American Airlines (AA, Dallas/Fort Worth) has lost an appeal in Europe's highest court to cancel rival Delta Air Lines' grandfathering rights at London Heathrow and Philadelphia International, two slots it surrendered to level competition on the route following its merger with US Airways (Phoenix Sky Harbor) in 2013.

On March 16, 2023, the European Union Court of Justice (EUCJ) in Luxembourg dismissed American Airlines' appeal to set aside a December 2020 judgement of the General Court of the European Union, which had denied the carrier's application for the annulment of an April 2018 decision by the European Commission to grant grandfathering rights to Delta when operating the city pairs. A grandfather clause is an exemption that allows for the continuation of activities approved before implementing new rules. The Commission and Delta had opposed the latest appeal, which was dismissed with costs.

In December 2014, American gave up the slots at Heathrow and Philadelphia to Delta to ensure competition on the route and get antitrust approval for its merger with US Airways. Delta intended to operate one daily frequency between the city pairs over six consecutive IATA seasons as of the Summer 2015 schedules. No other airline had applied for the slots, but the Commission found that Delta was unconnected with the merger and had exhausted its slot portfolio at Heathrow.

Under the slot release agreement, Delta was obliged to use the slots previously operated by US Airways Group to operate a non-stop scheduled service on the Heathrow-Philadelphia route. Once appropriate use of those slots had been made for the utilisation period, Delta would have grandfathering rights meaning it would retain the slots and be entitled to use them on any other city pair.

Delta started operating on the London-Philadelphia route at the beginning of the Summer 2015 schedule season. By September of that year, American complained to the European Commission that its rival was not operating the slots appropriately as stipulated per the agreement during the 2015 summer and 2015/2016 winter seasons. As such, those seasons could not count as acquiring grandfathering rights.

However, in 2018, the Commission found that Delta had appropriately used the slots and approved the grandfathering rights. It concluded that the term "appropriate use" should not be understood as "use in accordance with the bid" but that it should be interpreted as "absence of misuse" of the slots. In its initial appeal with the General Court, American argued that the EU competition watchdog had misinterpreted the term "appropriate use" and had not considered all the relevant factors for granting grandfathering rights to Delta.

On appeal with the Court of Justice on February 26, 2021, American asked that the General Court's decision be set aside and annulled, or alternatively that the case be remanded to the General Court for reconsideration.

Its appeal was based on three legal arguments, all of which were rejected by the highest court:

  • that the General Court had misinterpreted the term "appropriate use" of the slots;
  • that the General Court had erred when it concluded that "appropriate use" was equivalent to the "absence of misuse"; and
  • that the General Court had erred in interpreting Delta's commitments, specifically regarding the legal implications of the phrase "in accordance with the bid".

The European Union Court of Justice rejected American's appeal in its entirety and ordered the airline to bear its own costs and those of the Commission and Delta in the latest proceedings.