WHY are two autopilots required for a go-around...

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Re: WHY are two autopilots required for a go-around...

Postby flyboy2548m » Wed Apr 27, 2016 3:39 pm

I think you don't understand or believe me that I am perfectly ok with being wrong, it is a learning opportunity. Just convince me with something more than "because I say so", no matter who "I" is.
You're right, I don't believe you, because I've never seen you actually be OK with being wrong. You either backtrack Evanesque-style or keep pushing your position (which you don't have). It's one of numerous ways in which your words and actions are nowhere near in agreement.
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Re: WHY are two autopilots required for a go-around...

Postby Gabriel » Wed Apr 27, 2016 5:08 pm

You're right, I don't believe you,
And, whether you believe me or not, I could not care any less.
because I've never seen you actually be OK with being wrong.
Look harder. Or don't. That's none of my business. But your comments in this very thread make it very clear that you don't pay attention to what is being said, or that you apply a confirmation bias filter to what you read.

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Re: WHY are two autopilots required for a go-around...

Postby Not_Karl » Thu Apr 28, 2016 12:53 am

Guys, forget those damn autopilots, what I really want Verbie and PPilotie to explain is this (second picture, "Ground tracks") :shock: .
(Discovered by Evanie; my interwebs seem to be broken as I don't see updates newer than 25 april on AVHerald :? )
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Re: WHY are two autopilots required for a go-around...

Postby 3WE » Thu Apr 28, 2016 2:18 am

Guys, forget those damn autopilots, what I really want Verbie and PPilotie to explain is this (second picture, "Ground tracks") :shock: .
(Discovered by Evanie; my interwebs seem to be broken as I don't see updates newer than 25 april on AVHerald :? )
Don't ask stupid questions... when Evanie and Bradleyee said "I'm sure there's a very good reason"... well, look what happens with just one autopilot. :mrgreen:
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Re: WHY are two autopilots required for a go-around...

Postby PurduePilot » Sat Apr 30, 2016 4:00 am

Gabe, you're partially correct. It's both. I'm not going to get into the weeds on a complex airplane system that was designed over twenty years ago (even if I could share such information), but are you familiar with "Fail active" versus "fail passive"? You are correct that redundancy is indeed a factor. But you're also wrong about autopilot authority on fly-by-cable airplanes. Do you know what a servo is and how it works? Do you know how autopilot backdrive works on fly by cable? On fly by wire?

Bottom line is the autopilot is there to assist the pilot, not replace him. The airplane flies just fine without it.
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Re: WHY are two autopilots required for a go-around...

Postby PurduePilot » Sat Apr 30, 2016 4:04 am

Guys, forget those damn autopilots, what I really want Verbie and PPilotie to explain is this (second picture, "Ground tracks") :shock: .
(Discovered by Evanie; my interwebs seem to be broken as I don't see updates newer than 25 april on AVHerald :? )
Ground loop? I don't know.

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Re: WHY are two autopilots required for a go-around...

Postby ocelot » Sat Apr 30, 2016 7:07 am

Guys, forget those damn autopilots, what I really want Verbie and PPilotie to explain is this (second picture, "Ground tracks") :shock: .
wut

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Re: WHY are two autopilots required for a go-around...

Postby flyboy2548m » Sat Apr 30, 2016 3:12 pm

But you're also wrong about autopilot authority on fly-by-cable airplanes. Do you know what a servo is and how it works? Do you know how autopilot backdrive works on fly by cable? On fly by wire?
God, I hope he does, Bradley, he is, after all, one of our time's leading aerodynamicists.
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Re: WHY are two autopilots required for a go-around...

Postby 3WE » Sat Apr 30, 2016 3:51 pm

...I'm not going to get into the weeds on a complex airplane system that was designed over twenty years ago (even if I could share such information)...
Ok, thanks for your explanations, Bradley. I’m in worse shape than Gabriel on understanding these mechanisms. Unfortunately, I’m similar in my curiosity and PITA-ness on the subject.

So, when a 737 is on two autopilots, there’s more control authority…I dare say that the plane responds more quickly.
Fascinating! Is there any way for you to dumb it down for an aggie…A difference in transition time.

