SAF blend invariance and its limits: How feedstock economics and conversion yield govern airline-level carbon abatement

Sustainable aviation fuel (SAF) deployment is commonly judged by blend percentage, although blend fraction alone does not determine climate cost-effectiveness. This study retains a transparent plant-to-passenger framework that propagates SAF production economics through blend pricing, flight expenditure, annual airline exposure, passenger-normalized burden, lifecycle CO 2 e reduction, average abatement cost, and carbon-value break-even. The linear benchmark shows that when unit fuel price and lifecycle emission factor are independent of deployment scale and mix linearly with SAF fraction, average abatement cost is mathematically blend-invariant; this identity is treated as a baseline property of the accounting model rather than as a paradox or stand-alone novelty. The operational analysis is now anchored to the current 50% ASTM D7566 HEFA-SPK blend limit, with higher fractions used only as explicitly notional extensions where retained. At 50% SAF, the reference market case adds USD 3250 per flight and USD 15.29 per effective passenger while avoiding 6.50 tCO 2 e per flight; the average abatement cost remains USD 500 tCO 2 e −1 . The reference production model retains an LCOSAF of USD 2355 t −1 , while additional boundary tests show where the linear identity ceases to hold: a bounded feedstock-supply response that raises feedstock price by 15% at 50% blend increases modeled LCOSAF to about USD 2532 t −1 and average abatement cost to USD 628 tCO 2 e −1 . A coupled lifecycle sensitivity links yield, hydrogen carbon intensity, and electricity carbon intensity to the same production scenarios, co-product accounting is tested using revenue-credit, economic-allocation, and energy-allocation treatments, and Sobol indices confirm feedstock price, yield, and CAPEX as the dominant variance contributors. The resulting framework preserves the original plant-to-passenger transmission logic while distinguishing deployment scale from cost-effectiveness, identifying the assumptions that break linear blend invariance, and placing the cost–carbon results in a certification and policy context relevant to aviation deployment.