Ethiopian airliner down in Africa
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There are two big questions:
- why did he fail to recognize the stab trim runaway and activate the CUTOUT ?
- why did he hand control of the trim to the FO with such a critical malfunction occurring ?
As has been pointed out many times in the two main threads discussing the 737 MAX crashes, the LT610 PF counteracted uncommanded (MCAS) nose-down trim on 21 separate occasions, yet for whatever reason did not believe he was experiencing a stab trim runaway, and failed to activate the CUTOUT switches.
There are two big questions:
There are two big questions:
- why did he fail to recognize the stab trim runaway and activate the CUTOUT ?
- why did he hand control of the trim to the FO with such a critical malfunction occurring ?
So, even with 3 heads in the cockpit, one of whom was free from immediate control issues, it did not appear to that crew the Trim Runaway NNC was the obvious choice.
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Investigation into the Ethiopian Airlines Boeing 737 Max crash is likely to be scrutinised as much for impartiality and independence as for its analysis of technical and operational circumstances.
Under ICAO standards Ethiopian authorities are poised to lead the inquiry into the 10 March accident involving the Nairobi-bound flight ET302, which occurred just outside Addis Ababa.But Ethiopian investigators are likely to face pressure for full transparency – not just to satisfy concerns over the 737 Max, following the airline’s decision to ground the type, but to ensure there is no repeat of the controversy which tainted a previous fatal 737 accident probe involving the same carrier.
Lebanese investigators conducted an inquiry into the loss of Ethiopian flight ET409, a Boeing 737-800, which crashed into the sea just 4min after taking off from Beirut in February 2010.The inquiry concluded that the crew lost control of the jet as a result of “inconsistent” flight-control input and “mismanagement” of airspeed, altitude and attitude, adding that the aircraft was out of trim.It stated that the first officer failed to demonstrate sufficient assertiveness to intervene despite multiple warnings – including stick-shaker activation – and evidence that the captain, who was flying, was showing signs of disorientation and loss of situational awareness.The inquiry attributed the crash to a combination of “failure in basic piloting skills” by the captain, combined with inadequate crew resource management from the first officer, and queried the decision to pair the two.While the pilots met Ethiopian’s criteria for pairing the inquiry pointed out that their levels of experience “did not constitute a comfortable margin”, particularly for operation under demanding conditions.
Despite the in-depth analysis by the Lebanese investigators, both Ethiopian Airlines and the Ethiopian Civil Aviation Authority strongly condemned the conclusions.The ECAA claimed the inquiry report was an “unbalanced account” containing “factual inaccuracies, internal contradictions and hypothetical statements” which were not supported by evidence.In an extraordinary formal statement the authority insisted that the most probable cause of the crash was the break-up of the jet following an explosion – the result of sabotage, a lightning strike, or being shot down.The ECAA rejected the findings of crew mismanagement of the 737-800, claiming that flight-data recorder information revealed stabiliser and roll movements suggestive of damage to the tail section.....[etc]
Under ICAO standards Ethiopian authorities are poised to lead the inquiry into the 10 March accident involving the Nairobi-bound flight ET302, which occurred just outside Addis Ababa.But Ethiopian investigators are likely to face pressure for full transparency – not just to satisfy concerns over the 737 Max, following the airline’s decision to ground the type, but to ensure there is no repeat of the controversy which tainted a previous fatal 737 accident probe involving the same carrier.
Lebanese investigators conducted an inquiry into the loss of Ethiopian flight ET409, a Boeing 737-800, which crashed into the sea just 4min after taking off from Beirut in February 2010.The inquiry concluded that the crew lost control of the jet as a result of “inconsistent” flight-control input and “mismanagement” of airspeed, altitude and attitude, adding that the aircraft was out of trim.It stated that the first officer failed to demonstrate sufficient assertiveness to intervene despite multiple warnings – including stick-shaker activation – and evidence that the captain, who was flying, was showing signs of disorientation and loss of situational awareness.The inquiry attributed the crash to a combination of “failure in basic piloting skills” by the captain, combined with inadequate crew resource management from the first officer, and queried the decision to pair the two.While the pilots met Ethiopian’s criteria for pairing the inquiry pointed out that their levels of experience “did not constitute a comfortable margin”, particularly for operation under demanding conditions.
