Sorry, Wrong Number A Fresh Look at Avoiding Call Sign Mismatch Issues

--by Tom Hoffmann, FAA Safety Briefing

NextGen is the FAA-led modernization of our nation’s air transportation system that aims to improve aircraft separation standards and provide better safety to pilots and passengers. For a change of this size and magnitude to be successful requires NextGen’s various interconnected programs, systems, and procedures — each with their own moving parts and pieces — to flow together seamlessly and work in harmony. It’s a tall order, but one that the FAA has been assiduously working towards for well over a decade, with several critical milestones in its rearview mirror.

One of the unexpected “blips on the radar” with regard to integration efforts for ADS-B — a cornerstone technology for NextGen — has been an issue with aircraft identification inconsistencies, dubbed a 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. You may recall a previous article on the topic in our March/April 2017 issue, “What’s in a Name? How to Avoid a Call Sign Mismatch,” which outlined the issue and explored a few mitigation options to help prevent a CSMM. The intent here is to re-acquaint you with the issue and provide some important procedural changes and policy updates that are helping to resolve these mismatches.

Your Call (Sign) is Important to Us ...

Let’s start by reviewing why this issue matters. Simply stated, the call sign that an aircraft is broadcasting needs to match what an air traffic controller sees on the scope. The requirement for your ADS-B unit to transmit your aircraft identification is stated in Title 14 Code of Federal Regulations (14 CFR) section 91.227(d)(8). The result of a mismatch, especially in a busy sector, can lead to significant operational difficulties for ATC, including distraction and increased workload.

For example, were a CSMM to occur with the alerting function activated, a blinking CSMM alert would appear on either the controller’s or the ATC supervisor’s screen (depending on which platform is used) and require immediate action. FAA’s Order JO 7110.65, the chief guidance document for ATC procedures and phraseology, would require ATC to issue the following statement following a CSMM:


Because of the current frequency of CSMMs, the FAA has had to disable the feature that allows for flight ID matching. This step dilutes the system’s integrity and can affect a controller’s ability to detect a potential conflict. But it’s not just controllers who are affected by CSMMs.

“ADS-B In is negatively affected by incorrect FLT IDs [call signs], which at a minimum impairs a pilot’s situational awareness,” says Kerri Strnad, an Air Traffic Specialist with the FAA’s En Route Standards and Procedures Division. “It is imperative that operators set the correct call sign into their ADS-B, exactly matching their flight plan aircraft identification.”

You’ve Got the Wrong Number, Please Dial Again

In the previous CSMM article, we discussed how most GA pilots are not typically affected by this issue since the N-number they use as their call sign is almost always the same number entered into their ADS-B system. Recent data confirms this is still the case. As long as your ADS-B Out system is properly installed and configured to match your registration or N-number, you’re good to go. However, sometimes GA operators are simply unaware they have a mismatch, which is often the result of the installer “fat-fingering” the registration number into the ADS-B unit. But don’t worry, there’s an easy way to check that.

The FAA’s Public ADS-B Performance Report (PAPR) tool can verify that your system is configured properly. 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. If you suspect a typo is the cause of a CSMM, your repair shop should be able to help correct it. If the aircraft identification input on your unit can be manually configured, you should be able to update it yourself.

Beyond the “Call” of Duty

The more pervasive problem with CSMM lies with operators who use specialized call signs that differ from the aircraft’s registration number, like an Air Ambulance flight. By using a pilot-programmable ADS-B unit, an operator can easily sync the ADS-B call sign with the flight plan call sign to avoid a CSMM. However, some ADS-B transmitters do not have a pilot changeable call sign feature, and many air ambulance operators change their call sign depending on the need for priority handling. This sticking point has required some creative thinking to resolve.

FAA System Operations Security handles call sign policy and is the nexus between call signs and ADS-B equipment. Working with members of the air ambulance community, the FAA drafted new language in Advisory Circular (AC) 120-26M, Assignment of Aircraft Call Signs and Associated Telephonies, to resolve this issue. There were differing understandings in the community regarding the use of N-numbers, local call signs, and priority handling. Section 2.4 from the AC addresses priority handling for civilian air ambulance flights, clarifying that when the pilot states “MEDEVAC” before its FAA-authorized call sign or N-number, ATC will provide priority handling in accordance with FAA Order JO 7110.65, no exceptions. For any MEDEVAC flight, there is no additional flight plan filing requirement, such as entering “MEDEVAC” in the remarks section.

