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Interesting EFIS Observation

Ironflight

VAF Moderator / Line Boy
Mentor
Under the heading of ?knowing how your stuff works?, here?s a bit of trivia on the Dynon ?D?-series, Tru Trak ADI Pilot, and GRT Horizons. We recently had the pitot/static/xpnder/IFR certs done on both of our airplanes. For those who haven?t done this, it involves plugging into the pitot/static systems and using a vacuum pump and a calibrated test set to take the instrumentation system up to a simulated altitude of 20,000? (or greater, in some cases). This checks the static system for leaks, checks the altimeter for calibration, and makes sure that the transponder is reporting codes and altitudes properly. In our case, everything passed with flying colors once again (whew!), but I was able to make an interesting observation that tells me something about the way Attitude is calculated in the systems.

With the test set-up hooked to the RV-6 (Dynon D-10A & D-180, Tru Trak ADI), as we started ?up?, all three attitude indicators showed a significant pitch up ? about 15 degrees nose high with a 2,000 fpm climb rate. This was, of course, with the airplane sitting still, and not changing attitude. Clearly, this is not an expected flight condition, but it does tell me that pitot/static does play a part in the attitude solution for these instruments. A pitch down was indicated during ?descent?, but I don?t think it was quite as pronounced. When we transferred the test rig to the RV-8, with GRT equipment, there was no indication of a pitch change at all, even with a 3,000 fpm climb and descent. So apparently, the attitude algorithms that GRT uses don?t care about pitot/static. I have not seen the attitude determination software code for any of these systems, but I know this has been talked about before on the forums ? how the various electronic attitude systems are ?aided? to get their solution. So this isn?t really new, just an observation that you can see it in action with this kind of a test. Interesting from an engineering standpoint, and good to know that a static leak might affect the attitude solution with some equipment (although I doubt that in real life, it would be a huge error).

Paul
 
I was present (assisted the Tech actually) during the IFR cert on a dual AFS system installed in my RV-8 (yes, a dual cert costs more :eek:). The AFS system is like the GRT in that it did not indicate a pitch up or pitch down as Paul said happened with the Dynon system.
 
If connected to a GPS, the Dynon EFIS systems can use that as a backup to airspeed. So in flight, you would not see this behavior with a failure of the airspeed system. For engineering reasons, this is not active until we see airspeed on the GPS at least once, which is why it was not active during your test. If you did this test after a fast taxi or a full flight you would not have seen any motion.

As for the TruTrak ADI, all owners should read up on how it works, since it is very important to know that it is not an attitude device, it's a VSI with some very short term pitch rate biasing.
 
Of course they all have different algorithms driving their attitude display, different ways to aid/correct the affordable gyro's used in these systems, I am sure there are pros and cons of doing each way and in this case, who is to say which way is better?

I do know that some ways are better than others in regards to how fast a AHRS will recover from being restarted, saturated and tumbling and being able to recover in flight...It is my understanding that some take a long time to recover and some may never recover while in flight.

The Dynon can and will recover in flight in a small amount of time.

Like Dynon sez, the TT ADI is not a true attitude indicator and it works off of vertical speed in pitch so if you fake it into thinking it is going up or down, it will indicate pitch up or down. That being said, in most flying conditions it still works good enough to be a good backup for attitude.
 
If connected to a GPS, the Dynon EFIS systems can use that as a backup to airspeed. So in flight, you would not see this behavior with a failure of the airspeed system. For engineering reasons, this is not active until we see airspeed on the GPS at least once, which is why it was not active during your test. If you did this test after a fast taxi or a full flight you would not have seen any motion.

As for the TruTrak ADI, all owners should read up on how it works, since it is very important to know that it is not an attitude device, it's a VSI with some very short term pitch rate biasing.

I plumbed in a small valve into my pitot system to test the GPS aiding back when it was first introduced into the firmware. The GPS aiding works well, with one (very big) caveat.

The GPS aiding will only work IF your pitot pressure drops to static pressure (meaning that your IAS goes to 0). That is to say, for example if you are in the clouds and your pitot ices over and your pitot drain is open the GPS aiding will work well. If the drain is clogged, GPS aiding will NOT turn on.

Dropping the IAS to zero was the only way I could make the GPS aiding work. A partially clogged pitot or a significant pitot leak (tested with my valve temporarily plumbed in) resulted in pretty dramatic pitch deviations of the EFIS and no indication of GPS aiding.

As a result I added another small valve permanently to my pitot system (sort of like alternate static). If my pitot ever becomes partially clogged or leaky I can open up the pitot line and GPS aiding will kick in.