I continue to be amazed with flight directors- magical black boxes that position needles on an HSI display that- if followed relentlessly- will make the plane do damn near exactly what it needs to do to perform as the flight director is programmed (including tracking an ILS).
My dumbass thought is that the autopilot sort of just gives orders to make the AI match the FD needles…and that the order is not particularly different whether it comes from one or two autopilots…but…just like all those crazy MD-80 quirks I just learned about…I’m incredibly well qualified to be wrong 

I acknowledge your clear statement that there is a difference in “signal” and “control responsiveness” with one vs. two auto pilots.
However, I will still be a PITA like Gabe and ask what’s so expletive hard about doing a go around at 1500 to 2000 feet AGL? Is the lag so bad that the damn things going to pull up into a stall? (But that can’t be the problem- since pulling up into a stall at 2500 feet isn’t a particularly good thing to do either)…So, it’s too damn slow that it can’t transition from an 800 FPM descent to a 1 2 or 3000 FPM climb without burning 1000 feet of altitude? It takes a whole friggin two minutes to establish a good climb? That still bugs me because I can imagine doing ‘similar’ maneuvers during fat dumb and happy cruise-type flying without undue delay.

Going into big-time ass hat “what we should do mode”, Would the procedures be better if it was no single Otto ILS’s below 2000 feet whatsoever at all (except for ‘emergencies’) at which time your Evan memory checklist is that all go-arounds must be done manually. I will stand with Gabe and the objection that you can take it down to 200 feet on one autopilot, but are SOL if you hit TOGA at 1000 feet.

Yeah, that’s the way it is, STFU and go kill some weeds is acceptable to me. But aviation is cool and this stuff is interesting, and I’m keeping my crap off of PPRUNE ;-) .
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Re: WHY are two autopilots required for a go-around...

Postby flyboy2548m » Sat Apr 30, 2016 6:06 pm

So, when a 737 is on two autopilots, there’s more control authority…I dare say that the plane responds more quickly.
Fascinating! Is there any way for you to dumb it down for an aggie…A difference in transition time.

.
Actually, 3BS, there is a pretty direct aggie comparison. Take a look at this John Deere tractor pulling a massive planter attachment. Notice that in addition to the main tow bar, there are anti-sway bars connecting each side of the planter to the main tow bar. Another solution would be to use only one sway bar since the all parts of the planter are mechanically linked to each other (not entirely unlike the torque tubes linking elevators together). That would have probably worked well enough, but John Deere decided to use two sway bars. Let that sink in.

Now do you see what Bradley is driving at? Gabby-boy won't get it because it conflicts with his position (which he doesn't have), but you might.
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Re: WHY are two autopilots required for a go-around...

Postby 3WE » Sat Apr 30, 2016 8:14 pm

Good one.

I'm grasping the 73 go around logic about as good as I'm going too...however for farm impliments, tension works a lot better than compression , and I'm betting a steak dinner that one brace is inadequate if you snag something sticky at the wrong end of the planter...NOT that one brace is adeqate, but it's good to have backup nor that there's a speed of response difference if you change directions...

That being said, I'm seeing that the 737 is more of duplicate 'whole systems'...and that's OK I guess.

Definately no weirder than rear view mirrors for a compass and a sight gauge to adjust your seat.

There is however, a certain art to turning your head to check the planter without "bringing the steering wheel along with you". Actually what's interesting is that your planter has no marker attachments...if the GPS-RTK signal is lost, it's nearly impossible to drive with proper alignment...fortunately at 6 mph and an altitude of negative 1.5 inch AGL, corn seed usually doesn’t died.
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Re: WHY are two autopilots required for a go-around...

Postby flyboy2548m » Sat Apr 30, 2016 8:20 pm

Good one.

I'm grasping the 73 go around logic about as good as I'm going too...however for farm impliments, tension works a lot better than compression , and I'm betting a steak dinner that one brace is inadequate if you snag something sticky at the wrong end of the planter...NOT that one brace is adeqate, but it's good to have backup nor that there's a speed of response difference if you change directions...

That being said, I'm seeing that the 737 is more of duplicate 'whole systems'...and that's OK I guess.
Sigh...let me try again. Forget the planter, let's talk just in general. If you wanted to move both sides of something, would it not be better to move it FROM both sides?
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Re: WHY are two autopilots required for a go-around...

Postby 3WE » Sat Apr 30, 2016 8:31 pm

Good one.

I'm grasping the 73 go around logic about as good as I'm going too...however for farm impliments, tension works a lot better than compression , and I'm betting a steak dinner that one brace is inadequate if you snag something sticky at the wrong end of the planter...NOT that one brace is adeqate, but it's good to have backup nor that there's a speed of response difference if you change directions...

