Experiment & Questions

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Experiment & Questions

#1 Post by Learning2Fly » Mon Feb 26, 2018 11:32 am

I know most of the members here can replicate this scenario (or something similar) in a majority of their flights.

You level off at 10,000 feet.  ​While in level flight,  traveling a distance of 500 miles (no heading change),
​your pressure altitude remains at 10,000 feet.

​Why?

​Consider that your mechanical/STBY ADI.  The  ​aircraft attitude has not changed.

Shouldn't the altitude increase by "approximately" 166, 500 feet?  Meaning, altimeter reading of 176,500 feet?

If you read between the lines, you understand where this discussion might go, however I can't
​explain this and it's f*@#ing freaking me out.

Serious discussion only please.
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Re: Experiment & Questions

#2 Post by trey kule » Mon Feb 26, 2018 12:27 pm

Seriously, I have no idea whatquestion you are asking. Smarter people than me can maybe make sense of your numbers..
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Re: Experiment & Questions

#3 Post by photofly » Mon Feb 26, 2018 12:36 pm

He thinks he should be entering outer space.

Level flight isn’t laser-beam straight. Level means following the curvature of the earth, just like a water level. (cf. “Sea level”, sound familiar?)

If you were following a laser beam, after sufficient distance your attitude would have changed, because attitude is measured relative to the horizon. People in Australia are not actually standing on their heads, are they!
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Re: Experiment & Questions

#4 Post by Learning2Fly » Mon Feb 26, 2018 12:48 pm

trey kule wrote:
Mon Feb 26, 2018 12:27 pm
Seriously, I have no idea whatquestion you are asking. Smarter people than me can maybe make sense of your numbers..
Thanks for the response. Let me clarify:

When you level off and fly a specific heading, the aircraft is not climbing or descending with reference to sea level.
ADI remains at pitch/attitude (normally a few degrees positive).

The indicated pressure altitude is 10,000 feet throughout the flight.

According to USGS/CCMEO, there should be a change in altitude of about 166,500 feet over a 500 mile distance.

I am at a loss for explaining why the aircraft is not constantly corrected downward over the flight to compensate for
this change in landscape.
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Re: Experiment & Questions

#5 Post by Learning2Fly » Mon Feb 26, 2018 12:51 pm

photofly wrote:
Mon Feb 26, 2018 12:36 pm
He thinks he should be entering outer space.
No, that's not what I think, however I've been challenged with a question that I cannot answer.
Level flight isn’t laser-beam straight.
Agree, there are many variables effecting the flight.
Level means following the curvature of the earth, just like a water level. (cf. “Sea level”, sound familiar?)
Okay, now explain why the ADI never goes negative pitch to follow the curvature?

Over 500 miles, the expectation is to correct for 166,500 feet (or about 31 miles ) of 'altitude'. That is a significant change
during a relatively 'short' distance.
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Re: Experiment & Questions

#6 Post by Tips Up » Mon Feb 26, 2018 12:59 pm

Because it’s a flat earth with a ring of ice around it holding the oceans....
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Re: Experiment & Questions

#7 Post by Learning2Fly » Mon Feb 26, 2018 1:05 pm

Tips Up wrote:
Mon Feb 26, 2018 12:59 pm
Because it’s a flat earth with a ring of ice around it holding the oceans....
Is that what you believe, or can you explain the instrumentation?

I'd love to know the answer, because I can't explain the gyro behaviour, the ADI, or the constant altitude indicated.

I'm not even attempting to be a d*#k, this is truly frustrating.
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Re: Experiment & Questions

#8 Post by photofly » Mon Feb 26, 2018 1:57 pm

The gyro self- corrects to gravity, at the rate of about 5 degrees per minute. It has to, to meet TSO standards.

Your pitch change over a great circle route is a lot slower. Only if you were in low earth orbit (flying at about 15nm/second) would you outrun the AI, then your pitch rate would be 360 degrees (nose down) in about 84 minutes. Clearly you’re flying a lot slower than that, so the gyro fixes itself faster than you notice the pitch change.

