A formation flight of eight aircraft departing Livermore Municipal Airport (KLVK) and transiting San Francisco Bay drew attention on Reddit when observers noted that only two of the aircraft — N664V and N19CA — appeared on ADS-B tracking services. The remaining six aircraft were invisible to ground-based receivers and, by extension, to any ADS-B In-equipped traffic in the area. The observation, posted publicly with both admiration and genuine inquiry, raises substantive questions about ADS-B equipage compliance, formation flight transponder practices, and the procedural norms governing civilian formation operations in complex, congested airspace.
The San Francisco Bay Area presents one of the more demanding airspace environments in the contiguous United States for any general aviation or formation operation. KLVK sits within the lateral confines of SFO's Class B airspace structure, and a westbound routing over the Bay would almost certainly penetrate Class B shelves or transit within the 30-nautical-mile Mode C veil surrounding San Francisco International. Under 14 CFR 91.225, ADS-B Out is mandatory in all Class B and Class C airspace, within 30 NM of Class B primary airports up to 10,000 feet MSL, and in Class E airspace at and above 10,000 feet MSL. If these aircraft transited any of those areas — which a Bay crossing almost certainly entails — all eight were legally required to be broadcasting ADS-B Out, making the six silent aircraft a matter of regulatory concern rather than merely a technical curiosity.
Several plausible explanations exist for the gap between aircraft present and aircraft visible. Experimental amateur-built aircraft are not categorically exempt from ADS-B requirements in controlled or B/C airspace, though some legacy experimental registrations have deferred upgrades. A more operationally common factor in formation flying is the deliberate squawking of standby mode by wingmen to prevent TCAS/TCAD nuisance alerts — a practice borrowed from military formation doctrine where the flight lead holds the active squawk while wingmen go electronically quiet. While this is standard and accepted in military operations under instrument flight procedures, it is not an approved civilian workaround. In civil airspace, 14 CFR 91.215 requires an operating transponder in Class A, B, and C airspace and within the Mode C veil, with no provision for formation exemptions. An ADS-B Out unit that is powered down or set to standby satisfies neither the transponder nor ADS-B mandate.
From an operational risk standpoint, the practical consequence of six electronically invisible aircraft transiting the Bay Area is significant. Controllers at NorCal TRACON rely on a blended picture of radar returns and ADS-B data to sequence and separate traffic. Aircraft without functioning ADS-B Out are invisible to controllers' Traffic Information Service-Broadcast (TIS-B) uplink, meaning other ADS-B In-equipped aircraft in the area received no traffic advisory for the majority of the formation. Any IFR or VFR pilot receiving TIS-B traffic would have seen two targets where eight aircraft actually existed — a meaningful gap in situational awareness in airspace that routinely handles IFR departures and arrivals for SFO, OAK, and SJC simultaneously. TCAS II-equipped transport aircraft in the area would similarly have had incomplete threat geometry.
The broader trend this incident reflects is the uneven adoption and operational interpretation of the 2020 ADS-B Out mandate across the general and sport aviation community. While the mandate has been in effect for over five years, the formation and warbird communities have been slower to fully integrate ADS-B Out across aging fleets and experimental aircraft, and informal military-derived procedures for transponder management persist in civilian contexts without clear regulatory sanction. For professional pilots operating in the same airspace — particularly those flying corporate turboprops, business jets, or regional airline equipment — awareness that formation groups may be partially or wholly invisible to electronic conspicuity systems is a practical traffic management consideration. Filing and maintaining good visual scan discipline in areas known for formation activity, particularly around warbird-heavy fields like KLVK, remains a prudent supplement to TCAS and ADS-B traffic displays.