I have 250 hours on my Dynon D10 with a Garmin transponder eating grey code from the Dynon. This has worked perfectly until now. Recently ATC complain that I am reporting too low altitudes (typically 800-1200ft too low), often after a while (the reporting altitude is correct at takeoff and some time after). I was told by our local transponder workshop the fault was most likely a failed connection in one or more of the leads between the D10 and the transponder. This has been checked twice, with no indication of failing contact. The transponder has also been checked, and works as it should. Remaining is the D10 itself. Is the gray code in the Dynon prone to output incorrect after a while? Could this be a software problem, or how can I proceed the fault finding?
Transponder output incorrect
- Thread starter larsoyno
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The way gray code works, if one of the lines is broken or dead, you'd see altitude output that is sporadically really off, and sometimes quite close. A "shift" isn't really possible.
The first thing to check, though, is that the altimeter is basically right. On the ground, when you've set the BARO setting, is the displayed altitude correct?
Also, since the encoder output is pressure altitude - if you set the BARO to 29.92 in-flight, that is the exact altitude that is being reported to the transponder.
The first thing to check, though, is that the altimeter is basically right. On the ground, when you've set the BARO setting, is the displayed altitude correct?
Also, since the encoder output is pressure altitude - if you set the BARO to 29.92 in-flight, that is the exact altitude that is being reported to the transponder.
DBRV10
Active Member
LarsMCR
I may be about to tell you how to suck eggs but here goes just in case you have never been shown how the system works with encoders and transponders.
Assume you are at your airfield and its elevation is 100 feet AMSL. on a day when the QNH is 1013 (29.92) the transponder will be squawking to the ATC radar 100 feet and if you have a transponder that has a PA display it will also show 100 feet.
If however the actual QNH was higher say 1020 hectapascals you would adjust your QNH to 1020 and your altitude would read 100 feet again which is true as that is your aerodrome height. Now the catch is your transponder shows on its PA display a value of -100 feet. that is because the lapse in pressure is 30 feet per Hectapascal, so 7 points in QNH is 7 x30' = 210 feet and in the negative direction.
So even if you take off and fly around the Transponder will be fed an altitude of a "Pressure Alt" of -200 feet from your actual level. Now this is not a problem because EVERYONE is squawking theor own codes and Pressure ALT based on the 1013/29.92 datum so even if you enter the wrong QNH in your efis ATC and any TCAS equipped a/c know exactly what height you are at.
ATC use the local or area QNH (depends on what for ) to make an adjustment to your transmitted data and also everyone elses. This is why ATC confirm your QNH setting and ask you to verify level, that way the double check you are where you are and that your transponder agrees.
So if you do a check with ATC to verify your transponder MODE C output they will get you to confirm the QNH of the day, and your level. They will then confirm your apparent level. If you are at 2500' on your efis and they confirm that, yet your PA on the transponder says 2300' (or whatever the difference is that day) then you are all good.
If you are still in doubt go see an avionics shop who can test it all for you.
I would have thought that the Dynon uses one pressure sensor and the encoder output is the raw data un adjusted and the displayed data is that same raw data but corrected for QNH on the EFIS display.
I am sure Dynon would take a look at it for you if you send it to them.
By the way here in Oz we are required to have encoder/transponders calibration checked every two years.
Also we are required to use a TSO'd encoder, so we can not use the Dynon output which is a pity because I hav found the Dynon product to be far more accurate and more reliable that the TSO'd encoders which have failed a few times in my time of aviating (not that long 5 years or so).
Hope that helps.
DB
I may be about to tell you how to suck eggs but here goes just in case you have never been shown how the system works with encoders and transponders.
Assume you are at your airfield and its elevation is 100 feet AMSL. on a day when the QNH is 1013 (29.92) the transponder will be squawking to the ATC radar 100 feet and if you have a transponder that has a PA display it will also show 100 feet.
If however the actual QNH was higher say 1020 hectapascals you would adjust your QNH to 1020 and your altitude would read 100 feet again which is true as that is your aerodrome height. Now the catch is your transponder shows on its PA display a value of -100 feet. that is because the lapse in pressure is 30 feet per Hectapascal, so 7 points in QNH is 7 x30' = 210 feet and in the negative direction.
So even if you take off and fly around the Transponder will be fed an altitude of a "Pressure Alt" of -200 feet from your actual level. Now this is not a problem because EVERYONE is squawking theor own codes and Pressure ALT based on the 1013/29.92 datum so even if you enter the wrong QNH in your efis ATC and any TCAS equipped a/c know exactly what height you are at.
ATC use the local or area QNH (depends on what for ) to make an adjustment to your transmitted data and also everyone elses. This is why ATC confirm your QNH setting and ask you to verify level, that way the double check you are where you are and that your transponder agrees.
So if you do a check with ATC to verify your transponder MODE C output they will get you to confirm the QNH of the day, and your level. They will then confirm your apparent level. If you are at 2500' on your efis and they confirm that, yet your PA on the transponder says 2300' (or whatever the difference is that day) then you are all good.
If you are still in doubt go see an avionics shop who can test it all for you.