But if you want a real system that suffers from and has to be corrected for exactly the problem you are describing, look up both “transport error” and “apparent precession” in the context of inertial reference platforms, which maintain a reference attitude considerably more accurately than your average AI.

Plus any real spacecraft have to deal with this.

The Apollo spacecraft had a special box (“ORDEAL”) on its AI to rotate it in pitch to maintain a constant horizon attitude. When it left earth orbit that mode was switched off.
http://nassp.sourceforge.net/wiki/Orbit ... _And_Lunar
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Re: Experiment & Questions

#9 Post by Learning2Fly » Mon Feb 26, 2018 2:19 pm

photofly wrote:
Mon Feb 26, 2018 1:57 pm
The gyro self- corrects to gravity, at the rate of about 5 degrees per minute. It has to, to meet TSO standards.
I believe this to be true based on my research and minimal experience repairing flight instrumentation (at a very basic level).

However, while discussing this with a gentleman at a family function this weekend, he explains that the aircraft also imparts
forces on the gyro while banking, yawing, pitching, turbulence, etc. These forces can be much higher than gravity and do
not alter the performance of the gyro.

If I/we think about this, if an airplane flies over-seas at a distance of 6,500 miles, do we expect the gyro to be out of calibration
by 90 degrees? Meaning, the gimbal for the vertical plane would be offset 90 degrees...yet the ADI still indicates level flight.
How is this possible?

Even on a bench test, if the ADI was tilted 45 degrees, the indicator would not reset to 'level' due to gravity no matter how
long it was raised at an angle.

Another question for the high altitude cruisers:

How much ground distance would you cover to reach 35,000 feet? How much of that distance is relatively straight flight (no heading change)
after take-off vectoring, etc.?

Video of ADI Bench Test:
https://www.youtube.com/watch?v=9QKgYm1VRHI
[youtube]www.youtube.com/watch?v=9QKgYm1VRHI[/youtube]
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Re: Experiment & Questions

#10 Post by photofly » Mon Feb 26, 2018 2:41 pm

Learning2Fly wrote:
Mon Feb 26, 2018 2:19 pm
However, while discussing this with a gentleman at a family function this weekend, he explains that the aircraft also imparts
forces on the gyro while banking, yawing, pitching, turbulence, etc. These forces can be much higher than gravity and do
not alter the performance of the gyro.
The gentleman you spoke to at the wedding doesn't understand the mechanism inside the AI.

The mechanism inside the AI (the vanes that swing out of the way to enable air-jets to right the gimbal) isn't sensitive to the strength of a local force, only to its duration. The erection rate is 5 degrees per minute for the duration of the force, whether the force is very large or very small. So short duration bumps and accelerations have little effect, even if they're very strong.

Gravity maintains a constant pull, righting the gimbal at 5 degrees per minute, forever, until it's upright.

Steady turns cause relatively long duration non-vertical accelerations, but turns being what they are, the direction of the error turns too, so even in a turn of many minutes (through many multiples of 360 degrees) the accumulated error remains small. You will also note that holding patterns, in which aircraft perform steady turns through large heading changes often in instrument conditions, also include straight-and-level-portions during which the AI can reset some of the error accumulated over the preceeding sixty seconds of turning flight.

If you look up the requirements of TSO-5a (I think it is, from memory) and do some maths or some simulations, you'll see that the accumulated errors in a turn at the specified arspeed meet the requirement if the self-erection rate is no faster than 5 degrees per minute. On the other hand, an AI that erects much slower than that takes unnecessarily long to reset itself after a topple.

Moral: family weddings are bad places to learn how aircraft instruments work.


