High pressure days.
ADS-B inaccuracy and certificate action
- Thread starter birdtrick
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Eric Pauley
Pre-takeoff checklist
WGS84 altitude is based on GPS and unaffected by pressure. Regardless, KSAN altimeter was 30.06 that day, which puts your (likely wrong) baro altitude at 827ft.
Here's one where you buzzed the beach at 375ft: https://globe.adsbexchange.com/?ica...7.6&showTrace=2024-06-04×tamp=1717530210
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No one is perfect. They probably used an earlier altitude deviation letter as a template and the error was missed.
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This was a number of years before the compliance program.
No one is perfect, exactly. One thing I can show is that I am always trying to comply with the FARs, but have certainly made mistakes. A big one is trying to maintain exactly 1000’ which I’ve learned is flawed thinking. With higher terrain possibly within 2000’ horizontal distance, and possible altimeter errors, that’s an easy way to bust. In discussing the matter with the FAA that’s the main thing.. trying to comply. Going forward I’m just going to throw in at least a 200’ buffer for good measure.
Lindberg
Final Approach
Yes, the violation of 91.123.
Ok.. 1050’ is violating 91.123 but of course it would be petty (in my opinion) to go after that.
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If you really have a Notice of Proposed Certificate Action, you know whether the FAA is claiming that.
They’re not.
Ed Haywood
Final Approach
There is no such thing. Even the lowest, flattest coastal land is a few feet above sea level. Otherwise it would not be land. And there are almost always buildings, telephone poles, cell towers, etc. You definitely need a buffer of at least a few hundred feet over the charted altitude.
True, and that’s the biggest lesson I’m taking from this whole thing. I won’t accept requests for 1000’ and below anymore.
John Collins
En-Route
There are three possible sources of altitude information in your setup. Assuming you have a mode C transponder, it reports PRESSURE ALTITUDE to the nearest 100 feet. Your ADS-B Out system is required by 91.217(b) to report the PRESSURE ALTITUDE from the same source. Your ADS-B Out system also includes its own PRESSURE ALTITUDE source, but I assume it would only be used if the ADS-B Out system was installed without a transponder. The other altitude Broadcast by your ADS-B Out system is a form of GPS altitude called Geometric altitude. Geometric altitude is the altitude measured from a modeled surface of the earth using the WGS84 ellipsoid. WGS84 needs to be corrected for local variations in gravity. This correction would yield an approximate equivalent to MSL on a standard day. PRESSURE ALTITUDE needs to be corrected for a non standard altimeter setting to obtain MSL and to compare it with geometric altitude, a true altitude needs to be corrected for non standard temperature. PRESSURE ALTITUDEs can be compared with PRESSURE ALTITUDEs and geometric altitudes can be compared with geometric altitudes, but PRESSURE ALTITUDEs can't be compared with geometric altitudes, other than as a test of reasonableness. You should be able to obtain the ADS-B data for the flight from the FAA or from ADS-B exchange if they capture UAT data where your flight took place. To get transponder data, you would need to get secondary radar data. If you are able to get secondary radar data, the PRESSURE ALTITUDE should match what your ADS-B Out data is broadcasting. Your ADS-B Out geometric altitude data in the San Diego area needs to have about 115 feet added to obtain an approximate MSL altitude +/- 25 feet and a PRESSURE ALTITUDE would need to have approximately 210 feet +/- 100 feet added to obtain an MSL altitude.
This is fantastic info. Is the 210’ addition to the pressure altitude due to the 30.13 reported pressure I mentioned previously? Or some other factor?
Ed Haywood
Final Approach
I read somewhere there are also sources of error on the ATC side. For that reason, the ICAO standard of acceptable total system error is something like 350 feet.
Ed Haywood
Final Approach
You might consider taking that approach with the FAA. Show them you were making a good faith effort to comply with the FAR, and will revise your practices now that you are aware of this error in your understanding. An aviation lawyer might be able to negotiate the action down to retraining and a check ride. That is probably more likely to succeed with them than denial and defiance.