Despite the in-depth analysis by the Lebanese investigators, both Ethiopian Airlines and the Ethiopian Civil Aviation Authority strongly condemned the conclusions.The ECAA claimed the inquiry report was an “unbalanced account” containing “factual inaccuracies, internal contradictions and hypothetical statements” which were not supported by evidence.In an extraordinary formal statement the authority insisted that the most probable cause of the crash was the break-up of the jet following an explosion – the result of sabotage, a lightning strike, or being shot down.The ECAA rejected the findings of crew mismanagement of the 737-800, claiming that flight-data recorder information revealed stabiliser and roll movements suggestive of damage to the tail section.....[etc]
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Good questions, but as I said much much earlier, I think we can turn this one on its head and ask "what enabled the supernumerary Captain on the prior flight to recognise, at least in part, that CUTOUT was the solution?". I believe that, even then, electric trim was re-engaged albeit briefly.
So, even with 3 heads in the cockpit, one of whom was free from immediate control issues, it did not appear to that crew the Trim Runaway NNC was the obvious choice.
So, even with 3 heads in the cockpit, one of whom was free from immediate control issues, it did not appear to that crew the Trim Runaway NNC was the obvious choice.
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Good questions, but as I said much much earlier, I think we can turn this one on its head and ask "what enabled the supernumerary Captain on the prior flight to recognise, at least in part, that CUTOUT was the solution?". I believe that, even then, electric trim was re-engaged albeit briefly.
So, even with 3 heads in the cockpit, one of whom was free from immediate control issues, it did not appear to that crew the Trim Runaway NNC was the obvious choice.
So, even with 3 heads in the cockpit, one of whom was free from immediate control issues, it did not appear to that crew the Trim Runaway NNC was the obvious choice.
As to why it was not handled exactly per Trim Runaway NNC once recognised (re-enabling trim) one factor is that it did not match the at the time checklist perfectly since the 'runaway' was not continuous.
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AA To Have Pilots Do Sim Training on MAX: WSJ
By
Andy Pasztor
Updated April 15, 2019 5:33 p.m. ET American Airlines Group Inc., AAL +0.06% after saying for months that its pilots didn’t need additional ground-simulator experience on Boeing Co. BA +0.26% 737 MAX jets, now plans to include such instruction in training sessions for the aircraft, according to industry officials.
The decision, these officials said, means as soon as late summer, American 737 MAX pilots will start encountering some simulator scenarios tied to problems with an automated flight-control system, called MCAS, that has been implicated in two fatal nose-dives of the plane in less than five months.
The enhanced training also will deal with similar emergency situations in which pilots need to intercede to manually correct movement of flight-control surfaces on the jet’s tail.
American’s choice highlights growing differences between carriers—and in American’s case, with federal air-safety regulators—regarding the best way to ensure flight crews will be able to safely operate 737 MAX jets once they resume service.
At this point, the Federal Aviation Administration isn’t planning to mandate simulator training targeting potential MCAS misfires. American’s voluntary effort to go beyond minimum federal requirements hasn’t been reported before.
Southwest Airlines Co. and United Continental Holdings Inc., the other U.S. carriers with MAX aircraft, don’t intend to adopt similar training changes, the officials said. Some overseas carriers, however, have signaled they may opt for enhanced simulator training.
In the immediate wake of a Lion Air jet crash in Indonesia in October, American said it continued “to believe the Boeing 737 MAX aircraft is safe and that our pilots are well-trained and well-equipped to operate it.”
Investigators Blame Boeing Flight-Control System in Ethiopian Crash
An initial probe into the crash of the Ethiopian Airlines Boeing 737 MAX 8 found that the pilots followed emergency procedures. Investigators went on to recommend that the MAX fleet stay grounded until authorities validate changes that Boeing has made to the aircraft. WSJ’s Robert Wall explains. Photos: Getty ImagesAmerican’s pilots were critical of Boeing for not providing enough information initially about the MCAS system, but determined that they had learned enough about how MCAS worked to continue flying the plane without additional simulator training. But on Sunday, a spokesman for American Airlines said the carrier is “looking at the potential for additional training opportunities” in coordination with the FAA and representatives of the pilot union.
A Southwest spokeswoman said the airline’s current training covers operating in conditions present during “an MCAS misfire.” She added, “We briefed our pilots on MCAS post-Lion Air and emphasized the training for operating in unreliable airspeed conditions.”
On Monday, United said, “Our training is consistently refreshed and updated, and we will make any updates to our training necessary should the FAA decide more is required as part of their ongoing investigation.”
