Jeg skal prøve å forklare det igjen, på engelsk, slik er det enklere for meg :/
"operating air carrier" means an air carrier that performs or intends to perform a flight
Perform a flight. Not to "perform carriage" or to "carry passengers" but to perform a flight = departure, displacement by air and landing. Perform not arrange. This rules out code-share partners - for me that's clear and obvious, as code-share is just marketing.
OK, now who is actually performing the flight in case of Lease etc?
To perform a flight with passengers (= to carry out "air service") requires compliance with EC Regulation 1008/2008.
Art. 13 of EC Regulation 1008/2008 stipulates as follows:
1. Without prejudice to Article 4(c), a Community air carrier
may have one or more aircraft at its disposal through dry or wet
lease agreement.
The wording of said Article (and of entire Regulation 1008/2008) indicates that wet-leasing does not affect the identity of the carrier - the flight operated with wet-leased aircraft is still regarded as operated by the lessee.
Reg. 1008/2008 clearly says who is responsible for what when carrying out air services (performing flights with pax/cargo).
The one responsible must hold valid AOC and the one possessing AOC is the one performing flight under EU law.
If we read laws - EC261/2004, EC2111/2005, EC1008/2008 - it becomes clearer that there is only one operating carrier and it is AOC determining who is it (the one responsible for compliance towards Luftfartstilsynet or other regulatory body).
Now - check on FR24 if you have doubts, whose AOC is used
Norwegian Longhaul CPH-JFK -> NAX (= Norwegian Air Shuttle)
Swiss European ZRH-FLR -> SWR (=Swiss International Air Lines)
SAS CPH-AAL opb JetTime -> SAS (=Scandinavian Airlines System)
but
EasyJet Swietzerland GVA-RAK ->EZS (=not Easyjet, but Easyjet Switzerland)
WizzAir Ukraine IEV-LTN ->WAU (=not WizzAir, but Wizz Air Ukraine).
Interesting case is Meridiana -> trading as Meridiana, but actually the fleet is divided into Air Italy and MeridianaFly (not Meridiana). All the scheduled flights though are operated under "ISS" AOC (MeridianaFly).
Aegean Airlines by Olimpic Air: A3 381 Mykonos->Athens (AEE = Aegean Airlines), A3 7038 Athens->Astipalea (OAL38 = Olympic Air).
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The ECJ/CJ EU - in cases Rehder, IATA-ELFAA, TUI and others - always highlighted the division between Warsaw-Montreal Regime and EU regime.
Under Warsaw-Montreal regime, you can sue any carrier you encountered: contractual, operating, any of them in case of sequence of flights, for any damage within the limits, depending on the amount of loss.
Relatives of pax travelling on DL111 (=SR111), on ticket issued by SABENA, on that sad night in 1998, may have sued SABENA, Delta or Swissair depending on their choice, and the justified amount varied depending from case to case.
EU regime - on the other hand - provides for unified, instant help at the airport primarily. There is no time to deliberate who is responsible and what amount is fair.
They laid out a rule - one operating carrier pays fixed amount of money.
Beyond that system are the redress claims and questions of further compensation if 250 EUR is not enough.
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Now finishing. In this case Nemnda went beyond its competences, as under EU261, this claim should be examined by enforcement body of Germany (LBA), as the irregularity occurred on German territory.