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Single ADAHARS, Single big Screen and IFR
Seriously thinking about foregoing the double redundancy for PFD.
My thinking is, I'd have a single large screen (with a single ADAHARS and an EMS), a GTN650, and a small, inexpensive, self contained basic EFIS as a backup and maybe save a few dollars and some panel space. Is anyone else doing this? |
If you have a second independent EFIS you DO have redundancy. Have you thought about how you will decide if they disagree?
There is an on-going discussion about what is better: a well integrated system with multiple attitude sources where the system decides what is broken; or a system with independent, isolated attitude sources where the pilot must decide what if anything is broken. I think the jury is still out. |
That is exactly what I am installing in my aircraft. I have a Skyview Touch on the left side with the autopilot and control knob panels to the left. I have a D-6 mounted right above a Garmin 650 for a backup. Both the Skyview and D-6 have back-up batteries and are wired to the same VPX EFIS power circuit. This will allow me to download the EFIS systems to the backup batteries and save the battery for powering the Garmin 650 in the event of an alternator failure. I am mounting an IPAD on the right side of the panel, using all the functionality of Foreflight for the right seat.
Frank building an RV-7A dues paid for 2016 |
A different angle
I have a 7 inch Dynon planned centre stage of panel. Electric steam gauge turn and bank. Steam gauge airspeed altimeter. A AV8OR battery powered device on the right. Mission profile is Day/ Night VFR.
This should get me home with a failed electrical system. Some thought given to Dynon SLE but still like the moving map display in panel even though Foreflight does it all as well. Things changing so fast it is hard to keep up |
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I'm not aware of any system (Experimental glass)where the 'system' has the ability to decide which specific ADAHRS source is broken no matter how many you have, is there one out there that actually does that? I know they can show an error or mis-compare or in the case of a hard failed unit switch over to a backup source, but not actually throw out a unit that doesn't match the others (they don't agree), still up to the pilot to decide that one. |
do you have an auto pilot? that can keep you up right too. :)
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I currently have two independent EFIS systems in our panel, plus an A/P with the big blue button. Even with all that, I'm still giving consideration to a true 3rd attitude source. |
I'm planning for an IFR "capable" system. It seems like all the glass sales promotions are aimed at dual and triple redundancy. Yet, most IFR "capable" C-172s with vacuum systems do not have all this redundancy. And, they've trained how many thousands of instrument pilots?
I know this is an old debate. But, I'm really curios to know if there are any others out there flying IFR behind behind a big single display, instead of the airliner style full redundancy. I know with the steam gauges, you've got some redundancy with the fact that each gauge is independent and discreet. Whereas, if the EFIS display fails, you lose everything. That's why I would put a small basic self-contained unit, maybe a used D6, as a back up. |
[ more modern A/P's often contain a wonderful "gee whiz" feature called a "Straight and Level" button. If I had two attitude sources showing divergence, I would not hesitate to hit the big blue button and let the A/P act as a 3rd attitude reference. Wait and see which EFIS follows the airplane, then shut down the EFIS that shows the airplane is upside down! ]
Where does the straight and level information comes from that will drive the AP? Does the AP have it's own independent AHARS? Johan |
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Modern redundancy
In a prior RV, we went with a similar approach (though I wasn't IFR trained at the time) & it worked nicely overall. One big 8.4in GRT Hx screen with dual AHRS, plus a Dynon D10a for back up provided dissimilar redundancy. You could select AHARS 1 or 2 for that single screen. Though if that one display failed, now you lost two sources at once.
Nowadays, I think a single big screen (even without dual AHRS), a different brand/software back-up (think G5, GRT mini, Dynon 10a), plus an iPad having a separate source of attitude info (Stratus 2/2S) gives you a tie-breaker and enough redundancy. Many fly with an iPad any ways for multiple reasons, so it's not an added equipment cost for them/me. Even if some rare emergency destroys all electronics on board, this could provide basic attitude, altitude and ground speed to get down safely. Starting a flight fully charged, the battery life in the Stratus and iPad doesn't require keeping them connected to power except for the longest of XC days (more than 6-8 hours). More commonly, if one brand/model of primary flight data acts up, it gives you sufficient independent options to compare and decide which to believe. One downside -- I didn't like how I would lose the display of some desired information when displaying PFD, nav and engine information on the one screen. Specifically, if I remember correctly flap and trim indications come to mind. First world problem, I guess... I had to look over my shoulder to see flaps. Something to consider & assess with your specific model in mind for only one screen. Just my thoughts with the modern tools readily available to us these days. |
mis-compare
The mis-compare function is the key.
I would assume it takes 2 AHRS talking to the same system to get the mis-compare function. Is this correct? A single AHRS would simply give the display (and autopilot I imagine) bad info until it died. Could be wrong as I'm new to these systems. It seems to me 2 AHRS for a single screen and a independent stby would cover the tie-breaker problem. The question would be - is the backup AHRS being monitored when it isn't in use. I imagine so but not sure. |
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http://www.vansairforce.com/communit...d.php?t=139717 |
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Thanks for the link Walt. |
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OTOH, the digital altitude display and "bunch of dots" turn coordinator display make hand-flying using just the Trio display fairly difficult in all but calm weather. But if just used as a "tie breaker" between two EFIS boxes (one says wings level, one says turning), it easily tells you if you're turning, or not. |
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Think of it as insurance. It is better to have it and not need it than need it and not have it. I can't stress enough; Your life is worth more than "a few dollars and some panel space" :cool: |
It was easy for me.
I NEEDED a tie breaker. To me it's just another cost of safely flying IFR. 3 independent ADHRS was what I felt comfortable in the clouds with.
