Lion Air 737 MAX crashed

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Re: Lion Air 737 MAX crashed

Unread post by jimdavis » Sun Feb 10, 2019 10:43 pm

Breytie wrote:
Sun Feb 10, 2019 9:29 pm
Internal instruments (gyros / accelerometers etc) can give attitude information - where the nose is in relation to the horizon.
A vane or pitot-static based systems measure the actual airflow direction. The plane may be horizontal, but strong up and down drafts change the relative airflow direction. Same thing for GPS based ground speed vs airspeed. Like many planes now have systems that calculate and predict stall, older planes had horns that sounded when the wing actually started stalling, causing a change in pressure at the port on the wing.

Now a daft sounding question - If your plane uses a gyro based system for attitude indication - how does that system compensate for the curvature of the earth on long distance flights? You end up literally upside down when you land in Perth compared to when you took off from ORT.
Breytie, you are trying to pickle my brain. Do you suppose the gyro-based system is earth-tied/influenced as were the old vacuum gyros? And I don't really get how you are linking attitude indications with the stall, unless you are bringing in another variable.

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Re: Lion Air 737 MAX crashed

Unread post by StressMerchant » Mon Feb 11, 2019 10:43 am

Now a daft sounding question - If your plane uses a gyro based system for attitude indication - how does that system compensate for the curvature of the earth on long distance flights? You end up literally upside down when you land in Perth compared to when you took off from ORT.
Not such a daft question. Probably well off topic, but consider the following:
Suppose that you have a perfect gyro, ie it runs with no drift or errors. If it is free to move on a frictionless support, has its axis pointing straight down at the (local) ground, and is on the equator. Will it remain in the same orientation relative to the spot of ground beneath it, or will it precess at 15 deg/hr? And if it were at one of the Poles?
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Re: Lion Air 737 MAX crashed

Unread post by jimdavis » Mon Feb 11, 2019 10:59 am

StressMerchant wrote:
Mon Feb 11, 2019 10:43 am
Now a daft sounding question - If your plane uses a gyro based system for attitude indication - how does that system compensate for the curvature of the earth on long distance flights? You end up literally upside down when you land in Perth compared to when you took off from ORT.
Not such a daft question. Probably well off topic, but consider the following:
Suppose that you have a perfect gyro, ie it runs with no drift or errors. If it is free to move on a frictionless support, has its axis pointing straight down at the (local) ground, and is on the equator. Will it remain in the same orientation relative to the spot of ground beneath it, or will it precess at 15 deg/hr? And if it were at one of the Poles?
Interesting question, Stressors. Are we talking about actual precession, or apparent precession? 8)

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Re: Lion Air 737 MAX crashed

Unread post by StressMerchant » Mon Feb 11, 2019 11:23 am

Apparent - as seen by the observer.
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Re: Lion Air 737 MAX crashed

Unread post by Hop Harrigan » Mon Feb 11, 2019 11:51 am

I think it will end up inverted in Aus...we all know they’re down under and a little challenged 😀
No, I don’t know how that’s resolved...give us a clue
Here’s an idea...why not have a FIXED vane in the airflow and then measure the force on that vane instead of its position. Might also have mechanical issues I guess...
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Re: Lion Air 737 MAX crashed

Unread post by Burner » Mon Feb 11, 2019 11:52 am

Apparent wander/precession/drift is zero at the equator, and 15 degrees at the poles. Used to be part of the CPL and ATPL syllabuses when I wrote them 13 odd years ago. All this assuming a horizontal axis gyro.
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Re: Lion Air 737 MAX crashed

Unread post by rainier » Mon Feb 11, 2019 2:41 pm

Now a daft sounding question - If your plane uses a gyro based system for attitude indication - how does that system compensate for the curvature of the earth on long distance flights? You end up literally upside down when you land in Perth compared to when you took off from ORT.
As designer of many of these systems perhaps I can give a shortish description.

Start with the old vacuum driven artificial horizons. If you dissect them you will find a part called a "pendulous unit". This is really just a normal pendulum responding to gravity (or more accurately "acceleration"). If it is off kilter it opens vanes that force a change of airflow that spins the gyro so that over some time the gyro corrects.
This is also the reason why, if you would fly a continuous banked, balanced turn your horizon will eventually show that you are level even though clearly you are not.

Vacuum driven system where replaced initially with electrical gyros but a form of the pendulous unit would still be needed.

Modern AHRS systems started working in much the same way but relied on exceedingly accurate and drift free electronic gyroscopes to emulate what a good vacuum or electrical system could do and now used solid state accelerometer chips to emulate the pendulous unit.

