737MAX Stab Trim architecture
Join Date: Mar 2019
Location: Moon
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
flyingfalcon16,
Here are your answers:
Not necessarily. The nose down trim will create the tendency for the nose to pitch down. This "tendency" is better known as a "moment". There are many pitching moments acting at any given time. Which way the aircraft pitches, if at all, depends on what the sum of those moments is. If the sum was zero, for example, and nose down trim was applied, then yes, you would expect a pitch down result, but the application of nose-down trim does not guarantee a nose-down, or negative, pitch attitude.
Note that a pitch-down does not necessarily result in a "negative pitch attitude". In aerodynamics, the latter is considered to be a pitch attitude whose angle is below the horizon. An aircraft with a pitch attitude of positive 45 degrees, for example, could experience a pitch-down such that it's pitch attitude was now positive 30 degrees. Both these attitudes are considered positive pitch attitudes.
Here are your answers:
Not necessarily. The nose down trim will create the tendency for the nose to pitch down. This "tendency" is better known as a "moment". There are many pitching moments acting at any given time. Which way the aircraft pitches, if at all, depends on what the sum of those moments is. If the sum was zero, for example, and nose down trim was applied, then yes, you would expect a pitch down result, but the application of nose-down trim does not guarantee a nose-down, or negative, pitch attitude.
Note that a pitch-down does not necessarily result in a "negative pitch attitude". In aerodynamics, the latter is considered to be a pitch attitude whose angle is below the horizon. An aircraft with a pitch attitude of positive 45 degrees, for example, could experience a pitch-down such that it's pitch attitude was now positive 30 degrees. Both these attitudes are considered positive pitch attitudes.
Trim changes create changes to the pitching moment. This is true for everything on the aircraft. Extending undercarriage and flaps, increasing or decreasing thrust, etc - all create pitching moments. At any time in flight, there are many pitching moments. Whether the aircraft pitches up or down, and at what rate, depends on the sum of all those moments. If the sum is zero, for example, then there will be no pitching.
Join Date: Mar 2019
Location: Moon
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
@flyingfalcon
I get the impression you are mixing AoA and pitch attitude.
If you are flying level (not descending or climbing) and there are no significant up/downdrafts then pitch attitude and AoA are linked.
But when you are descending fast you can have a high AoA while having a negative pitch attitude.
So in that situation MCAS would still activate and if the AoA value is correct it would indeed be correct for it to activate.
Assuming the pilot does not pull further on the stick compensating the nose down trim, MCAS will make the plane pitch down.
I get the impression you are mixing AoA and pitch attitude.
If you are flying level (not descending or climbing) and there are no significant up/downdrafts then pitch attitude and AoA are linked.
But when you are descending fast you can have a high AoA while having a negative pitch attitude.
So in that situation MCAS would still activate and if the AoA value is correct it would indeed be correct for it to activate.
Assuming the pilot does not pull further on the stick compensating the nose down trim, MCAS will make the plane pitch down.
We typically see it described as preventing a stall, I guess that's why it seems odd to see it working when the plane is in a dive. In this case I suppose it would still be keeping the plane from pitching upward to quickly.
I wonder if the erratic oscillating pitching up and down movement we see in the recent crash scenarios could be MCAS over activating or not resetting properly to flight scenarios of both negative and positive pitch attitude and high AoA?
Join Date: Mar 2019
Location: Moon
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
So while high AoA can happen while having negative pitch attitude, isn't it saying in the manual MCAS is only activated at pitch up conditions only? Or does this just mean the pilot is pulling up causing a pitch up condition which could occur during negative pitch attitude of the craft?
Join Date: Apr 2008
Location: Australia
Posts: 669
Likes: 0
Received 0 Likes
on
0 Posts
So isn't it accurate to say, since MCAS creates a pitching down moment by adjusting trim, it is effecting the final attitude of the aircraft...
Consider the aircraft being hand-flown in straight and level flight with the thrust at idle. In this situation, the speed is decaying, but to maintain level flight, the pilot is steadily increasing the rearward pressure on the control column. The aircraft is maintaining altitude but approaching the stall. Note that the pitch attitude is increasing (becoming more positive, or "nose high").