I have presented exactly this to them. I’m hopeful it will be moved to a compliance program instead. The 1-year suspension seems crazy to me considering the 30 or 45 day suspensions I hear about for pilots buzzing houses as close as 50’.
Ed Haywood
Final Approach
If this is your business or profession, get an aviation lawyer, preferably one who worked in the office of general counsel at the FAA. They will understand the agency internal decision making process better.
Yes
Ed Haywood
Final Approach
Good luck then, I hope you sort it out and reach an accomodation that is acceptable to you and the FAA.
Thank you. I appreciate it.
John Collins
En-Route
The 210 addition is based on the altimeter barometric non standard value of 30.13. Temperature will also have an effect on determining the true altitude. at 15C 1000 indicated = 1000, 40C 1000 indicated = 1079 true, at 0 C 1000 indicated = 946 true.
Two things to add. If you are going to claim the ADSB altitude is in error you better have a logbook entry and test/repair attempt. The other question that seems to be getting dodged is did ATC tell you where to hold? If not why did you not simply hold out over the water to the NW where no legality issues would arise. Trying to hold over Mission Bay at 1000 feet is a virtual guarantee the FAA phones are going to light up and frankly poor headwork.
Lindberg
Final Approach
It's worth noting that ATC doesn't expect you to do this math. They expect you to have your altimeter adjusted to the local altimeter setting and comply with their instructions based on what it indicates in the cockpit. But complying with that instruction, especially when they haven't instructed you to stay in a particular location, doesn't relive you from compliance with 91.119.
I was told to hold there. They needed me as close to my operating area as possible so that I could get in and out quickly between arriving airlines. Normally they would ask for that hold to be 1 mile north of the tower, but their workload that day was such that they wanted me just outside the border of the Bravo.
I don't believe there is any regulation stating that I need to be checking my ADSB accuracy regularly. If there is, I would certainly like to know about it. Of course after receiving the letter I have done a lot of testing.
If you haven't already done so... Go take a flight and note the time and take pictures of your Altimeter where you can see the Baro setting. Take pics of any other device you want, but the Altimeter is the must have. Call ATC and confirm our Altitude OR go into a shop and get your Altimeter checked so you have a report. ALSO... Go to https://adsbperformance.faa.gov/paprrequest.aspx and get a report on your ADS-B Out. If there is no difference, you have your proof. If there is a difference, you need to get it addressed and have the paperwork to show you had it fixed once you were aware of the issue per the FAA letter.
And you've already posted numerous details on this, so if you can get the proof that there is no discrepancy or the documentation that the discrepancy has been addressed, then post that!
Eventually I think the goal is to go to ADS-B as the Primary and radar as the Backup. But as far as ATC goes, I think TIS-B is still in what I'd refer to as the Beta Testing stage.
Same with commercial flights. At the moment, an RA comes only comes from TCAS. They use the ADS-B only for additional safety information and they may act upon the ADS-B information, but it is not the same type of mandated directive as an RA.
EDIT: Fixed where I kept using "TIS-B" above when I should have switched to the ADS-B (air-to-air) in comparison to TCAS.
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Uh, not so fast there.... My first home, in Kenner, La. was -2.8' below sea level. My realtor was excited because it meant the lot had been raised 2' and qualified for, at the time, a 'B' rating for insurance purposes. I looked at her like she had three eyes and exclaimed that it was a little crazy to be excited that my new home's slab was 2.8' below sea level.
Well, D-list celebrity at best, 20 years ago. Anyway, yes, back to the thread.
Interestingly, I still haven't gotten an answer to my original question regarding so called "automated ADS-B altitude discrepancy alerts". The FAA is claiming that if my Mode C reported altitude differs by a certain amount from my ADS-B reported altitude, there will be an automatic alert sent to the FSDO for that region and I will get a letter as well. This is their reasoning for not accepting my defense that there is a significant difference between my ADS-B reported altitude and my altimeter reading. I asked them to give me more technical info on these automated alerts, specifically what altitude difference triggers an alert. They responded that it is 125' per 91.217(a)(2). This same regulation states that the pressure altitude reported for Mode C and ADS-B must be derived from the same source. It would seem that the 125' test/alert is to make sure the same pressure source is indeed being used. But there are quite a few ADS-B installations, notably the ones from uAvionix, that use different pressure sources for ADS-B and Mode C. Mine is one of those. So it would seem I am violating that regulation with my system.