The simplest fix would be to add a menu option to manually switch to GPS aiding mode.
 
Like Dynon sez, the TT ADI is not a true attitude indicator and it works off of vertical speed in pitch so if you fake it into thinking it is going up or down, it will indicate pitch up or down.

Agreed, but this could also be construed as better than a true attitude indicator in some ways? Trutrak's underlying philosophy seems to be that pitch attitude and magnetic heading are not as important as your actual path through the air (vertical speed and track). Most instrument systems require that the pilot interpret how the former is affecting the latter, and this hasn't fundamentally changed since the late 1920s. The ADI gives you flight path directly, which in principle should make piloting easier. This is not how we were trained however...

In several hundreds flight hours I have yet to see the ADI indicate an incorrect (up vs. down) attitude. It would require very low airspeed to fake it out, at which point a warning is flashed. A leak in the static system might cause problems though?
 
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It could also be argued that the ADI is a better flight instrument than an attitude indicator. When flying instruments with a traditional attitude indicator (or even an EFIS attitude indicator) the attitude indicator is not the primary pitch instrument in straight and level flight. Why? Because it changes based on the loading and speed of the airplane. The ADI, however, indicates a level attitude the same way, regardless of speed or AOA.
 
Jamie,
It is true that GPS aiding will not turn on until airspeed goes to zero. We did this because we found very little error in our testing when airspeed errors were high. So if you are going 160KTS and your ASI is 75 KTS, the Dynon will still be quite accurate and will never diverge (show you level when you are not). Because of this testing, we only use GPS aiding when the pathological case occurs and all airspeed info is lost.
 
Jamie,
It is true that GPS aiding will not turn on until airspeed goes to zero. We did this because we found very little error in our testing when airspeed errors were high. So if you are going 160KTS and your ASI is 75 KTS, the Dynon will still be quite accurate and will never diverge (show you level when you are not). Because of this testing, we only use GPS aiding when the pathological case occurs and all airspeed info is lost.

Interesting. That's definitely at odds with my findings. When straight and level, yes, it would stay there but I found that turning 45 degrees it never would recover properly until IAS dropped to zero or I closed off the 'leak' in the pitot system.

It was a while back when I did this, but if I remember correctly I tested it at around 100kts IAS while I was doing 167kts TAS (my typical cruise speed).
 
Under the heading of ?knowing how your stuff works?, ... Interesting from an engineering standpoint, and good to know that a static leak might affect the attitude solution with some equipment (although I doubt that in real life, it would be a huge error).

Paul, excellent write up.

I think this phenomenon shows potentially a more significant problem as the attitude platform appears not to be very stable under any kind of failure condition. Clearly Dynon provides great equipment for VFR operations, but for regular IFR? To achieve their price point perhaps Dynon have had to make savings somewhere, perhaps this is one area? In my view the attitude solution should not require pitot or GPS aiding to remain stable.

Pete
 
I think this phenomenon shows potentially a more significant problem as the attitude platform appears not to be very stable under any kind of failure condition. Clearly Dynon provides great equipment for VFR operations, but for regular IFR? To achieve their price point perhaps Dynon have had to make savings somewhere, perhaps this is one area? In my view the attitude solution should not require pitot or GPS aiding to remain stable.

Even the mighty Garmin G-1000/G-900X uses GPS aiding and pitot aiding as a backup.
 
Your opinion stated here shows that you are clearly uninformed on how these things work...there are many post in the archives that will educate you on why the affordable Gyro's used in these experimental EFIS systems need aiding of some sort or how aiding is used to augment the system for better performance.

Like Jamie said above even the very expensive Garmin gear uses aiding...is it not suited for IFR????

The algorithms used in these AHRS's are very complex...You cannot judge a AHRS's performance from testing it on the ground while manipulating one or two of it's inputs without following the rules of physics for all its other inputs. The laws of physics for an airplane moving through space are much different than those of a box sitting on the ground with one or two of its inputs being manipulated unnaturally....

For Dynon (and others) pitot or GPS data is used in the attitude solution. If you wildly vary this input without the other natural reactions that should be there in real life, you will see an error.

Another thought:

Aiding is also used to decrease the time it takes for an AHRS to recover from being restarted of from being saturated and tumbling....

Which is better in IMC, an EFIS that cannot be restarted in flight or one that can?
Which is better in IMC, an EFIS that takes a very long time to recover from being restarted/being saturated/tumbling or one that only takes a few seconds?

Aiding can be a good thing to have in IMC not bad...



Paul, excellent write up.

