Airlines established in Spain must be majority owned and effectively controlled by EU states and/or nationals of EU states, unless otherwise provided for through an international agreement to which the EU is a signatory. Cabotage is reserved to national airlines. Aircraft may be registered in the national registry by natural or legal persons who enjoy Spanish or EEA nationality.
"Article 4 - Conditions for granting an operating licence:
An undertaking shall be granted an operating licence by the competent licensing authority of a Member State provided that: [...]
(f) Member States and/or nationals of Member States own more than 50 % of the undertaking and effectively control it, whether directly or indirectly through one or more intermediate undertakings, except as provided for in an agreement with a third country to which the Community is a party; [...]" (Regulation (EC) No 1008/2008 on common rules for the operation of air services in the Community, Article 4(f))
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"Article 18
The following are registrable in the Aircraft Registration Registry of the Spanish State:
1.º Those belonging to natural or legal persons who enjoy Spanish nationality or one of the member countries of the European Economic Area.
2. At the request of the lessee, the aircraft leased to those who possess Spanish nationality or those of a member country of the European Economic Area.
3.º Aircraft for private use owned or leased to natural or legal persons from third States that have, respectively, their habitual residence or a permanent establishment in Spain.
[...]
Article 68 [...]
Cabotage traffic includes all transport of passengers, cargo or mail that takes place between places of Spanish sovereignty, even if for this purpose it flies over the territory or jurisdictional waters of another State.
[...]
Article 71
Cabotage services will be reserved for national companies.[...]" (Air Navigation Law (48/1960, of July 21), last amended on 30 December 2020, Articles 18, 68 and 71 [google translation])
Regulation (EC) No 1008/2008 on common rules for the operation of air services in the Community, Article 4(f)
Air Navigation Law (48/1960, of July 21), last amended on 30 December 2020, Articles 18, 68 and 71
[<a href="https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX%3A02008R1008-20201218">Reg</a>],[<a href="https://www.boe.es/buscar/act.php?id=BOE-A-1960-10905&tn=1&p=20201230">Law</a>]