That being said, I'm seeing that the 737 is more of duplicate 'whole systems'...and that's OK I guess.
Sigh...let me try again. Forget the planter, let's talk just in general. If you wanted to move both sides of something, would it not be better to move it FROM both sides?
Yes, yes... I get it...the autopilots operate 'an actuator each'... and thanks to you and Brad for the explanation. If I have a position on this it's that this stuff is interesting and I recognize that my position doesn't matter and that the 737 is a good airplane...almost as good as your 'bus.
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Re: WHY are two autopilots required for a go-around...

Postby flyboy2548m » Sat Apr 30, 2016 8:43 pm

Yes, yes... I get it...the autopilots operate 'an actuator each'...
A servo each, but you're close enough for government work.
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Re: WHY are two autopilots required for a go-around...

Postby 3WE » Sun May 01, 2016 2:34 am

Yes, yes... I get it...the autopilots operate 'an actuator each'...
A servo each, but you're close enough for government work.
This is all making sense. Staying on an ILS generally requires small control inputs. A go around requires a bigger input...that is marginal, or perhaps inadequate from a single servo.

Of course I still like you monitoring ILSs and go arounds regardless of the number of autopilots.

Thanks.
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Re: WHY are two autopilots required for a go-around...

Postby flyboy2548m » Mon May 02, 2016 3:13 pm

This is all making sense. Staying on an ILS generally requires small control inputs. A go around requires a bigger input...that is marginal, or perhaps inadequate from a single servo.
It's really not that complicated, is it? Unless, of course, your specialty is absolutely refusing to see the forest for the trees like some around here.
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Re: WHY are two autopilots required for a go-around...

Postby 3WE » Mon May 02, 2016 6:36 pm

It's really not that complicated, is it?
Getting you GD insiders to spell out the answer in a simple, clear, jargon-free sentence, not requiring 60+ posts in a thread is pretty damn complicated.

But rest assured, we are appreciative and entertained.
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Re: WHY are two autopilots required for a go-around...

Postby Gabriel » Tue May 03, 2016 3:47 am

Gabe, you're partially correct. It's both. I'm not going to get into the weeds on a complex airplane system that was designed over twenty years ago (even if I could share such information), but are you familiar with "Fail active" versus "fail passive"? You are correct that redundancy is indeed a factor. But you're also wrong about autopilot authority on fly-by-cable airplanes. Do you know what a servo is and how it works? Do you know how autopilot backdrive works on fly by cable? On fly by wire?

Bottom line is the autopilot is there to assist the pilot, not replace him. The airplane flies just fine without it.
PP, it took me some time to answer because I was trying to do some research over the internet. But I didn't find anything in more detail that what is there in smartcockpit.com (where the link about the autopilot system in my first post here came from).

I do understand that, depending on the AP technology, loosing a single AP can lead to a non-fail-passive situation, which could be extremely dangerous especially if it happens very low.

I do know what is fail passive (that the plane will do nothing strange after a system fails and the automation is lost) and fail operational (the automation will not be lost after a system fail). I don't know what is fail active but I guess it is that it doesn't meet the fail passive requirements. I also know what a servo is (a device that can move and control its position with precision). There are servomotors, servo-valves, servomechanisms...

I do not know how autopilot backdrive works on fly- by-cable or on fly-by-wire.

As I said, I do not know the details of the 737 systems. But I do know that the 737 NG is neither fly- by-cable nor fly-by-wire. So I am very surprised and confused by you bringing that to the table.

The control columns of the 737 are connected to each other with a torque tube, and with control cables that control the hydraulic PCUs. To control the PCUs requires minimum force: You are just moving a control valve, not applying the force directly to the surface controls. This is similar to moving a light dimmer: You control how much power goes to the light, but you don't have to do that power yourself. The electrical power powers the light according to your command, and in the 737 the hydraulic systems power and move the elevator according to your command. So much is that, that the plane is fitted with an artificial feel system that creates artificial loads on the control column to provide feedback to the pilot so it "feels as" he was manipulating the controls directly, so it feels as if it was a flight-by-cable airplane. Because there is no real force feedback path from the elevators to the control columns, in the same way as there is no feeling of power in a light dimmer.

Now, the 737 does have a manual reversion for the elevators, in case both hydraulic systems fail. In this manual reversion, you ARE controlling the elevators directly with the control columns, via a mechanical linkage, you do have to do a real force, and the elevators will feed real force back to the control columns according to the aerodynamic loads.

I don't know how this manual reversion is geared. I couldn't find that information. The elevator of the 737 is huge so, even when the information I found warns that the loads will be quite greater than in normal operation, it cannot be simple direct linkage like in a smaller plane without any other provision. I do note that there are tabs in the trailing edge of the elevator, and I suspect that they have something to do with this (they cannot be trim tabs because the trim is provided by the stabilizer, electrically or manually, so it works even in case of full hydraulic and even full electrical failure).