Within weeks of the Lion Air crash, American already was working behind the scenes on possible training changes. Without any prodding from the FAA, the carrier’s safety and training experts began considering possible additional simulator training, according to internal FAA documents reviewed by The Wall Street Journal.
According to one email at the time from a senior FAA inspector, the carrier was developing new simulator scenarios for the MCAS system malfunctions and potential consequences. The email added that FAA and American officials determined “it would be better to wait for further guidance” from the plane maker and agency certification experts before proceeding to develop full-blown simulator scenarios.
The carrier, according to one industry official familiar with the details, didn’t follow through with the proposal at the time because managers decided there was too much uncertainty about the cause of the Lion Air crash. But now, after a second fatal 737 MAX crash—in Ethiopia in March—in which MCAS was implicated, American is working in earnest to implement extra simulator training for MCAS-related events, this official said.
Andy Pasztor
Updated April 15, 2019 5:33 p.m. ET American Airlines Group Inc., AAL +0.06% after saying for months that its pilots didn’t need additional ground-simulator experience on Boeing Co. BA +0.26% 737 MAX jets, now plans to include such instruction in training sessions for the aircraft, according to industry officials.
The decision, these officials said, means as soon as late summer, American 737 MAX pilots will start encountering some simulator scenarios tied to problems with an automated flight-control system, called MCAS, that has been implicated in two fatal nose-dives of the plane in less than five months.
The enhanced training also will deal with similar emergency situations in which pilots need to intercede to manually correct movement of flight-control surfaces on the jet’s tail.
American’s choice highlights growing differences between carriers—and in American’s case, with federal air-safety regulators—regarding the best way to ensure flight crews will be able to safely operate 737 MAX jets once they resume service.
At this point, the Federal Aviation Administration isn’t planning to mandate simulator training targeting potential MCAS misfires. American’s voluntary effort to go beyond minimum federal requirements hasn’t been reported before.
Southwest Airlines Co. and United Continental Holdings Inc., the other U.S. carriers with MAX aircraft, don’t intend to adopt similar training changes, the officials said. Some overseas carriers, however, have signaled they may opt for enhanced simulator training.
In the immediate wake of a Lion Air jet crash in Indonesia in October, American said it continued “to believe the Boeing 737 MAX aircraft is safe and that our pilots are well-trained and well-equipped to operate it.”
Investigators Blame Boeing Flight-Control System in Ethiopian Crash
An initial probe into the crash of the Ethiopian Airlines Boeing 737 MAX 8 found that the pilots followed emergency procedures. Investigators went on to recommend that the MAX fleet stay grounded until authorities validate changes that Boeing has made to the aircraft. WSJ’s Robert Wall explains. Photos: Getty ImagesAmerican’s pilots were critical of Boeing for not providing enough information initially about the MCAS system, but determined that they had learned enough about how MCAS worked to continue flying the plane without additional simulator training. But on Sunday, a spokesman for American Airlines said the carrier is “looking at the potential for additional training opportunities” in coordination with the FAA and representatives of the pilot union.
A Southwest spokeswoman said the airline’s current training covers operating in conditions present during “an MCAS misfire.” She added, “We briefed our pilots on MCAS post-Lion Air and emphasized the training for operating in unreliable airspeed conditions.”
On Monday, United said, “Our training is consistently refreshed and updated, and we will make any updates to our training necessary should the FAA decide more is required as part of their ongoing investigation.”
Within weeks of the Lion Air crash, American already was working behind the scenes on possible training changes. Without any prodding from the FAA, the carrier’s safety and training experts began considering possible additional simulator training, according to internal FAA documents reviewed by The Wall Street Journal.
According to one email at the time from a senior FAA inspector, the carrier was developing new simulator scenarios for the MCAS system malfunctions and potential consequences. The email added that FAA and American officials determined “it would be better to wait for further guidance” from the plane maker and agency certification experts before proceeding to develop full-blown simulator scenarios.
The carrier, according to one industry official familiar with the details, didn’t follow through with the proposal at the time because managers decided there was too much uncertainty about the cause of the Lion Air crash. But now, after a second fatal 737 MAX crash—in Ethiopia in March—in which MCAS was implicated, American is working in earnest to implement extra simulator training for MCAS-related events, this official said.
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I disagree. It appears it occurred to someone (the jump seat pilot) pretty quickly that it was a runaway. They got pitch under control and hit the cutouts. My guess is they re-enabled stab trim in a troubleshooting / curiosity activity (lets see if its still doing it?) because clearly they did not need the electric trim to fly the plane.
