If I have two AHRS can I get rid of my vacuum system?

[Ragsf15e]

9 hours ago, PT20J said:

I believe this is correct. As I understand it, a basic AHRS sensor has 3-axis accelerometers and 3-axis gyros. These are accurate for short periods but will drift over time and the amount of drift depends on the acceleration. So, they are better straight and level than maneuvering. Adding 3-axis magnetometers greatly improves accuracy with time and maneuvering. AHRS sensors are available with built-in magnetometers. I believe this is what Kelly uses in the RCA 2610 which does not need GPS or pitot/static inputs. If you have a magnetometer, GPS and pitot/static, you have a belt and suspenders approach and it should be very reliable. 

Garmin told me that the GTX 345 uses the same AHRS as other Garmin units. However, the GTX 345 has a GPS input but no pitot/static and no magnetometer inputs. My GTX 345 linked to Foreflight shows an accurate bank angle, but a 2 degree nose up in a level left turn and a 2 degree pitch down in a level right turn. If I turn off the GPS, it performs reasonably well in a left turn but pitches down 20 deg or so in a level right turn. 

Skip

 

unfortunately, removing power from the system (total electrical failure) leaves the gmu11 magnameter off although the G5s have 4 hours or so of battery.  They have to survive only with internal gps or air data if those exist.  It almost seems that a battery backup on the gmu would be a good option.

[aviatoreb]

9 hours ago, PT20J said:

I believe this is correct. As I understand it, a basic AHRS sensor has 3-axis accelerometers and 3-axis gyros. These are accurate for short periods but will drift over time and the amount of drift depends on the acceleration. So, they are better straight and level than maneuvering. Adding 3-axis magnetometers greatly improves accuracy with time and maneuvering. AHRS sensors are available with built-in magnetometers. I believe this is what Kelly uses in the RCA 2610 which does not need GPS or pitot/static inputs. If you have a magnetometer, GPS and pitot/static, you have a belt and suspenders approach and it should be very reliable. 

Garmin told me that the GTX 345 uses the same AHRS as other Garmin units. However, the GTX 345 has a GPS input but no pitot/static and no magnetometer inputs. My GTX 345 linked to Foreflight shows an accurate bank angle, but a 2 degree nose up in a level left turn and a 2 degree pitch down in a level right turn. If I turn off the GPS, it performs reasonably well in a left turn but pitches down 20 deg or so in a level right turn. 

Skip

 

Any gyroscope and any accelerometer, whether an old school mechanical device or a mems device, will drift over time and also in an environment where there are accelerations.  The question is if the mems devices in our ahrs perform same, better, or worse, than the rotating mechanical gyros they compete against.  I don't know.  Also, again I don't know what the answer is as to what they actually do but you described what is mechanically minimal.  2 pairs of 3 axis accelerometers separated by a distance, a rod, can be used to make a gyro sensor.  Each accelerometer in a mems I believe is an mini oscillating beam whose period of oscillation is sensed since it will change with respect to oscillations.  But if once the chip etching process is fired up, is it relatively cheap to etch more and more single axis accelerometers?  If so, if I were designing this thing, I would etch many many accelerometers of all sorts of displacements and orientations and then an assimilation method (some sort of averaging) could be designed to make observations from an over-sensed measurement more accurate than the minimal sensing alone.  The principle is that m-crappy measurements can be combined to make 1 more reliable measurement.  m-good measurements can be combined to make 1 very good measurement.

 

[PT20J]

1 hour ago, Ragsf15e said:

unfortunately, removing power from the system (total electrical failure) leaves the gmu11 magnameter off although the G5s have 4 hours or so of battery.  They have to survive only with internal gps or air data if those exist.  It almost seems that a battery backup on the gmu would be a good option.

Well, we're all forced to guess a bit about how all this stuff actually works because Garmin doesn’t explain it in any detail. The best description I’ve found is in the G3X Pilot’s Guide.

Also, keep in mind that the “up to” 4 hour battery capacity for the G5 is the marketing description. According to the G5 Maintenance manual, the battery doesn’t have to be replaced until the annual capacity test shows less than one hour. I have no idea how long these batteries last. They cost $219 at Aircraft Spruce.

Skip

 

 

[RobertGary1]

3 hours ago, PT20J said:

 

Also, keep in mind that the “up to” 4 hour battery capacity for the G5 is the marketing description. According to the G5 Maintenance manual, the battery doesn’t have to be replaced until the annual capacity test shows less than one hour. I have no idea how long these batteries last. They cost $219 at Aircraft Spruce.

 

 

 

my shop says they’ve tested the g5 and gi-275 battery. Supposedly the g5 battery last longer but they said it was pretty much identical. The real difference is in cerification. The g5 was developed for experimental aircraft and the FAA approved it on the basis that not approving it would be more dangerous than what we have now. The 275 was designed as a certificated instrument from The start. 

