Aer Lingus never committed to keeping a base at Belfast International Airport for a set period of time, the High Court in Belfast heard on Wednesday.
Counsel for the airline claimed it only ever agreed to a commercial arrangement where it would pay a price to fly out of Aldergrove.
BIA has lodged a £20 million lawsuit over Aer Lingus’s decision to switch its Belfast operation.
The decision to move to George Best Belfast City Airport in 2012 was in breach of a binding 10-year contract, it alleges.
Proceedings centre on a dispute over the terms of a deal said to have been reached by the two sides back in the summer of 2007.
It followed months of negotiations as the airline sought to establish a base outside the Irish Republic.
Issues under discussion were said to include charging rates and £900,000 in launch support for the three Airbus A320s over the first three years.
BIA’s lawyers contend that a letter the airport’s former managing director sent to Aer Lingus in June 2007 contained the terms of their arrangement.
But the airline’s barrister confirmed its case was that only price to be charged per passenger and launch support was involved.
Stephen Shaw QC claimed the “true contract” emerged following a further conversation and internal processes.
“It’s a pricing agreement and no more,” he said.
At one stage Mr Justice Weatherup said he was “astounded” at the alleged way the deal was reached.
Mr Shaw insisted, however, that those unfamiliar with the aviation industry may be surprised at how such arrangements work.
“The central point we advance is that this airline did not commit to a specific time period,” he said.
“The true agreement was of a simpler kind - we would come, we would fly our aircraft in and out and we would pay for that.”
Referring to the claim that Aer Lingus pledged to stay for 10 years, he added: “There’s no such term in the letter, and there’s no such term in the internal papers.”
The case continues.