I would have thought that the Dynon uses one pressure sensor and the encoder output is the raw data un adjusted and the displayed data is that same raw data but corrected for QNH on the EFIS display.
I am sure Dynon would take a look at it for you if you send it to them.
By the way here in Oz we are required to have encoder/transponders calibration checked every two years.
Also we are required to use a TSO'd encoder, so we can not use the Dynon output which is a pity because I hav found the Dynon product to be far more accurate and more reliable that the TSO'd encoders which have failed a few times in my time of aviating (not that long 5 years or so).Hope that helps.
DB
David
Thank you for your engagement, but unfortunately this is not helping me.
I know how the transponders and encoders work.
I fly from a controlled airfield, where of course I always set my QNH before takeoff.
If you read my first post carefully, you will see I write "Recently ATC complain that I am reporting too low altitudes (typically 800-1200ft too low), often after a while (the reporting altitude is correct at takeoff and some time after)".
Note that the error occurs after a while, and it is usually closer to 1000ft, much more than you would find if you forgot to adjust for changing QNH while airborne anyway.
No - the problem is not due to incorrect setting of QNH pressure, I can assure you of that.
Lars
Thank you for your engagement, but unfortunately this is not helping me.
I know how the transponders and encoders work.
I fly from a controlled airfield, where of course I always set my QNH before takeoff.
If you read my first post carefully, you will see I write "Recently ATC complain that I am reporting too low altitudes (typically 800-1200ft too low), often after a while (the reporting altitude is correct at takeoff and some time after)".
Note that the error occurs after a while, and it is usually closer to 1000ft, much more than you would find if you forgot to adjust for changing QNH while airborne anyway.
No - the problem is not due to incorrect setting of QNH pressure, I can assure you of that.
Lars
I know you said that you checked the wiring but I would almost bet that it is an intermittent connection problem...
Gray code will do all sorts of crazy stuff if it is loosing one or more of the signal lines or if a couple of them get shorted together.
I would take all the connectors apart and check for any sign of an issue, clean the contact points, check shorts between pins ect. Don't skip the female connectors.
You say that your alt reading is always correct on the screen & Dynon sez they output the value for whatever your altimiter is reading at 29.92 so the only way it could be in the Dynon is for the Dynon to have a bad output on the gray code converter built into the unit. If that were the case, I would suspect that it would be a fixed issue and not come and go.
Bet it is the wiring or connections!
Gray code will do all sorts of crazy stuff if it is loosing one or more of the signal lines or if a couple of them get shorted together.
I would take all the connectors apart and check for any sign of an issue, clean the contact points, check shorts between pins ect. Don't skip the female connectors.
You say that your alt reading is always correct on the screen & Dynon sez they output the value for whatever your altimiter is reading at 29.92 so the only way it could be in the Dynon is for the Dynon to have a bad output on the gray code converter built into the unit. If that were the case, I would suspect that it would be a fixed issue and not come and go.
Bet it is the wiring or connections!
PhantomPholly
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Ok, I'm a dumb software guy and I really don't "know" the gray code scheme - but I THINK I remember it is like a base-2 system with each wire being "on" or "off." Found this link explaining the gray code.
If this is true and if each point on the gray code is represented by it's own wire, we can calculate which wire would result in a discrepancy of 1000 feet and simply pull those pins and replace with a new wire to see if the problem "goes away?" An intermittently bad wire would always show "off" when the "Problem" occurs....
So the question is - what altitude were you AT when they called you 800' too low? The B4 line seems to alternate from 1 to 0 at 1000' intervals - dead center in your "800-1200' off" example. If you were climbing and descending, at some altitudes you would show the "correct" altitude and at others you would be "off by 1000'"
If this is true and if each point on the gray code is represented by it's own wire, we can calculate which wire would result in a discrepancy of 1000 feet and simply pull those pins and replace with a new wire to see if the problem "goes away?" An intermittently bad wire would always show "off" when the "Problem" occurs....
So the question is - what altitude were you AT when they called you 800' too low? The B4 line seems to alternate from 1 to 0 at 1000' intervals - dead center in your "800-1200' off" example. If you were climbing and descending, at some altitudes you would show the "correct" altitude and at others you would be "off by 1000'"
Yep,
Using this chart:
http://www.airsport-corp.com/modecascii.txt
And knowing what your altimiter reads at 29.92 and what is being reported by your transponder, you will be able to find which pin or pins are causing this error.
Grey code is a base-2 system where only one bit at a time changes no matter what. In our case it has a 10 bit resolution and a range that gives us 100ft steps.
Using this chart:
http://www.airsport-corp.com/modecascii.txt
And knowing what your altimiter reads at 29.92 and what is being reported by your transponder, you will be able to find which pin or pins are causing this error.
Grey code is a base-2 system where only one bit at a time changes no matter what. In our case it has a 10 bit resolution and a range that gives us 100ft steps.
Now we are talking!
Thanks to Brantel and PhantomPholly for good input!
I agree completely with Brantel concerning a loose/bad connection, that is why I have checked all the connections twice, tugged the wires while ohm-ing and cleaned the connectors and everything I could think of. Still, I suspect you are right in your assessment.
