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What’s in a Name?

How to Avoid an ADS-B Call Sign Mismatch

Source:, by Tom Hoffmann

“What’s in a name? That which we call a rose by any other name would smell as sweet.”
— William Shakespeare

This enduring line from Shakespeare’s Romeo and Juliet deftly underscores Juliet’s effort to interpret the significance of names as mere labels and not let the “Montague” moniker obstruct true love. While that certainly leaves one to ponder the superfluous nature of names, that same logic doesn’t exactly apply in the realm of Automatic Dependent Surveillance-Broadcast (ADS-B) technology. A name, or aircraft registration number/call sign in this case, is critical to the integrity of the ADS-B Out system and pretty much defines who you are in the National Airspace System (NAS).

Names Matter

ADS-B is the principal workhorse behind the FAA’s new GPS-based surveillance system that aims to improve aircraft separation standards and provide better safety to pilots and passengers. This more accurate system naturally requires more precise data including the aircraft’s identification. The aircraft’s identification can be the aircraft name (approved call sign) or FAA registration number. A large number of operational inconsistencies with ADS-B Out so far result from a naming problem, or Call Sign Mismatch (CSMM). This issue occurs any time the aircraft identification listed in a flight plan does not exactly match the ADS-B transmitted identification. The requirement for your ADS-B to transmit your aircraft identification is stated in Title 14 Code of Federal Regulations (14 CFR) section 91.227(d)(8).

Having these two IDs match might seem simple, but in real world aviation operations, naming conventions aren’t always as straightforward as you would expect. Adding to that is the complexity of a new technology and getting used to all the new procedures it requires. It’s worth noting that a CSMM can lead to significant operational difficulties for air traffic controllers, including distraction and increased workload, so it is important to be aware and fully understand these issues.

The good news is that for most GA flyers, CSMM shouldn’t be an issue. Whew! The problem stems more from operators who use specialized call signs, like an Air Ambulance flight (more on that later). For the average GA pilot however, the N-number is always the call sign. So, if you own your own aircraft and your ADS-B Out system was properly installed and configured to ensure your registration or N-number mirrors what your ADS-B unit is transmitting, you’re good to go.

The best way to verify this is to check your system with the FAA’s Public ADS-B Performance Report (PAPR) tool at Simply fly in an area of ADS-B coverage and then submit a request. PAPR reports are typically delivered within 30 minutes and can verify if your system’s call sign is matched properly with your aircraft as well as detect any other operational deficiencies with your ADS-B transmitter. Some CSMM issues are caused by a simple typo when the technician is first configuring the ADS-B unit. If that’s the case, your repair shop should be able to help correct it. If the aircraft identification input on your unit can be manually configured, you may be able to update it yourself.

A Tale of Two Signs

Where the CSMM issue tends to be more frequent is with operators who use specialized call signs during a flight that differ from the aircraft’s registration number. These could be used to help designate specialty operations like the previously mentioned Air Ambulance life flights or air taxi operators (either of which may also require a modified N-number), or with volunteer humanitarian flights that involve transporting hospital patients, veterans, or pets. For example, one of the more common special call signs, “Compassion,” is used by the many public benefit flying groups that make up the Air Care Alliance (e.g., Angel Flight, Pilots N Paws, etc.). These call signs help expedite and improve pilot-ATC communications, signify to ATC the type of operation or mission being conducted, and facilitate priority handling if warranted. Many flight planning companies will also permit customers to use a specialized call sign when you contract for flight planning assistance. However, when used in an aircraft with ADS-B Out, there are some things you’ll need to consider.

For those that might not be used to using special call signs and their associated telephonies, here’s a quick primer. There are three types of call sign designators and telephonies authorized by the FAA:

  1. International Civil Aviation Organization (ICAO) three-letter designator (3LD)
  2. U.S. Special call sign designator
  3. Local call sign designator

The ICAO 3LD is typically used by part 121 and 135 operators, as well as corporations, government agencies, and charitable organizations. Using the earlier example with Air Care Alliance flights, their ICAO 3LD is “CMF” and associated telephony (for radio voice communications) is “Compassion” plus the last three or four digits of the aircraft’s registration number. The operator would need to use this code (e.g., CMF1234) in the aircraft identification block of the flight plan and make sure the ADS-B transponder transmits that same code to avoid a CSMM. On the return or ferry leg, the pilot should use the aircraft registration number as the call sign and ensure that ADS-B is transmitting the registration number.