If I/we think about this, if an airplane flies over-seas at a distance of 6,500 miles, do we expect the gyro to be out of calibration
by 90 degrees? Meaning, the gimbal for the vertical plane would be offset 90 degrees...yet the ADI still indicates level flight.
How is this possible?
Because, as I explained, the AI self-erects to follow gravity, at a rate of 5 degrees per minute.
Even on a bench test, if the ADI was tilted 45 degrees, the indicator would not reset to 'level' due to gravity no matter how
long it was raised at an angle.
If you tilt the AI, you're tilting the case, which is not the appropriate test. To test the self erection mechanism you'd have to tilt gravity by 45 degrees. One way to do that is to fly very quickly to a different part of the globe (c.f. "transport error", which I told you about earlier.) If you could travel 6,500 miles in a few seconds, then the AI would indeed be out of calibration by 90 degrees. Although it would probably lock first - mostly they don't work past 60 degrees of pitch, unless you have a specialist one.

Alternatively, you could, while the gyro is spinning, reach in and push the gimbal to tilt the axis to 45 degrees. If you do, do it slowly, to prevent the precessional forces damaging the instrument. Once you let go, it will indeed reset 'level' due to gravity. And from 45 degrees, it will take about 9 minutes, if the AI meets its specification. Something similar happens to the AI after every time I gimbal-lock it practicing spins in a particular Cessna 172.

However, consider an AI starting up from stationary, that just happened to be at 45 degrees, at rest. While the gyro is spinning slowly it still resets to level (it has to, otherwise it wouldn't be any use) but because it's spinning slowly it does so much quicker.
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Re: Experiment & Questions

#11 Post by Learning2Fly » Mon Feb 26, 2018 3:02 pm

photofly wrote:
Mon Feb 26, 2018 2:41 pm

The mechanism inside the AI (the vanes that swing out of the way to enable air-jets to right the gimbal)
Air-jets? We're talking about an electrically powered ADI, correct? I've never worked with a vacuum powered ADI, so I cannot
comment.
Because, as I explained, the AI self-erects to follow gravity, at a rate of 5 degrees per minute.
Why is this not an issue during climb then? The aircraft is pitched up for a long duration of time; if the level flight is corrected
to follow the curvature, how do we explain the climb angle? This is why I asked the question about high altitude cruising prior
to this post.
If you tilt the AI, you're tilting the case, which is not the appropriate test. To test the self erection mechanism you'd have
to tilt gravity by 45 degrees. One way to do that is to fly very quickly to a different part of the globe (c.f. "transport error", which
I told you about earlier.)
I'm not sure I follow. By tilting the case you are effectively simulating the aircraft angle because the case is fixed to the aircraft
frame.

We're at the following stalemate now:
- The ADI can be corrected in level flight by gravity, but not during long duration ascend or descend?

Again, I'm not trying to be difficult, I want to understand this perhaps by playing devil's advocate if necessary. I'd love for anyone
here to place a level on the glare-shield and observe the bubble to determine if the ADI is being reset by gravity while the airframe
(nose) maintains constant (or near constant) attitude.

The few couple of times that I've flown (right seat), I don't recall feeling nose down during the cruising phase.
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Re: Experiment & Questions

#12 Post by photofly » Mon Feb 26, 2018 3:11 pm

Learning2Fly wrote:
Mon Feb 26, 2018 3:02 pm
Air-jets? We're talking about an electrically powered ADI, correct? I've never worked with a vacuum powered ADI, so I cannot
comment.
The self-erection mechanism for a vacuum-powered AI is part of the PPL syllabus, which I teach. I also have a non-functioning AI with which to teach it, so I'm very familiar with it. An electric AI will have a different mechanism but must follow the same general idea because it's subject to the same TSO certification limits. An AI only works if the self-erection mechanism is sensitive to the duration but not the strength of local accelerations. That's how it can both self-erect over a longish period, and still function as an attitude reference that doesn't go wonky in turbulence or turns.
Why is this not an issue during climb then? The aircraft is pitched up for a long duration of time; if the level flight is corrected
to follow the curvature, how do we explain the climb angle? This is why I asked the question about high altitude cruising prior
to this post.
The aircraft is pitched nose up but the gyro axis remains vertical. Since the self-erection mechanism is attached to the gyro axis and not to the aircraft, there's no change in the force direction as felt by the self-erection mechanism and no error, regardless of how long the aircraft pitch is non-level.