I think this phenomenon shows potentially a more significant problem as the attitude platform appears not to be very stable under any kind of failure condition. Clearly Dynon provides great equipment for VFR operations, but for regular IFR? To achieve their price point perhaps Dynon have had to make savings somewhere, perhaps this is one area? In my view the attitude solution should not require pitot or GPS aiding to remain stable.

Pete
 
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As just another note, I continually monitor both Ground speed and IAS during flight. The type of issue Jamie reported should never be a surprise, and if it starts to happen in-flight it should immediately be "Pitot Heat On" and "if no quick restoration, land immediately." I know approximately what my IAS should be at a given altitude and power setting, and so should every pilot who flies IFR.

The worst that is likely in this situation is to have a system not responding quite perfectly but trends accurately. If you aren't flying aerobatics in the soup, this should not be an issue, IMHO.
 
I have no knowledge of what algorithms are ACTUALLY used in the experimental EFIS's. I would think they are using the Kalman filter or one of the plethora of newer algorithms though. These algorithms can detect when an input source is giving bad data, and then the software can cut that source out of the loop.

Another observation is that recovering from tumbling means the unit has to determine it's current orientation. This is greatly eased by another source of data, such as magnetic heading, GPS data, air speed data, or altitude data. Each of these can provide a good indication of one element of the solution which helps the algorithm to orient itself. GPS would be fastest as it provides more than one of the solution element at once, ie velocity, heading, altitude all together.

I'm assuming that when it is turned on, and there is no motion detected from any of these inputs, then logically it can be assumed that the airplane is mostly level, no significant pitch or roll. But then how does it work with a Cozy, which can have the nose gear up and a significant down pitch on the aircraft when systems are powered?
 
Cuz they can sense gravity....

Never seen a modern EFIS that uses whatever position it is turn on in as its straight and level position...

I'm assuming that when it is turned on, and there is no motion detected from any of these inputs, then logically it can be assumed that the airplane is mostly level, no significant pitch or roll. But then how does it work with a Cozy, which can have the nose gear up and a significant down pitch on the aircraft when systems are powered?
 
... even the very expensive Garmin gear uses aiding...is it not suited for IFR????

The algorithms used in these AHRS's are very complex...You cannot judge a AHRS's performance from testing it on the ground while manipulating one or two of it's inputs without following the rules of physics for all its other inputs. The laws of physics for an airplane moving through space are much different than those of a box sitting on the ground with one or two of its inputs being manipulated unnaturally....

For Dynon (and others) pitot or GPS data is used in the attitude solution. If you wildly vary this input without the other natural reactions that should be there in real life, you will see an error.

Another thought:

Aiding is also used to decrease the time it takes for an AHRS to recover from being restarted of from being saturated and tumbling....

Which is better in IMC, an EFIS that cannot be restarted in flight or one that can?
Which is better in IMC, an EFIS that takes a very long time to recover from being restarted/being saturated/tumbling or one that only takes a few seconds?

Aiding can be a good thing to have in IMC not bad...

Which EFIS is better, one which only displays data when it knows it to be valid, or one that displays anything that is available?

I am aware of aiding and Kalman filters and have worked with systems that use these devices for many years. The whole point of a well designed Kalman filter is that it really doesn't care in the short term if one of the inputs is lost - once the filter is up and running (which can take a while) it can cope with losing inputs, for example position without GPS should be reasonably accurate for at least 15 minutes.

A system that shows an immediate deviation in output when one input is excited doesn't sound to me like a well written filter. Of course it is often difficult to tell exactly what is going on by just observing outputs, and none of these companies will divulge what is going on behind the screen.

If you want to trust your life to one box that must re-boot quickly for you to maintain control then that is your choice. I prefer to install an independent attitude indication (and ASI & Alt) that I can use if my EFIS should fall over (and I can also use to cross compare the EFIS output), and to let my EFIS come back on line when it has carried out proper airborne alignment.

Dynon provides a great system for VFR and occasional (life-saving?) IFR. Please understand the limitations of all the systems installed in your airplane and operate accordingly.

Pete
 
Keep in mind these comments I have been making have nothing to do with any particular manufacturer. They are all different and all have their limitations. Even the very high dollar units.

One brand will turn monochrome if it detects that the attitude solution is out of whack, others put up a big red X, others do something similar. The problem is how does the EFIS know when its attitude solution is bad? This is not an easy thing to determine 100% of the time. Even the units with dual AHRS and realtime crosschecking have a problem with deciding which one is correct. Too sensitive and it will be red x'd more than it needs to be, too insensitive and it may not let you know it has a problem when it should. There are always tradeoffs to be dealt with.