I decided to do a test today by plugging the static source on my ADS-B altitude encoder and going for a short flight. My ADS-B pressure altitude reported around 150' most of the time while I climbed all the way up to 6000'. Then I ran the PAPR report and there were no failures. Will the local FSDO be getting an alert? Time will tell.
I still do not understand why the FAA is using only my ADS-B uncorrected pressure altitude to cite the violation. It seems they absolutely should be using Mode C radar data.
Interestingly, I still haven't gotten an answer to my original question regarding so called "automated ADS-B altitude discrepancy alerts". The FAA is claiming that if my Mode C reported altitude differs by a certain amount from my ADS-B reported altitude, there will be an automatic alert sent to the FSDO for that region and I will get a letter as well. This is their reasoning for not accepting my defense that there is a significant difference between my ADS-B reported altitude and my altimeter reading. I asked them to give me more technical info on these automated alerts, specifically what altitude difference triggers an alert. They responded that it is 125' per 91.217(a)(2). This same regulation states that the pressure altitude reported for Mode C and ADS-B must be derived from the same source. It would seem that the 125' test/alert is to make sure the same pressure source is indeed being used. But there are quite a few ADS-B installations, notably the ones from uAvionix, that use different pressure sources for ADS-B and Mode C. Mine is one of those. So it would seem I am violating that regulation with my system.
I decided to do a test today by plugging the static source on my ADS-B altitude encoder and going for a short flight. My ADS-B pressure altitude reported around 150' most of the time while I climbed all the way up to 6000'. Then I ran the PAPR report and there were no failures. Will the local FSDO be getting an alert? Time will tell.
I still do not understand why the FAA is using only my ADS-B uncorrected pressure altitude to cite the violation. It seems they absolutely should be using Mode C radar data.
mandm
Avionics Upgrade
Sounds like you need to speak to a supervisor or something, I don’t think the person handling your case understands the equipment.
Both Mode C and your ADS-B are "uncorrected" pressure altitude. When you change your Altimeter, that is for what you see. What the computers see from your plane, they correct at their end.
You would think if you’d been sent letters, they could show you said letters. Also, if a lack of response to said letters will result in an enforcement action against you, I would assume those letters would be delivered by certified mail at the minimum.
This whole situation sounds like a prank or a setup to me.
I'm guessing those neighborhood groups that have been sending out letters may have upped their game and are making them sound like a real FAA letter.
KMYF has noise abatement procedures. It could be that someone complained about a plane circling overhead.
King Schools even made a video. It’s pretty old, but give an interesting overview of the area.
Edit: not sure if the file attached. Here is the URL for the file I attempted to attach.
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John Collins
En-Route
I am not sure you mean TIS-B. TIS-B is a ground station broadcast service that provides ADS-B like position broadcasts for aircraft targets that are not ADS-B Out equipped, but are detected by secondary radar Mode C. So only about 20% of aircraft are not ADS-B Out equipped. If such an aircraft has a transponder that supports mode C and it flies within the 15 NM/+/-3500 feet (aka the hockey puck) of a client ADS-B Out equipped aircraft, then the ground station converts the secondary radar target into a latitude/longitude/pressure altitude and broadcasts that information in a TIS-B message that is formatted much like an ADS-B Out message. There are more conditions required for a TIS-B including that the client and the target must both simultaneously be in a TIS-B SSV and a secondary radar SSV.
Ah yes, thanks for the catch. Just fixed it.
TIS-B in the since of the first step towards replacing Radar. But the Air-To-Air component of ADS-B for the comparison of of TCAS and RAs.
flyingcheesehead
Taxi to Parking
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ATC instructions do not absolve you from obeying FARs. If they give you an illegal instruction, you say "unable" and tell them why and it's on them to come up with a different plan for you.