Same 737 Max 8 model :-/
Nobody has any idea yet, but the grounding of MAX aircraft has started. Boeing postponed the commercial presentation of the 777X which is technically of course totally unrelated but not psychologically.
Hoppie
We need to get to the bottom of this, why both accidents have occurred. What was the cause? Was it preventable, rather than a major flaw with the airplane itself (that could open a new Pandora's Box for Boeing and regulators).
However, it is very intriguing that both, relatively very young air-frames, have crashed just a few minutes after the takeoff. I thought that after the reported issue with sensors and/or MCAS following the LionAir crash, Boeing/airlines/regulators would be more prudent in enlightening airlines/pilots/crews in differences between NGs and MAXs systems. I hope that lessons learned from both accidents, will benefit the flying public.
Call me crazy, but this is a first time in my life, that I question, whether to fly MAX or not. Is it paranoid or just precautionary feeling, I don't know.
On a side note, this crash is little more personal. My brother's former colleague, who became a public figure, has lost his wife, daughter, and son. My friend lost a friend on that flight. This is 4 people from Slovakia that have died. It is quite strange and somber feeling. Words can not express my grief for all souls lost, but this one is a little more "special". :(
In regards the postponement of the commercial presentation of the 777X it makes sense. The timing is not right now. Boeing needs to mitigate current risk, that is 737MAX. I was looking forward to this presentation though.
How reliable is the FlightRadar data? According to that the aircraft's vertical speed was very unstable (going up and down) while the speed keeps increasing up to about 380 knots. Isn't this indication at least somewhat similar to the Lion Air crash?
I wonder if they will ever introduce cameras into the cockpit. There have been accidents where it would show important evidence not recorded (or difficult to deduce) on the CVR or FDS.
One thing that comes from the flight radar tracking websites is that even though some airlines have grounded the 737 Max 8, there are still dozens in the air at any given time.
Quote from: kryten on Mon, 11 Mar 2019 17:47
I wonder if they will ever introduce cameras into the cockpit. There have been accidents where it would show important evidence not recorded (or difficult to deduce) on the CVR or FDS.
The pilot unions, at least stateside, will never allow that.
Quote from: ahaka on Mon, 11 Mar 2019 11:17
How reliable is the FlightRadar data?
FlightRadar and other tracking sites store and reproduce data received by a network of ground-based ADS-B IN receivers. Just as with personal weather stations, you can buy one of these and put it on your roof and feed into their network to get some perks. Enthusiasts like it. So the data is as reliable as what the ADS-B squitters spit out. If the data shows wild fluctuations, the aircraft spit out wildly fluctuating data.
The networks also have other data input sources, but the vast majority is ADS-B receivers.
Hoppie
https://flightaware.com/adsb/flightfeeder/
Quote from: Jeroen Hoppenbrouwers on Mon, 11 Mar 2019 07:49
Nobody has any idea yet, but the grounding of MAX aircraft has started. Boeing postponed the commercial presentation of the 777X which is technically of course totally unrelated but not psychologically.
Hoppie
Australia has just joined the list of countries banning the MAX from its airspace.
"This is a temporary suspension while we wait for more information to review the safety risks of continued operations of the Boeing 737 MAX to and from Australia." CASA chief executive and director of aviation safety, Shane Carmody, said.
"CASA regrets any inconvenience to passengers but believes it is important to always put safety first."
CASA's announcement comes after Singapore's aviation regulator today completely banned the use of the MAX aircraft in the country's airspace.
It joined China, Indonesia, South Korea and Mongolia in grounding the jets.
Ethiopian Airlines, Cayman Airways, Singapore Airlines' subsidiary SilkAir and carriers across China and Indonesia have grounded all MAX 8 planes in response to the fatal Ethiopian Airlines crash at the weekend.
Southwest, Air Canada and American Airlines are several carriers who continue to use the aircraft."Australian media story here:
https://www.news.com.au/travel/travel-updates/incidents/virgin-pilot-body-says-it-has-utmost-confidence-in-boeing-max-8/news-story/2680f4379969f1a73b5351f51f17c0e6
Quote from: Jeroen Hoppenbrouwers on Mon, 11 Mar 2019 23:18
Quote from: ahaka on Mon, 11 Mar 2019 11:17
How reliable is the FlightRadar data?
FlightRadar and other tracking sites store and reproduce data received by a network of ground-based ADS-B IN receivers. Just as with personal weather stations, you can buy one of these and put it on your roof and feed into their network to get some perks. Enthusiasts like it. So the data is as reliable as what the ADS-B squitters spit out. If the data shows wild fluctuations, the aircraft spit out wildly fluctuating data.
Thanks for the info. So I assume this data comes from the same source as the one recorded by the FDR, but perhaps at a slightly different rate (Hz). However if FlightRadar interpolates for gaps (missing data) then it might not be as accurate.
For the sake of completeness, note that e.g. FR24 also utilizes other methods to acquire data.
The two other important methods are MLAT (Multi Lateration), basically using many receivers and monitoring the time differences between arrival of Mode-S signals (at individual receivers) to interpolate a position, and, particularly in Germany/Europe, FLARM, which is a custom-made ADS-B system designed for gliders and light aircraft (taking into account e. g. circling in thermals for collision prediction + avoidance).
And of course, North Atlantic / Pacific data from the FAA (at least: used to).
Typically, the more modern and/or large airliners transmit ADS-B, older/smaller regional jets / turbo probs (or 737 classics, for that matter), are tracked via Mode-S MLAT, and some of the light aircraft via ADS-B or FLARM.
Markus
Quote from: ahaka on Tue, 12 Mar 2019 10:16
Thanks for the info. So I assume this data comes from the same source as the one recorded by the FDR, but perhaps at a slightly different rate (Hz). However if FlightRadar interpolates for gaps (missing data) then it might not be as accurate.
I am not sure how it does it, but FlightRadar does some weird tricks. If you look at the take off profile of this flight, it starts at 0 feet MSL. Then in the space of less then a minute, where you see the ground speed accelerating from zero upwards, it jumps from 0 to 8000 feet. I believe the airport elevation was somewhere around 7200 feet. Somehow during the take off run, flightradar records 0 feet and then jumps up to a more realistic altitude.
No idea what the sampling time is and how it deals with connecting the dots.
Jeroen
https://www.ndtv.com/india-news/boeing-737-max-aircraft-grounded-by-civil-aviation-watchdog-dgca-2006656?pfrom=home-topscroll
The item I find most unnerving about this at the moment is Boeing's announcement they are preparing a modification of the MCAS software. Given the uncertainty about what role the current MCAS has played in these crashes, I find it hard to believe that they could test and verify new flight software adequately, while the investigation is just starting.