Whether the aircraft is in level flight, or climbing, or descneding, If the aircraft travels a sufficient distance around the globe then the fixed gyro axis isn't parallel with the new local vertical any more (pretty much the definition of "transport error") and the self erection mechanism gets to work.

By the way altitude is quite irrelevant.
I'm not sure I follow. By tilting the case you are effectively simulating the aircraft angle because the case is fixed to the aircraft
frame.
That's correct. The gyro axis a) remains fixed in space in the short term and b) slowly the mechanism corrects it to be vertical to local gravity. The orientation of the aircraft and the instrument case is entirely relevant to the gyro or the self-erection mechanism (as long as it doesn't lock) so aircraft movements (roll, pitch, yaw) don't cause AI errors. You can fly in a sideslip with one wing down all day, the gyro won't tilt.

What causes a an AI to indicate incorrectly is a longish duration steady acceleration, such as on the runway at takeoff, or during a turn. The technical specifications for an approved attitude indicator are worked out so that in normal flight the aggregate error compounded over various manoeuvres remains small and in any case is always being decreased, at 5 degrees of error per minute, when the accelerations stop, i.e. whenever the aircraft is in unaccelerated flight. That makes the instrument useful as an attitude reference in IMC. Useful, but not perfect.
We're at the following stalemate now:
- The ADI can be corrected in level flight by gravity, but not during long duration ascend or descend?
There's no stalemate. Long duration ascents and descents don't involve accelerations. The gyro is designed to self-erect to follow accelerations, not the orientation of the case.
Again, I'm not trying to be difficult, I want to understand this perhaps by playing devil's advocate if necessary.
You're not being difficult, you're just missing the point of how an AI works.
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Re: Experiment & Questions

#13 Post by photofly » Mon Feb 26, 2018 3:40 pm

Here's the excerpt from TSO-C4c which gives the acceptable performance error in turn for a certfiied gyro attitude reference (electric, or pneumatic, doesn't matter):
Screen Shot 2018-02-26 at 5.36.54 PM.png
Screen Shot 2018-02-26 at 5.36.54 PM.png (51.48 KiB) Viewed 1428 times
Useful, not perfect.

And here's a graph of the accumulated pitch and bank error for a simulated AI in a 360 degree standard rate turn, with a self-erection rate of 5 degrees per second (if I remember right) although at 100 knots. I guess I should redo the simulation for 180mph, really.
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Re: Experiment & Questions

#14 Post by Zaibatsu » Mon Feb 26, 2018 4:42 pm

The reason we don't wind up in outer space is there is a pressure and gravity gradient we have to deal with.

Supposing your attitude indicator froze at a fixed point in space. Or maybe a point that rotated 360 degrees every 23 hours and 56 minutes. Your aircraft would climb and while doing so slowly start losing speed, losing power, and eventually stall unless you brought the nose back to the natural horizon.

We also have gravity helping us through the stability of the aircraft, since your weight vector will change direction at the same rate the horizon will.
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Re: Experiment & Questions

#15 Post by Learning2Fly » Mon Feb 26, 2018 6:35 pm

photofly wrote:
Mon Feb 26, 2018 3:11 pm

By the way altitude is quite irrelevant.
I have to disagree. The altitude is the entire problem with the instrumentation over a curve of 8 inches per mile squared.

What you explained earlier is that a gyro in level flight with little (no) accelerations, would self-correct due to gravity. By doing
so, the altitude corrected by the ADI alone is 166,500 feet over 500 miles.

By the same function, while the aircraft is ascending or descending at a relatively fixed pitch (little to no accelerations), the ground
distance covered during this stage of flight is well over 500 miles from a 35,000 foot altitude. That means, by only observing the ADI
for 'corrections', the altitude is compensated by 166,000 feet.

