ADS-B inaccuracy and certificate action

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birdtrick

Pre-Flight
Joined
May 24, 2014
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Troy Hartman
I received a letter from the FAA recently proposing a 1-year certificate suspension due to a couple of holds over sea level populated areas where my ADS-B reported altitudes as low as 800'. I was told by ATC to hold at 1000' or below, and I was holding 1000' exactly. Of course I was shocked to receive this letter and I started testing out my ADS-B in flight to compare to my altimeter. On the ground, the ADS-B reported 100' low and once in the air in a level hold it would read as much as 300' lower than my altimeter. I recorded this on video and submitted it to the FAA. Their response was that if my ADS-B was that far off from my Mode C, they would have received a PAPR alert at the local FSDO and would have contacted me. Is this true?
 
Do you have flight data logging in your aircraft? Or track logs in Foreflight that would show GPS altitude?
 
birdtrick said:
I received a letter from the FAA recently proposing a 1-year certificate suspension due to a couple of holds over sea level populated areas where my ADS-B reported altitudes as low as 800'. I was told by ATC to hold at 1000' or below, and I was holding 1000' exactly. Of course I was shocked to receive this letter and I started testing out my ADS-B in flight to compare to my altimeter. On the ground, the ADS-B reported 100' low and once in the air in a level hold it would read as much as 300' lower than my altimeter. I recorded this on video and submitted it to the FAA. Their response was that if my ADS-B was that far off from my Mode C, they would have received a PAPR alert at the local FSDO and would have contacted me. Is this true?
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Assuming this was over a congested area, even at 1000' exactly, you'd violate 91.119, which requires you to be at "an altitude of 1,000 feet above the highest obstacle within a horizontal radius of 2,000 feet of the aircraft." What altitude were you at before ATC told you to maintain at or below 1000'? And can you find the LiveATC recording or the ADSB track?
 
I was probably at 1050' since I would naturally hold the needle slightly above the 1000' marker than slightly below. I was over a residential area so I was likely 1000' over the top of the houses.

But the issue here is how far off my ADS-B altitude reporting is from what my altimeter is showing. I made a video of the Foreflight altitude on my iphone next to my altimeter, starting from taking off with the altimeter set to field elevation. The ADS-B altitude is already reading 100' low on the ground and then gets worse when I'm in the air.

The FAA is saying that if my ADS-B altitude differs a certain amount from my Mode C altitude, the local FSDO office will automatically be notified. That is their basis for saying they will not take into consideration the video showing the difference. I've never heard of automated alerts being sent to the FAA when an ADS-B system is differing from the Mode C encoder data. It's weird to me.

I'm not sure why they are even using the ADS-B data. I was holding 2 miles from KSAN so why not use their radar/Mode C data?
 
I'm curious what got the FAA even looking in the first place. I was under the belief they were not allowed to use ADS-B data to trigger enforcement action, only to use it as evidence after a complaint is reported.
 
midwestpa24 said:
I'm curious what got the FAA even looking in the first place. I was under the belief they were not allowed to use ADS-B data to trigger enforcement action, only to use it as evidence after a complaint is reported.
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Same here. My guess, someone in San Diego has nothing better to do than watch flight aware all day. Then made a complaint of a low circling plane.
 
birdtrick said:
But the issue here is how far off my ADS-B altitude reporting is from what my altimeter is showing. I made a video of the Foreflight altitude on my iphone next to my altimeter, starting from taking off with the altimeter set to field elevation. The ADS-B altitude is already reading 100' low on the ground and then gets worse when I'm in the air.
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Foreflight is reporting GPS altitude. ADSB reports both GPS and pressure aptitude but I'd expect FAA to use pressure. I don't know though.
birdtrick said:
The FAA is saying that if my ADS-B altitude differs a certain amount from my Mode C altitude, the local FSDO office will automatically be notified. That is their basis for saying they will not take into consideration the video showing the difference. I've never heard of automated alerts being sent to the FAA when an ADS-B system is differing from the Mode C encoder data. It's weird to me.
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Yes, that's definitely a thing.
 