Sure it cost's a bit more, but me and my wife are worth every penny.. I'm sure there are any number of ways to do it. Just always leave yourself an out when, not if, but when things go south. ![]() |
Tie Breakers
Much discussion in this thread seems to imply that we need 3 fully independent AHRS/ADAHRS to have a "tie breaker'. However, just like a traditional 6 pack has adequate information to "tie break" in the event of instrument failure, so do glass panels with only 2 attitude systems. In my RV for example, I have a single AHRS dual display EFIS as well as analog airspeed, altimeter, artificial horizon, and vertical card compass for back up. I also have a Truetrak Vizion AP. In my case, a disagreement in bank between the iron gyro and efis can be resolved by observing GPS (GNS430W) track or the compass. Pitch information disagreement can be resolved by air speed and altitude trends. Heck, I learned to fly a Citabria "under the hood" including unusual attitude recoveries with only "needle, ball, and airspeed". And as others have stated, an independently functioning autopilot (Truetrak in my case) can also be used to stabilize the aircraft to determine which attitude source is correct. And yes, my plane has periodically been flown hard IFR, as well as many hours under the hood.
Skylor RV-8 |
I'm Just Spit Ballin Here,
but it seems like most posters are talking about completing an Instrument flight after they lose a primary instrument. BAD CHOICE.
In the "old days", when we lost our sole vacuum source, we advised ATC (sometimes declaring an emergency) and looked for an exit out of IMC or a quick landing using our turn coordinator, compass and pitot static instruments. You don't need three or four or five independent ADAHARS sources to safely fly instruments, but you do need to know what to do if the dominos start falling, and from my experience, it's seldom to "carry on". Terry, CFI RV9A N323TP |
Both absolutely correct
Both Skylor and Terry above are correct. It's a comfort level and what you are trained proficient too. In any emergncy or abnormal, fly the plane and work yourself out of it.
We had all rather be on the ground wishing we were up there than vice versa in a bad situation. |
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Seems to me that we should look at the most likely failure mode of an electronic/MIMS/whatever ADAHRS, which I suspect is probably a complete failure (which would show as a failed component/red X/etc. on the corresponding display). Second most likely might be a complete failure of a sensor in one axis, which perhaps would be caught by the EFIS or the ADAHRS and flagged (I dunno...maybe not, but it seems if a sensor is out and not providing data, the Kalman filter can't operate properly and find a solution?). Third, and probably least likely, is an *incorrect* reading from a sensor...which is what all the "tie-breaker" discussion has really centered on. So I'd be interested in real-world data on failure modes and likelihoods, and then one could do a proper FMEA to figure out how to mitigate those risks. Just saying "failure" is not specific enough. In the case of an incorrect attitude, I should be able to first see the difference between the two systems, then resolve it by comparing it to GPS track, airspeed, altimeter, ROC indicator, etc. Just as an example...sort of like how it has always been done in the case of a failed mechanical gyro or vacuum pump for a "traditional" AI, right? |
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If airspeed is unavailable, the SV manual says that GPS groundspeed is used as an attitude aid ("GPS Assist" is displayed). But with a partial blockage, I suppose that both of them would show the same incorrect value. Better install a 2nd pitot tube to avoid this SPF. :eek: |
Two Engines?
I've been reading all of the posts regarding multiple ADAHARS and the worry of losing one or two and not being able to complete the IFR flight.
I've only got a single IFR G3X/GTN625 installation. I would be more concerned about losing the engine due to mechanical failure than losing the electronics. I've flown in actual IFR with only needle ball and airspeed in the past, before the days of ipads and Foreflights, and Garmin Pilots. If I had had these tools years ago I would have felt blessed. To anyone who has not flown actual IFR using only the needle ball and airspeed, I recommend it as a great confidence builder. You'll stop worrying about your backup ADAHRS agreeing. Steve |
Single screen ifr
Ken
Thanks for your question. I'm contemplating same scenario. My engine install is almost done then I'll be doing avionics. I fly IFR regularly in 6-pack Cherokee. I'm thinking for my RV7 I'll do Dynon Skyview plus a Dynon D2 pocket EFIS. But I'm also thinking and electrical Attutude Indicator-maybe. Plus I'll probably have Garmin 430 or equivalent. And I'll use VP-X but the AI would be wired outside of VP-X. With my 6-pack if I loose vacuum pump I lose AI, DG and autopilot. Turn and bank is almost worthless backup gauge because it is too wiggly. But compass and Garmin 430 tells me if there is a direction change. Altimeter and GPS tells me if there is a pitch change. So I think there is a lot of backup in using the GPS, and the compass such that a third EFIS isn't necessary. And also plenty of info to decide if a gauge is wrong. And iPAD with Garmin Pilot gives a lot of functionality that is working very reliably for me. PS I'm in Anacortes. |
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Can't comment on a blocked/partially blocked pitot but an intermittently blocked static port (rain/water) will cause an erratic pitch display on the Skyview.
Fin. 9A |
Air does flow in the pitot tube - because of the drain hole. As the pitot opening gets reduced (bug, ice) down close to the size of the drain hole, the line pressure drops. And the airspeed reads low but not zero.
I know the GRT Horzon EFIS will show the horizon correctly without either airspeed or GPS data. |
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IMHO this makes for great theoretical discussion(s), but not much for a practical application. :cool: |
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OK, that's *one* branch down the failure tree...still looking for inputs on likelihood of the failure modes for an ADAHRS itself. |
I've had one experience with an airspeed malfunction while flying through rain. D10A with AoA in cruise flight the airpeed suddenly jumped to 500 knots and AoA display went nuts. Attitude remained stable throughout - until the display went black a few minutes later (unrelated failures).
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