Mode S transponder codes
- Thread starter Tom-D
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Let'sgoflying!
Touchdown! Greaser!
It will be used to cross-reference with a number each US-certificated airman will soon have, on a microchip implanted under our skin.
It is a part of the coming Parsed Airmen Identification and eNumeration system, (P.A.I.N.) brought to us by the TSA.
It is a part of the coming Parsed Airmen Identification and eNumeration system, (P.A.I.N.) brought to us by the TSA.
Joe B
Pre-takeoff checklist
Tom,
Every airframe is assigned a unique 24-bit Mode S code. The code is used to maintain the technical side of Mode S communications. For example, when one is tooling along receiving TIS advisories the ground computers are talking to "Airframe 12345678", not "NC19143". The Mode S standard also has your aircraft broadcasting its N-number. Why the powers that be felt the aircraft registration identifier wasn't good enough for the computers is one of those mysteries we'll just have to live with.
New Zealand has a pretty good write-up of the scheme here:
http://www.caa.govt.nz/Advisory_Circulars/ac91-2.pdf
Every airframe is assigned a unique 24-bit Mode S code. The code is used to maintain the technical side of Mode S communications. For example, when one is tooling along receiving TIS advisories the ground computers are talking to "Airframe 12345678", not "NC19143". The Mode S standard also has your aircraft broadcasting its N-number. Why the powers that be felt the aircraft registration identifier wasn't good enough for the computers is one of those mysteries we'll just have to live with.
New Zealand has a pretty good write-up of the scheme here:
http://www.caa.govt.nz/Advisory_Circulars/ac91-2.pdf
poadeleted20
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Probably because N-numbers change, or are reassigned. The Mode S code, like a serial number, stays with that particular aircraft forever.
John Collins
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Actually, the Mode S ICAO identifier changes when the N number changes. They are related by an algorithm.
RotorAndWing
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John Collins
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There are 24 bits in the ICAO mode S address. The USA is assigned a block of codes starting with Axxxxx, where A is the first Hexadecimal digit and xxxxx are 5 Hexadecimal digits unique to the aircraft. There are 1,048,576 codes assigned to the USA under this scheme. According to my calculations, there are 915,399 possible N numbers that can be assigned, so each possible N number can have its own unique address.
Tom-D
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OK,,,,, I won't re-register my N number, now what? They whole data base gets AFUed.
My old N number gets assigned to a new 787 and ATC is wondering how I got my 24 to 30,000'
Two bureaucrats not thinking how one rule effects the other. and the more they play with the worse it gets.
poadeleted20
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I don't remember my Mode S code being changed when I changed my N-number. What did we miss?
John Collins
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Ron,
Based on your current N number in the FAA registry, your mode S code should be 50360052 in octal or A1E02A in Hexadecimal.
What was your old N number? If your mode S N number wasn't reentered into your transponder, you are still transmitting your old ID.
poadeleted20
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N26977. And I don't remember the Airworthiness Inspector who checked the new numbers and signed the new Airworthiness Certificate asking about Mode S.
Tom-D
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Why would they? transponder isn't required for airworthiness.
poadeleted20
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Well, if what John says is true, a change of N-number requires a change of Mode S code, and it seems pretty odd that an FAA Inspector wouldn't mention that as part of the N-number change process.
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Bob Noel
What's the algorithm?
RotorAndWing
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Why would he not be telling the truth?
So if you had the Mode S transponder when you had your registration changed to the vanity number, did the avionics shop go into the transponder and also change the registration number there?
John Collins
En-Route
I wouldn't doubt that. It is possible that when your transponder test was last accomplished, the technician verified if the correct code was loaded. See the attached excerpt from the GTX330 install manual to display and change the ID if it is incorrect.
View attachment US Tail number entry GTX330.pdf
John Collins
En-Route
I don't know. I have tried to obtain this information before, but was told that the algorithm was not in the public domain.
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Can you say who told you there was an algorithm?
