Case T-351/02: Judgment of the Court of First Instance of 5 April 2006 — Deutsche Bahn v Commission (State aid — Competitor's complaint — Directive 92/81/EEC — Excise duties on mineral oils — Mineral oils used as fuel for the purpose of air navigation — Exemption from duty — Letter from the Commission to a complainant — Action for annulment — Admissibility — Challengeable act — Regulation (EC) No 659/1999 — Concept of aid — Imputability to the State — Equal treatment)