Also, I would never trust my life to any single AHRS while in IMC no matter what brand it is, that just ain't very smart.

I do however kindly disagree that you can take any one of the current mainstream experimental EFIS maker's platforms (Garmin, GRT, AFS, Dynon, MGL) and label it as no good for IMC. Each of them have their own pros/cons but any of them can be used safely in an IMC environment where a proper panel has been designed with the appropriate backups. There are many ways to come up with what appropriate backups. How many backups you need depends on how much risk you are willing to accept.

So one or more manufacturers use pitot info as primary for their aiding with GPS backup. While others use one or a combination of pitot, GPS, magnometer in some way in their solutions for aiding.. manipulate one or more of these inputs randomly and see what it does to the attitude indication. They all can be tricked into wrong indications if you give them inputs that do not follow the laws of physics that the algorithms are based on....


Which EFIS is better, one which only displays data when it knows it to be valid, or one that displays anything that is available?

I am aware of aiding and Kalman filters and have worked with systems that use these devices for many years. The whole point of a well designed Kalman filter is that it really doesn't care in the short term if one of the inputs is lost - once the filter is up and running (which can take a while) it can cope with losing inputs, for example position without GPS should be reasonably accurate for at least 15 minutes.

A system that shows an immediate deviation in output when one input is excited doesn't sound to me like a well written filter. Of course it is often difficult to tell exactly what is going on by just observing outputs, and none of these companies will divulge what is going on behind the screen.

If you want to trust your life to one box that must re-boot quickly for you to maintain control then that is your choice. I prefer to install an independent attitude indication (and ASI & Alt) that I can use if my EFIS should fall over (and I can also use to cross compare the EFIS output), and to let my EFIS come back on line when it has carried out proper airborne alignment.

Dynon provides a great system for VFR and occasional (life-saving?) IFR. Please understand the limitations of all the systems installed in your airplane and operate accordingly.

Pete
 
Dynon provides a great system for VFR and occasional (life-saving?) IFR. Please understand the limitations of all the systems installed in your airplane and operate accordingly.

Pete

Is there some particular reason you are singling out Dynon for your criticism?

Your opinion is your business, but realize that the arguments you give are not consistent with the relative merits / limitations of our other available choices. Vacuum attitude indicators are considered adequate for IFR and have NO corrective input to actual horizon. If a vacuum instrument tumbles IFR, you are likely toast. Thus, ANY of the current generation of experimental EFIS units, when functioning, are likely superior to that solution.

As to reliability, I've had several vacuum pump and instrument failures over my flying lifetime - I'll take an electric backup vs. a second vacuum instrument any day.
 
It was just an observation I was sharing guys....didn't mean it to be a grenade tossed in the room! ;)
 
Is there some particular reason you are singling out Dynon for your criticism?

Because this thread is about Dynon. Clearly this is a very emotive subject (I have to say I'm a little surprised how emotive), also I have not supported vacuum systems - I too regard them as rather unreliable. But I do disagree that a vacuum failure = death. If your instrumentation system consists of vacuum gyros and an electic T&B then it strikes me that you should be competent to fly to clear air on the T&B - otherwise you're betting your life on a vacuum pump which doesn't seem very smart.

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
 
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
 
Its all good civil discussion...so far :rolleyes:

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.

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

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
 
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?

I would only fly IFR with 3 of them.
Pete
 
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.
 
TSO allows an AHRS 3 degrees of error after a 180 degree rate one turn.

That is terrible :eek:.

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

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.
 
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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.
 
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
 
ADI test

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.
 
This system is similar to the new GPS based attitude you can select on our EFIS systems either as sole attitude or as reference for a real AHRS.
Taking advantage of the Swiss made u-Blox GPS receiver the EFIS has access to internals way beyond what is available on NMEA strings, including a very accurate 3D velocity vector.
This means you can create a reasonable AHRS display (very reasonable actually) that is not unlike the aiding information used with some high-end aided gyro based systems (except you ditch the gyros and look directly at the aiding information).
This system too, describes your path though the air and is not capable of showing pitch up near the stall. Of course - that could be faked but we decided to not after our many test flights. The reason for this is that it started making sense rather quickly. It's a case of the pilot understanding what he is seeing and then using it correctly.
In order to avoid infringing patents our system adds other sensor inputs as well and takes advantage of the fast operation of a modern GPS receiver.
As a cross-check to an existing gyro based AHRS it is great (and you don't need an expensive EFIS for this, our little sub-$1000 XTreme can do this as well).
The main reason it is great as a cross check is that it is based on completely different technology and neither suffers from the potential problems of the other.

Rainier
CEO MGL Avionics


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.
 