I know a few non-FAA Mode S users and none of them have any algorithm for assigning their Mode S codes (that I'm aware of).
I'm just curious.
John Collins
En-Route
I don't remember. I would probably have tried to get the information from the Mode S manufacturer or an ADSB manufacturer as they use the same address. The algorithm needs to come up with a unique solution for each possible N number. Other countries use a database to assign the codes to the registration number. When we get to ADSB, then the ADSB Out equipment needs to match any mode S transponder in the aircraft, which may cause one more place to screw up.
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John Collins
En-Route
According to Appendix F of FAA part 43, there is supposed to be a test of the mode S using the correct address and incorrect addresses to verify proper operation. If I was designing the test equipment, I would have the technician enter the N number into the test equipment and automate the process. If so, if the unit was not configured properly, it would produce an error that would have to be corrected. I will check my avionics shop to see if they have to enter the N number or how they verify the address on Monday. My guess is that your shop dealt with it at that time.
gismo
Touchdown! Greaser!
If that's true, what do the transponder codes in the aircraft registration DB get used for? I was under the impression that the mode S transponder code is indeed fixed to the aircraft SN and both the code and the registration number get programmed into the transponder for transmission.
John Collins
En-Route
Read the section of the install manual I attached to post #17. In the GTX330, you enter either the 24 bit ICAO address or the N number and the other is determined automatically. Not all mode S transponders have the automatic conversion and the DB provides the installer with the correct value.
There are three potential pieces of data, the N number, the ICAO 24 bit address that is directly related to the N number, and the Flight ID.
In addition to the 24 bit ICAO address, a Flight ID is transmitted by the mode S transponder. It is the N number if it is fixed as would be the case for most of our GA aircraft. If there is a need to use a flight ID that can change each flight (for example in the case of an airliner flight number), then the 8 character flight ID is entered at startup and the transponder is configured accordingly. Regardless, the transponder 24 bit ICAO code can be converted to an N number, since there is a 1 to 1 correspondence.
poadeleted20
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Unless the FAA Inspector who signed off on the registration number change said something about this issue, why would anyone have any idea they needed to get the avionics shop involved in the registration number change? Seems to me that's a question the Inspector should be asking before signing the paperwork.
RotorAndWing
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So you are saying you had no idea that your Mode S transponder displayed your aircraft's registration number to ATC?
Isn't the owner responsible for aircraft maintenance as well as inspections?
poadeleted20
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Without responding to your purely inciteful statement, it seems to me that an FAA Inspector signing off the registration change would check this before signing the paperwork. And if an FAA Airworthiness Inspector doesn't know about this, it's quite absurd to expect an aircraft owner to know it, especially since it's not part of any FAA pilot publication of which I am aware that registration number changes affect Mode S codes. Perhaps you can point out where it says that in the AIM, FAR's, Pilot's Handbook of Aeronautical Knowledge, etc.So you are saying you had no idea that your Mode S transponder displayed your aircraft's registration number to ATC?
Isn't the owner responsible for aircraft maintenance as well as inspections?
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Tom-D
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In replacing the Airworthiness certificate the inspector cared only if the aircraft met its type design, or its properly altered condition.
Ever see transponders/radios on a type certificate? if they were you would be required to get an STC or field approval to change them.
Who is responsible for getting the transponder installed and certified? 91.405 will get ya started.
OBTW, your ASI does not care who gets the violation from ATC, so why should they care what code the S is assigned.
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Tom-D
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Isn't the owner responsible for aircraft maintenance as well as inspections?
I wonder how many owners will sell their aircraft and not change the "S" info.
And then whine about getting busted when they weren't flying it.
that brings up who is going to get the busted for not complying with 91.405 in getting that info up to date during the sale. New owner? seller?
My guess is the new owner, they are the ones 91.405 applies to as soon as the bill of sale is signed.
denverpilot
Tied Down
It looks more to me like the Mode S is only "tied" to the airframe's tail number by the loosest of connections... an arbitrary database entry by humans. 