"I thought the right switch killed auto but allowed manual electric trim"
tries various combinations of switch positions, ending with both up..
"There it goes again"
"That's strange, I have the conversion notes in my carry on, let me go get them"
Possibly they were trying to understand the (changed) function of the cutout switches:
"I thought the right switch killed auto but allowed manual electric trim"
tries various combinations of switch positions, ending with both up..
"There it goes again"
"That's strange, I have the conversion notes in my carry on, let me go get them"
"I thought the right switch killed auto but allowed manual electric trim"
tries various combinations of switch positions, ending with both up..
"There it goes again"
"That's strange, I have the conversion notes in my carry on, let me go get them"
Except I **believe** that s/he had already gotten the documentation out. It is interesting that the (altered) function of the cutout switches appears to have required clarification on this forum.
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Not sure it has been confirmed either way on when the library run occurred relative to cutout.
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Possibly they were trying to understand the (changed) function of the cutout switches:
"I thought the right switch killed auto but allowed manual electric trim"
tries various combinations of switch positions, ending with both up..
"There it goes again"
"That's strange, I have the conversion notes in my carry on, let me go get them"
"I thought the right switch killed auto but allowed manual electric trim"
tries various combinations of switch positions, ending with both up..
"There it goes again"
"That's strange, I have the conversion notes in my carry on, let me go get them"
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I disagree. It appears it occurred to someone (the jump seat pilot) pretty quickly that it was a runaway. They got pitch under control and hit the cutouts. My guess is they re-enabled stab trim in a troubleshooting / curiosity activity (lets see if its still doing it?) because clearly they did not need the electric trim to fly the plane.
It's obvious from the FDR traces that is not the case, and that they actually fought MCAS for about 5 minutes before using the stab trim cutout switches. For the Ethiopian flight, 4 minutes after they experienced the first MCAS activation they were already hitting the ground.
And the Ethiopian crew used the cutout switches about 40 seconds after MCAS first activated. So about 7 times faster than the Lion Air crew. It still didn't save them. In fact it may have made things worse for them, because it appears that they could not use either the manual electric trim or the trim wheels after using the cutout switches.
I was trying to understand why the Ethiopian pilots have retracted the flaps, which allowed MCAS to activate. Looking at their FDR traces it seems to have happened when they were getting close to 250 knots. The VFE on the 737-800 is 250 KIAS at flaps 1, 2 and 5, and it's probably similar on the MAX. So it's not surprising they retracted them. About 20 seconds later, when the flaps completed retraction, they were over 250 knots. I wonder what would have happened if they didn't retract them and they got close to 340 knots. Would they fully retract automatically at some point to prevent damage to them?
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Originally Posted by Maninthebar 
This
Except I **believe** that s/he had already gotten the documentation out. It is interesting that the (altered) function of the cutout switches appears to have required clarification on this forum.
ThisExcept I **believe** that s/he had already gotten the documentation out. It is interesting that the (altered) function of the cutout switches appears to have required clarification on this forum.
We have been reliably informed that nobody was told about MCAS - but a pilot before any crashes not only had knowledge of procedure (cut out Stab Trim) which saved the aircraft; but also had documentation of the effect on MCAS in his carryon?
Ian, I don't believe that this is a claim that MCAS was in the documentation, the suggestion is that a) the difference in CUTOUT switches was and b) that it took another head in the cockpit to suggest trying the CUTOUT as if there was runaway trim. As the previous poster indicates, it took them a while to get there and they subsequently reported STS running in REVERSE (and not MCAS)
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No need to know about MCAS (why) to use the cutout switches to kill uncommanded trim (what).
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By the time of the ET flight it was known that autopilot was a protection against MCAS, from one of the ASRS reports:
It was my leg, normal Ops Brief, plus I briefed our concerns with the MAX issues, bulletin, MCAS, stab trim cutout response etc. I mentioned I would engage autopilot sooner than usual (I generally hand fly to at least above 10,000 ft.) to remove the possible MCAS threat.
It also appears to have pitched, and trimmed, them down when apparently set to climb, this I do not understand. I cannot find any autopilot-engage preconditions other than "no stick force", but if the autopilot actually cares about AOA why on earth would it engage at 74deg - if that is true there is no way you are flying. On the other hand if it didn't care about AOA what on earth was it doing with the pitch when set to climb? One of several WTFs on this one.