To PhantomPholly, the table you linked to is really helpful. I have no consistent recording of my problems, because I am always told by ATC to switch to mode A when this problem occurs to avoid confusing their systems. However, I can insist on recording 5-10 different readings at different altitudes before going to mode A to see if there is any consistancy. Good suggestion!
Thanks to Brantel and PhantomPholly for good input!
I agree completely with Brantel concerning a loose/bad connection, that is why I have checked all the connections twice, tugged the wires while ohm-ing and cleaned the connectors and everything I could think of. Still, I suspect you are right in your assessment.
To PhantomPholly, the table you linked to is really helpful. I have no consistent recording of my problems, because I am always told by ATC to switch to mode A when this problem occurs to avoid confusing their systems. However, I can insist on recording 5-10 different readings at different altitudes before going to mode A to see if there is any consistancy. Good suggestion!
Also, I think I remember that these things use low=true logic so for each 1 in that table, it means that pin has been grounded. This is done inside the Dynon with a transistor I bet.
Double check your grounding on both the Dynon and the transponder, make sure there is no ground differential between the two units.
Double check your grounding on both the Dynon and the transponder, make sure there is no ground differential between the two units.
Ahhh Brantel, thanks again.
I know I have good and common ground.
I also always transmit too low altitude when the fault occurs. I figured that that must eliminate a shorting to ground for any of the pins, but if what you say is true, and that a 1 is really a grounded wire, this could also be my problem.
Looks like I also should check for intermittent shorting to ground (in addition to a test flight, which will be the funnest part)
I know I have good and common ground.
I also always transmit too low altitude when the fault occurs. I figured that that must eliminate a shorting to ground for any of the pins, but if what you say is true, and that a 1 is really a grounded wire, this could also be my problem.
Looks like I also should check for intermittent shorting to ground (in addition to a test flight, which will be the funnest part)
PhantomPholly
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Don't forget it IS POSSIBLE (although unlikely) that a bad transistor could cause a faulty output by the Dynon unit - they could verify that on their bench if everything else fails.
So far in my project, every "problem" has been traceable to the human component (me)...
So far in my project, every "problem" has been traceable to the human component (me)...
DBRV10
Active Member
is your harness shielded and grounded at one end.
I had a problem from a AK350 to Transponder a year or so back and it was a harness problem.
RF from the radio in the 118-120MHZ range would do strange thing to the altitude.
Also Cell phones will do it. So maybe your harness needs replacing altogether, and make sure shielded/screened cable is used and grounded at one end only. The dynon end is best.
DB
I had a problem from a AK350 to Transponder a year or so back and it was a harness problem.
RF from the radio in the 118-120MHZ range would do strange thing to the altitude.
Also Cell phones will do it. So maybe your harness needs replacing altogether, and make sure shielded/screened cable is used and grounded at one end only. The dynon end is best.
DB
Whoaa!
I flew the test flight today from 4000ft (my home base) up to 10000ft in 500ft increments. Sometimes the readback from ATC corresponded with my altitude, sometimes it was 500ft off, sometimes it was WAY off. For example my altitude at 7000ft read back 10500ft!
Coming back and crunching the numbers, it became crystal clear. When the transponder was off , it was alway the B1 bit, and only that one, that was inverted, always showing a 1 where it should read a zero.
Thanks for your help, B and PP! Now I know exactly which cable to replace/debug, I am going for the B1 cable. And thanks to Brantel, I know it is a loose connection or cable fracture, and not a connection to GND on B1!
You have made a nerd/pilot/homebuilder happy!
I flew the test flight today from 4000ft (my home base) up to 10000ft in 500ft increments. Sometimes the readback from ATC corresponded with my altitude, sometimes it was 500ft off, sometimes it was WAY off. For example my altitude at 7000ft read back 10500ft!
Coming back and crunching the numbers, it became crystal clear. When the transponder was off , it was alway the B1 bit, and only that one, that was inverted, always showing a 1 where it should read a zero.
Thanks for your help, B and PP! Now I know exactly which cable to replace/debug, I am going for the B1 cable. And thanks to Brantel, I know it is a loose connection or cable fracture, and not a connection to GND on B1!
You have made a nerd/pilot/homebuilder happy!
DBRV10
Active Member
Good work!
Some folk do not have the patience for that. Well Done!
DB
Some folk do not have the patience for that. Well Done!
DB
PhantomPholly
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Congrats on a positive diagnosis - fixin's a lot easier than knowing WHAT to fix!
Let us know how it turns out...
Let us know how it turns out...
Dumb question here...
Why are you using "grey code" and not serial (RS232)?
Also, as "grey code" is pull to ground (for a "1" signal), it's highly unlikely that any RF signal would cause a problem, especially a very low power cellphone...
I would suspect a problem that shows up when you key the mic will be a bad ground for the encoder and/or com radio.
Why are you using "grey code" and not serial (RS232)?
Also, as "grey code" is pull to ground (for a "1" signal), it's highly unlikely that any RF signal would cause a problem, especially a very low power cellphone...
I would suspect a problem that shows up when you key the mic will be a bad ground for the encoder and/or com radio.
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The original EFIS-D10 only had gray code out.