop) was to try to wind the trim wheel (unsuccessfully, for the reasons already discussed in the thread), followed by his statement that it wouldn't move.
DaveReidUK is offline  
Old 2nd Jul 2019, 14:29
  #939 (permalink)  
 
Join Date: May 2019
Location: Somewhere over the rainbow...
Posts: 0
Likes: 0
Received 0 Likes on 0 Posts
Lot’s of very interesting and detailed discussion here. I’ve learned quite a bit about electric motors and their control circuits, so thanks. I should point out, however, that all this is far, far above the level of detail that would ever be included in the FCOM.

Now back to the elephant in the room. All of us are basically amateurs when it comes to accident investigations. Many of us like to know how things work, or should work, and enjoy puzzling things out from the limited information that is available. Perhaps some of us just like a good mystery. Perhaps some don’t have the patience to wait for the investigative process to run its course or are uncomfortable with what is perceived as an information void. I absolutely get the psychology at work.

While I consider myself fairly knowledgeable about the 737 and it’s operations, I will acknowledge that the information and resources I (or anyone else here) possess is absolutely dwarfed by those available to the collective body of professional accident investigators, multiple certificate authorities representing dozens of nations, subject matter experts, and other interested parties such as the aerospace-centric media (i.e. Aviation Week and the like) and the massive army of tort lawyers lining up to sue Boeing.

Is anyone here seriously suggesting that it has not occurred to any of these parties that they should investigate the Main Electric Trim system to see if there were any issues? Please speak up, because I would really like to hear your thinking here.

In the past couple of months, we have heard a litany of items that various certificate authorities want to see addressed before the MAX is cleared to fly. Conspicuously absent is any mention of the Main Electric Trim system. No suggestion of failures, stalls, overheats, or anything else. No one is asking for any redesign or replacement of switches, relays, wires, or motors. If this was still early in the investigation, sure, absolutely valid questions have been raised. And all those questions have been found wanting of actual hard, verifiable evidence to back them up.

So really, folks, why the continued obsession with this system? No party who actually has all the data, resources, and experts is saying anything about it. Do you really think you are more knowledgeable than the collective body of all these very smart people? There is far too much grasping for straws that simply aren’t there.

Time to move on.

Last edited by yoko1; 2nd Jul 2019 at 21:24.
yoko1 is offline  
Old 2nd Jul 2019, 15:04
  #940 (permalink)  
 
Join Date: Jan 2008
Location: Weedon, UK
Age: 77
Posts: 125
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by yoko1
Lot’s of very interesting and detailed discussion here.
// snip //
So really, folks, why the continued obsession with this system? No party who actually has all the data, resources, and experts is saying anything about it. Do you really think you are more knowledgeable than the collective body of all these very smart people? There is far too much grasping for straws that simply aren’t there.

Time to move on.
No party who actually has all the data, resources, and experts is saying anything about anything. Maybe you could explain why you are so determined to shut this aspect of the discussion down. What harm can it do?
sooty655 is offline  

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.