I've been using the TruTrak ADI with track as my primary attitude indicator for two+ years and lots of bad weather flying. I especially like its instantaneous response in both pitch and bank. Excellent instrument & love it for solid IFR work. If I had room on the panel I suppose another one just like it would be a good back-up, but there's not much to break or wear out with these things.
 
I've been using the TruTrak ADI with track as my primary attitude indicator for two+ years and lots of bad weather flying. I especially like its instantaneous response in both pitch and bank. Excellent instrument & love it for solid IFR work. If I had room on the panel I suppose another one just like it would be a good back-up, but there's not much to break or wear out with these things.

Yes, quite right.
Systems like this have been getting bad publicity, usually from people that have not used them or don't quite understand them.
I was one of them, I am ashamed to say.
It's only after we started experimenting with alternative "pseudo attitude cum flight path" indications that I started realizing that these are just as valuable a pilot aid than anything else in the cockpit and, as usual, if you are a pilot trained in proper use of something like this, you will have zero problems interpreting what this instrument is telling you.
The only thing that needs to be clear is that you are not seeing the aircraft's attitude in the way a gyroscope based system would show but you are rather seeing where you are going using a graphic interpretation, quite regardless of where the aircraft is pointing (the two happen to coincide as long as you fly nicely, but only in this case).

The danger is that some may think that this kind of thing is an attitude indicator. Of course it is not. It's a flight path indicator. As an attitude indicator it is dangerous. Switch your mind so that you see it as flight path and immediately it becomes useful.

Rainier
CEO MGL Avionics
 
The danger is that some may think that this kind of thing is an attitude indicator. Of course it is not. It's a flight path indicator. As an attitude indicator it is dangerous. Switch your mind so that you see it as flight path and immediately it becomes useful.

Rainier - I would agree with this; the ADI tells you very accurately where you're going in 3D, but much less about attitude.

Regarding the Xtreme with GPS only (no AHRS), does it provide climb/descent information or just turn?
 
That's good info Ranier, as always.

As these units mature, I believe we will start seeing messages informing us of the status of each of the inputs, as well as warnings when one or more inputs are, or are suspected of being, degraded. With multiple aiding inputs, a color "condition indicator" may be the simplest way to present overall condition, with a button / page / whatever to inform us of why the EFIS considers current information to be less than optimal when that occurs.

Having the system automatically compare flight parameters between systems could generate cautions and warnings which the pilot could later use to build a set of parameters for their specific aircraft. For example, if your aircraft AHARS attitude at 120 IAS is typically within 15 degrees of GPS flight path, a deviation to 20 degrees might generate a warning of unexpected divergence.

As the systems get more complex, the information to diagnose current status also becomes more complex - and the good old "OFF" flag simply isn't sufficient for determining whether or not an instrument is reliable.
 
Rainier - I would agree with this; the ADI tells you very accurately where you're going in 3D, but much less about attitude.

Regarding the Xtreme with GPS only (no AHRS), does it provide climb/descent information or just turn?

The full monty, so to speak - but it is not dependent on static pressure for this and can in fact operate just fine completely without pressure sensors.
Of course it needs a GPS lock but it differs from a normal GPS system in that it is able to track phase shifts of the received satellite signals relative to another and using a very fast correlator will iteratively work out how your position is changing in 3D space so you have a real 3D vector of very high resolution and is available to good accuracy very quickly after you have changed direction. Due to the tight coupling with the EFIS system it becomes possible to add the other sensor inputs as available to augment the information and derive a pretty convincing "picture" of what the aircraft is doing. The interface between EFIS and GPS has become blurred and in this case they are just two components of the same thing.

Rainier
CEO MGL Avionics
 
When I'm in cloud and the ADI says I have a nose up attitude, I accept it as a nose up attitude and act just as I would when using vaccuum gyro instruments. I've not ever had to give the slightest thought to interpreting anything the ADI presents me with. The ADI blends perfectly with the other attitude instruments and actually has eliminated the DG from my scan except as a secondary reference.

> Systems like this have been getting bad publicity, usually from people that have not used them or don't quite understand them.

I've never read anything in any of the forums to make me think there is any bad publicity out there. On the contrary, TruTrak customers seem to be consistently enthusiastic about the products and actually rave about the customer service.

************

Later,

I see the concern is about behavior near the stall. Picturing that in a worst case scenario a plane could be gradually accumulating ice with an iced-up pitot and pilot was paying no attention to GPS speed and the canopy was iced up too - a little red light on the ADI comes on when nearing the stall that would get ones attention. I'm pretty good about not letting things get that far :)
 
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