By Karen Jacobs and Victoria Bryan

FARNBOROUGH, England (Reuters) - Airbus has won a potential $4.2 billion order for its A350 passenger jet, its first major deal at a subdued Farnborough Airshow, where a faltering global economy is casting clouds as dark as the skies over southern Britain.

The deal with Hong Kong airline Cathay Pacific is a major boost for the European planemaker, which has been struggling to sell its A350-1000 mini-jumbo and make a dent in Boeing's hold on a lucrative corner of the jet market, just below 400 seats.

Separately, Boeing announced a deal to sell 100 of its next-generation narrowbody 737 planes to aircraft leasing firm GECAS, worth around $9.3 billion at list prices.

That is Boeing's second big deal for the revamped plane in as many days, bolstering its fightback against Airbus's A320neo in the top-selling short-haul segment of the market.

Both deals, however, were well flagged in advance of the show, where there have so far been no major surprises.

Boeing and Airbus, which battle for the bulk of a jet market estimated at $100 billion a year, played down expectations ahead of the aerospace industry's showcase gathering, arguing their order books are already bulging.

Despite stuttering economies, they say demand remains strong as airlines modernize fleets to survive high fuel costs and the balance of growth shifts towards Asia, with Boeing raising its long-term industry forecasts last week.

However, some investors say the euro zone debt crisis and slowing economic growth in China could see orders delayed or canceled, and that some recent deals suggest Airbus and Boeing are descending into a price war.

"We can't see how it could be a very successful airshow this year," Cheuvreux analysts said in a research note, predicting the Farnborough week will produce a combined 300-400 orders for Airbus and Boeing, less than half the number in Paris last year.

DIFFERENT SHAPES AND SIZES

Airbus, owned by European aerospace giant EADS, said that as well as planning to buy 10 new A350-1000 aircraft, worth around $3.2 billion at list prices, Cathay Pacific would convert an existing order for 16 A350-900 jets to the larger model.

The carbon-composite A350 spans two categories of aircraft, aiming to challenge the Boeing 787 Dreamliner as well as the U.S. company's most profitable plane, the 777 mini-jumbo.

Boeing, meanwhile, said GECAS had committed to buy 75 of its 737 MAX 8 planes as well as 25 of its next-generation 737-800.

That deal leaves Boeing well ahead of Airbus in terms of orders at the rain-soaked airshow.

The GECAS deal was widely anticipated. The leasing firm's parent company General Electric has a joint-venture with France's Safran to produce the LEAP engines that will power the 737 MAX.

Earlier in the day, Boeing unveiled long-awaited details on the payload and range of the 737 MAX, arguing it would fly farther and offer more revenue potential than both its predecessor and its main competitor.

"The 737 MAX models will have the capacity to fly more than 3,500 nautical miles (6,482km) ... This will allow our customers the flexibility to open up new markets," it said, adding the 737 MAX 8 model would have an 8 percent per-seat operating cost advantage over Airbus's A320neo.

Separately, Canada's Bombardier said it had signed a letter of intent with Air Baltic Corp for the Latvia-based airline to buy 10 CS300 aircraft worth $764 million at list prices, with the possibility of adding a further 10 planes that would lift the value to $1.57 billion at list prices.

(Writing by Mark Potter; Editing by Peter Graff)