I still don't know the details of how the the autopilot mechanism works, but I am quite sure that it is not via the manual reversion system, but via the PCUs and hydraulic systems. So much, that the AP is inop if the hydraulic systems are both inop. (as a side note, the feel feedback would also be inop because it depends on hydraulics too, but you would have the real-thing aerodynamic feedback in manual reversion).

This is why Flyboy's John Deere analogy is horrible. The two anti-sway bars are the things firmly grabbing the planter. In the airplane, that would be the two things that move the elevator, and that is the hydraulically powered PCUs, which would be both still there, alive, kicking and moving the elevator regardless of whether there are two channels or one channel engaged and giving control commands. Unlike the Flyboy's example, there is no direct mechanical link between the autopilot servos and the elevator. So how about this other analogy. In my car, the forward-left blinking light is commanded by the turn signal lever and the hazard light button. Guess what, it doesn't blink any faster or brighter if I engage both controls simultaneously.

In this scenario, the only way that I can imagine (and there can be other ways that I could not imagine) for a dual channel AP having more elevator authority than a single channel AP would be if (all of these) a) the artificial feel system is still active while the AP is engaged, b) the AP has to "fight" against the artificial feel system (instead of, for example use it to move the control columns, which do move when the AP gives elevator commands), and c) each servo associated withe each AP channel doesn't have enough force to "win" the fight against the artificail feel.

I don't know if that's the case. I found this which, depending on the interpretation, may (or may not) mean that only one AP is activelly moving the elevator at each instant (although which one is doing it can be changing all the time):
Approach (APP) Mode Dual A/Ps
Approach mode allows both A/Ps to be engaged at the same time. Dual A/P operation provides fail–passive operation through landing flare and touchdown or an automatic go–around. During fail passive operation, the flight controls respond to the A/P commanding the lesser control movement.
Bottom line: I still don't know if two APs have more elevator authority than one. I tend to think that they don't, but I cannot say that that is the case. If they do, PP's explanation didn't provide enough information to help me understand (posing questions that either I could not find the answer, or that the answer is not useful for explaining that), and I found it even confusing, as explained at the beginning. So I still have one or two questions:

Why does the AP and procedures allow for flying really low on a single channel AP for some cases (approach: procedure: down to 200ft, AP: all the way to the ground) but not for others (go-around: AP won't fly the GA on single channel below 2000ft)?

If it is, as PP said, because one AP doesn't provide enough elevator authority for the necessary pitch maneuver needed to transition from approach to go-around, then why is it that the case that one AP lacks the authority but to have it? I have a candidate explanation for that (the one given above), but I have no idea if it is correct.

http://www.smartcockpit.com/aircraft-re ... light.html
http://www.smartcockpit.com/aircraft-re ... trols.html
http://www.smartcockpit.com/aircraft-re ... ulics.html
http://www.smartcockpit.com/aircraft-re ... rsion.html

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Re: WHY are two autopilots required for a go-around...

Postby Ancient Mariner » Tue May 03, 2016 6:39 am

I'm with Gabriel on this one. I could pull out some analogies from a hat, but I will not.
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Re: WHY are two autopilots required for a go-around...

Postby flyboy2548m » Tue May 03, 2016 4:04 pm


This is why Flyboy's John Deere analogy is horrible. The two anti-sway bars are the things firmly grabbing the planter. In the airplane, that would be the two things that move the elevator, and that is the hydraulically powered PCUs, which would be both still there, alive, kicking and moving the elevator regardless of whether there are two channels or one channel engaged and giving control commands. Unlike the Flyboy's example, there is no direct mechanical link between the autopilot servos and the elevator. So how about this other analogy. In my car, the forward-left blinking light is commanded by the turn signal lever and the hazard light button. Guess what, it doesn't blink any faster or brighter if I engage both controls simultaneously.
On some vehicles, interestingly enough, if one blinker bulb is out, the remaining one on that side DOES blink twice as fast.

At some point, Gabby-boy, you'll have to accept that your understanding of some things is not as excellent as you think. I realize you'd rather die, but still...
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Re: WHY are two autopilots required for a go-around...