I find it bizarre ATC gave you a hold which was impossible to comply with legally. Any land area has an elevation of at least 1 foot MSL. Populated areas are going to have buildings, telephone poles, etc at least 20-30 feet tall. So absolute best case, if you are exactly at 1000' MSL as directed, then you are no more than 975' above obstacles.
 
Ed Haywood said:
I find it bizarre ATC gave you a hold which was impossible to comply with legally. Any land area has an elevation of at least 1 foot MSL. Populated areas are going to have buildings, telephone poles, etc at least 20-30 feet tall. So absolute best case, if you are exactly at 1000' MSL as directed, then you are no more than 975' above obstacles.
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Would a hold be considered landing and thus not privy to that restriction?

Whole thing is bizarre and feels like we're missing something
 
I just ran the PAPR for the flight in question. I see nothing in there that compares my ADS-B altitude to my altitude encoder reported altitude. Where would I find that?
 
It’s extremely odd and frustrating. ATC gives instructions all the time over the city, when in close proximity to a major airport, to hold at 1000’ or below. You’re in trouble if you drift in either direction by the slightest. Of course some will say that the pilot should simply state that they cannot comply. It would really make it difficult for everybody in busy airspace if pilots continually did that.

For sure somebody on the ground got tired of me circling overhead for 5 minutes and reported me.

What’s really frustrating is I cannot get an answer to my question anywhere. I’ve spoken to lawyers and they don’t understand the PAPR. I’m posting everywhere to see if somebody out there has the technical knowledge to help.

Again, the FAA is using my ADS-B data, which I have proven is reporting up to 300’ lower than my altimeter. They aren’t considering my Mode C data from ATC because their claim is that the local FSDO would have already been notified of the altitude discrepancy. I ran the PAPR for the flight and only see a Kinematic section which doesn’t seem to contain the information that the FAA claims it does.

Somebody must know something about this.
 
You can check what they saw using the replay function on ADSBexchange.

…and yes, they’re not supposed to use ADSB to initiate enforcement action. Did they confirm they received a specific complaint?
 
birdtrick said:
It’s extremely odd and frustrating. ATC gives instructions all the time over the city, when in close proximity to a major airport, to hold at 1000’ or below. You’re in trouble if you drift in either direction by the slightest. Of course some will say that the pilot should simply state that they cannot comply. It would really make it difficult for everybody in busy airspace if pilots continually did that.
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Can you provide some more context, like where you were, how you got there, and what you were trying to do?
 
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birdtrick said:
I received a letter from the FAA recently proposing a 1-year certificate suspension due to a couple of holds over sea level populated areas where my ADS-B reported altitudes as low as 800'. I was told by ATC to hold at 1000' or below, and I was holding 1000' exactly. Of course I was shocked to receive this letter and I started testing out my ADS-B in flight to compare to my altimeter. On the ground, the ADS-B reported 100' low and once in the air in a level hold it would read as much as 300' lower than my altimeter. I recorded this on video and submitted it to the FAA. Their response was that if my ADS-B was that far off from my Mode C, they would have received a PAPR alert at the local FSDO and would have contacted me. Is this true?
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Yikes! Can you share the letter, of course blocking your personal details? Did ATC give you any verbal warnings or ask you to copy down a number? Or was this out of the blue? It’s weird, maybe ask to speak with a supervisor or lawyer up. AOPA too maybe?!?
 
You need to speak face to face with inspectors. I can’t imagine anything more than a nothing burger from this unless something else is actually off. Most I’ve personally met are very reasonable humans.
 
birdtrick said:
. I was told by ATC to hold at 1000' or below,
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Might be worth pointing out that you did as you were told?

Ask if they expected you to violate an atc instruction.

I can think of only three times you can refuse an atc instruction, and they all involve only safety of flight.
 