[PT20J]

I posted this in another thread, but it is also germaine here:

Here is Garmin's description of COMPARISON ALERTS from the G3X Pilot’s Guide. This seems to have been written by a software engineer because it shows the typical error trap response: throw up a cryptic message and leave it to the human to determine what to do. So, if you get an attitude miscompare between the G3X and G5 in IMC you’re screwed because you are supposed to somehow figure out which ADAHRS is good and switch to it by some method that I cannot find in the manual anywhere. I really think one needs a third attitude source as a tie breaker.  Maybe the GTX 345 Bluetoothed to an iPad is enough, since it is independent UNLESS it’s possible that the source of the error is the GPS common to all three. I’ve got lots of panel space available. I may just keep the vacuum system and put in a vacuum attitude indicator which would be less expensive than other solutions.
Skip

[FlyingDude]

The turn coordinator backs up artificial horizon. DG is backed up by compass. You can always get electric DG. The Brittain tc100evs can run on electric power indefinitely. This way you could remove your vacuum pump. A turn coordinator and/or independent DG + a steam gauge altimeter would break the tie between two conflicting AHRSs ...

 

[RobertGary1]

10 minutes ago, FlyingDude said:

The turn coordinator backs up artificial horizon. DG is backed up by compass. You can always get electric DG. The Brittain tc100evs can run on electric power indefinitely. This way you could remove your vacuum pump. A turn coordinator and/or independent DG + a steam gauge altimeter would break the tie between two conflicting AHRSs ...

 

I think I’d rather have a back up attitude than a turn coordinator. Why keep an old turn coordinator around?

although if you follow the Garmin example you throw all that out and your 6 pack is reduced to two 275’s

[FlyingDude]

What I mentioned are the primary and secondary instruments for ifr flight ...

[RobertGary1]

25 minutes ago, FlyingDude said:

What I mentioned are the primary and secondary instruments for ifr flight ...

Garmin says the top 275 is primary and the bottom is secondary. When the top fails the bottom reverts. 

[carusoam]

1 hour ago, FlyingDude said:

What I mentioned are the primary and secondary instruments for ifr flight ...

For ‘ideal’ IFR flight…

They never mention OHing the instruments… to keep them current.  Or knowing when to OH the TC…

The TC is the instrument that can leave a pilot disappointed when it becomes necessary to use it….

 

I trained for my IR chasing a TC in bumpy weather…

The school planes had unknown to me… TCs.

Best regards,

-a-

[Ragsf15e]

12 hours ago, PT20J said:

Well, we're all forced to guess a bit about how all this stuff actually works because Garmin doesn’t explain it in any detail. The best description I’ve found is in the G3X Pilot’s Guide.

Also, keep in mind that the “up to” 4 hour battery capacity for the G5 is the marketing description. According to the G5 Maintenance manual, the battery doesn’t have to be replaced until the annual capacity test shows less than one hour. I have no idea how long these batteries last. They cost $219 at Aircraft Spruce.

Skip

 

 

Skip, their diagram and description seems to indicate that if you lose gps and magnameter, attitude is lost no matter if you have air data… But you lose both of those in power failure.  Am I missing something with that?  Battery wouldn’t help other than show its failed?

[PT20J]

5 minutes ago, Ragsf15e said:

Skip, their diagram and description seems to indicate that if you lose gps and magnameter, attitude is lost no matter if you have air data… But you lose both of those in power failure.  Am I missing something with that?  Battery wouldn’t help other than show its failed?

That's the way I read it, also. But, since the G3X doesn't have a backup battery, it doesn't really matter. I wonder what the failure tree looks like for a G5 since it does have a battery backup?

Skip

[Ragsf15e]

12 hours ago, PT20J said:

That's the way I read it, also. But, since the G3X doesn't have a backup battery, it doesn't really matter. I wonder what the failure tree looks like for a G5 since it does have a battery backup?

Skip

Yes, exactly.  It must be different, but that would be a nice diagram to have!

[PT20J]

It appears to me from reading various manuals that Garmin has put quite a bit of thought about redundancy into the G3X/G5 combination. But there are still a lot of details missing.

According to the G3X Touch AFMS Emergency Procedures, the G3X will will revert to using the G5 ADAHRS for an outright failure of the GSU 25 ADAHRS. However, it is unstated what constitutes a failure or how a failure is detected. Additionally, Section 3.8, Erroneous Air Data or Attitude Information, implies that there are failure modes that the G3X cannot detect and must somehow be detected by the pilot and require pulling the ADAHRS C/B to force the reversion. It is interesting the the approved STC installation powers both the GSU 25 and the GMU 11 through this C/B which means that the backup G5 would be denied magnetometer input in this case. Also, the G3X/G5 miscompare is not described in the AFMS at all. Hopefully any ADAHRS issue when IMC would be clear enough to determine which had gone rogue. If it were a subtle failure, it would be hard to detect without a tie breaker.