Cheers
Torrence
My theory: already before the Lionair crash, somebody pointed at MCAS as working fine unless one of the sensors failed, after which MCAS would not have the logic to quit. Much like the single faulty radio altimeter that made Turkish flare in midair near Schiphol. This is not a software fault, as it works perfectly as designed, but a design safety flaw that should have been caught by the system safety assessment. It's possible that this was considered a point of improvement and work had been started. Lionair may have pushed it forward, but such a change in a safety-critical system isn't easy to hasten. Ethiopian most definitely did not have an effect, it's way too recent.
Still all my theory. I'm not at all even near this system.
Hoppie
Edit: it seems that this point of work start was August 2018, after reading more stuff down below.
Quote from: Jeroen Hoppenbrouwers on Tue, 12 Mar 2019 23:44
My theory: already before the Lionair crash, somebody pointed at MCAS as working fine unless one of the sensors failed, after which MCAS would not have the logic to quit. Much like the single faulty radio altimeter that made Turkish flare in midair near Schiphol. This is not a software fault, as it works perfectly as designed, but a design safety flaw that should have been caught by the system safety assessment. It's possible that this was considered a point of improvement and work had been started. Lionair may have pushed it forward, but such a change in a safety-critical system isn't easy to hasten. Ethiopian most definitely did not have an effect, it's way too recent.
Still all my theory. I'm not at all even near this system.
Hoppie
+1.
It **SEEMS** to have NO redundancy, NO cross-checking of data, NO ability to fault, and instead, just works regardless if it is correct or not.
Someone said something VERY interesting this evening about MCAS: it adjusts the slats to maintain a constant pitch attitude to minimize drag. This is a very curious comment.
QuoteSomeone said something VERY interesting this evening about MCAS: it adjusts the slats to maintain a constant pitch attitude to minimize drag. This is a very curious comment.
I think that
someone is confusing MCAS with something else.
From PPRuNe:
QuoteWhile there is not yet sufficient information to draw a clear link between the ET accident that is the subject of this thread and MCAS I am deeply troubled by the amount of misinformation regarding MCAS that is being spread here. Reluctantly I offer the explanation below without any suggestion that this system contributed to the tragedy in Ethiopia this last weekend. The truth will be revealed by the recorder data and the full investigation. I strongly implore those who do not know MCAS details to stop providing incorrect information here or anywhere else. Posing questions is fine, but please do not state as fact that about which you are not sufficiently knowledgeable.
MCAS Operation Clarification
MCAS is triggered when all of the following are true:
A. Sensed AOA exceeds a flight condition based activation threshold
B. Flaps are fully retracted (i.e., up)
C. Autopilot is not engaged
When triggered, MCAS commands nose down stabilizer as a function of how much AOA has exceeded the activation threshold and the current Mach number. For large exceedence of the MCAS activation AOA threshold, MCAS will command 2.5 degrees of stabilizer at low Mach number but less than 1/3rd of that at cruise Mach number (gradual Mach number based schedule between). For a lesser exceedence of the MCAS activation AOA threshold the size of the stabilizer increment will be proportionally less. MCAS stabilizer command will be stopped immediately upon pilot activation of pitch trim. (Pilot trim input also serves as MCAS reset - see next paragraph.)
Once MCAS has commanded one increment of stabilizer motion, it will not command more until it has been reset. MCAS is reset if any of the following occur:
1. Pilot makes a manual trim command. (MCAS will not re-activate until there have been 5 continuous seconds without pilot trim command.)
2. AOA drops below MCAS activation threshold and MCAS has run stabilizer in the airplane nose up direction taking out the increment of airplane nose down command it inserted earlier.
3. Autopilot is engaged and then disengaged.
Without pilot trim input, MCAS will not run the stab more than one increment (up to 2.5 degrees) unless MCAS is reset via either 2 or 3 above.
Talk of MCAS running the stabilizer for 10 seconds, pausing for 5 seconds, and then running it again repeatedly without pilot trim input are patently incorrect.
I would have been surprised if such a system existed.
I was very surprised at the very specific suggestion due to the apparently shifted CoG due to the engines being further forward than normal.
Using an aerodynamic approach rather than adding ballast was at the very least intriguing.
I was also considering the fact that both accidents occurred shortly after takeoff, where configuration changes involving the slats seems reasonable, changing AoA and possibly upsetting the MCAS.
Quote from: United744 on Wed, 13 Mar 2019 11:39
I was very surprised at the very specific suggestion due to the apparently shifted CoG due to the engines being further forward than normal.
My understanding is that there is no CG shift on the 737 MAX even though these big pods sit more forward. I rather think the pitch momentum is greater because the engines are
higher. The "pendulum" is shorter.
Imagine a clock with a pendulum that is 2 feet long.
Move its lower tip sideways by 1 inch. The pendulum will turn by an angle of one degree or so.
Imagine a clock with a pendulum that is 1 inch long.
Move its lower tip sideways by 1 inch. The pendulum will turn by more than 30 degrees.
The shorter the pendulum arm, the greater the rotation when the tip is moved sideways by 1 inch.
The motion sideways in this illustration is analog to the thrust vector. The pendulum length is analog to the distance between wing and engine. And the pendulum angle is analog to the aircraft pitch momentum.
The engines sit higher, the pendulum is shorter, the effect of a thrust increase on the nose-up momentum is greater. So the stab-trim-nose-down feature will compensate it.
If the aircraft was nose heavy, why should MCAS make it even more nose heavy?
I might be wrong here because I do not have all the necessary knowledge, but here is what I understood from various articles about the Indonesia accident.
In order to fit these new big engines on the 737, which was never designed for those, they had indeed to move them to a rather awkward position... Which moved, the CG, but also the center of thrust.
I read a few topics here about the speed trim system on the 747. Actually, if I am correct, the 737NG also has a speed trim system, to correct minor deficiencies of the aerodynamics of the aircraft, as for the 747 and this is not a problem. MCAS is something different (although the MCAS acronym might include the speed trim functions on the MAX, I don't know that for sure).
From what I got, the MCAS system is there because they feared that if the pilot found itself at high AOA, and suddenly increased thrust to get out of this situation, the forward center of thrust could generate of big nose up momentum which would in turn amplify the high AOA situation. MCAS was supposed to alleviate this.
I feel Boeing was a bit on the edge here as it looks like an economic decision. After deciding not to launch a replacement to the 737, Airbus announced the NEO which sold very well. So Boeing was a bit forced to react and launched the MAX. Unfortunately, the 737 is despite all its qualities an old design, and it feels they found themselves with a problem rather difficult to solve, and did it the quick and dirty way.
This is just my personal understanding of it. I am by no means an Airbus fanboy.
Charles
Has anyone determined conclusively whether the A/P was in use during either of these crashes?
This resourceful website provides interesting information on the MCAS.
http://www.b737.org.uk/mcas.htm (http://www.b737.org.uk/mcas.htm)
Cheers,
Thanks for this link.
So it would seem it is an aerodynamic effect from the nacelle that would be causing concern and not the center of trhust.
Nevertheless, I have the feeling that had they designed a new airplane, this would have been a no go.
But as they found themselves "forced" to update the 737 with the latest generation of engines, they validated this bad aerodynamic configuration and put a bandaid on it.
