Sanctions usually work slowly. You cannot buy this aircraft, you cannot get that part, and a fleet quietly ages into uselessness over twenty years. What the US Treasury Secretary described on Monday is something faster and blunter.

“On September 23, all the Iranian airlines will be shut down around the world,” Scott Bessent told CNBC. Wednesday, in other words. Tomorrow.

The threat is not aimed at the airlines. It is aimed at everyone who touches them.

How you ground an airline without touching it

An airliner on an international route is not a self-sufficient machine. It needs someone to sell it a ramp slot, someone to push a fuel bowser up to it, someone to load the bags, someone to file the flight plan, and somewhere to sell the seats. Almost every one of those services is provided by a company that would very much like to keep its access to the US dollar.

That is the whole design of a secondary sanction. Washington is not telling Iran Air it cannot fly. It is telling a ground handler in a third country that fuelling an Iranian aircraft will cost it the ability to bank in dollars, which for most of the aviation supply chain is the same as telling it to stop trading. Faced with that choice, the handler stops. The aircraft sits.

The practical effect, if it works as described, is that Iranian carriers lose the ability to operate anywhere they need foreign services, which is anywhere outside Iran.

Operation Economic Outcast

Monday's statement did not come out of nowhere. On 24 August the Treasury announced what it called Operation Economic Outcast, and internally dubbed “Economic D-Day”, issuing determinations against critical sectors of the Iranian economy including aviation.

On 8 September, OFAC followed through. It sanctioned 36 targets supporting Iran's aviation sector and designated 27 Iranian airlines under Executive Order 13902, a list that reads like a directory of the country's commercial aviation: Iran Aseman, Qeshm Air, Zagros, Taban, Saha, Kish, Ata, Iran Air Tour, Varesh, Sepehran and seventeen more. Mahan Air did not need adding; it has been designated since October 2011 for supporting the Islamic Revolutionary Guard Corps Quds Force, with a further State Department designation under the counter-proliferation order in December 2019.

The overflight clause

Buried in the same action is a measure that will be felt by airlines with no Iranian connection at all. OFAC suspended three Iran-related aviation authorisations, including ones permitting overflights and allowing non-US airlines to fly US-origin or US-controlled commercial aircraft into Iran.

Think about what that second clause covers. Almost every widebody in the world is either American-built or contains American-controlled technology, which is precisely why sanctions bite so hard in aviation. Withdrawing that permission means foreign carriers flying Boeings, or Airbuses with American systems, lose their legal cover for operating into Iranian airports. Treasury said it will consider aviation safety-related requests case by case.

Three 777s, two countries, one paper trail

The most revealing part of the September action is not the airline list. It is the procurement chain.

According to Treasury, Mahan Air received at least three Boeing 777s during summer 2026, diverted through the United Arab Emirates and Oman. Each aircraft came out of a retired fleet, passed through a UAE company called ECT Aviation Support, where it picked up a temporary registration, and moved on. A Türkiye-based firm, Sky Phoenix, acted as the other intermediary. Treasury noted that the route closely mirrored previous sanctions-evasion operations for the same airline.

OFAC also designated cargo agents and general sales agents servicing Mahan Air's international flights in Türkiye, Malaysia and Kazakhstan, one of which it says coordinated shipments of uncrewed aerial vehicle components destined for Iran.

That is the argument the US makes for treating Iranian commercial aviation as a security problem rather than a transport one: that the same airlines moving passengers are moving other things.

A fleet that was already dying slowly

Whatever happens on Wednesday, Iranian civil aviation was in trouble long before it. Decades of sanctions have made it very hard for Iranian carriers to buy new aircraft or obtain spare parts and maintenance services legally, which is why their fleets are routinely described as among the oldest in commercial service anywhere.

The 2015 nuclear agreement briefly changed that. In December 2016 Iran Air signed for 100 Airbus aircraft and 80 Boeings, with a further 20 ATR turboprops. Almost none of it arrived: one A321, two A330-200s and 13 ATR 72s were delivered before the United States withdrew from the agreement in May 2018 and the export licences were revoked. Iranian carriers went back to keeping A300s and Fokker 100s flying on parts bought through networks of exactly the kind OFAC spent September dismantling.

So the honest way to read Wednesday is not that a healthy airline industry is being switched off. It is that a long strangulation is being accelerated to its conclusion.

The question is China

Bessent's comments came a day after he met Chinese Vice Premier He Lifeng, ahead of a Trump meeting with Xi Jinping later this week. He said Chinese officials had been “very engaged” in the pressure campaign and described positive talks with Chinese financial authorities, including People's Bank of China Governor Pan Gongsheng, about sanctions compliance.

That is the load-bearing detail. Secondary sanctions only work where the dollar matters, and Iran's most important remaining aviation partners are in places where Washington's leverage is weakest. Mahan Air has flown to Chinese airports for years. Whether a Chinese fuel supplier fears OFAC more than it values the business will determine whether Wednesday is a genuine worldwide shutdown or a shutdown of everything except the routes that were keeping the airline alive anyway.

We will know by Thursday.

News coverage of the Treasury Secretary’s warning that Iranian airlines will be shut out worldwide from 23 September.

Sources: the US Department of the Treasury, the Office of Foreign Assets Control, FinCEN, CNBC, Al Jazeera, AFP, Reuters.