A merger between United Airlines and American Airlines would bring together two of the biggest airlines – not just in the U.S., but the world.

For that reason – sheer size alone – industry experts say any UA-AA merger would be unlikely to fly past regulatory approvals.

However just last week U.S. Secretary of Transportation Sean Duffy said there’s room for consolidation in the U.S. airline industry, as reported by Reuters.

Duffy told CNBC: “If there was a merger between some of the larger airlines, they would have to peel off some of their assets. I am not going to pre-commit to anything. … Is there room for some mergers in the aviation industry? Yeah, I think there is.”

Regulators, consumer advocates and labour unions would come at any deal with intense scrutiny, say antitrust law experts in today’s Reuters coverage.

There are all sorts ways to gauge airline size – number of passengers carried, available seat miles (ASMs), fleet size, network size. No matter the metric, United and American are consistently in the top three biggest airlines in the world, along with Delta Air Lines.

Industry insiders say United CEO Scott Kirby put the merger idea out there during talks with U.S. President Donald Trump in late February.

Since then the Iran war’s impact on jet fuel costs and jet fuel supply have taken airlines and travellers on a wild ride, leading to higher airfares, surcharges and warnings about jet fuel supply shortages.