It is this situation (and similar) that Boeing had in mind when they conceived of the MCAS. Certification standards require the nose-down moment (or control column forces?) to linearly increase in this scenario, but Boeing found this wasn't happening satisfactorily and needed some way to impose more down moment - hence the MCAS.
In this same scenario, but with the MCAS working as designed, the pilot must exert a steadily increasing back pressure on the column in order to maintain level flight. The aircraft is now more stall resistant. This is what Boeing had in mind.
This example shows that there is no hard and fast link between MCAS operation and downward changes of pitch attitude. In fact, the pitch attitude has increased throughout (become more positive, or "nose up"), with the effect of MCAS being to increase the control column forces being experienced by the pilot.
Or put very shortly: MCAS effects the pitch attitude of the plane.
Join Date: Mar 2019
Location: Moon
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
Right, but it's effecting pitch attitude. So maybe the actual pitch attitude doesn't change, though I think considering it's design it would be hoping the pitch did decrease as it's primarily meant to become active in a stall scenario. From MCAS' point of view, it is effecting the pitch attitude by adding nose down trim. It is "trying to pull the nose down" whether that results in a net effect of the pitch changing or not. Put another way, I would expect that when it's active, the nose would pitching down more than if it was not.
Instead of giving MCAS authority to control the aircraft, I propose a solution... When MCAS activates, just allow it to illuminate an annunciation in big red flashing letters...'AoA High, Stall Warning'
and let the pilots perform the correction..
and let the pilots perform the correction..
Join Date: Dec 2015
Location: Cape Town, ZA
Age: 62
Posts: 424
Likes: 0
Received 0 Likes
on
0 Posts
Last edited by GordonR_Cape; 19th Mar 2019 at 20:31.
Join Date: Feb 2009
Location: Seattle
Posts: 379
Likes: 0
Received 0 Likes
on
0 Posts
That's not a solution at all. The B737 already has a stick shaker and stall warning. The problem is that the elevator does not have sufficient authority, and the stabiliser needs to be rapidly trimmed down, just at the time when the pilot already has a high workload.
Join Date: Apr 2008
Location: Australia
Posts: 669
Likes: 0
Received 0 Likes
on
0 Posts
Right, but it's effecting pitch attitude. So maybe the actual pitch attitude doesn't change, though I think considering it's design it would be hoping the pitch did decrease as it's primarily meant to become active in a stall scenario.
From MCAS' point of view, it is effecting the pitch attitude by adding nose down trim. It is "trying to pull the nose down" whether that results in a net effect of the pitch changing or not. Put another way, I would expect that when it's active, the nose would pitching down more than if it was not.
It can pull the nose down, as we now know, but that was never Boeing’s intention.
Last edited by FGD135; 20th Mar 2019 at 07:05.
Join Date: Mar 2019
Location: Moon
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
No, the MCAS is absolutely not there for the stall scenario. It is to improve manual handling characteristics at flight near the stall. It is not for stall recovery. If Boeing had wanted that, they would have installed a “stick pusher”, which is a simple system, common to many airliners.
No, it is not trying to pull the nose down. It is trying to change how the plane feels when manually flown at speeds near the stall.
It can pull the nose down, as we now know, but that was never Boeing’s intention.
No, it is not trying to pull the nose down. It is trying to change how the plane feels when manually flown at speeds near the stall.
It can pull the nose down, as we now know, but that was never Boeing’s intention.
The original Boeing document provided to the FAA included a description specifying a limit to how much the system could move the horizontal tail — a limit of 0.6 degrees (turned out to be actually 2.5 deg), out of a physical maximum of just less than 5 degrees of nose-down movement (20+deg with the new 2.5 deg rotation (don't forget to add it can reset indefinitely)). From this statement, it's pretty clear that the intention was to pull the nose down hence the wording, "5 degrees of nose-down movement".
Since MCAS changes the position of the horizontal stab it is absolutely effecting the pitch attitude of the plane. Or if you rather, it is effecting the pitch down forces on the plane.
