A

apatti

Well Known Member
Ok, I've been told by someone (who shall remain unnamed because he lurks here) that my plumbing for my dual AHRS is ugly and will cause my plane to fall out of the sky within the first 10 hours! :eek: :) So, do any of you GRT dual AHRS users have any pics of how you plumbed your's?

Thanks,
 
Firstly, as long as the connection to your AD/AHRS is reliably airtight, what difference does it make how it looks?

Secondly, at the risk of hijacking your thread and starting a minor controversy, I'm wondering about the value of a dual AHRS system. It seems to me that having dual AHRS offers little value for most failures --outside of an outright failure. AHRS being solid state systems without any moving parts, I would think outright failures are the exception and discrepancies the norm. Typically if you want redundancy for this type of system, you would need THREE AHRS! Having a third AHRS unit would provide a tie-breaker. Without a tie breaker you would need some extraordinary algorithm (read guess) to determine which unit is at fault.

So it seems the only reason for a vendor to promote a dual AHRS system is because they are not confident in the quality of their AHRS product. I think this is why all of the certified systems are certified with either ONE or THREE (think Boeing) AHRS. With a single AHRS, it is either working, on not. If not, you rely on external backup instruments. With DUAL AHRS, how exactly, outside of a FULL failure, can you be sure which AHRS is giving you the correct information and which one is not? While it is good to have a back-up for most items in aviation, somehow, DUAL AHRS doesn?t pass the surface test. DUAL AHRS seem to make sense on the surface, when you peer behind the curtains however, it reveals more questions than answers.

How does a DUAL AHRS system arbitrate between units to determine which is correct without a tie breaker? It seems the only value of a dual AHRS system is if there is a clear-outright failure of one unit. Otherwise, any algorithm is only guessing as to which unit is correct when there is a discrepancy. Potentially this is a worse scenario than if your single AHRS had an outright failure. Without a third arbitrator, you would be using information of a bad AHRS or ignoring information of a good ADHRS, or integrating good data with bad data. So what am I missing?
 
w1curtis said:
Firstly, as long as the connection to your AD/AHRS is reliably airtight, what difference does it make how it looks?

Secondly, at the risk of hijacking your thread and starting a minor controversy, I'm wondering about the value of a dual AHRS system. It seems to me that having dual AHRS offers little value for most failures --outside of an outright failure. AHRS being solid state systems without any moving parts, I would think outright failures are the exception and discrepancies the norm. Typically if you want redundancy for this type of system, you would need THREE AHRS! Having a third AHRS unit would provide a tie-breaker. Without a tie breaker you would need some extraordinary algorithm (read guess) to determine which unit is at fault.

So it seems the only reason for a vendor to promote a dual AHRS system is because they are not confident in the quality of their AHRS product. I think this is why all of the certified systems are certified with either ONE or THREE (think Boeing) AHRS. With a single AHRS, it is either working, on not. If not, you rely on external backup instruments. With DUAL AHRS, how exactly, outside of a FULL failure, can you be sure which AHRS is giving you the correct information and which one is not? While it is good to have a back-up for most items in aviation, somehow, DUAL AHRS doesn?t pass the surface test. DUAL AHRS seem to make sense on the surface, when you peer behind the curtains however, it reveals more questions than answers.

How does a DUAL AHRS system arbitrate between units to determine which is correct without a tie breaker? It seems the only value of a dual AHRS system is if there is a clear-outright failure of one unit. Otherwise, any algorithm is only guessing as to which unit is correct when there is a discrepancy. Potentially this is a worse scenario than if your single AHRS had an outright failure. Without a third arbitrator, you would be using information of a bad AHRS or ignoring information of a good ADHRS, or integrating good data with bad data. So what am I missing?
Click to expand...

Very good points William - "The man with two watches never knows what time it is..." and all that. While I have not studied how the various dual AHRS systems are implemented, one thing you might be missing is using BITE (Built In Test Equipment) for the box to self-check it's own health. No, BITE will not catch all failures, but it can catch lots of them - things like power supplies, voltage problems, or inconsistancies. In my "big, fast" airplane, we have BITE tests on all three IMU's, and it is as common for an IMU to annunciate a self-failure as it is for one to get voted out by the others.

Of course, this has to be implemented properly - and having an independent autopilot "platform" is a nice way to add a third leg of redudnancy.

Paul
 
Yes, you are 100% correct.
Interestingly all of the low cost gyros provide a built in test so the system can do a basic go/no go test - but that is not really good enough to cater for actual dynamic issues in flight.
Most gyro failures result in bias or gain errors of a gyro (you have three). This results in a measurement error which is very difficult to detect (and often cannot be detected reliably). This directly results in a serious degrading of the system as a whole.
The higher cost gyros (like the types we use in our SP-5 product) which have been designed by British Aerospace (MEMS ring gyros) do not have a means of self test - but I have yet to see one of them fail in any way.

There is a way to build a AHRS using four servos (a forth one at an angle to the other three) - as long as you have a means of detecting a failure of any one gyro (this has to be reliable of course and this is where the rub is) - you can still have a perfectly functioning system.

But to cap it off, your answer of ether "one" or "three" is the real and only correct answer.

Rainier
CEO MGL Avionics

w1curtis said:
Firstly, as long as the connection to your AD/AHRS is reliably airtight, what difference does it make how it looks?

Secondly, at the risk of hijacking your thread and starting a minor controversy, I'm wondering about the value of a dual AHRS system. It seems to me that having dual AHRS offers little value for most failures --outside of an outright failure. AHRS being solid state systems without any moving parts, I would think outright failures are the exception and discrepancies the norm. Typically if you want redundancy for this type of system, you would need THREE AHRS! Having a third AHRS unit would provide a tie-breaker. Without a tie breaker you would need some extraordinary algorithm (read guess) to determine which unit is at fault.

So it seems the only reason for a vendor to promote a dual AHRS system is because they are not confident in the quality of their AHRS product. I think this is why all of the certified systems are certified with either ONE or THREE (think Boeing) AHRS. With a single AHRS, it is either working, on not. If not, you rely on external backup instruments. With DUAL AHRS, how exactly, outside of a FULL failure, can you be sure which AHRS is giving you the correct information and which one is not? While it is good to have a back-up for most items in aviation, somehow, DUAL AHRS doesn?t pass the surface test. DUAL AHRS seem to make sense on the surface, when you peer behind the curtains however, it reveals more questions than answers.

How does a DUAL AHRS system arbitrate between units to determine which is correct without a tie breaker? It seems the only value of a dual AHRS system is if there is a clear-outright failure of one unit. Otherwise, any algorithm is only guessing as to which unit is correct when there is a discrepancy. Potentially this is a worse scenario than if your single AHRS had an outright failure. Without a third arbitrator, you would be using information of a bad AHRS or ignoring information of a good ADHRS, or integrating good data with bad data. So what am I missing?
Click to expand...
 
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