Autopilot pitching down unexpectedly was also a feature of some of the MAX ASRS reports, including the one I quoted - issues may or may not be related.
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I posted this yesterday morning, but I think it got buried awaiting moderator approval (hopefully I'll be off probation soon 
)
Procedurally, it does not matter what the switches do or how they are labeled. You could call the switches "Hank" and "Frank", and it does not matter. From Boeing's perspective, you don't have to know what these switches are connected to. Whether the NG or the MAX, you always cutoff BOTH switches when called for in the NNC.
It wasn't always so on the NG. A while back, the 737NG stab runaway procedure was changed so that BOTH cutout switches are always selected together, and we no longer try to isolate the offending circuit. I was kind of curious why the change, but I was simply told that Boeing thought this was a better to handle runaway trim. The conspiracy theorist in me now says that Boeing did this because they were looking down the road at the MAX certification and were looking for any opportunity to harmonize procedures.
)Procedurally, it does not matter what the switches do or how they are labeled. You could call the switches "Hank" and "Frank", and it does not matter. From Boeing's perspective, you don't have to know what these switches are connected to. Whether the NG or the MAX, you always cutoff BOTH switches when called for in the NNC.
It wasn't always so on the NG. A while back, the 737NG stab runaway procedure was changed so that BOTH cutout switches are always selected together, and we no longer try to isolate the offending circuit. I was kind of curious why the change, but I was simply told that Boeing thought this was a better to handle runaway trim. The conspiracy theorist in me now says that Boeing did this because they were looking down the road at the MAX certification and were looking for any opportunity to harmonize procedures.
Interesting info on the runaway procedure change, hard to see what advantage it would have other than keeping things simple at the cost of eliminating the possibility of using manual electrical trim.
And from a later post:
Am a 737 pilot.
Reason I was given was to provide redundancy in case of a rare instance of switch (or actually relay) welding. That phenomenon occurs when a relay remains in a set position for so long that it basically welds the contacts closed. I'm not an electrical engineer, so I don't know how to evaluate this statement, but it is not unlike the rationale for the split thumb switches on the yoke.
I suspect another unspoken reason was to maintain the look and feel of the 737NG switches. As long as there were two switches that were always used together, then Boeing could take the position that no additional training was required.
Reason I was given was to provide redundancy in case of a rare instance of switch (or actually relay) welding. That phenomenon occurs when a relay remains in a set position for so long that it basically welds the contacts closed. I'm not an electrical engineer, so I don't know how to evaluate this statement, but it is not unlike the rationale for the split thumb switches on the yoke.
I suspect another unspoken reason was to maintain the look and feel of the 737NG switches. As long as there were two switches that were always used together, then Boeing could take the position that no additional training was required.
Toggle switches are very unlikely to get stuck, especially when infrequently operated, the contacts are positively driven, not relying on a spring to open as in the case of a relay or momentary contact switches such as thumb switches on the yoke.
The previous (NG) switches had multiple paths to disable trim with the left disconnecting the main power to the motor via a (single) relay, while the right (autopilot) disabled inputs from the autopilot. The max has 2 switches in series, either of which kills the main power via the same single relay.
Last edited by MurphyWasRight; 16th Apr 2019 at 22:51. Reason: Added MAX single relay; time >trim
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I'm SLF but have enough of a technical background to add something here. My understanding is that the control column stab trim switches have two series contacts specifically to minimise the probability of single switch contact fault causing a stabilizer runaway. The first contact operates an arming relay regardless of the trim direction and the second contact provides the trim direction control. Signals from both contacts are required to be present to operate the stabilizer electrical trim. (the schematic diagram in #3882 shows this nicely). The arrangement is a sound and common principle which I think was derived from bitter experience.
As for the Stab Trim Cutout switches on the Max, if the schematic is correct then selecting the Main cutout switch to CUTOUT stops all electrical trim (including MCAS) by, amongst other things, de-energising the main trim control relay which cuts power to the stab actuator. The Backup switch is in series and since a series switch has never been needed on any previous 737 seems to be superfluous so I'd agree that Boeing put it there because there have always been two switches.
With both switches selected to CUTOUT as they seem to have been on ET302 then re-selecting the Backup switch to NORMAL would have no effect on anything. If the Main switch is then selected to NORMAL then both main electrical trim and MCAS would be active. If only the cutout switch nomenclature and not the new functionality on the Max was communicated by Boeing then the cockpit of ET302 must have been a bad place to be learning about it.