When you see what they are doing with the landing gear of the MAX10, it looks kind of the same.
They are once again "forced" by the market (meaning the A321NEO LR which is selling very well) to make a bigger 737 to compete. But once again, the aircraft was never designed for such a length and they are forced to come with "strange" solutions to say the least.
Charles
Quote from: emerydc8 on Wed, 13 Mar 2019 14:04
Has anyone determined conclusively whether the A/P was in use during either of these crashes?
I haven't yet - and I was looking for some information on the AP since, as I understand the checklist for Unscheduled Stab Trim (at least on the 747), it includes turning off the autopilot as well as setting the cutout switches to Cutout. I assume the autopilot state for both MAX accidents may be one of the things they may get from the black boxes.
Torrence
If I am putting this together correctly, the Lion Air MAX8 had the MCAS activate because a faulty sensor gave incorrect information about the angle of attack.
Do we have any evidence that something similar happened (faulty AOA information leading MCAS to trim nose-down) in the Ethiopian MAX8?
There hasn't been any report I've seen of what the AOA was doing in the Ethiopian case. As far as I can see from published reports the strongest suggestion that something similar was going on is the similarities of the records of altitude variations.
US as well now.....
https://edition.cnn.com/world/live-news/boeing-737-max-8-ethiopia-airlines-crash/index.html
fc
Now we'll see how political Boeing, the FAA and NTSB are. It has been weeks since the Atlas crash and we haven't really heard jack squat even though they have the recorders. Watch how fast they figure out what happened on the Ethiopian crash and come up with a fix for it, since there's big money at stake. Remember this when you hear them tell you it takes a long time for the investigators to put all the pieces together and we have to be patient, like has been said of the Atlas crash.
This sounds pretty bad ....
(taken from CNN)
Speaking with reporters on a conference call, acting FAA Administrator Daniel Elwell said the grounding of the 737 Max 8 and 9 will remain in effect pending new information including from the flight data recorder and voice recorder.
"Since this accident occurred we were resolute that we would not take action until we had data," Elwell said.
"That data coalesced today."
Elwell said the new data was "added fidelity -- missing pieces that we did not have prior to today." It aligned the Ethiopian flight data to the Lion Air incident.
Andrea
Re: Lion Air and Ethiopian Air 737 MAX crashes
Tragic as the circumstances are, I'm beginning to think the investigations of these incidents may mark an important watershed moment in crash investigations – driven primarily by the rapid availability of key flight data in near real time from ADS-B and related capabilities. Hoppie and others have been discussing this on the Forum for some time. Although not yet all-flight-data-in the cloud-all the time, the current capabilities (i.e. Flightradar24 and similar) are a long way from waiting days/weeks for black box recovery, more weeks/months for transport to specialized facilities for forensic analysis, more months/years for 'final' accident reports. Although not definitive by themselves these data showed alarming similarities and prompted (finally) the US to ground the MAX aircraft temporarily. Personally, these groundings may inconvenience me for this weekend's upcoming flight to a science conference, but I'll put up with it happily if it means lower probability for further loses (the old nautical 'souls on board' for the passenger count should be remembered - and yes, 3 SOB for the Atlas accident is just as tragic).
Torrence
Quotedriven primarily by the rapid availability of key flight data in near real time from ADS-B and related capabilities.
It's good if we can agree on the data that is being sent to us by ADS-B. Do all flight tracking programs show the same data? With the old ATC systems, the transponders sent altitude data based on standard baro (1013mb), not corrected (local) baro, and the ATC centres applied corrections as necessary.
Do ADS-B flight tracking programs use IRU/GPS groundspeed (not computed airspeed)? Do flight tracking programs use corrected baro? All programs interpolate data, so the data displayed is not always accurate.
Wouldn't the MCAS system be a sub-part of the larger SMYD - stall management yaw damper system ?
I haven't seen any info on how this is hooked up internally - typically the AOA sensors feed directly to the SMYD computers.... edit: perhaps they feed multiple systems, adc etc....
The 737 appears to have an Elevator Feel Shift Module, which increases the stick force in a stall.. I don't think the 744 has this device.....
http://www.737ng.co.uk/B_NG-Flight_Controls.pdf
QuoteStall Identification
Stall identification and control is enhanced by the yaw damper, the Elevator Feel Shift (EFS) module and the speed trim system. These three systems work together to help the pilot identify and prevent further movement into a stall condition.
During high AOA operations, the SMYD reduces yaw damper commanded rudder movement.
The EFS module increases hydraulic system A pressure to the elevator feel and centering unit during a stall. This increases forward control column force to approximately four times normal feel pressure. The EFS module is armed whenever an inhibit condition is not present. Inhibit conditions are: on the ground, radio altitude less than 100 feet and autopilot engaged. However, if EFS is active when descending through 100 feet RA, it remains active until AOA is reduced below approximately stickshaker threshold. There are no flight deck indications that the system is properly armed or activated.
As airspeed decreases towards stall speed, the speed trim system trims the stabilizer nose down and enables trim above stickshaker AOA. With this trim schedule the pilot must pull more aft column to stall the airplane. With the column aft, the amount of column force increase with the onset of EFS module is more pronounced.
fc
This sounds like a very complicated version of the 767 "nudger." Boeing seems to be at a loss for words when explaining the details of its operation in the FCOM. I have never seen a demonstration as to how it works in the sim. I doubt it's even programmed into the sim. Maybe it's just on a "need-to-know" basis and I guess the flight crews don't need to know. No doubt a certification Band-Aid.
(http://www.hoppie.nl/forum/var/nudger(1).jpg)
This may be unrelated to the tragic incidents with Ethiopian & Lion Air, however a recent media report in Australia made these claims regarding the MAX:
"It comes hours after at least four pilots made reports following the October crash of a Lion Air flight in Indonesia shortly after takeoff, all complaining that the aircraft suddenly pitched downward, according to documents reviewed by AFP on a flight safety database. The incidents seem to involve the flight stabilisation system designed to prevent the aircraft from stalling, the "MCAS," which was implicated in the fatal accident in the Lion Air crash that killed 189 people shortly after takeoff.
One pilot logged an incident in November 2018, just weeks after the Lion Air crash, saying the plane "pitched nose down" two to three seconds after engaging the autopilot following takeoff, according to the report on the Aviation Safety Reporting System, maintained by NASA.
"The captain immediately disconnected the autopilot and pitched into a climb," the report said. "The rest of the flight was uneventful." The report said the flight crew reviewed the incident "at length ... but can't think of any reason the aircraft would pitch nose-down so aggressively." Another pilot on a flight in November said the crew discussed the concerns about the aircraft and "I mentioned I would engage the autopilot sooner than usual." But again once engaged, there was a quick automated warning of "DONT SINK DONT SINK!"
"I immediately disconnected the AP (autopilot) ... and resumed climb," the officer said. But after review, "frankly neither of us could find an inappropriate set up error."
