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G3X Pitot Data required for Attitude Solution

RhinoDrvr

Well Known Member
This came up in another thread…hoping G3Xpert can chime in.

In my G3X setup, as well as the Dynon Skyview that was in my previous airplane, it seems airspeed is required for the system to display an attitude solution. Recently I had a pitot line break, so I had zero airspeed in flight. To my surprise the artificial horizon drifted to the point of being unusable for instrument flight (I was VFR at the time).

The concern here is obviously that an airspeed failure could lead to loss of control if in actual IMC. Is this a known failure mode? I would assume there should be some cross check that would provide the ‘red x’ over the airspeed indicator and tell the G3X to disregard the airspeed input (and perhaps use GPS ground speed as a backup?).

The G5 was of no use, as it was affected by the common pitot line failure.

As is, this gives me serious reservations about using this instrument suite for actual IMC operations…

Thoughts?
 
Both Dynon and Garmin software requires a speed - either airspeed or gps ground speed - to get long term attitude stability. The switch over from one to the other is supposed to be automatic. For IFR use I consider supplying both airspeed and gps data to be mandatory. OR,

Or get a GRT EFIS. It doesn’t need either airspeed or gps data to reach a stable long term attitude solution.
 
Both Dynon and Garmin software requires a speed - either airspeed or gps ground speed - to get long term attitude stability. The switch over from one to the other is supposed to be automatic. For IFR use I consider supplying both airspeed and gps data to be mandatory. OR,

Or get a GRT EFIS. It doesn’t need either airspeed or gps data to reach a stable long term attitude solution.

I thought the Garmin folks posted in the past that airspeed was not required for AI, although it is more accurate when it is supplied. Interested for them to chime in here.
 
I thought the Garmin folks posted in the past that airspeed was not required for AI, although it is more accurate when it is supplied. Interested for them to chime in here.

That was my understanding as well, although my experience was that without airspeed the attitude was not reliable enough to maintain aircraft control in IMC. This would be less of a problem if the display indicated a malfunction in any way, but it did not. The airspeed tape remained at zero, but not x’d out, and the attitude indicator was inaccurate but not x’d out or blanked…
 
That was my understanding as well, although my experience was that without airspeed the attitude was not reliable enough to maintain aircraft control in IMC. This would be less of a problem if the display indicated a malfunction in any way, but it did not. The airspeed tape remained at zero, but not x’d out, and the attitude indicator was inaccurate but not x’d out or blanked…

Interesting. I would have assumed that if the unit saw pitot airspeed at 0 and GPS airspeed of 150, it would revert to GPS speed, as it is an obvious failure. Hoping Garmin can shed some light on this.

Larry
 
I flew my RV10 for the first time this week.
2xGDU, 2XAHRS, G5, GTN, GNC
Because I'm not too sharp, I assumed I didn't need the GDU GPS antennas, as I have the GTN and a GNC in the system and this is the install verbiage:

"NOTE
For installations w/out a GPS 20A, failure of a single GDU or GPS antenna (in a multi- display/multi-antenna installation) would cause the system to use GPS information from the remaining functional GDU. If no GPS data is available from any operating GDU, the remaining GDUs will use GPS position data from an external GPS navigator (GNS 4XX/ 5XX, GNS 480, or GTN 6XX/7XX series unit, see Section 27.2 or Section 28.2). Accuracy will be degraded when using an external GPS navigator."

This misconception was reinforced when I had no issues calibrating the AHRS and the magnetometer. The magnetometer calibration page indicated that there was GPS data available. I calibrated the AHRS indoors so expected no green tick for GPS data.
On startup the GDU messaged that there was no GPS data to AHRS 1 and 2. I put that down to the GTN taking a few minuted to fix at start up each time.
So plenty of missed signs...

Anyway, without a GDU GPS signal to the AHRS, the resultant attitude solution is not useable. Certainly not adequate for IMC. I got continuous miscompares for both heading and attitude and sporadic AHRS reversion CAS messages. Meanwhile the G5 is rock solid. So I'm reasonably relaxed so far and start thinking through the issue.

Now here's where it gets interesting.
I decide to pull the breakers for both AHRS 1 and 2 to put the G5 in charge of everything.
nothing changes! Misbehaving AHRS1 is powered through the GAD27 keep alive as well.
I thought about pulling the GAD27 breaker as well but given this was only flight 2 I already had enough on my plate buzzing about over the airport flat out breaking in the engine. So I decided to park the problem and just enjoy the view for a bit.

So this now has me completely reconsidering my failure modes. It appears that the GDU/GSU25 with a failed GPS/Pitot or whatever will not take itself offline. So you'd better have a foolproof way of shutting it down when you realise its gone south.

Also, having your AHRS dependent on a $50 consumer grade black GPS antenna getting baked on the glare shield year in year out may not be so smart either. Or scheduled or unscheduled GPS outages.
Im curious as to why the GSU25 can't take GPS data from my GTN without reducing the "accuracy" significantly.

Just to be clear, it seems that my issues are mostly of my own making, but the questions it raises around reliability and redundancy I think are worth discussion.
 
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