Erratic Altitude Indications

[NASA515]

I have had problems with erratic altitude readings since day 1.  Three different ADAHRS.  Currently at 277 hrs. 

I set the correction via the Hardware Menu item.  The instructions tell you to wait until the unit has warmed up for 30 minutes.  I obey that requirement.  Further, since there seems to be a temperature sensitivity, I do most of my adjusting on warm summer days.  ADAHRS is definitely warm.

I do my checking at surveyed locations.  I primarily like to use Bravo 10 at Boeing Field - a nice flat field, that I use a lot.  Elev is 21 feet.  When I set to the current ATIS altimeter setting, I have an offset.  The offset can be plus or minus about 125 ft, commonly 60 - 100 ft.

If I check, with the system warm, current altimeter and known elevation, I might get, say minus 60 ft.  I go into the Menu and reset to get plus 20 ft.  Maybe, the next day, same spot, same conditions, the Altitude might be Plus 80 ft.  Maybe Plus 120 ft.  I reset.  Maybe, sometimes, it's correct at 20 ft.   But not too often.

I'm chasing this - a lot.  Boeing Field downwind departures to the NW requires you to be at 800 ft.  The Class Bravo floor sits right above you at 1100 ft.  SeaTac heavy inbounds overfly Boeing Field just above  the traffic pattern.  There's not much margin for having a sloppy altitude.  The Tower closely monitors your altitude.

To compensate, where I'm unsure of a precise elevation, or don't want to go through the reset procedure, I'll just ignore the altimeter setting and reset to the Field Elevation.

Bottom Line:  There seems to be a lot of variability in the system and a degree of hysterisis.  I'm not happy or confident with my indicated altitudes.

Bob Bogash
RV-12
N737G

 

[swatson999]

FWIW, I've never seen any such variation...maybe 40-60' difference for just a few minutes as the system warms up, but by the time I get to the run-up area, it's dead-nuts on.

That it happened with 3 different ADAHRS units tells me it's not the ADAHRS...static system leak? Something else? Do you have a second display you can try connecting it, or perhaps put your ADAHRS into a buddy's system to see if your *EFIS* is at fault (somehow...though that's hard to see how...)

 

[NASA515]

No second display, and no ready airplane to swap units with.

It's hard to see how a static system leak could have any impact, since the airplane is unpressurized, the tailcone is pretty much wide open and the airplane is sitting on the ground.

I haven't done a great job of documenting this problem, mostly just lived with it as a nuisance. I think I'll start to keep a data log of daily readings and my tweaks so that I can understand this more definitively.

Bob

 

[DBRV10]

You know there are many Dynon folk that live around there…the factory is there….go ask them!

 

[Dynon]

Hi Bob. Agreed that you shouldn't have that much variability. Being local to the area, we're definitely familiar with the challenges at BFI (I learned there and used to rent airplanes based at the north parking, myself). Please do log things, but all that said, if you sync up with our technical support staff, we could get you an advanced ADAHRS swap going to minimize your downtime. But getting some data logs and other information would probably be helpful for us (and you) before we have you swap things out.

 

[NASA515]

Yeah, OK, i'll call Steve. Flew into BFI twice yesterday with the same variability mentioned in my first message. The second time was after 4+ hours flying and it was a hot day, so the unit was definitely "warmed up." When I went into the setup screen and set the correct field elevation, the indicated altitude slowly but steadily increased even though the airplane was stationary.

Changing ADAHRS on the RV-12 is sort of like getting all your uppers pulled while you get root canals on all the lowers. So, I live with a lot of things!

Bob

 

[NASA515]

An update to hopefully close the loop on this item:

Replaced the ADAHRS with replacement unit from Dynon during my Annual.  I also checked the pitot and static lines for kinks or obstructions and blew them out completely.  No discrepancies were noted.

Have made four flights since, including a flight into BFI where I performed the same altitude checks as I described at the beginning of this thread.

The altitude indications definitely appear to be much improved with greater accuracy and improved repeatability.  I will continue to monitor and report any further discreps.

Note:  I made several flights with the static line disconnected at the ADAHRS at Dynon's suggestion.  The result was not what I expected.  The airspeed was somewhat erratic, and read up to 20 kts high, which made for interesting flying!  Clearly the static pressure seen inside the fuselage of an unpressurized airplane is not the same as seen by the static ports on the sides of the airplane!

Bob Bogash
RV-12
N737G

 

[Dynon]

Thanks for the report. And yeah, a lot of people think that they can just run static to the cabin. And even if altitude is pretty close, airspeeds, especially at the lower end, are particularly sensitive to VERY small pressure changes.

 

[ricardo_godoy]

humity in static line?????