(Reuters) -Planemakers, airlines and suppliers are combing through billions of dollars worth of contracts to check their exposure to tariffs after a major U.S. supplier sparked debate over who should pay for an emerging trade war, industry sources said on Monday.

Reuters reported on Friday that U.S. supplier Howmet Aerospace ( HWM ) had declared a "force majeure event," effectively claiming the right to halt shipments if they were affected by U.S. President Donald Trump's tariffs.

Howmet makes engine parts, fuselage fasteners and other components.

Analysts said Howmet's rare and unexpected declaration that it can legally avoid contract obligations due to unavoidable circumstances will amplify debate about who should bear the cost for chronic disruption to parts supplies, including new tariffs.

So far, the answer has been passengers, as higher costs ripple along an $800-billion-plus supply chain from parts to aircraft and airlines, and ultimately to higher fares.

Trump's 20% tariffs on European Union products including Airbus planes, and likely EU retaliation against U.S.-based Boeing ( BA ), may put that to the test.

"Howmet has made a chess move, declaring force majeure and threatening to halt shipments. We all know it takes just one nut or bolt to stop the ... supply chain," Jefferies analyst Sheila Kahyaoglu wrote.

"This ultimately is an effort to pass on rising input costs in areas not already protected ... In our view, the potential to stop shipments creates leverage for Howmet, but also is pitted against its customers' willingness to cave."

Howmet declined comment.

Aside from an 18-month transatlantic tariff war over Airbus and Boeing ( BA ) subsidies in 2020 and 2021, the industry has broadly operated under a 1979 treaty on zero-duty trading in aerospace that includes the U.S. and Canada, but not Mexico.

"By definition, aviation is a global market for both buyers and sellers. When you introduce friction like this for such large sums of money, you have instant chaos, not only for airplanes but also for engines and multiple spares from avionics to seats," said aviation adviser Bertrand Grabowski.

A European lawyer said some aerospace clients had already made enquiries about how to avoid the tariff fallout.

"I don't think any of my clients have got to the stage yet of specifically wanting to (trigger) contractual provisions. But it may not take very long," he said, asking not to be identified because of potential client conflicts.

DEVELOPMENT CARROT

For now, shortages have given many suppliers the upper hand.

But planemakers have one key point of leverage. In an industry with long cycles, suppliers are already sharpening technologies for the next generation of jets.

How they approach tariff negotiations will not go unnoticed as decisions on those developments are made in coming years, several industry sources said. Airbus and Boeing ( BA ) did not respond to requests for comment.

A parallel tug of war pits jetmakers against airlines.

In the 2020-2021 trade dispute, Michael O'Leary, head of European budget airline Ryanair, asked Boeing ( BA ) to 'eat' EU counter-tariffs. Other airlines without such clout may have to try to defer deliveries, executives said.

Aircraft contracts typically call on the buyer and seller to pay their respective taxes. But Howmet's stand is being watched for signs the Trump tariffs can be construed as a special case. Aircraft manufacturers are expected to argue they cannot.

"Everyone will look at this with a fine-tooth comb, contract by contract," Grabowski said, referring to airplane contracts.

Inflation-adjustment clauses will not generally help much with tariffs as they are usually limited and slow to react, sources said.

Much of the industry also revolves around maintenance, given the compulsory replacement cycle for safety-critical parts.

It is not uncommon for engines to cross borders several times when being traded and repaired.

Lessors, who rent to airlines and make up half the world's fleet, are also scrambling to analyse the implications.

"Everyone is calling each other, looking for clarity," Grabowski said of the aviation industry as a whole.

(Additional reporting by Rajesh Kumar Singh; editing by Rod Nickel)