Postby Gabriel » Tue May 03, 2016 6:38 pm

On some vehicles, interestingly enough, if one blinker bulb is out, the remaining one on that side DOES blink twice as fast.
Exactly. That, if anything, proves my point. That ridiculous, irrelevant, inapplicable analogies lead to nowhere. The combination of the 737 NG's flight control system, artificial feel system, and autopilot system (including both the logic and actuators) is nothing alike a blinker or a planter.
At some point, Gabby-boy, you'll have to accept that your understanding of some things is not as excellent as you think. I realize you'd rather die, but still...
Are you kidding? My participation in this whole thread was about me not understanding things and seeking for help. I know, you will say that help was provided and that I would not listen. That's fair, I've already explained that part. Let just say that the help provided so far is more in the range of belief than of understanding. Believing what PP said will not help me understand why, which, as you can note, is the first word in the title of this thread, it is highlighted by all-caps, and it was not written by me.

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Re: WHY are two autopilots required for a go-around...

Postby 3WE » Wed May 04, 2016 2:19 pm

...ridiculous, irrelevant, inapplicable analogies lead to nowhere...
The more I think about it, Flyboy's REALLY WIDE planter is an interesting analogy. You have to turn off your black and white thinking and get beyond the fact that the planter is intended to be basically a rigid hitch...

Consider that three's a hinge on the primary tongue at the tool bar (and there is since it 'folds' for transport...Here's this ONE "cross brace" that controls the angle between the tongue and the tool bar...And given that those cross braces are actually a bit close in, the far ends of the planter have quite the mechanical advantage to push and pull against those braces.

With a little imagination, I see that one of them has half the 'authority' to adjust that angle at the hinge....but when you put the second one there, you have double the 'authority'.

It also illustrates the 737 autopilot system...the brace on the left is controlled by autopilot one, the brace on the right, by autopilot two.

The turn signal thing also contains frightening wisdom...different systems have different designs. :shock:

Flyboy is a sneaky, subtle SOB.

(Probably comes from memorizing all those damn procedures while still remembering how the fundamentals come into play)
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Re: WHY are two autopilots required for a go-around...

Postby Gabriel » Wed May 04, 2016 3:13 pm

With a little imagination, I see that one of them has half the 'authority' to adjust that angle at the hinge....but when you put the second one there, you have double the 'authority'.

It also illustrates the 737 autopilot system...the brace on the left is controlled by autopilot one, the brace on the right, by autopilot two.
Except that this is not the case in the 737 autopilot system, where each and every autopilot channel uses all the hydraulic PCUs that are available (i.e any and each of them gives orders to both crossbars of your planter).

The reason is quite simple: I don't know how this is geared, but the autopilot DOES move the control columnS, which are mechanically linked and they move together, so pushing or pulling one control column moves all control columns, all PCUs, and all elevators out there.

What moves the elevator is not the autopilot channel but the hydraulic PCUs. Sure, the AP channels give the orders to the PCUs, but I don't see why (except in the case mentioned in my previous post) having two channels giving orders to the PCUs would make the PCUs move any faster or farther. The PCUs are powered by hydraulic systems that have a given pressure and flow rate independently of how many autopilot channels are asking them to move.

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Re: WHY are two autopilots required for a go-around...

Postby flyboy2548m » Wed May 04, 2016 5:12 pm

Except that this is not the case in the 737 autopilot system, where each and every autopilot channel uses all the hydraulic PCUs that are available (i.e any and each of them gives orders to both crossbars of your planter).
No, it doesn't, at least not in approach mode. The onside autopilot only works the onside servo, which, in turn, works only the onside actuator(s). This is a huge part of Fail-Operational redundancy. "We" wouldn't want bad AP commands going to both servos now, would "we"?
The reason is quite simple: I don't know how this is geared, but the autopilot DOES move the control columnS, which are mechanically linked and they move together, so pushing or pulling one control column moves all control columns, all PCUs, and all elevators out there.
You're close, Gabby, but not quite there. The control columns move not because of the autopilot, but because of the "backdrive" that Bradley referred to (not that you believe him either). In other words, there is a delay involved. Just like there is a delay involved when one side of the elevator commanded by its servo commanded by its AP moves the offside elevator through the torque tube. It's not a huge delay, but it's there.

Hope this helps (I'm not holding by breath though).
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Re: WHY are two autopilots required for a go-around...

Postby flyboy2548m » Wed May 04, 2016 5:22 pm

I'm with Gabriel on this one. I could pull out some analogies from a hat, but I will not.
Per
It's up to you, of course, but of all possible times to be "with Gabriel", I'm not sure this is the best one. At the end of the day there are only two possibilities here: either Gabby is full of shit or a company that's been building airplanes for a century (building specifically 737s for half of that century) doesn't know what they're doing.

So, you know, choose your sides carefully.
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