Source (may be unreliable): https://www.news.com.au/travel/travel-updates/incidents/us-resists-pressure-to-ground-boeing-planes-despite-canada-banning-planes-pilots-concerns/news-story/734948a8165a4af53d4c6053f4b55bea
If there are two main factors -- AOA sensor system and MCAS software -- I ask myself which one is more likely to be the cause of the problem in all this similar incidents and accidents?
If 5000 737s have no problems with this AOA sensor system, and 50 737s fitted with a special software have a problem, then common sense tells me the problem probably lies in the software rather than in the AOA sensor. Especially if the software operates when it shouldn't (A/P engaged).
Quote from: Steve Hose on Thu, 14 Mar 2019 07:45
This may be unrelated to the tragic incidents with Ethiopian & Lion Air, however a recent media report in Australia made these claims regarding the MAX:
[...][¡]Another pilot on a flight in November said the crew discussed the concerns about the aircraft and "I mentioned I would engage the autopilot sooner than usual." But again once engaged, there was a quick automated warning of "DONT SINK DONT SINK!"
"I immediately disconnected the AP (autopilot) ... and resumed climb," the officer said. But after review, "frankly neither of us could find an inappropriate set up error."[/i]
The aural ,,DON'T SINK" alert also implies that the flaps were out of up in this case (immediately after takeoff). That's also a condition where the MCAS should not be active. It is either a different issue with the automatic pitch trim or another (poorly documented) protection system chimed in.
It is sad to read what Boeing has done with their incredible reliable 737 product line.
Dirk
I think one of the problems lies in the fact that only one AOA is used [EDIT: at a time] in the MCAS system. There are no comparisons/mixing/etc.
AOA sensors don't break very often (other than by physical damage from birds, ground equipment, etc). A bent AOA may cause an airspeed disagree. Airspeed disagrees can be resolved by the crew. MCAS does what it wants to do based on one AOA position.
I thought I read somewhere that the engineers didn't calibrate the AOA properly prior to the accident, but I can't find any calibration procedures in the AMM (for the standard NG)
QuoteIt is sad to read what Boeing has done with their incredible reliable 737 product line.
I think the 737 has had its fair share of dramas ... rudder hardovers, fire systems crosswired, primitive fault annunciation systems (on the earlier aircraft), etc, causing quite a few catastropic incidents.
Possible ?
I mean..if reality, then it's a shame....
https://www.businessinsider.com/boeing-ethiopian-crash-government-shutdown-delayed-plane-software-fix-2019-3?r=US&IR=T
according to this:
http://www.b737.org.uk/mcas.htm
QuoteThe AoA source
I had assumed that the AoA source for MCAS was always the Captains AoA probe but the following explanation from an engineer suggests that it alternates between AoA probes each flight:
MCAS is implemented within the two Flight Control Computers (FCCs). The Left FCC uses the Left AOA sensor for MCAS and the Right FCC uses the Right AOA sensor for MCAS. Only one FCC operates at a time to provide MCAS commands. With electrical power to the FCCs maintained, the unit that provides MCAS changes between flights. In this manner, the AOA sensor that is used for MCAS changes with each flight.
so i guess only one sensor reading at a time....
fc
Worth a subscription
https://youtu.be/AgkmJ1U2M_Q
Interesting article.
https://www.washingtonpost.com/investigations/how-the-faa-allows-jetmakers-to-self-certify-that-planes-meet-us-safety-requirements/2019/03/15/96d24d4a-46e6-11e9-90f0-0ccfeec87a61_story.html?noredirect=on&utm_term=.e5c696b49450
Regards,
Mariano
I found this NY Times article on the 737 MAX problems to be one of the more informative general news media articles on this set of issues.
https://www.nytimes.com/2019/03/16/business/boeing-max-flight-simulator-ethiopia-lion-air.html
Quote from: Hardy Heinlin on Wed, 13 Mar 2019 12:44
My understanding is that there is no CG shift on the 737 MAX even though these big pods sit more forward. I rather think the pitch momentum is greater because the engines are higher. The "pendulum" is shorter.
Maybe this picture explains it better:
(http://aerowinx.com/downloads/hhPod01.jpg)
Quote from: Hardy Heinlin on Tue, 19 Mar 2019 21:15
Quote from: Hardy Heinlin on Wed, 13 Mar 2019 12:44
My understanding is that there is no CG shift on the 737 MAX even though these big pods sit more forward. I rather think the pitch momentum is greater because the engines are higher. The "pendulum" is shorter.
Maybe this picture explains it better:
I see what you are saying, Hardy - but don't forget that with a shorter arm the force required to achieve the same angular displacement will be much higher (imagine the difference between tightening a nut with a spanner that is, say, 5cm long and one that is 10 or 20cm long).
In any event, the requirement for MCAS as I understand it is aerodynamic rather than thrust couple related: however it is the fact that the nacelles are further
forward of the CG which gives rise to the issue (at high AoA the lift created by the nacelle has a longer arm to work with ahead of the CG and this gives a greater pitch-up effect).
This is incompatible with the longitudinal static stability requirements in FAR25 which requires stick forces to increase at a given rate as airspeed is reduced away from the trimmed airspeed. Hence MCAS - in the problem region some nose down trim is introduced, the pilot has to pull back harder to maintain or reduce airspeed and everyone is happy, until the AoA data used to compute the MCAS demand is faulty...
Quote from: skelsey on Tue, 19 Mar 2019 23:38
... don't forget that with a shorter arm the force required to achieve the same angular displacement will be much higher ...
Yes, I take this factor into account. But I have difficulties to believe that this factor will eliminate the angular force amplification by 100%? The thrust vector is much greater than the aerodynamic and inertia forces of the counter-acting aircraft structure. This engine doesn't push a drill through granite; it's a rocket that pushes a tin can through the air ...
|-|ardy
QuoteYes, I take this factor into account. But I have difficulties to believe that this factor will eliminate the angular force amplification by 100%?
I think it would be hard to analyse unless we knew the height changes and thrust changes (between different engines).
Does engine
weight and position at different body angles factor into the equation?
I haven't yet understood the effect of the more forward located engine, but if that also shifts the average CG more forward, I understand that this aircraft provides less elevator maneuverability just by that forward CG alone, no matter whether it comes from the engine weight or cockpit weight.
It's my interpretation from various sources that:
.) the lower arm ( due to higher engine "center of force" ) would actually contribute to a lower pitching moment but....
.) the nacelle design itself made it more "aerodynamic" and lift generating than the older nacelles and...
.) the shift forward of the "center of force" ( because AFAIK the engine was also shifted fwd ) all contribute to
the increased pitching moment when power is increased from a steady state situation.
Quote from: jcomm on Wed, 20 Mar 2019 07:51.) the shift forward of the "center of force" ( because AFAIK the engine was also shifted fwd ) all contribute to the increased pitching moment when power is increased from a steady state situation.