Since electronic gyros that are good enough for this are still exceedingly expensive ways to use lower cost gyros have been created. Since these gyros tend to drift very fast on their own it has now become common practice to "aid". In principle we now use these gyros, accelerometers and something that tells us the velocity of the aircraft (true airspeed or a 3D velocity vector from a GPS or the usual sources). If we know the velocity and we know the rates of rotation around the three major axis (the gyros give us that with some random error) we can work out the acceleration forces acting on the aircraft caused by rotation and changes of the velocity. All the while our accelerometers give us the same information PLUS the gravity vector - we don't know by looking at the accelerometers where the direction of Earth is unless you are flying nicely straight and level. So now we can do a nifty little trick - we subtract the velocity derived acceleration force measurement from that of the real accelerometers and out pops the 1G vector that points to the Earth. It's "noisy" but the average is useful. We use that to correct our gyros continuously. This form of AHRS can show a correct bank angle regardless of how long you stay in a banked turn even if it is not balanced.

Now regardless of any of the above methods - we are correcting our gyro derived artificial horizon to detect and follow gravity. Since we are rotating at 15 degrees per hour (24*15=360) all we need to ensure is that we correct our gyros at a rate that exceeds this and we no longer have to worry too much about the Earth rotating.

Of course this only applies to those that are not members of the flat Earth society. For the esteemed members that do, no correction is ever required and any mention of such is at best a poor attempt of using observation to prove a fact.
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Re: Lion Air 737 MAX crashed

Unread post by jimdavis » Mon Feb 11, 2019 8:09 pm

Burner wrote:
Mon Feb 11, 2019 11:52 am
Apparent wander/precession/drift is zero at the equator, and 15 degrees at the poles. Used to be part of the CPL and ATPL syllabuses when I wrote them 13 odd years ago. All this assuming a horizontal axis gyro.
Ha ha ha, sorry Burner you didn't RTFQ. I fell into the same trap when I originally looked at it. They sneakily mentioned that the axis was vertical! This means zero apparent drift at the poles.! :lol:

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Re: Lion Air 737 MAX crashed

Unread post by Burner » Mon Feb 11, 2019 8:19 pm

jimdavis wrote:
Mon Feb 11, 2019 8:09 pm
Burner wrote:
Mon Feb 11, 2019 11:52 am
Apparent wander/precession/drift is zero at the equator, and 15 degrees at the poles. Used to be part of the CPL and ATPL syllabuses when I wrote them 13 odd years ago. All this assuming a horizontal axis gyro.
Ha ha ha, sorry Burner you didn't RTFQ. I fell into the same trap when I originally looked at it. They sneakily mentioned that the axis was vertical! This means zero apparent drift at the poles.! :lol:

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AAARGH then yes it would be inverse!
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Re: Lion Air 737 MAX crashed

Unread post by jimdavis » Mon Feb 11, 2019 8:22 pm

rainier wrote:
Mon Feb 11, 2019 2:41 pm
Now a daft sounding question - If your plane uses a gyro based system for attitude indication - how does that system compensate for the curvature of the earth on long distance flights? You end up literally upside down when you land in Perth compared to when you took off from ORT.
As designer of many of these systems perhaps I can give a shortish description.

Start with the old vacuum driven artificial horizons. If you dissect them you will find a part called a "pendulous unit". This is really just a normal pendulum responding to gravity (or more accurately "acceleration"). If it is off kilter it opens vanes that force a change of airflow that spins the gyro so that over some time the gyro corrects.
This is also the reason why, if you would fly a continuous banked, balanced turn your horizon will eventually show that you are level even though clearly you are not.

Vacuum driven system where replaced initially with electrical gyros but a form of the pendulous unit would still be needed.

Modern AHRS systems started working in much the same way but relied on exceedingly accurate and drift free electronic gyroscopes to emulate what a good vacuum or electrical system could do and now used solid state accelerometer chips to emulate the pendulous unit.

Since electronic gyros that are good enough for this are still exceedingly expensive ways to use lower cost gyros have been created. Since these gyros tend to drift very fast on their own it has now become common practice to "aid". In principle we now use these gyros, accelerometers and something that tells us the velocity of the aircraft (true airspeed or a 3D velocity vector from a GPS or the usual sources). If we know the velocity and we know the rates of rotation around the three major axis (the gyros give us that with some random error) we can work out the acceleration forces acting on the aircraft caused by rotation and changes of the velocity. All the while our accelerometers give us the same information PLUS the gravity vector - we don't know by looking at the accelerometers where the direction of Earth is unless you are flying nicely straight and level. So now we can do a nifty little trick - we subtract the velocity derived acceleration force measurement from that of the real accelerometers and out pops the 1G vector that points to the Earth. It's "noisy" but the average is useful. We use that to correct our gyros continuously. This form of AHRS can show a correct bank angle regardless of how long you stay in a banked turn even if it is not balanced.