As for the Stab Trim Cutout switches on the Max, if the schematic is correct then selecting the Main cutout switch to CUTOUT stops all electrical trim (including MCAS) by, amongst other things, de-energising the main trim control relay which cuts power to the stab actuator. The Backup switch is in series and since a series switch has never been needed on any previous 737 seems to be superfluous so I'd agree that Boeing put it there because there have always been two switches.
With both switches selected to CUTOUT as they seem to have been on ET302 then re-selecting the Backup switch to NORMAL would have no effect on anything. If the Main switch is then selected to NORMAL then both main electrical trim and MCAS would be active. If only the cutout switch nomenclature and not the new functionality on the Max was communicated by Boeing then the cockpit of ET302 must have been a bad place to be learning about it.
Last edited by Europa01; 17th Apr 2019 at 20:25. Reason: correction single spelling error in first paragraph (volume - column)
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Ian, I don't believe that this is a claim that MCAS was in the documentation, the suggestion is that a) the difference in CUTOUT switches was and b) that it took another head in the cockpit to suggest trying the CUTOUT as if there was runaway trim. As the previous poster indicates, it took them a while to get there and they subsequently reported STS running in REVERSE (and not MCAS)
there's just so much mud flying around
it seems, a little clarity is very welcome 👍
Psychophysiological entity
Single relay!? That would seem to obviate the safety factor that the series switch modification gives. Well, at least in part. I would be horrified to learn there was truly only one point of cut off to that vital flying surface motor.
It wouldn't surprise me to learn it stayed on with a delayed opening, to avoid stop-starts as different inputs to the HStab came in quick succession.
The last relay I had fail was on a Mercedes. It left the secondary air fan on, which wrecked it as it was not continuously rated. When I finally found the darn thing the welded points took a lot of parting. In the car case the fan runs at start-up and shut down. So regular use. On the Boeing, I don't know what current that isolation relay would have to switch during (I presume) pre-flight checks. There was talk in this thread of the motor running - and the clutches choosing the driving logic. I have never found out just how long the H-Stab motor runs for during normal ops.
It wouldn't surprise me to learn it stayed on with a delayed opening, to avoid stop-starts as different inputs to the HStab came in quick succession.
The last relay I had fail was on a Mercedes. It left the secondary air fan on, which wrecked it as it was not continuously rated. When I finally found the darn thing the welded points took a lot of parting. In the car case the fan runs at start-up and shut down. So regular use. On the Boeing, I don't know what current that isolation relay would have to switch during (I presume) pre-flight checks. There was talk in this thread of the motor running - and the clutches choosing the driving logic. I have never found out just how long the H-Stab motor runs for during normal ops.
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Single relay!? That would seem to obviate the safety factor that the series switch modification gives. Well, at least in part. I would be horrified to learn there was truly only one point of cut off to that vital flying surface motor.
It wouldn't surprise me to learn it stayed on with a delayed opening, to avoid stop-starts as different inputs to the HStab came in quick succession.
The last relay I had fail was on a Mercedes. It left the secondary air fan on, which wrecked it as it was not continuously rated. When I finally found the darn thing the welded points took a lot of parting. In the car case the fan runs at start-up and shut down. So regular use. On the Boeing, I don't know what current that isolation relay would have to switch during (I presume) pre-flight checks. There was talk in this thread of the motor running - and the clutches choosing the driving logic. I have never found out just how long the H-Stab motor runs for during normal ops.
It wouldn't surprise me to learn it stayed on with a delayed opening, to avoid stop-starts as different inputs to the HStab came in quick succession.
The last relay I had fail was on a Mercedes. It left the secondary air fan on, which wrecked it as it was not continuously rated. When I finally found the darn thing the welded points took a lot of parting. In the car case the fan runs at start-up and shut down. So regular use. On the Boeing, I don't know what current that isolation relay would have to switch during (I presume) pre-flight checks. There was talk in this thread of the motor running - and the clutches choosing the driving logic. I have never found out just how long the H-Stab motor runs for during normal ops.
The NG diagram has less detail in the motor block.
This relay is controlled by the cutout switches but is normally on (energized).
There are also a number of 'enable' as well as direction input shown connected to the motor block, on the NG the autopilot signals are interrupted by the right hand switch. The left switch kills power to the power relay. On MAX either switch kills power (and control inputs)..
Note both of these are labelled "functional description" so don't include all details.
