Interesting EFIS Observation

[Doug]

Paul,

I think you will find that the factor causing the pitch change was the airspeed not the altitude or rate of climb.

Here is the simplest explanation I can offer - hopefully I have not made too many errors by oversimplifying.

The AHRS algorithm must correct for the drift in the gyroscope estimate of pitch and roll (P&R). Accelerometer data is used to make the correction.

Pitch and Roll is of course referenced to the gravitational vector. The acceleration vector seen by the AHRS is the sum of the gravitational vector and the acceleration of the aircraft in flight.

The aircraft's acceleration vector is computed by differentiating its velocity vector in time (hence the significance of the ASI or GPS input), this is subtracted from the AHRS acceleration before being used to correct the gyro drift.

The Kalman (or other eg PID) filter is used in giving a stable P&R from the gyroscope data when informed by the adjusted accelerometer data.

The actual algorithms used are closely guarded intelectual property - at the heart of these companies' product lines, so each may employ their own variations.

Doug Gray

[Brantel]

Its all good civil discussion...so far

I wish we had more of these type of civil debates on how our units really work. It has a way of flushing out all the bad information and replacing it with the truth. Too bad that most of the time it goes too far on the wrong side of the rules and ends up getting shut down. Glad to see this one has not went to that place.

Originally Posted by Ironflight

It was just an observation I was sharing guys....didn't mean it to be a grenade tossed in the room!

[Brantel]

No problem with that. It is a good thing we all have a choice!

Agree with you 100% on knowing your equipment and its limitation. The hard part is truly understanding what those limitations are since some of the manufacturers are less than free flowing with the info....

Originally Posted by penguin

Brantel - I agree with most of what you wrote, but we will have to agree to disagree about the suitability of all of the manufacturers you list - I would only fly IFR with 3 of them.

My basic point is that we should understand the risks inherent with the equipment we choose to fit to our airplanes and accept those risks knowingly, rather than in blind ignorance.

Pete

Pete

[Brantel]

Pete,

In the spirit of keeping this a good friendly discussion while trying to keep this neutral in regards to brands.....
I truly value your stance but I would like to fully understand your decision on this issue. What are the biggest things in your mind that would cause you to limit your choices to just three of those choices?

You mentioned that a system should be able to maintain accurate attitude for 15 minutes without its input that it uses for aiding. My question is how accurate are we talking and under what conditions (smooth IMC flying or wild acro flying). Second question on this one is are there any of the systems mentioned in this thread capable of meeting your requirements?

Originally Posted by penguin

I would only fly IFR with 3 of them.
Pete

[IowaRV9Dreamer]

Originally Posted by Brantel

My question is how accurate are we talking and under what conditions

The ideal ARHS would at least meet the TSO for attitude indicators under all conditions.

Note that modern MEMs based sensors will significantly exceed the performance levels referenced in this document, when they are aided. Check the date on the technical document to see why. Yep, it is TSO number 4, written for spinning metal gyro based reference systems, and still in full force.

Some sensors also meet the TSO unaided. Of course any that carry the TSO will meet it. Some that are uncertified may meet the TSO. It would be an interesting question to ask any vendor.

This isn't a question of quality, but of design. There are certainly quality and design assurance aspects to consider as well.

And as many have noted, an overall system approach can mitigate a lot of risk. Two dissimilar sensors, aided via different means (one by GPS and one by air data for example) might go a long way to making the overall attitude display function more available and less misleading. Or so I've heard

Not intending to add to any flame war - but I thought some might appreciate the technical document. Fun to talk work stuff on the VAF forums.

[Jamie]

On a very related note, I have been keeping an eye on the RCA 2600 for a while now as a backup. The most interesting thing is self-contained and it doesn't appear to use any aiding all.

http://www.aircraftspruce.com/catalo...es/rca2600.php

[Rainier Lamers]

TSO allows an AHRS 3 degrees of error after a 180 degree rate one turn.

That is terrible .

I'm willing to bet that any modern non-certified AHRS will easily meet this requirement.

There is another requirement that is much harder to get right - the phugoid test. Unfortunately it is a static test intended to symulate dynamic conditions (bad turbulence). Trouble is, it is dead easy to "fake" this with an modern AHRS system so you pass easily yet in real conditions fall over instantly.

No, honestly - don't look at this TSO requirement. That is so yesterday I'd be embarrased to sell you an AHRS that only meets this spec.

As far as the comment "Manufacturers don't tell you how their systems work" goes - partly not true. First, if you do a search on VAN's you will see lengthy e-mails from me describing both general operation of these systems as well as specifics to our own system.
Second - if you are really interested, all of this is basic physics and well known. Just grab a copy of Titterton's "Strapdown inertial navigation technology" ISBN 0 86341 260 2. This is a good book and you will see one on the shelves of any AHRS designer.

The only "secret" there may be is exactly how you get a cheap and nasty gyro to perform well (many ways, aiding is just one of them and not strictly required in all cases - our SP-4 and SP-5 AHRS systems are solid proof of that).
Any halfway reasonable electronics and software nut can figure this stuff out. There is no black magic involved. None !

Rainier
CEO MGL Avionics

Originally Posted by IowaRV9Dreamer

The ideal ARHS would at least meet the TSO for attitude indicators under all conditions.

Note that modern MEMs based sensors will significantly exceed the performance levels referenced in this document, when they are aided. Check the date on the technical document to see why. Yep, it is TSO number 4, written for spinning metal gyro based reference systems, and still in full force.