Now regardless of any of the above methods - we are correcting our gyro derived artificial horizon to detect and follow gravity. Since we are rotating at 15 degrees per hour (24*15=360) all we need to ensure is that we correct our gyros at a rate that exceeds this and we no longer have to worry too much about the Earth rotating.

Of course this only applies to those that are not members of the flat Earth society. For the esteemed members that do, no correction is ever required and any mention of such is at best a poor attempt of using observation to prove a fact.
Thanks Rainier, very neatly explained. So, in a nutshell, you have substituted the brass gyro wheel and the gravity tied pendulous unit, with fiendishly clever electronics which simulate the gyro and the pendulous unit. And even more devilish electronics to take the long term turning error out of the pendulous unit.

I have to tell you I am hugely impressed by clever guys like you who work miracles with minute things that possibly don't exist!

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Re: Lion Air 737 MAX crashed

Unread post by Hop Harrigan » Mon Feb 11, 2019 9:29 pm

Hi rainier,
Thanks for the insight. I really like the nifty trick to determine the 1G vector.
Still doesn’t explain why the Aussies are so odd mind you 😂
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Re: Lion Air 737 MAX crashed

Unread post by Mfezi » Tue Feb 12, 2019 8:06 am

rainier wrote:
Mon Feb 11, 2019 2:41 pm
As designer of many of these systems perhaps I can give a shortish description.

[snipped excellent explanation]
Good explanation, Rainier. One thing that I am surprised about is that you don't mention the use of a magnetometer? I work mostly with military systems, but have had some exposure to civilian ones and even developed smaller systems for flight testing and UAV control systems. All the systems that I use also include a 3-axis magnetometer. This then gives you two reference vectors: The local earth magnetic vector and the gravity vector (which, as you mentioned also, is not quite the gravity vector once you accelerate in any way). You really need both reference vectors for a good, consistent, and 1:1 attitude reference. Both are susceptible to disturbances, so obviously they get combined in some optimal way with other sensors. We usually combine the gyro, accelerometer, magnetometer and GPS data using extended Kalman filters or one of a number of alternative complementary filters in order to get really high accuracy attitude and position estimates. Usually temperature measurements are also used as part of the calibration or compensation process - especially in helping to correct for the gyro bias and drift errors. In principle, I believe that is what most AHRS also do.
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Re: Lion Air 737 MAX crashed

Unread post by StressMerchant » Tue Feb 12, 2019 11:46 am

Well, the preceding few posts do show that there are no daft questions, and that there is a lot to learn from the collective minds on here.

To pick up on Mfezi's and Rainier's posts, you can get a lot of info from the fusion of accelerometer, gyro, and magnetometer data. The combo is often called 9DOF, meaning 9 degrees of freedom (fancy talk for 9 independent variable: 3 axis accelerometer, 3 axis gyro, and 3 axis magnetometer). 10DOF just adds an altimeter.

The issues is that none of the inputs are perfect. Accelerometers are often noisy, gyros drift (and precess), magnetometers are subject to the vagaries of the earth's magnetic field. So they get cmbined, as Mfezi dais, usually using a Kalman filter. The Kalman filter isn't actually a filter, it's a method of predicting an outcome given uncertainties in the input data. Originally used for the space program. Complimentary filter is a similar idea, and also not really a "filter" in the classic sense.

So the interesting comparison, from the perspective of the current debate, is the safety aspect if you use it for AoA. Consider that the IMU system may give the wrong answer if any one of the inputs is faulty - and in a 9DOF system you potentially have 9. In the vane, you basically have one. So sometimes it is easier to have a vane system, with a system that can be handled by a procedure if the vane fails. Endless debate can ensue..... ;-)
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Re: Lion Air 737 MAX crashed

Unread post by richard C » Wed Feb 13, 2019 8:46 am

my mind is officially blown
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Re: Lion Air 737 MAX crashed

Unread post by heisan » Wed Feb 13, 2019 9:46 am

StressMerchant wrote:
Tue Feb 12, 2019 11:46 am
So the interesting comparison, from the perspective of the current debate, is the safety aspect if you use it for AoA. Consider that the IMU system may give the wrong answer if any one of the inputs is faulty - and in a 9DOF system you potentially have 9. In the vane, you basically have one. So sometimes it is easier to have a vane system, with a system that can be handled by a procedure if the vane fails. Endless debate can ensue..... ;-)
And don't forget that for an AoA calculation, you still need a pitot reading too... So make that 10 points of failure, and one of them is also subject to icing ;) .

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