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I'm also wondering why the focus on Mode-C vs. ADS-B data. My understanding is that, in most cases, the transponder pulls this data from the same altitude encoder. You should be trying to verify whether your altitude encoder and altimeter read similarly, perhaps corroborated by GPS altitude data from saved track logs.
 
Eric Pauley said:
I'm also wondering why the focus on Mode-C vs. ADS-B data. My understanding is that, in most cases, the transponder pulls this data from the same altitude encoder. You should be trying to verify whether your altitude encoder and altimeter read similarly, perhaps corroborated by GPS altitude data from saved track logs.
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Great question and headed down the path of what I am trying to solve here.

I am using a uAvionix system that has its own barometric pressure sensor and does not use the altitude encoder. My testing has shown the altitude reports from this are way off. The FAA won’t accept that testing, however. They are claiming they would have already been notified by a PAPR if the altitude was off.
 
birdtrick said:
Yes, I had a pre-coordinated photo flight with KSAN and they requested that I hold outside of the class B at 1000’ or below until they could accommodate me.
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OK - now we’re getting somewhere.

So - it was a photo flight. Had you done this before and in the same area? Was it over/near a stadium or something? Is that your profession?

I may be wrong but I’d be surprised the FAA would send a letter threatening a 1-year suspension without having listened to the tapes (and they have access to stuff at least as good as LiveATC), reviewing the ADS-B data, pulling a PAPR, and (maybe wrong) but at least talking with you. There are still some key details missing, I think - probably unintentionally on your part.
 
Llewtrah381 said:
OK - now we’re getting somewhere.

So - it was a photo flight. Had you done this before and in the same area? Was it over/near a stadium or something? Is that your profession?

I may be wrong but I’d be surprised the FAA would send a letter threatening a 1-year suspension without having listened to the tapes (and they have access to stuff at least as good as LiveATC), reviewing the ADS-B data, pulling a PAPR, and (maybe wrong) but at least talking with you. There are still some key details missing, I think - probably unintentionally on your part.
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A Letter of Investigation would be a normal first contact if a violation was being investigated.

As a photo flight, it was not “for the purpose of takeoff or landing,” there’s nothing ATC can do to waive the 1000 ft rule. Unless the OP was 2 miles west of KSAN, I don’t really see where it would be possible to maintain 1000’ above any obstacle within 2000’ horizontally at 1050 feet. A discrepancy between ADS-B and Mode C is largely irrelevant.
 
Llewtrah381 said:
OK - now we’re getting somewhere.

So - it was a photo flight. Had you done this before and in the same area? Was it over/near a stadium or something? Is that your profession?

I may be wrong but I’d be surprised the FAA would send a letter threatening a 1-year suspension without having listened to the tapes (and they have access to stuff at least as good as LiveATC), reviewing the ADS-B data, pulling a PAPR, and (maybe wrong) but at least talking with you. There are still some key details missing, I think - probably unintentionally on your part.
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The actual photo flight was in a different area at a higher altitude. This was during an ATC-requested hold outside of the class B while waiting for a gap in arrivals that would allow for the photo flight. I only mention the photo flight because somebody else in the thread speculated that the event occurred during the photo flight.
 
MauleSkinner said:
A Letter of Investigation would be a normal first contact if a violation was being investigated.
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Having no experience with the snail mail process (knock on wood) and only a “call this number” that was very constructive and uneventful, would “a letter from the FAA recently proposing a 1-year certificate suspension” be the first contact? That seems different than a letter of investigation but maybe that’s how they’re worded. Maybe the OP’s letter was worded more along the lines of “we’re looking into this and, if true, could result in a 1 year suspension” or something. Again - I’ve not seen one of these.
 