Join Date: Apr 2008
Location: Australia
Posts: 669
Likes: 0
Received 0 Likes
on
0 Posts
MCAS is qualified in many sources as fundamentally a stall prevention system.
From this statement, it's pretty clear that the intention was to pull the nose down hence the wording, "5 degrees of nose-down movement".
Check how MCAS is described by insider sources. Here is one such source:
737 MAX - MCAS
Since MCAS changes the position of the horizontal stab it is absolutely effecting the pitch attitude of the plane. Or if you rather, it is effecting the pitch down forces on the plane.
Join Date: Mar 2019
Location: Wakefield, RI
Posts: 23
Likes: 0
Received 0 Likes
on
0 Posts
2 other flights
Here are 2 more accidents that may bear looking into. Both happened with 737 - 800 and 737 - 8AS aircraft and both soon after takeoff. In both cases the cause of the crash was ruled pilot error. You can look them up on Wikipedia as i cannot post URLs
5 May 2007 Kenya Airways Flight 507
25 Jan 2010 Ethiopian Airlines Flight 409
5 May 2007 Kenya Airways Flight 507
25 Jan 2010 Ethiopian Airlines Flight 409
Join Date: Mar 2019
Location: Moon
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
All of those sources would be media, seeking to explain the system to the layman as simply as possible. For the media, trying to explain in terms of pitch-down moments would be way too technical. You won't hear Boeing, the FAA, or anybody on the inside describing MCAS as being for stall prevention.
No. A system that pulls the nose down would be for stall prevention/recovery. If certification standards required that on the MAX, then Boeing would have simply added a stick pusher. The stick pusher is a vastly more simple and reliable system and has been in use on airliners for many decades now. Why would Boeing have adopted the new and unproven MCAS when they could have simply used a stick pusher?
Check how MCAS is described by insider sources. Here is one such source:
When you say "effecting", I think you mean "affecting". That first sentence is false. The second is true. For the first, if you had said, "influencing the pitch attitude", then it would have been true.
No. A system that pulls the nose down would be for stall prevention/recovery. If certification standards required that on the MAX, then Boeing would have simply added a stick pusher. The stick pusher is a vastly more simple and reliable system and has been in use on airliners for many decades now. Why would Boeing have adopted the new and unproven MCAS when they could have simply used a stick pusher?
Check how MCAS is described by insider sources. Here is one such source:
When you say "effecting", I think you mean "affecting". That first sentence is false. The second is true. For the first, if you had said, "influencing the pitch attitude", then it would have been true.
Right that's better wording. MCAS affects pitch attitude which can, in the scenario of avoiding a stall, effect the pitch attitude. Since it's changing the stab position it has incredible amount of pitch authority, so I think it isn't really that inaccurate to say "effects". Or MCAS has an effect on pitch attitude. It may not if the pilot uses the elevators to counteract the pitch down force. But that's technically adding another force outside the MCAS system itself. All other things being equal, if MCAS activates, the pitch attitude will change / the nose will drop (more so than if it wasn't active).
Put another way, your disagreement would be like saying the aerodynamic forces on the tail have no effect on pitch attitude. They only have an affect. I'm not sure that's more accurate but if it is it seems pedantic.
From the link you pasted (which I've read at length before): "as the nacelle is ahead of the CofG this causes a pitch-up effect which could in turn further increase the AoA and send the aircraft closer towards the stall. MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching stall."
I don't think it's inaccurate to think of MCAS as a system to avoid a possible stall. Effectively, pulling the nose down by changing stab position.
I would post a good video of an airliner pilot explaining it as essentially a stall prevention system but I can't because I don't have enough posts to paste links. His take is the engines change the stall characteristics of the plane such that under a stall condition the elevator forces may not be enough to bring the nose down, so an adjustment to the stab is required.
So nose down trim affects pitch attitude. Which can and certainly in the latest crashes effect the pitch. By describing it the way you do you seem to imply MCAS has no pitch authority. Changing the position of the entire "tail wing" has a profound effect on the lift forces generated by the tail and the final pitch attitude of the plane.
Last edited by flyingfalcon16; 21st Mar 2019 at 13:14. Reason: inaccurate wording
Salute!