Here’s an example of how MEDEVAC is now used during a flight using a local call sign:

NO PRIORITY: For radio transmissions, use “MID-ATLANTIC Three;” when filing a flight plan, file as MA3.
PRIORITY HANDLING REQUESTED: For radio transmissions, use “MEDEVAC MID-ATLANTIC Three;” when filing a flight plan, file as MA3.

While this policy change allows air ambulance operators without programmable ADS-B units to still receive priority handling, using a radio transmission to make this request does not automatically communicate this special status to controllers downstream from where the flight is operating. The National Air Traffic Controllers Association (NATCA) and the FAA are currently looking at possible solutions to this problem, which include exploiting data from the remarks section (Field 18) from the ICAO Flight Plan to convey a MEDEVAC status to other controllers. We’ll follow up on their progress in a future issue.

AC 120-26M also provides additional clarity on programmable ADS-B equipment and strongly recommends it for anyone using new or existing FAA-authorized call signs (ICAO Three-letter Identifier, U.S. special, or local) in the NAS. However, if an aircraft operator does not have a pilot programmable ADS-B transponder, the FAA-authorized call sign may be used by having a mechanic or other qualified person preset the call sign in the ADS-B equipment before flight. Otherwise, the pilot must use the aircraft registration number as the aircraft identification when filing a flight plan and for use during radio communications.

To sum up, if the pilot is able to ensure that the local or other FAA-authorized call sign is programmed into the ADS-B equipment before flight, the pilot can continue to use that FAA-authorized call sign. If the pilot is unable to do so, use of the N-number is required.

Answering the Call

In our 2017 CSMM article, we reported a monthly snapshot of about 44,000 flights with mismatch events in the lower 48 states, with general aviation (GA) accounting for about 30-percent of that number. As of October 2018, that total number of CSMMs has been reduced to just less than 17,000. GA operators not using call signs dropped by almost half to only 16-percent of the total. When you discount non-operator related CSMMs caused by technical and procedural errors, that number drops to 14,000 events. This change accounts for slightly more than 1-percent of the total number of flights for aircraft with ADS-B, down from a previous rate of 4.5-percent.

“I think we’re making a lot of progress, but we still have a ways to go,” says Jim Kenney, an aviation safety inspector with the FAA’s Flight Technologies and Procedures Division in Flight Standards. Kenney, who is on the frontline of the CSMM issue, is leading an effort to help educate everyone from GA pilots to air carrier pilots on the need to properly align the aircraft identification they transmit. “Most people have been very understanding and they get it corrected right away,” says Kenney. “But obviously as we get more and more new aircraft to equip, we have to keep the pressure on since we’ll have more folks using the system.”

According to Kenney, there are several minor and easily preventable mistakes that can lead to a CSMM. “Maybe the technician forgot to put the N in front of the registration number or mistakenly entered a serial number during the ADS-B installation,” he says. “Or perhaps the pilot hasn’t been properly trained on the unit, or simply forgot to enter the right call sign.”

To help prevent these mistakes, Kenney advocates getting familiar with your ADS-B system and getting training on its features. Check to see if it has a pilot-programmable call sign and if that feature was activated during the installation. If it is programmable, be sure to integrate the call sign update task into your normal preflight checklist so it’s not forgotten.

Finally, and as we mentioned earlier, be sure to run an ADS-B Performance Report, or PAPR request, after installation and at regular intervals thereafter to ensure everything is working properly. “You might want to consider a PAPR request when it comes time for your annual,” says Kenney. “Then if there’s a problem, you can hand it to your mechanic and they can handle it during the inspection.”

Have any questions, comments or feedback on the ADS-B Call Sign Mismatch issue? Send us an email at 9-AWA-AVS-ADS-Programs-AFS@FAA.gov.

Three Tips to Help Prevent a CSMM

1. Get a PAPR report after install and at regular intervals.
2. Take the time to carefully enter your ADS-B call sign when using a programmable unit, or better yet, integrate the procedure into your preflight checklist.
3. Seek out some training on the features of your ADS-B unit, including how to program your call sign.

Learn More

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

This article was originally published in the January/February 2019 issue of FAA Safety Briefing magazine.
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