That may or may not be the case, but either way it is not the reason for MCAS -- it is NOT a thrust coupling issue. The FAR25 longitudinal stability tests are done at
constant thrust, so what happens to the nose when thrust is increased is irrelevant.
All aircraft with underslung engines will exhibit some degree of pitch up moment when thrust is applied; this is not unique to the Max.
Quote from: skelsey on Wed, 20 Mar 2019 08:20
Quote from: jcomm on Wed, 20 Mar 2019 07:51.) the shift forward of the "center of force" ( because AFAIK the engine was also shifted fwd ) all contribute to the increased pitching moment when power is increased from a steady state situation.
That may or may not be the case, but either way it is not the reason for MCAS -- it is NOT a thrust coupling issue. The FAR25 longitudinal stability tests are done at constant thrust, so what happens to the nose when thrust is increased is irrelevant.
All aircraft with underslung engines will exhibit some degree of pitch up moment when thrust is applied; this is not unique to the Max.
Not saying it's unique to the MAX, of course, just saying it's potentiated in the MAX due to various factors that can aggravate the pitching moments due to thrust...
Quote from: skelsey on Tue, 19 Mar 2019 23:38
In any event, the requirement for MCAS as I understand it is aerodynamic rather than thrust couple related: however it is the fact that the nacelles are further forward of the CG which gives rise to the issue (at high AoA the lift created by the nacelle has a longer arm to work with ahead of the CG and this gives a greater pitch-up effect).
Is this because the thrust vector (turbine axis) is also slightly pitched up in relation to the fuselage axis?
If it were exactly parallel to the fuselage, I wouldn't understand why the more forward position would have a greater pitch-up effect.
Edit: Of course, when the aircraft AOA is high, the engine AOA is high as well. But that engine AOA doesn't change just by shifting the engine location more forward. I mean geometrically.
I imagine an engine with zero weight located 3 miles in front of the wings. If the thrust vector is parallel, it won't pitch up the aircraft more than it would when installed just 1 mile away. But if there is a vertical force component, it will. The longer the arm, e.g. 5 miles, the more easily it can work.
|-|ardy
I believe the problem is due to the somewhat larger inlet cowls on the new gen-X engines. There will be more drag and nose up lift at higher angles of attack (as there would be close to stall speed)
Further very informative update .....
https://youtu.be/9Ts_AjU89Qk
Quote from: Hardy Heinlin on Wed, 20 Mar 2019 14:24
If it were exactly parallel to the fuselage, I wouldn't understand why the more forward position would have a greater pitch-up effect.
Edit: Of course, when the aircraft AOA is high, the engine AOA is high as well. But that engine AOA doesn't change just by shifting the engine location more forward. I mean geometrically.
I imagine an engine with zero weight located 3 miles in front of the wings. If the thrust vector is parallel, it won't pitch up the aircraft more than it would when installed just 1 mile away. But if there is a vertical force component, it will. The longer the arm, e.g. 5 miles, the more easily it can work.
|-|ardy
I'm not sure I follow, Hardy; the pitching moment caused by thrust is a product of the thrust force x the vertical distance from the CG.
When the aircraft is trimmed for level flight at a given thrust setting, there will be X Nm of nose-up torque which is counteracted by the sum of the other nose-down moments on the aircraft to give 0 resultant nose-up moment.
If the AoA is then increased (classically, by pitching the nose up and reducing airspeed) and all other factors remain unchanged then the nose-up moment due to thrust is also unchanged -- the thrust force has not changed, the thrust line remains in the same place relative to the aircraft's longitudinal axis, the CG of the aircraft has not changed, therefore the vertical distance from the thrust line to the aircraft CG has not changed either, therefore the pitching moment due to thrust must also be unchanged.
It is the lift from the nacelles (not the thrust couple) which is the factor; in the case of the Max we have a different nacelle design (which in itself may produce greater lift at certain higher AoAs) and, crucially, the nacelles are located further forward of the CG than in previous variants.
This is significant because the pitch-up moment due to lift from the nacelles is a product of the nacelle lift force and the
horizontal distance between the nacelles' CofP and the aircraft CofG.
Greater horizontal distance from nacelle CofP to aircraft CofG = larger pitching moment.
(https://www.dropbox.com/s/nfe32aj28i03mcn/737ng_thrustcouple.PNG?raw=1)
(https://www.dropbox.com/s/sewo2yfeo8jfh4o/737max_thrustcouple.PNG?raw=1)
Ah, you mean the nacelle body alone, its pure aerodynamic function as part of the wing surface, similar to that of a leading edge flap, in the sense that the larger nacelle area and its more forward location increases the overall "wing surface" at the leading edge side?
So the effect also occurs when the engines are shutdown?
I was misunderstanding you in the discussion about "thrust increase" and "thrust couple". I did understand that you don't mean the "thrust increase" momentum. But I didn't get that you exclude the "constant thrust" idea as well.
QuoteIf I am putting this together correctly, the Lion Air MAX8 had the MCAS activate because a faulty sensor gave incorrect information about the angle of attack. Do we have any evidence that something similar happened (faulty AOA information leading MCAS to trim nose-down) in the Ethiopian MAX8?
Well, it looks like we have that evidence now. Reuters and the Wall Street Journal are reporting that the Ethiopian 737MAX was also indicating a stall in the final moments.
It sounds like both flights may have succumbed to a very aggressive MCAS action in the setting of faulty AoA data.
https://www.pprune.org/10433110-post179.html
Interesting.... MCAS overrides the stab trim column cutout switches. Another line of defence removed from the 737Max.
Quote from: John H Watson on Sat, 30 Mar 2019 00:11
https://www.pprune.org/10433110-post179.html
Interesting.... MCAS overrides the stab trim column cutout switches. Another line of defence removed from the 737Max.
I KNEW IT!! Boeing are in so much s**t.
I've been saying this from day 1, but people rubbished me...
Quote from: John H Watson on Sat, 30 Mar 2019 00:11
https://www.pprune.org/10433110-post179.html
Interesting.... MCAS overrides the stab trim column cutout switches. Another line of defence removed from the 737Max.
Yes and no.
Have to be careful about the nomenclature here. MCAS does bypass the
control column cutouts (that stop you trimming opposite column movement) because that is the whole raison d'etre of MCAS - to provide a nose-down force to pull against. It couldn't do that if it were inhibited from trimming nose down when you were pulling back, so for it to work it
has to bypass the column switches.
As far as I have seen, however, it does
not bypass the cutout switches on the
pedestal.
New rumours about the Ethiopian crew. Apparently they initially used the cutout switches as per procedure and then, for some reason, reactivated the electric trim system which also reactivated MCAS and drove them into the ground. It is not yet clear why the reactivated. You can manually wind the tailplane pitch up and down on the 737. They did not need the electrical trim.
Hoppie
Possibly with a significant nose-down trim and running out of altitude they felt they would not able wind the trim off manually fast enough so in desperation put the elec system back in. Anyone who has seen those trim wheels on the 737 fly around when electrically operated would appreciate there is quite a speed difference between elec/manual operation.