@flyingfalcon
One thing that confuses many folks is that airplanes and the folks in them are operating in a different geometrical/spatial frame of reference than when sitting in a chair in front ot a screen or reading a book. They are also operating in a different dynamic frame of reference, as they are usually moving with the pointy end forward! Heh heh.
So you have to leave your chair behind in the computer room or living room, huh? You have to put yourself in the "aerospace" vehicle and your new frames of reference You have your body coordinates and the Earth's space coordinates. The body coordinates are related to your chair - up/down, left/right and front/back. And motion is along those axis plus rotation about them. The six degrees of freedom we hear about, huh?
Your plane moves your body coordinates within the space coordinates. It's up to you to deal with both if you wanna get someplace or put on a great acro routine. Your "attitude" is your coordinate system's relationship to the space coordinates. It's how people outside see your vehicle. And AoA is related to your velocity vector WRT the air in those space coordinates. So are all the functions of the other appendages of your vehicle .
With a neutral, fixed elevator , changing the AoA of the horizontal stabilizer definitely changes the AoA of the main wing and the whole vehicle. Hell, every fighter and even many civilian planes have no elevator! They use that big stab to control AoA. So maybe you are still in your living room chair reference frame and see the effects of that change in AoA as "attitude".
That's the impression we are getting from your posts. Watch a big plane land and it's obvious that the thing is not "pointing" along its flight path because of its AoA. Nose up attitude, downward flight path.
Gums sends...
@flyingfalcon
One thing that confuses many folks is that airplanes and the folks in them are operating in a different geometrical/spatial frame of reference than when sitting in a chair in front ot a screen or reading a book. They are also operating in a different dynamic frame of reference, as they are usually moving with the pointy end forward! Heh heh.
So you have to leave your chair behind in the computer room or living room, huh? You have to put yourself in the "aerospace" vehicle and your new frames of reference You have your body coordinates and the Earth's space coordinates. The body coordinates are related to your chair - up/down, left/right and front/back. And motion is along those axis plus rotation about them. The six degrees of freedom we hear about, huh?
Your plane moves your body coordinates within the space coordinates. It's up to you to deal with both if you wanna get someplace or put on a great acro routine. Your "attitude" is your coordinate system's relationship to the space coordinates. It's how people outside see your vehicle. And AoA is related to your velocity vector WRT the air in those space coordinates. So are all the functions of the other appendages of your vehicle .
With a neutral, fixed elevator , changing the AoA of the horizontal stabilizer definitely changes the AoA of the main wing and the whole vehicle. Hell, every fighter and even many civilian planes have no elevator! They use that big stab to control AoA. So maybe you are still in your living room chair reference frame and see the effects of that change in AoA as "attitude".
That's the impression we are getting from your posts. Watch a big plane land and it's obvious that the thing is not "pointing" along its flight path because of its AoA. Nose up attitude, downward flight path.
Gums sends...
Last edited by gums; 20th Mar 2019 at 19:59. Reason: added/subtracted
Join Date: Mar 2019
Location: Moon
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
I am not too sure how many correspondents in this thread have ever flown an aircraft, but the first lesson pilots learn, is that trimming the tailplane controls the speed of the aircraft. This trimming is either done at the front of the all moving tailplane with the jackscrew, or at the rear with the elevator.
Even in the simplest form of aircraft, a glider, you can bimble around for hours looking for thermals, with the trim set towards the rear, for 40 knots. But to give some increase in control for landing you actually need to speed up to 60 knots which equates to putting the trim just a bit further forward than mid-way. If you put the trim fully forward the speed will increase even further.
So The Lion Air pilots must have seen their trim going towards the very high speed end of the range, but unfortunately did nothing to disconnect the automatics, and fly manually. Maybe they thought if they switched off the electrics, they would also loose power to their trim switches.
Even in the simplest form of aircraft, a glider, you can bimble around for hours looking for thermals, with the trim set towards the rear, for 40 knots. But to give some increase in control for landing you actually need to speed up to 60 knots which equates to putting the trim just a bit further forward than mid-way. If you put the trim fully forward the speed will increase even further.