Llewtrah381 said:
Having no experience with the snail mail process (knock on wood) and only a “call this number” that was very constructive and uneventful, would “a letter from the FAA recently proposing a 1-year certificate suspension” be the first contact? That seems different than a letter of investigation but maybe that’s how they’re worded. Maybe the OP’s letter was worded more along the lines of “we’re looking into this and, if true, could result in a 1 year suspension” or something. Again - I’ve not seen one of these.
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Yes, it is an investigation and proposed 1-year suspension. I have gone back and forth with the FAA multiple times about it (and yes, they are very cordial) and shown them the altitude reporting discrepancies. I thought that would be enough, but they are standing firm that I violated the altitude minimums. There are many more tests I am going to perform and conversations to be had with technical experts, but I figured somebody in these forums might know of something helpful.
 
Send a letter/email to the FAA to save the ATC communications for this date for future legal evidence. Be specific about which ATC you are requesting, and be accurate.

Use Live ATC recording for your own reference, but that is not admissible in most courts. It is important to have those instructions, and a written copy, to put the onus on the FAA to prove you did not comply with an ATC instruction.

If they do not save the tapes, they have a serious legal problem, and your Live ATC becomes admissible.

Not a lawyer, but have won in Federal Court without one. :)
 
birdtrick said:
The actual photo flight was in a different area at a higher altitude. This was during an ATC-requested hold outside of the class B while waiting for a gap in arrivals that would allow for the photo flight. I only mention the photo flight because somebody else in the thread speculated that the event occurred during the photo flight.
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What altitude and where were you at when ATC told you to hold? And did ATC say, "Hold," or just, "Remain outside of the Class B"? Had ATC put you at the altitude you were at in the place that you were at to begin with?
 
geezer said:
Send a letter/email to the FAA to save the ATC communications for this date for future legal evidence. Be specific about which ATC you are requesting, and be accurate.

Use Live ATC recording for your own reference, but that is not admissible in most courts. It is important to have those instructions, and a written copy, to put the onus on the FAA to prove you did not comply with an ATC instruction.

If they do not save the tapes, they have a serious legal problem, and your Live ATC becomes admissible.

Not a lawyer, but have won in Federal Court without one. :)
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Very helpful. Thank you.
 
Never been to the area but that seems really low to hold, is that a common instruction out there? Sounds like a helicopter instruction maybe. I'd say this hold must have taken place over a home owned by somebody "important".
 
Lindberg said:
What altitude and where were you at when ATC told you to hold? And did ATC say, "Hold," or just, "Remain outside of the Class B"? Had ATC put you at the altitude you were at in the place that you were at to begin with?
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I have to review the tapes still to get these specifics. I just know they said 1000' or below since it is a common request.
 
Again I'm wondering what was the initiating event of this investigation. The FAA is not supposed to be reviewing ADS-B data looking for violations, although it can be used as evidence in support of an investigation. If ATC believed there was a deviation, you should have received a Brasher warning, which does not seem to have occurred.

The only other option is a neighborhood Karen using ADS-B tracking was tracking you and reported you. I think the FAA needs to start answering some questions on this one.
 
Do you have the actual data (ADS-B and Mode-C) from the FAA for the time in question?
If not, why not? You can't do anything until you understand what data is being used against you.
Does the Mode C from your transponder match the baro altitude from the ADS-B?
Does the GPS altitude correlate to either reported baro altitude?
What was the altimeter setting at the time in question?
 
There are sharper tools than I in this toolbox but I still think a NASA report would be useful, if only to show you’re trying to do the right thing. I’d even note that you weren’t aware of a potential violation or anything until you got the letter.

It may be too late for a true “get out of jail” tool but there’s no downside I can see.
 
Capt. Geoffrey Thorpe said:
Do you have the actual data (ADS-B and Mode-C) from the FAA for the time in question?
If not, why not? You can't do anything until you understand what data is being used against you.
Does the Mode C from your transponder match the baro altitude from the ADS-B?
Does the GPS altitude correlate to either reported baro altitude?
What was the altimeter setting at the time in question?
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I don’t know if the Mode C data is available anymore. I’ve put in a request. The letter came many months after the event.
 
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