Skip

 

[EricJ]

Just now, PT20J said:

It appears to me from reading various manuals that Garmin has put quite a bit of thought about redundancy into the G3X/G5 combination. But there are still a lot of details missing.

The more I hear about this from you and others it is apparent that Garmin put a lot of thought into it, but it is not evident that it was coordinated thought that didn't leave a bunch of holes and undesirable failure modes.

 

[201Mooniac]

1 hour ago, EricJ said:

The more I hear about this from you and others it is apparent that Garmin put a lot of thought into it, but it is not evident that it was coordinated thought that didn't leave a bunch of holes and undesirable failure modes.

 

While I'm hopeful that Garmin thought this out well and designed the system to handle all the faults they identified, the concern for me is that by not documenting their fault analysis and remediation, there is no way to know how best to respond to an issue.  I would hope Garmin would publish this data so users will know what to expect and how to respond.

[PT20J]

1 hour ago, 201Mooniac said:

While I'm hopeful that Garmin thought this out well and designed the system to handle all the faults they identified, the concern for me is that by not documenting their fault analysis and remediation, there is no way to know how best to respond to an issue.  I would hope Garmin would publish this data so users will know what to expect and how to respond.

Exactly. 

[Fly_M20R]

On 6/27/2021 at 10:10 AM, Ragsf15e said:

unfortunately, removing power from the system (total electrical failure) leaves the gmu11 magnameter off although the G5s have 4 hours or so of battery.  They have to survive only with internal gps or air data if those exist.  It almost seems that a battery backup on the gmu would be a good option.

I do not know what the G5 does regarding powering the GMU11. However, the GI275 does power the GMU11 even from its own internal battery if the master switch is off as you can see from this picture. Master switch is off and turned on the primary ADI with its internal battery. The GMU11 is connected to the primary ADI and heading comes up once the AHRS has aligned.

[Ragsf15e]

26 minutes ago, Fly_M20R said:

I do not know what the G5 does regarding powering the GMU11. However, the GI275 does power the GMU11 even from its own internal battery if the master switch is off as you can see from this picture. Master switch is off and turned on the primary ADI with its internal battery. The GMU11 is connected to the primary ADI and heading comes up once the AHRS has aligned.

Interesting.  Ill play with that some more…

[jlunseth]

I have a Flightstream 210 but would not rely on that as a backup in the event of a failure in the Garmin system. In any event, a major problem seems to be the potential for failure of the electrical system, which would take out all the inputs to the 275s even if they continued to run on internal battery, and would also shut down the Flightstream as well as any guidance that could be expected from the GPS (i.e. heading on a 750/650). I assume that inputs that would ordinarily be analog, like airspeed being read as a difference between static and pitot pressure, are translated into an electrical signal to be passed to the Garmin units, and thus the signal would fail. Failure of the electrical would also shut off the TC indications in the 275 or G5's and any electrical, old-style TC. This would particularly effect all the models with one alternator and one battery, but even models with that redundancy can have a failure of the electrical system although the chances are smaller.

Am I wrong here? If the OPs failure mode (lack of pitot input?) can terminate a Garmin dual-275 system (or possibly a G5 system?), then the only real means of protection would be (1) to keep the vacuum AI which would run off the engine, or (2) go with something like the RC Allen  AI's and DG's that are a different technology (electrical-mechanical) and have the option of adding a backup battery, maybe put them on the co-pilot's side as a backup. 

[Fly_M20R]

1 hour ago, jlunseth said:

I have a Flightstream 210 but would not rely on that as a backup in the event of a failure in the Garmin system. In any event, a major problem seems to be the potential for failure of the electrical system, which would take out all the inputs to the 275s even if they continued to run on internal battery, and would also shut down the Flightstream as well as any guidance that could be expected from the GPS (i.e. heading on a 750/650). I assume that inputs that would ordinarily be analog, like airspeed being read as a difference between static and pitot pressure, are translated into an electrical signal to be passed to the Garmin units, and thus the signal would fail. Failure of the electrical would also shut off the TC indications in the 275 or G5's and any electrical, old-style TC. This would particularly effect all the models with one alternator and one battery, but even models with that redundancy can have a failure of the electrical system although the chances are smaller.

Am I wrong here? If the OPs failure mode (lack of pitot input?) can terminate a Garmin dual-275 system (or possibly a G5 system?), then the only real means of protection would be (1) to keep the vacuum AI which would run off the engine, or (2) go with something like the RC Allen  AI's and DG's that are a different technology (electrical-mechanical) and have the option of adding a backup battery, maybe put them on the co-pilot's side as a backup. 

