Aviation accounts for roughly two to three percent of global carbon dioxide emissions, and its full climate impact—including high-altitude effects such as contrail-induced cirrus and nitrogen oxide chemistry—may be substantially larger. As mandatory sustainability disclosure expands across major economies—through European Union reporting requirements, the International Sustainability Standards Board (ISSB) climate baseline, the GHG Protocol, and international aviation schemes such as CORSIA—organisations face a practical problem: the proliferation of calculation methodologies produces materially different emissions estimates for the same flight, undermining comparability and auditability. This study asks which aviation greenhouse gas methodologies best support transparent, assurance-oriented corporate disclosure, and what drives the divergence in reported results. We assess ten widely used or policy-relevant methodologies through a three-part design: a structured documentation review with explicit version locking; controlled computations over standardised short-, medium-, and long-haul flight profiles under each method's native assumptions for the eight methodologies with publicly reproducible factors; and a documentation-based multi-criteria analysis scoring transparency, boundary clarity, non-carbon-dioxide treatment, data granularity, and auditability on a four-point ordinal scale, with a double-coded protocol and sensitivity analysis. Estimates converge at short range but diverge markedly beyond about five thousand kilometres, driven primarily by lifecycle boundary choice (tank-to-wake versus well-to-wake) and by how non-carbon-dioxide effects are represented; methods including upstream fuel-chain emissions and a radiative forcing multiplier report systematically higher totals. The analysis reveals no universal link between documentation quality and emission level: higher scores coincide with higher estimates only when driven by boundary completeness. We therefore propose a minimum-viable reporting schema—explicit energy-scope declaration, documented non-carbon-dioxide treatment with sensitivity disclosure, versioned emission factors, and traceable allocation rules—enabling comparability without mandating a single tool. We do not claim physical accuracy: some implementations are partly proprietary, and airline fuel-burn ground truth is unavailable.