So The Lion Air pilots must have seen their trim going towards the very high speed end of the range, but unfortunately did nothing to disconnect the automatics, and fly manually. Maybe they thought if they switched off the electrics, they would also loose power to their trim switches.
Salute!
from scifi:
If they thot that, then they were correct. Might be because using the wheel switches they could get nose up trim, ya think? The stab switches on the pedastal turn off electric power to the stab for all of the pseudo-manual functions. Then they have to crank that manual trim wheel ten turns just to get what HAL did in a half a second!!!
Gums sends...
from scifi:
Maybe they thought if they switched off the electrics, they would also loose power to their trim switches.
Gums sends...
Join Date: Mar 2019
Location: Moon
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
I am not too sure how many correspondents in this thread have ever flown an aircraft, but the first lesson pilots learn, is that trimming the tailplane controls the speed of the aircraft. This trimming is either done at the front of the all moving tailplane with the jackscrew, or at the rear with the elevator.
Even in the simplest form of aircraft, a glider, you can bimble around for hours looking for thermals, with the trim set towards the rear, for 40 knots. But to give some increase in control for landing you actually need to speed up to 60 knots which equates to putting the trim just a bit further forward than mid-way. If you put the trim fully forward the speed will increase even further.
So The Lion Air pilots must have seen their trim going towards the very high speed end of the range, but unfortunately did nothing to disconnect the automatics, and fly manually. Maybe they thought if they switched off the electrics, they would also loose power to their trim switches.
Even in the simplest form of aircraft, a glider, you can bimble around for hours looking for thermals, with the trim set towards the rear, for 40 knots. But to give some increase in control for landing you actually need to speed up to 60 knots which equates to putting the trim just a bit further forward than mid-way. If you put the trim fully forward the speed will increase even further.
So The Lion Air pilots must have seen their trim going towards the very high speed end of the range, but unfortunately did nothing to disconnect the automatics, and fly manually. Maybe they thought if they switched off the electrics, they would also loose power to their trim switches.
Moderator
I'm trying to respond to qualify my remarks but they won't approve many of my posts. This is very heavily curated forum
The site has a requirement that new posters with less than how ever many posts have each each new post invigilated and then released to view. Once you get to the magic number, your posts are treated as OK by default. This is to put a check on out of left field inappropriate posts by new folk and, more importantly, spammers. Unfortunately, like most sites, we get our share of the latter but you don't get bothered by them because of the mod invigilation requirement.
Unfortunately, that means that one of us has to have a look at the post and, given that we are part time mods, that takes a little while longer than instantly on most occasions. So, in respect of your last post, I have just done my run through the queue and released it a few minutes ago. Next time someone might coincidentally review a post near immediately following its drafting .. all depends on the luck of the draw. However, you should expect to see your new posts appear in a reasonable timeframe, if not instantaneously.
I assure you that this forum is very relaxed in its moderation activity. As a general rule, the folk who frequent TL, in the main, are "nice" people and we don't have much in the way of problems ....
The site has a requirement that new posters with less than how ever many posts have each each new post invigilated and then released to view. Once you get to the magic number, your posts are treated as OK by default. This is to put a check on out of left field inappropriate posts by new folk and, more importantly, spammers. Unfortunately, like most sites, we get our share of the latter but you don't get bothered by them because of the mod invigilation requirement.
Unfortunately, that means that one of us has to have a look at the post and, given that we are part time mods, that takes a little while longer than instantly on most occasions. So, in respect of your last post, I have just done my run through the queue and released it a few minutes ago. Next time someone might coincidentally review a post near immediately following its drafting .. all depends on the luck of the draw. However, you should expect to see your new posts appear in a reasonable timeframe, if not instantaneously.
I assure you that this forum is very relaxed in its moderation activity. As a general rule, the folk who frequent TL, in the main, are "nice" people and we don't have much in the way of problems ....
Join Date: Dec 2015
Location: Cape Town, ZA
Age: 62
Posts: 424
Likes: 0
Received 0 Likes
on
0 Posts
I have been told more than once that this kind of statement contains several misperceptions. I leave it to others to provide the correct answer.