I have simulated electrical failure of the avionics connected to my GI275's by turning them off and they worked very well, in spite of having no pitot input. I have the video that hope to have finished editing soon and will post when done. I did share pictures of the scenarios I set up. 

I found that the attitude indicator in the FS210 AHRS is a bit off in my plane and shows slight left bank (about 3 deg) and slight down pitch (about 2.5 deg) on my Garmin Pilot app and which I have not been able to correct through the app on my iPad. However, since it also shows GPS altitude and GS as well as VSI as long as the GPS navigator is alive it should be flyable with some mental correction.  If not flyable then can go to the Navigation split screen to show analog equivalents of ASI, ALT, VSI, HSI. Since the FS210 gets its inputs from the GPS navigator (GNS, GTN, etc) once those go out because of electrical failure the FS210 would become useless and not be powered itself. At that point I would rely on my iPad's internal GPS and use the Map page with Navigation split screen if for some crazy reason I also lost BOTH GI 275's.

Regarding pitot and static inputs to the GI 275's, the tubing goes directly into the GI 275's without any electrical "analog to digital converter" in between. The conversion gets done inside the 275's with its own battery power. Therefore you would not lose ASI or ALT (or VSI) unless you had an iced up pitot and static ports. If you have an iced up static port you can engage the ALT STATIC source. With iced up pitot you may not have valid IAS and it would be flagged as such as in the pictures I showed. You would still have TRK info from the backup VFR GPS of the GI 275. The VFR GPS antenna is an option which I HIGHLY RECOMMEND having installed and connected to the backup ADI since it would be your only GPS TRK source in case you had a full electrical failure. I recommend GMU11 connected to the Primary ADI and VFR GPS to the MFD (or HSI) Standby ADI.

Failure of electrical system does not shut off the TC in the 275 since it is powered by its own internal backup battery. If you were to lose either IAS or internal GPS TRK then you would lose Standard Rate turn bugs as well as TAS calculation but NOT bank. 

Since I have eliminated my six-pack and replaced with the dual 275's without adding an additional backup electrical AI with own battery, I would eventually have to depend on my iPad GPS and its Navigation split screen to get me out of trouble if both 275's were to shut off. If the iPad fails, I have the same app on my phone and another iPad. How likely is that to happen for both GI 275's to fail in addition to full electrical failure? Extremely unlikely. Lightning? The GI 275's are designed with strong lightning protection and would very very likely not be affected in spite of the rest of the avionics going out.

I am aware that we all have our personal limits and levels of comfort and many on the forum will have very different views. For me, after flying 90+ hours with the 275's and having done several LIFR approaches as well as quite a few hrs of IMC I am very confident of their robustness. 

[PT20J]

There are some interesting differences in the AFMS Limitations sections between the G3X and the GI 275. Note that the GI 275 requires GPS and pitot inputs whereas the G3X requires GPS or pitot. I have no idea why.

GI 275

 

G3X

Also, the fact that the GI 275 apparently powers the GMU 11 magnetometer probably accounts for the reduced battery operating time compared to the G5. 

[Fly_M20R]

6 minutes ago, PT20J said:

There are some interesting differences in the AFMS Limitations sections between the G3X and the GI 275. Note that the GI 275 requires GPS and pitot inputs whereas the G3X requires GPS or pitot. I have no idea why.

GI 275

 

G3X

Also, the fact that the GI 275 apparently powers the GMU 11 magnetometer probably accounts for the reduced battery operating time compared to the G5. 

Crazy enough the only scenario which I did not try is the one where having lost air data (pitot input) I did not eliminate all  GPS signals (530 and internal VFR GPS) in addition to the magnetometer. I had thought about it but unfortunately overlooked it completely. I know how to set that up if I get another pitot failure. I have a very strong feeling that at that point I will have a RED-X over the attitude indicator. 

I did not have an AHRS failure when in addition to not having pitot input I eliminated all GPS sources but continued to have magnetometer.

In my test the AI stayed alive without air data (pitot) or GPS as long as it had HDG. Go figure!!

[Fly_M20R]

Here is a very short video on how the primary ADI powers the GMU11 and what happens if the primary ADI is off and the MFD/Standby ADI is on.

 

 

 

[Ragsf15e]

1 hour ago, PT20J said:

There are some interesting differences in the AFMS Limitations sections between the G3X and the GI 275. Note that the GI 275 requires GPS and pitot inputs whereas the G3X requires GPS or pitot. I have no idea why.

GI 275

 

G3X

Also, the fact that the GI 275 apparently powers the GMU 11 magnetometer probably accounts for the reduced battery operating time compared to the G5. 

Failure tree must be different for the G5s for some reason if they can carry on without the gmu11?  Weird.