FAA Supersonic Flight Rule Faces Legal Challenge Over Weak Scientific Basis for Noise Threshold

The FAA's proposed rule to repeal its 1973 ban on civil supersonic flight over the continental United States would replace the prohibition with a noise-based standard permitting supersonic flight where sonic boom overpressure at the ground remains at or below 0.11 pounds per square foot. The agency plans to finalize this rule and a companion rule covering takeoff and landing noise by mid-2027, ahead of Boom Supersonic's planned 2029 commercial introduction of its Overture aircraft, for which Boom Supersonic holds orders and options from United Airlines, American Airlines, and Japan Airlines.

The public comment period on the proposed rule, published in July, closed August 17. Earthjustice filed comments arguing that the FAA's chosen 0.11 psf threshold lacks adequate scientific support, noting that the agency's proposal cites a decades-old study by the National Transportation Systems Center that was originally designed to characterize atmospheric scattering patterns of secondary sonic booms from offshore Concorde flights rather than to establish a human exposure threshold. That study recorded pressure readings ranging from 0.01 psf to 0.69 psf, a spread Earthjustice argues does not support selecting 0.11 psf as the specification.

The Eleventh Circuit Court of Appeals vacated the Department of Transportation's order terminating the Delta-Aeromexico joint venture on the grounds that the agency "did not reasonably explain" why its analysis departed from its own historical standards. Boom Supersonic's XB-1 demonstrator achieved Mach 1.18 in January 2025 without a discernible boom at the surface, a result the FAA cited in support of the proposal. Aviation Week's August 24 assessment described industry feedback on the proposed rule as "largely negative."

Rulemaking that leans on a study built for a different question is a familiar exposure in aviation regulation, and it tends to surface only once a comment record forces the agency to defend its methodology in public. The Eleventh Circuit's Delta-Aeromexico ruling gives opponents a template: courts can vacate agency action when the reasoning does not connect the chosen standard to the record supporting it, regardless of how far implementation has already progressed.

Overland supersonic economics depend on a stable noise threshold that airframers, engine makers and airlines can design and certify against years in advance. A threshold that moves after aircraft development is committed forces costly redesign or re-certification, which is why the mid-2027 finalization date matters as much as the number itself.

The FAA can respond to Earthjustice's comments, add supporting data, or adjust the 0.11 psf figure before finalizing the rule, and any of those paths could push the timeline past mid-2027. Whether United Airlines, American Airlines and Japan Airlines hold their 2029 network plans depends on how quickly the agency closes that evidentiary gap.