RotorAndWing
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Since you are avoiding what you consider an "inciteful" statement:
I'll ask once again. When you as the aircraft owner purchased and agreed to have a shop install your Mode S transponder certainly you were aware of it's features, such as transmitting the Registration number to ATC, weren't you?
And to take it a step further I'm sure the shop provided literature on the installed transponder.
The CFR's are very clear that the owner is responsible for maintenance as well as inspections.
Sec. 91.405 — Maintenance required.
Each owner or operator of an aircraft— (a) Shall have that aircraft inspected as prescribed in subpart E of this part and shall between required inspections, except as provided in paragraph (c) of this section, have discrepancies repaired as prescribed in part 43 of this chapter;
(b) Shall ensure that maintenance personnel make appropriate entries in the aircraft maintenance records indicating the aircraft has been approved for return to service;
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John Collins
En-Route
Mode S transponders are not the only equipment that should be updated when the N number changes, the same would also apply to an ADSB Out unit. Also, if you have a 406 ELT, you are supposed to update the SARSAT registration data. Failure to do the latter can lead to a very large fine.
Also, from what I have been told by controllers, the N number from the mode S transponder is available to the ATC computer, but that it is not displayed to the controller. Since I have no first hand knowledge to back this hearsay up, if you are a controller, feel free to correct me on this.
Also, from what I have been told by controllers, the N number from the mode S transponder is available to the ATC computer, but that it is not displayed to the controller. Since I have no first hand knowledge to back this hearsay up, if you are a controller, feel free to correct me on this.
John Collins
En-Route
Selling the airplane would not generate a requirement to change the N number or ICAO 24 bit address in the Mode S transponder unless the N number was changed as well. I would expect that the owner of the aircraft at the time of the change would be responsible for updating the equipment.
I don't see how 91.405 would apply to this situation. The only regulation I can find that would apply is in 43 Appendix F.
John Collins
En-Route
I talked to the avionics shop and they said that their avionics test equipment would detect that the wrong value is loaded and they would correct it. The manager said that they often come across this when the N number has been changed. He commented that 99% of the time, when the N number is changed, this step is forgotten.
John Collins
En-Route
One can only repair a discrepancy that one is aware of. There would be no missing appropriate log entries for return to service as the log entry for return to service only applies to the work performed. The transponder ID update was not work performed nor noted as a discrepancy. As far as I can see, there is no regulation enforcing the Mode S transponder requiring it to be set properly, with the exception of Appendix F of part 43.
RotorAndWing
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I'll respectfully disagree with you on this. If the owner knows there is a discrepancy with an installed piece of equipment then it is up to the owner to comply with 91.405.
Feigning ignorance on an installed piece of equipment or trying to blame the FAA does not absolve the owner from compliance.
John Collins
En-Route
I have no disagreement with your statement. My post assumed the owner was not aware of a discrepancy and had not been informed of one.
You can argue that the owner should have been aware of the discrepancy, but prior to this discussion, where you aware of this? I suggest that the vast majority of pilots and IA's are totally unaware of the consequences I pointed out.
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n12365
Pre-takeoff checklist
I have heard this before, both for 406 ELTs and 406 PLBs, but have never completely understood this. Do you know where this requirement comes from? Is it an FCC or FAA requirement? I wouldn't think the FAA would have anything to say about PLBs.
Tom-D
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This is true if the S only links the N number, the bill of sale will cause the FAA Data base to change ownership. buy that could take up to 90 days
one more reason the seller should be diligent of the bill of sale being sent in when the check cashes.
Tom-D
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My assumption is that the S only links the N number and ATC must send any violations to the registered owner. If this is correct, they might be mistaken if a sale is in progress and the FAA data base is not up to date as to the sale.
In this case I'm sure there is a recourse.
poadeleted20
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Since you're not an Airworthiness Inspector, I guess you can't answer my question. Thanks again for your always helpful responses.