I wonder how many turns of the trim wheel it would take to return from a full-nose down to a neutral trim position??
<<Edit>> A couple of internet sources quote 250 to 268 manual turns (on the 737-700) for full range trim movement?? Surely not!!. Even a 10th of that would be incredibly difficult to perform in such dire circumstances.
Anyone know why the trim wheel in 737 was designed to have such a wide range of movement? Would it be too sensitive otherwise?
QuoteAnyone know why the trim wheel in 737 was designed to have such a wide range of movement?
It's a purely mechanical system. It's a small wheel driving a large control surface which will probably have heavy airloads on it. You need sufficient mechanical advantage. On the ground, the torque required to turn the wheel is somewhere between 22 and 62pound-inches (2.5 and 7 newton-meters). In the air, the crew may have to use excess elevator to unload the mechanism.
Quote from: ahaka on Thu, 4 Apr 2019 05:24
Anyone know why the trim wheel in 737 was designed to have such a wide range of movement? Would it be too sensitive otherwise?
Gearing to make it physically possible to move the stab I would imagine - I believe the 737 trim wheels literally move the stab directly via cables and pullies (not via hydraulics) so I imagine there would need to be a substantial ratio reduction to allow the aerodynamic forces on the stab to be overcome by hand.
<Edit: I see John has beaten me to it! >
I recently read about 40 turns for the full range. Still a lot. That is why there is a flip-out handling knob on the trim wheels, like some truck drivers have on their steering wheel.
(https://qph2.c7.quoracdn.net/main-qimg-7efd9e28308cc52c39e2ead71d7987eb)
Is the 737 the only airliner produced today that uses this kind of mechanical trim backup? In 777 the trim lever is not mechnically operated I think. And why doesn't the 744 have any? Elevators are powerful enough to fly the plane even if the trim is in extreme position?
737 is the oldest... cheapest... simplest...
Prelimanary report
http://www.ecaa.gov.et/documents/20435/0/Preliminary+Report+B737-800MAX+%2C%28ET-AVJ%29.pdf/4c65422d-5e4f-4689-9c58-d7af1ee17f3e
I'm reading about the Ethiopian report saying the pilots followed Boeing's procedures. Presumably, that refers to the pilots using the stab cutout switches.
I doubt Boeing's procedures say to re-engage them after suspected runaway trim. Right?
What is taught about stab cutout switches? Are they like circuit breakers, where you can try once to re-engage them? Or is the teaching that once you've identified a problem that needs cutout switches, it's best to never turn them on again? Or is it left up to the pilots?
Quote from: Will on Thu, 4 Apr 2019 21:02
I'm reading about the Ethiopian report saying the pilots followed Boeing's procedures. Presumably, that refers to the pilots using the stab cutout switches.
I doubt Boeing's procedures say to re-engage them after suspected runaway trim. Right?
Correct...
...BUT I've not seen it stated, definitively, that the switches were re-engaged at any point.
The report is explicit that the switches were placed to cutout. But it doesn't say anywhere that they were returned to normal. People have inferred that, but I wouldn't say it is possible to consider that to be a fact yet.
Still -- ultimately if the aircraft was uncontrollable in pitch and it was not possible to move the trim by hand (which is inferred from the FO's responses) what do you do -- fly the aircraft in to the ground with the switches in cutout because that's the SOP or attempt to use the electric trim (almost certainly having a good idea that the issue is MCAS and not the stab trim itself running away per se) to recover the aircraft?
Damned if you do, damned if you don't...
This is the most worrying part:
QuoteAt 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.
What exactly did he try? Did he really attempt to wind the mechanical wheel? If so, @#%$@%#$@. But what is the chance that, as a low-hour FO, he never before wound the mechanical wheel and grossly underestimated the number of turns required to get there? Would the airloads on the stabilizer already be so high (with the increasing forward airspeed) that manual winding was as good as impossible?
If the times above are correct, the FO tried for about four seconds to manually trim. Only a fully jammed wheel would cause this near-immediate response that it is not working, I believe.
All the rest, though alarming, is exactly what you expect when your left AoA goes haywire. Their decision to cut the stab trim was timely and right on. The missing bit is why the manual winding up apparently didn't work. And it is such a pity that they did not keep trimming up electrically -- MCAS clearly waited 5 full seconds every time before undoing the nose-up. They could have won, by simply persisting. Up... up... up... up... would have stopped MCAS logic. We saw the same pattern with LionAir. MCAS won because the pilots stopped. The big question is why they stopped.
Not much assurance to clear the MAX for flight, yet...
Hoppie
Why should a higher airspeed make the manual stabilizer rotation impossible? Isn't the stabilizer rotational axis more or less in the middle between the leading edge and the trailing edge? One half of the stabilizer surface rotates into the ram air, and the other half on the opposite side of the axis rotates with the ram air. If the ram air force ratio is 50:50, the ram air cannot rotate the stabilizer surface. Maybe it's not exactly 50:50. But the axis is certainly not in the leading edge, is it?
|-|ardy
The hinge is about 3/5's back from the leading edge, just forward of the elevator hinge line.
However, the horizontal stabiliser is swept back. I'm not sure where the centre of pressure would be.
737NG Stabiliser (http://www.members.iinet.net.au/~b744er@ozemail.com.au/744er/737NGStabilizer.gif)
I don't know if the surface is a negative airfoil like the 744.
QuoteThe report is explicit that the switches were placed to cutout. But it doesn't say anywhere that they were returned to normal. People have inferred that, but I wouldn't say it is possible to consider that to be a fact yet.
QuoteAt 05:43:11, about 32 seconds before the end of the recording, at approximately 13,402ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU [airplane nose up] direction from 2.1 units to 2.3 units.
Strange. This suggests the cutout switches were
returned to the non-cutout position.
QuoteAt 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 [airplane nose down] direction from 2.3 to 1.0 unit in approximately 5 seconds.
This sounds like the MCAS system acting again. This is a second indication that the cutout switches were in the non-cutout position.
QuoteThe 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.
Is the 737 the only Boeing type whose elevators cannot maintain level flight when the stabilizer is completely mistrimmed?
QuoteAt 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
from https://avherald.com/h?article=4c534c4a/0022&opt=0
There's some ref to Master Caution Anti-Ice going off as well...
and
QuoteThe last recorded pressure altitude was 5,419 ft on the left and 8,399 ft on the right.
thats a big diff between L & R
fc
QuoteThere's some ref to Master Caution Anti-Ice going off as well...
... due to a Left AOA sensor malfunction. I'm not sure about the Max, but the standard 737NG AOA sensors have integral heaters for both the vanes and the cases (and the heater system has separate wiring and electrical plugs to the angle measuring system).
It sounds like the AOA was physically damaged in some way. There were theories about birdstrikes, but the CVR records didn't detect the sound of a birdstrike or comments on one.
Quotethats a big diff between L & R
Seems a lot. Do the AOA's really affect the pitot-static system that much?
