Evidence is emerging of a growing tension between airlines and travel search engines over how prices for air tickets are displayed to users in results.
At the core of the issue is the contract that a number of airlines are trying to impose on metasearch brands in order to improve their visibility when a user looks for fares and, inevitably, the likelihood of them booking with the carrier.
Whilst there are lots of options for an airline to improve their position in metasearch engine results, such as paid placement in ad units and so on, carriers are looking - in the words of one insider - for a more "heavy-handed tactic".
The practice in question goes to the heart once more of the direct-vs-intermediary battle being played out publicly in the hotel industry - airlines want to "own the customer", so therefore want to stop travellers booking with an online travel agency.
Some airlines are now alleged to be trying to force metasearch engines to sign a contract that says only the carrier's fares can be shown in a search result to consumers, with prices for the same ticket from online travel agencies excluded.
The strategy is known as a "redistribution restriction" and has been known to cover airline's data and schedules, as well as the more controversial area of pricing.
Such tactics first came to light in the middle of 2015 in the US, when a paper from the Travel Technology Association outlined similar tactics apparently being deployed by some of the country's carriers.
These included, again, prohibiting a metasearch engine from displaying an OTA's prices for a fare and not providing the site with schedule information unless other OTAs are banished from search results.
Delta was one of the carriers singled out by the report following coverage on Tnooz into its activities to squeeze out OTAs and search sites.
Now, 12 months on, "redistribution restrictions" in contracts from European carriers are gradually being felt amongst metasearch engines.
Yet whilst the US end of the saga has inevitably stalled, European metasearch engines are not taking the issue on the chin.
Earlier this year, European Commission regulators began a wide-ranging study into airline distribution, carried out under the auspices of the Directorate-General for Competition (known as DG COMP).
The investigation asked airlines, GDSs, intermediaries and others to respond to a series of questions about the state of the market and how it works from a distribution perspective.
Responses contained in those questionnaires regarding the "redistribution restriction" clauses are now understood to have caught the attention of DG COMP and could, in due course, come under further scrutiny and, eventually, be outlawed.
Unsurprisingly, given the sensitivity of the issue and because agreements are confidential, few within the industry on either side of the debate are willing to go on the record about it, unless simply to say that they will not comment on their contracts with customers.
EC officials are also tight-lipped, remarking in a statement:
"The European Commission is closely following the market for the distribution of airline tickets."
The strategy is generally being pushed forward by full-service carriers, although some of their low-cost counterparts are also known to have tried.
Some metasearch engines have so far resisted the contracts that airlines try to put on them that include the restrictions.
But there is increasing concern that airlines are attempting to "stifle distribution" to benefit their own direct-model strategy, although such tactics are seen as essentially removing the very idea of why metasearch engines exist - so travellers can shop for tickets based on a selection of prices.
Another, somewhat darker angle highlighted recently by some is how airlines within alliances are using similar language in their contracts and attempting to serve the "redistribution restriction" at around the same time.
Such collaborative efforts have the potential to raise concerns over attempted collusion by airlines to force their strategy as a group on metasearch engines.