Special call signs are mainly used to enable priority handling by ATC. These might include civil aircraft used for law enforcement, supporting medical emergencies or disasters, or organized events. Operators flying civilian Air Ambulance flights, for example, might use the call sign “Medevac” or “Lifeguard” and coordinate with ATC on any expeditious handling required. Note: you may see some pilots operating medical emergency flights use the “L” Lifeguard designator before their registration number on their flight plan (e.g., LN777PW). Similarly, air taxi operators who do not use a call sign should prefix their registration number with the phonetic word Tango, and file “TN ...” in their flight plan.

Finally, there are also local call signs which are used only for local flight operations as specified in a letter of agreement (LOA) between the local ATC facility and the requesting aircraft operator. Some larger flight schools might have an agreement to use a local call sign in order to reduce confusion and ambiguity among several similar sounding aircraft operating in close proximity. This practice can benefit both the pilots and ATC.

Call Sign of the Times

As you can see, there are several useful reasons for call signs other than your registration number in the GA arena. However, when ADS-B enters the mix, there’s a potential disconnect on how aircraft are identified.

“When the average GA pilot is authorized to use a special call sign, they don’t always realize that what they use as a name on their flight plan has to match what their ADS-B unit transmits,” says James Kenney, an aviation safety inspector with the FAA’s Flight Technologies and Procedures Division in Flight Standards. “If you’re transporting rescue dogs and using the call sign ARF234, that’s great. But just remember you have to change your ADS-B aircraft identification to match that call sign. If your ADS-B doesn’t allow you to update the name, you’ll have to revert to using your N-number instead.”

Kenney, who is the FAA point of contact for CSMM, is leading an effort to help educate everyone from GA pilots to air carriers on the need to properly align the aircraft identification they transmit. Regular data feeds from ATC help Kenney identify those involved in CSMMs. A single phone call to an air carrier or corporate flight department can often go a long way in preventing future occurrences. “But with single GA operators, it’s much more time consuming to track these folks down,” says Kenney. Instead, the agency now hopes to rely more on public outreach and industry briefings to educate pilots on how to avoid CSMM issues.

A 30-day snapshot of U.S. air traffic data in July 2016 revealed a total of 44,226 flights with a CSMM. Most (67-percent) were from part 121, 135, and 129 commercial operators, but GA accounted for nearly 30-percent of CSMMs in the study. Many of the GA aircraft were improperly programmed during installation.

“We’re currently in a proactive, helpful mode with the industry,” says Kenney. “We realize this is new technology and that there will be a learning curve for some of these procedures. However, the sooner we can get CSMM issues down to a more manageable level, the sooner we can move forward with enabling a more complete suite of features and services for ADS-B and begin gathering more comprehensive operational feedback.”

What’s Your Sign?

So what can pilots do to prevent CSMM? For starters, Kenney suggests that pilots involved in specialty flying should consider an ADS-B unit that has a pilot programmable call sign feature. “If you go this route, you’ll probably also want to integrate the call sign update task into your normal preflight checklist so you don’t forget it,” adds Kenney.

Those who have already purchased an ADS-B unit can check to see if it has a pilot programmable call sign and if that feature was activated during the installation. You will need to use your registration number if it isn’t programmable, but you can still use the remarks section of the flight plan and approved telephonies to advise ATC of any specialty flying you plan to conduct.

“Flight schools that use a local sign may want to consider purchasing configurable units, or else make the best of what equipment they have,” says Kenney. Another option may be to program the ADS-B transmitter to align with the call sign, for example RDDL23. This option works well when the call sign is assigned to a particular aircraft as opposed to a particular flight, but may present a problem if the pilot desires to use the registration number as a call sign and the ADS-B transmitter is not pilot-programmable. This is an area the FAA is currently examining and policy changes may be forthcoming.

“We’re actively looking at solutions and alternatives for segments of the industry that use specialized call signs or modified N-numbers,” says Kenney.
Regardless of what alternatives and flexibilities may be offered, pilots need to be aware that when an aircraft is equipped with ADS-B, the ADS-B call sign must match exactly with the flight plan call sign.

Unlike for Romeo and Juliet, names here do matter. But follow this step and you’ll be smelling like a rose!

Have any questions, comments or feedback on the ADS-B Call Sign Mismatch issue? Send us an email at

Tom Hoffmann is the managing editor of FAA Safety Briefing. He is a commercial pilot and holds an A&P certificate.

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