This study systematically evaluates greenhouse gas (GHG) emissions reporting practices of European airline groups, covering both mandatory and voluntary key performance indicators (KPIs) under evolving regulatory frameworks. By analysing annual and sustainability reports from 16 major airline groups, the research identifies significant progress in the reporting of core metrics, with Scope 1 CO2 totals reported by 94 % and emissions intensity by 88 %, reflecting growing regulatory alignment and stakeholder expectations. However, persistent gaps remain: Scope 2 and Scope 3 reporting appears in only 56 % and 50 % of cases, respectively, while non-CO2 emissions are disclosed by just 38 %, despite forthcoming European Union Emissions Trading System (EU ETS) monitoring requirements. Reporting on sustainable aviation fuels (SAF) life-cycle emissions is limited (19 %), and CO2 offsetting disclosures are rare (6 %), complicating verification of decarbonisation claims and readiness for ReFuelEU Aviation and Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). The proliferation of voluntary KPI disclosures further complicates comparability due to a lack of standardization and clear definitions. These challenges are compounded by risks of greenwashing, where airlines selectively report favourable data such as emissions intensity, and greenhushing, where substantive achievements are under-communicated. The study concludes that while regulatory frameworks such as the Corporate Sustainability Reporting Directive (CSRD), the EU ETS, CORSIA, and ReFuelEU are driving improvements, further harmonization and methodological clarity are required to ensure transparency, comparability, and genuine progress toward aviation's climate goals.