During take-off with a positive pitch of 10-15 degrees, the ADI will continue to indicate 10-15 degrees while the altitude changes at 500- 1000 ft./min
(for example). How is the rate of climb steady as the ground curvature is changing at a rate higher than 500 ft./min? This velocity is
not linear, and will increase as the distance increases. ie: at 1000 miles, the curve is 651,201 feet (123 miles).

In that sense, the altitude makes all the difference; I'll create or find a diagram to illustrate what I'm trying to convey.

Are you, or anyone else here able to put a small fluid level on the glare-shield and video the result during a climb, level flight, and descent
to show the attitude of the nose in all phases of flight?

I understand that local, instantaneous forces will cause the gyro to deflect slightly and that gravity will act upon the gyro to self-correct.
That is not my issue.

The fluid level would corroborate the pitch and whether the ADI is following the gyro errors, or gravity.

Do you agree with this?
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Re: Experiment & Questions

#16 Post by photofly » Mon Feb 26, 2018 7:16 pm

I confess I no longer have any idea what you’re talking about.

Could pdw explain it to me?
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Re: Experiment & Questions

#17 Post by Learning2Fly » Mon Feb 26, 2018 8:18 pm

Here's a quick diagram of what I'd like to discuss. I'll fill in the math tomorrow as it's getting late, but this
should get the wheels turning for now.

Without specifying values, based on your explanation of the ADI and gravity, will the horizon indicate higher or lower
when the aircraft is in a continuous climb at point B (compared to point A)?

Consider rate of climb, altitude, attitude, control surfaces (trim, yoke, etc.) as the aircraft moves along the brown line or yellow line.


Image
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Re: Experiment & Questions

#18 Post by youhavecontrol » Mon Feb 26, 2018 8:46 pm

I would think the aircraft would be climbing in a slightly curved trajectory, as the pitch maintained is not as important as the rate of climb and airspeed.
Regardless, the pendulum vane, as designed to keep the attitude indicator level relative to gravity (and Not rigidity in space), would also cause the aircraft to climb in a curved trajectory while following a constant pitch.
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Re: Experiment & Questions

#19 Post by photofly » Mon Feb 26, 2018 9:09 pm

The attitude indicator indicates a plane locally tangent to the surface of the planet.

The higher you are, the lower the real (visible) horizon is. (cf “dip of the horizon”)

So the higher you are, the more the AI diverges from the real horizon. This seems like a sensible place to start:
https://aty.sdsu.edu/~aty/explain/atmos_refr/dip.html

To maintain the same aircraft attitude at any altitude you should follow the guidance provided by the AI, or else aim above the real visible horizon by an amount that increases with altitude.

You can tell me what the horizon dip is at 3000 feet and FL300. (I calculate about 0.7 and 3 degrees respectively.)

Does that answer the question?
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Re: Experiment & Questions

#20 Post by Learning2Fly » Mon Feb 26, 2018 9:24 pm

Thanks for the link, I've bookmarked it and will read it tomorrow.

This US Navy training video explains the function of the gyro different from what is posted here (time stamped for brevity).

[youtube]https://youtu.be/JnKloSdUJLo?t=4m00s[/youtube]
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Re: Experiment & Questions

#21 Post by photofly » Mon Feb 26, 2018 9:34 pm

The video is brief about self-erection mechanisms, but consistent with what I wrote.

From https://www.faa.gov/documentLibrary/med ... 65-15A.pdf
81C837E2-A03B-4187-888B-96C1CD847A35.jpeg
81C837E2-A03B-4187-888B-96C1CD847A35.jpeg (161.45 KiB) Viewed 1306 times
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Re: Experiment & Questions

#22 Post by ahramin » Mon Feb 26, 2018 9:39 pm

What would be really strange to me, is if it really did read 176 500 feet. How would the plane magically compensate for the earth's rotation and the orbital speed of the earth travelling about the sun? Not to mention we're way out here on an arm of the milky way as the galaxy spins around. You'd think the centripedal force would be so great that the gyro would collapse in on itself.
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Re: Experiment & Questions

#23 Post by Chris M » Tue Feb 27, 2018 5:09 am

What you don't seem to be grasping is that the AI only cares about the local gravitational field of the Earth. It isn't an astro-navigational instrument that is stabilized relative to the solar system or galaxy. The axis of the AI will always, via the self-erecting mechanism Photofly has so well explained (much thanks by the way, I'm learning a lot here too), point to the center of mass of the Earth. Straight down, in simplified terms.