Wild speculation: no, they don't, but the existing air data software (not MCAS) does not stop taking garbage when the AoA goes to extreme values not obtainable in flight and keeps correcting.
If one AoA measures 75 degrees, which is complete nonsense, and the software dutifully calculates what the corrected static pressure would be given the airspeed measured at that angle of attack, it is possible you do get very silly things. This is pure speculation, as static pressure is not intuitively dependent on AoA, but once computers get stupid data and don't throw in the towel by themselves, all bets are off.
Hoppie
From AV Herald:
Quote
On Apr 4th 2019 The Aviation Herald received a video to demonstrate, how long it takes to trim from full nose down (trim position 0 units) to a normal trim setting (around about 5 units) via manual trim: the captain needed about 30 seconds to get the trim back into normal range and reported being tired afterwards, the first officer tried and failed needing one minute for one unit and then being exhausted. The source added: "This video was taken on a parked B738 which is the same physical centre console system as the B38M. The importance was to demonstrate how acutely aware crews must be of their Stab Trim position. In the case of ET302, with the trim passing 2.8 Units towards 0 due to suspected MCAS faults, with aerodynamic loads by increased relative wind, it is even more demanding, time and energy consuming and thus dangerous to recognise a mis-trim too late. In case of low level flight or severe CFIT (controlled flight into terrain) risk, the outcome is predictable."
http://avherald.com/h?article=4c534c4a/0023
QuoteThis is pure speculation, as static pressure is not intuitively dependent on AoA,
The block diagram in a schematic of a 744 ADC shows a line going from the AOA processor block/box to the "Static Source Error Correction block/box.
This makes sense on a 744 as the
primary static sensors are on the pitot tubes, not on the side of the fuselage. Pitot tubes
do change their angle to the wind (with variations in body pitch angle).
Quote from: AV Herald... with aerodynamic loads by increased relative wind, it is even more demanding ..."
I still don't get why the increased ram air should make manual trimming more demanding.
(1) If the surface axis is well-balanced, ram air will not produce any rotational force at all.
(2) If the surface axis is more aft (which isn't the case), ram air tries to increase any non-neutral trim and thus a brake or the trim gear friction needs to stop that "runaway" force. (Bad design, and not applied, obviously.)
(3) If the surface axis is more forward (which seems to be the case), ram air tries to center any non-neutral trim and thus a brake or the trim gear friction needs to stop that centering force. This means that increased ram air actually increases the stab trim centering force.
I conclude system (1) or (3) is applied.
If (3) is applied, the gear must incorporate enough friction to stop that self-centering force. And in that case I understand that the friction must be increased at higher airspeeds. And if so, it's not the increased airspeed per se that requires greater human muscles but it's the artificially increased gear friction. Such a design, all in all, doesn't look reasonable to me, and therefore I return to idea No. (1) -- or a mix of (1) and (3).
|-|ardy
Quote from: Hardy Heinlin on Fri, 5 Apr 2019 15:04
Quote from: AV Herald... with aerodynamic loads by increased relative wind, it is even more demanding ..."
I still don't get why the increased ram air should make manual trimming more demanding.
Don't forget that at the trailing edge of the stab is the elevator.
When deflected in an aircraft nose up position, the aerodynamic force will be in effect pushing the TE of the stab down... the same direction as a nose down trim input.
I would be unsurprised if, at even moderate airspeeds, this force renderered manual stab movement difficult to impossible without relaxing the back pressure to unload the stab.
Not much of an option at 1000ft AGL...
In my comment above I'm intentionally excluding the elevator component -- just trying to understand the principle of the rotational balance.
What force or brake keeps the stabilizer surface in its set position? (When the hydraulics are not in use.)
I guess it's basically the same as on other airliners: A "screw thread". (Not sure if that's the correct English term.) And the gear ratio is so high that the ram air cannot turn the screw. The advantages of the high gear ratio is that it makes manual control possible and it also holds the surface in its position. So that question is possibly answered. But what remains is the question on the rotational balance -- be it with or without elevator inputs.
On the 747, two hydraulic systems move the stab trim faster than 1 hyd sys does. This means one can notice a trim speed variation when the resistance force is higher than the hydraulic force. I conclude that a trim speed variation would also occur when the stab trim is far off-center, because of the increased resistance. But as far as I know (on the 747) the trim speed (of the current trim mode) is pretty constant along the entire trim range. That's why I assume the air load is very well balanced at any stab trim angle.
P.S.: When just 1 hydraulic system runs the stab trim, is the trim speed reduction by 50% really caused by the reduced hydraulic force or is there automatically a lower gear ratio in use to assure the single system can drive the trim?
QuoteWhat force or brake keeps the stabilizer surface in its set position? (When the hydraulics are not in use.)
On the 744, there are two types of brake. The primary brake is a purely mechanical double ratchet/clutch setup, but I don't understand how it works. The secondary brake is a hydromechanical brake. You need hydraulics to push against a spring-loaded brake to release it. The 737 stabiliser isn't driven by hydraulics. It has an electric trim actuator. It just uses the ratchet type.
QuoteP.S.: When just 1 hydraulic system runs the stab trim, is the trim speed reduction by 50% really caused by the reduced hydraulic force or is there automatically a lower gear ratio in use to assure the single system can drive the trim?
The two motors drive the jackscrew via a gearbox differential. Mechanical magic.
I
Quoterecently read about 40 turns for the full range. Still a lot. That is why there is a flip-out handling knob on the trim wheels, like some truck drivers have on their steering wheel.
Rumour has it that the MAX manual trim wheel is smaller than the normal NG trim wheel :P
Are more turns required... or do you just have to apply more force?
Is there any word on the AOA failures on both aircraft? Apart from the MCAS not using a comparator for the L and R AOA, the problem seems to have begun with the AOA failure.. (The L one in both cases?)
It seems odd that the same unit would fail in 2 accidents, in 2 different areas, at more or less the same time of the flight phase...
Some wiring issue, or RF/EMF interference? Perhaps an issue with the A to D conversion inside the boxes? Incorrectly installed? - the unit is the same type of synchro used at various places on the aircraft.. If i recall, the Lion air LHS sensor was replaced before the fatal flight, as the previous crew reported issues - can 2 sensors be faulty in a row? Possible... but strange if true..
https://www.washingtonpost.com/business/economy/sensor-cited-as-potential-factor-in-boeing-crashes-draws-scrutiny/2019/03/17/5ecf0b0e-4682-11e9-aaf8-4512a6fe3439_story.html?noredirect=on&utm_term=.41b3feb6df1d
https://edition.cnn.com/2019/04/04/us/boeing-sensor-investigation/index.html
fc
QuoteIf i recall, the Lion air LHS sensor was replaced before the fatal flight, as the previous crew reported issues - can 2 sensors be faulty in a row? Possible... but strange if true..