Some sensors also meet the TSO unaided. Of course any that carry the TSO will meet it. Some that are uncertified may meet the TSO. It would be an interesting question to ask any vendor.

This isn't a question of quality, but of design. There are certainly quality and design assurance aspects to consider as well.

And as many have noted, an overall system approach can mitigate a lot of risk. Two dissimilar sensors, aided via different means (one by GPS and one by air data for example) might go a long way to making the overall attitude display function more available and less misleading. Or so I've heard

Not intending to add to any flame war - but I thought some might appreciate the technical document. Fun to talk work stuff on the VAF forums.

[breister]

Agree with Ranier on several points, and also for reasons not mentioned.

The ugly truth about TSO for gyro attitude indicators?

a) Very few mechanical gyros, certainly not most in rental planes, will actually remain accurate for 15 minutes after loss of suction in bumpy weather.
b) For those few that actually MIGHT meet that specification after, say, a few hundred hours of operation - that capability is moot as soon as you realize that most pilots won't notice zero suction immediately. How long has it been? How much time do I have left to descend to clear air? Is it still giving me attitude, or is it just reporting the force vector (e.g. I pull back on the stick and it shows level flight)?
c) And of course, any other reason for losing the gyro accuracy other than loss of suction will NOT meet the TSO (e.g. tumbled; broken bearing; etc.).

It has even been debated (for nearly a century) that the TSO is a BAD thing because it may give a pilot a false sense of confidence in a bad gyro.

The bottom line? Have multiple ways of cross checking attitude references in flight, and as soon as any of them become suspect make a hard call whether or not to continue the mission. Not to selves: There is no peacetime mission that requires we make it to our destination today at the risk of arriving dead.

Given the above info, I believe the quality most important in an attitude system is this: The pilot must be able to determine at any time, by cross referencing other instruments, whether a particular attitude reference is suspect or operating in a degraded mode. Based on that criteria, I'll take any of the mentioned Experimental EFIS units as my PRIMARY PFD any day of the week, and use the vacuum instrument as one of my cross-reference instruments.

Turn and Bank? Sole reliance on this dinosaur is rarely or never practiced by most pilots, and its use in a high performance aircraft is tricky on a GOOD day. It's nice to prove that you're still a "man's man" (or a "woman's woman," as the case may be) - but "Needle, Ball, and Airspeed" has got to be one of the biggest causes of fatal accidents in weather not attributable to engine failure.

[penguin]

Originally Posted by Brantel

Pete,

In the spirit of keeping this a good friendly discussion while trying to keep this neutral in regards to brands.....
I truly value your stance but I would like to fully understand your decision on this issue. What are the biggest things in your mind that would cause you to limit your choices to just three of those choices?

You mentioned that a system should be able to maintain accurate attitude for 15 minutes without its input that it uses for aiding. My question is how accurate are we talking and under what conditions (smooth IMC flying or wild acro flying). Second question on this one is are there any of the systems mentioned in this thread capable of meeting your requirements?

Its difficult to describe in a few words why I would (or would not) install equipment from any manufacturer in my airplane. I tend to take a pessimistic view in that I want manufacturers to show me that their claims are reasonable, rather than believing the advertising, and really would like a solid track record for a few years. That's not to say I won't be an early adopter - I had Emag #10 - when I have adequate back-ups. I really like to meet the designer and talk to him(her) to understand what the design drivers were for the equipment, and to understand a little of his background. The important things are what is going inside the box abd the quality of the components used to make the system - it is difficult to figure that out just by looking at the outside. Talking to the designer and discussions like this help inform that view.

On the 15 min front, I'm not saying that it is my personal requirement that any system should be able to 'survive' for 15 mins with an input failed, just that systems that I have worked with would achieve that. Clearly as soon as an input is failed there is some level of degradation, which will increase with time. But a well designed system should be useable for its basic purpose for around 15 minutes after the failure of any one input (how the slow failure or drifting of one input is detected is another question, but again a well designed filter should mitigate that situation to a reasonable degree). The more the aircraft is manoeuvred the quicker any degraded solution will drift. Of course this is one area where tests in a lab really struggle to simulate reality (at a minimum the gyro platform is by-passed), so real world results on how systems degrade are difficult to come by.

I think it is the degraded performance that we are really talking about here. I think we all agree that with everything working we would all rather be flying behind an EFIS than a vacuum panel. Its what happens when something with-in the system, or an important input (pitot, static, GPS, etc) goes down that we are really interested in. I suppose we are talking about insurance policies against those failures impacting our ability to remain in control of the airplane - and that insurance could extend to fitting a completely separate 2-axis autopilot. If anyone has any real worls flight data I for one would be interested in looking at it.

Pete

[Alan Carroll]

Yesterday I went out and tested how a static leak would affect the ADI, by opening the alternate static source I have installed (which vents to the cockpit). The sudden decrease in static pressure caused a pitch up, as Paul noted in the OP. However after about 10-12 seconds the pitch indication stabilized as the instrument adjusted to the new lower pressure. So, ultimately a static leak doesn't seem to pose a problem so long as it is a stable leak. What the ADI cares about it rate of change.

On the other hand I found I could rather easily "fake out" the ADI at speeds as high as 80 knots, by using a very low power setting. This results in nose-up attitude but a nose-down indication on the ADI (due to loss of altitude). The ADI continues to accurately indicate the actual flight path though. Like someone said earlier, you need to understand what this instrument is telling you.