What this means is that it doesn't matter how far around the globe you travel. The axis of the AI is always pointed directly at the center of the Earth and a local, level flight attitude is always perpedicular to it.

With regards to the diagram you drew, the second point should not still be 15 degrees pitch attitude. You're measuring that angle against an imaginary horizontal plane bisecting the planet. You should be drawing a radial line from the center of the planet and measuring the angle to that, which will get you a lot more than 15 degrees.
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Re: Experiment & Questions

#24 Post by Learning2Fly » Tue Feb 27, 2018 5:46 am

Chris M wrote:
Tue Feb 27, 2018 5:09 am
What you don't seem to be grasping is that the AI only cares about the local gravitational field of the Earth. It isn't an astro-navigational instrument that is stabilized relative to the solar system or galaxy. The axis of the AI will always, via the self-erecting mechanism Photofly has so well explained (much thanks by the way, I'm learning a lot here too), point to the center of mass of the Earth. Straight down, in simplified terms.
This is not what the US Navy video explains. The Gyro self corrects to maintain the original phase of the spinning rotors.

However, there are more issues besides this point which I'll be updating shortly.

With regards to the diagram you drew, the second point should not still be 15 degrees pitch attitude. You're measuring that angle against an imaginary horizontal plane bisecting the planet. You should be drawing a radial line from the center of the planet and measuring the angle to that, which will get you a lot more than 15 degrees.
Again, the US Navy video contradicts this paragraph. Once we detail the curve of the earth vs. climbing pitch, and VSI the problem becomes more apparent.
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Re: Experiment & Questions

#25 Post by photofly » Tue Feb 27, 2018 5:59 am

The AI is fixed in space over the short term, and adjusts itself to point “down” over the longer term. “Down” is the only acceleration that’s consistent in direction, so averaging the directions of the accelerations felt by the instrument over a sufficiently long period gives an adequate reference, and enable the instrument to right itself at startup and fix its own precessional errors during operation.

Reasons to erect faster are to recover from topples quicker, and to reduce the effect of errors due to precession and transport effects.

A reason to erect slower is to reduce the compounded error introduced during accelerating flight, that is, during turns, and while the aircraft is speeding up or slowing down, or pulling out of a dive. Speeding up and slowing down, and pulling out of dives doesn’t usually last very long. Turns generate bigger pitch and roll errors.

Slow turns are generally worse for errors than fast ones. In a rapid turn the direction of error reverses itself quickly. In a slow turn there’s enough time for the error to build up.

Attitude errors that occur during turns appear as both both pitch and roll errors. Imagine an airplane flying north that rolls into a right turn. The gyro senses an acceleration to the east and slowly starts to builds a bank error due to the non-vertical local acceleration. After some seconds the aircraft has turned ninety degrees and the initial east tilt error in the gyro appears as a pitch-up error, incorrectly showing a nose up attitude when the aircraft is level. Alternatively the aircraft will be flown nose down if held level according to the AI.

As the aircraft turns through east the error continues to accumulate, the direction of the error always to the inside of the turn. The biggest aggregate error occurs shortly after 180 degrees of turn.

If the aircraft continues to fly a circular path for long enough, eventually the AI error settles down to a steady combined pitch-and-bank error, showing. too much nose-up and under-representing the bank.

I have some graphs somewhere showing how the error varies with rate of turn. An AI that meets TSO-4c has sufficiently small errors still to be useful as an attitude reference during turns.
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