They changed the wrong one. In regards to the MCAS function, the Left AOA feeds into the Left Flight Control Computer, the Right AOA feeds into the R Flight Control Computer. The maintenance engineers (and the pilots) didn't understand the automatic selection process for the AOA sensors.
It's probably something like the data selection process on the 744. e.g. first autopilot in command.
QuoteP.S.: When just 1 hydraulic system runs the stab trim, is the trim speed reduction by 50% really caused by the reduced hydraulic force or is there automatically a lower gear ratio in use to assure the single system can drive the trim?
In hydraulic system, with parallel pumps, with all things being equal, the rate at which an actuator moves is a direct function of the volume of hydraulic fluid being moved by the pump (s). With two similar pumps running, switching to one pump, the actuator will move at half the speed.
Or am I missing something?
Jeroen
QuoteOr am I missing something?
Hardy is talking about fixed hydraulic fluid/pressure driving individual hydraulic stabiliser drive motors,
not hydraulic pumps moving fluid.
There is only one jackscrew which is being driven by a single shaft coming from a differential gearbox. The gearbox gearing/differential is being driven by the two hydraulic motors on either side of the differential gearbox.
Just asking because I am curious.
If, by defect or mistubing or miswiring, both hydraulic motors would turn their input shaft to the differential gearbox in opposite directions -- then the output shaft would remain basically static, right? All kinds of planetary wheels whirring around inside but no actual work being done?
Hoppie
Quote from: John H Watson on Fri, 5 Apr 2019 23:05
The 737 stabiliser isn't driven by hydraulics. It has an electric trim actuator. It just uses the ratchet type.
I see. So to get back to my original question, -- just referring to electromagnetic force now instead of hydraulic force -- would the trim speed decrease when the electromagnetic force remains constant while the ram air force is increasing due to the stabilizer surface rotating towards max angle?
The core of my idea was this: If the trim speed (in the current trim mode) remains constant along the entire trim range (which seems to be the case), one can assume that the stabilizer surface axis is pretty well balanced so that increasing airspeeds, i.e. ram air in general, have only little effect on the work required to turn the surface.
Or, in other words: Is the (motor driven) trim speed lower at higher airspeeds?
(Work is work, be it a working human muscle or a motor.)
|-|ardy
If there is a weathervane effect on the stabilizer surface (and no brake), I conclude:
When a
higher trim force is required to turn the surface into a
bad angle, a
lower trim force is required to return the surface into a
good angle.
If there is
no weathervane effect on the stabilizer surface (and no brake), I conclude:
The required trim force is nearly the same at any angle and airspeed.
Lastly, if the gearbox has a ratio that is sufficiently large and the gears/jackscrew are well greased or with a ball bearing nut, then it is possible that the difference in required force at different airspeeds is low compared to the torque of the motor.
It isn't currently clear whether the Ethiopian pilot could not fight the air loads, or was "just" not used to the always heavy manual trim, pulling mechanics around by steel wires 50 meters long. I still think that manually winding 737 trim wheels may not be part of the training... and you get a very nasty surprise when the moment comes, especially when already in dire straits.
Hoppie
Just to complete the picture with Skelsey's suggestion, assuming the elevator position adds further air load to the stabilizer rotational force: If so, I conclude ...
When the elevator is in a command-nose-down position, the command-nose-down trim speed decreases.
When the elevator is in a command-nose-up position, the command-nose-up trim speed decreases.
In other words, when the elevator's tail goes down, the elevator pushes the stabilizer's tail up.
If this effect really happens and the pilots feel this effect in manual flight, the required nose-up trim force increases when the elevator is pulled and the airspeed increases.
@Hoppie
QuoteIf, by defect or mistubing or miswiring, both hydraulic motors would turn their input shaft to the differential gearbox in opposite directions -- then the output shaft would remain basically static, right? All kinds of planetary wheels whirring around inside but no actual work being done?
Sounds plausible in an unlikely"what if" situation )
QuoteLastly, if the gearbox has a ratio that is sufficiently large and the gears/jackscrew are well greased or with a ball bearing nut, then it is possible that the difference in required force at different airspeeds is low compared to the torque of the motor.
Beyond my knowledge, but I assume the 3 phase electric motor would have sufficient force, with the aid of gearing, to move the jackscrew under all conditions. However, the aircraft was exceeding max airspeed at relatively low altitudes.
QuoteIt isn't currently clear whether the Ethiopian pilot could not fight the air loads, or was "just" not used to the always heavy manual trim,
Not clear, but there are lots of posts on PPRuNe talking about how to unloading the force on the stabiliser by inputting elevator (to allow easier manual adjustment). However, no pilot is going to unload the stabiliser by pushing the stick forward at 1~2,000 feet.
https://www.youtube.com/watch?v=rxPa9A-k2xY NG jackscrew in motion
https://www.youtube.com/watch?v=-IlsPnMPYHE - 787 - here the motor seems to be fixed top down... and appears to move faster... ?
https://www.youtube.com/watch?v=mFq06tGuxak - 777
for the 737:
QuoteThe stabilizer trim actuator gets 115 volt, three-phase, 400 Hz
AC power. The stabilizer trim
actuator has an internal AC to DC converter that changes input
power to 270v dc necessary for the brushless DC motor.
Nor sure if the MAX is the same....
fc
Summary of new MCAS software
https://www.pprune.org/10443754-post3824.html
Quote from: skelsey on Fri, 5 Apr 2019 17:20
Quote from: Hardy Heinlin on Fri, 5 Apr 2019 15:04
Quote from: AV Herald... with aerodynamic loads by increased relative wind, it is even more demanding ..."
I still don't get why the increased ram air should make manual trimming more demanding.
Don't forget that at the trailing edge of the stab is the elevator.
When deflected in an aircraft nose up position, the aerodynamic force will be in effect pushing the TE of the stab down... the same direction as a nose down trim input.
I would be unsurprised if, at even moderate airspeeds, this force renderered manual stab movement difficult to impossible without relaxing the back pressure to unload the stab.
Not much of an option at 1000ft AGL...
Oh dear... New stuff, mostly original research by Aviation Herald.
http://avherald.com/h?article=4c534c4a/0045
QuoteExcessive airloads on the stabilizer may require effort by both pilots to correct the mis-trim. In extreme cases it may be necessary to aerodynamically relieve the airloads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually."
This procedure was first mentioned in Boeing's publication "Airliner" published in May 1961 stating (in relation to other Boeing aircraft, the 737 first flew in 1967): "To trim the stabilizer manually while holding a high stick force on control column. As the airplane changes altitude, crank in the desired trim change. Correct airplane attitude after a few seconds with elevators. Relax stick force again and crank in more trim. Repeat this procedure as necessary until proper 'trim' position of stabilizer is established."
http://knkt.dephub.go.id/knkt/ntsc_aviation/baru/2018%20-%20035%20-%20PK-LQP%20Final%20Report.pdf
Final Report for Lion Air .....
Seems like this report didn't go through some serious proof reading... Synopsis page has a date of 29th October 2019... They need to be a little more attentive than this...