* Farnborough Airshow runs from July 14-20
* Doubts over whether F-35 will make international debut
* Airbus to launch revamped and rebranded A330 - sources (Adds U.S. Navy grounding maintained, analyst quote)
By Tim Hepher, Andrea Shalal and Jack Stubbs
LONDON, July 12 (Reuters) - Suspense over the appearance of America's newest combat jet, a diplomatic chill between the West and Russia and the re-launch of an Airbus jet with a surprise tweak in its name could make the Farnborough Airshow one of the least predictable for years.
While the world's largest aerospace event is traditionally a cauldron for new technology and dazzling flying displays, technical problems forced the F-35 Joint Strike Fighter to withdraw from a warm-up event and Farnborough's aircraft list is already crossed through with several high-profile no-shows.
The July 14-20 gathering will go ahead without the brand-new Bombardier CSeries or popular Russian fighter displays. Qatar Airways has withdrawn its Airbus A380 superjumbo, saying the still undelivered plane is not yet ready.
At the July 11-13 Royal International Air Tattoo (RIAT) in England, visitors were disappointed after Lockheed Martin Corp's F-35 fighter jet failed to make the military air show.
An engine fire has triggered a fleetwide grounding of the Pentagon's newest warplane - and the world's single most expensive weapons programme - but officials are still hoping the stealth fighter will be cleared to fly at Farnborough.
RIAT visitors drawn by posters making it one of the most anticipated aviation events had to settle for a "stress ball" shaped like an F-35 but saw a dazzling display of other planes.
"It was the main reason for us to come here - to see the F-35; it is a big disappointment," said Jan Corstjens, a retired Dutch pharmaceutical executive with a private pilot's licence.
The U.S. Navy said on Friday it was not yet ready to lift the flying ban on F-35B and F-35C models.
Tensions over the crisis in Ukraine have also depleted this year's Farnborough show with Russia adopting a scaled-down presence, focusing on civil displays and the space industry.
"It's obvious that because of tensions with Western governments over differences between Russia and Ukraine, the military presence is really scaled down at this year's Farnborough," said Alexander Velovich, sales representative for Russian and Ukrainian clients at the Farnborough Airshow.
Many Russian visitors have struggled to obtain British visas and on Friday more than 100 of the expected 450 delegates were yet to receive clearance, said Velovich.
Somewhat more predictable is the drumbeat of civil orders led by leasing firms, despite fears of airline overcapacity and a peak in the aerospace cycle that have caused the U.S. sector to underperform wider stocks by around 10 percent this year.
Airbus and Boeing are expected to grab more than 500 orders worth well over $50 billion, including a major 100-plane order for Airbus aircraft from leasing company SMBC, reported by Reuters on Thursday.
Qatar Airways may finalise an order for 50 Boeing 777X jets and order Airbus small A320 jets, while Boeing could confirm the defection of UK's Monarch Airlines from Airbus as both planemakers pursue a relentless battle for market share.
But analysts are likely to scrutinise the announcements more closely than ever to see how much is genuinely new business.
"With the usual Airshow games, we could see some previously announced orders being firmed up, and unidentified orders being identified," RBC analyst Rob Stallard said in a note.
Most attention will be on Airbus as it kicks off the show by launching a revamped and rebranded version of the A330 jetliner.
"There is a strong likelihood it will be announced at Farnborough," a senior airline industry source said.
Airbus hopes to generate more than 1,000 new sales with new engines and curved aerodynamic winglets, but is also preparing to back away from the newer A350-800 which it once hoped would replace the A330 and underpin its new A350 jet family.
Airbus has only 34 A350-800s left on order, including Hawaiian Airlines which is being courted for the revamped A330.
The airline has previously been reluctant to change to a different model, but a spokeswoman told Reuters it was ready to examine the A330neo since the A350-800 may not be built.
Boeing officials say the change demonstrates its rival's wavering strategy and that the market will be both far smaller than Airbus hopes - closer to 400 aircraft - and short-lived.
Still, the launch is expected to lead to stiff competition as Boeing tries to limit any damage to margins and sales of its carbon-fibre 787.
In a last-minute switch, the upgraded model may be named A330-800/900neo - instead of the -200/-300 tags used in pre-marketing - in an effort to smooth the end of the A350-800 and add a new tier in Airbus's portfolio.
Unmanned vehicles will also be a big topic at the show as the United States wrestles with export policy while Europe struggles to close a gap with Israel and the United States.
Few will be able to forget that the air show is also taking place under the shadow of Malaysian Flight MH370.
The reasons for the jet's disappearance remain a mystery, but the fruitless international search could fuel interest in tracking technology and more maritime surveillance aircraft, with U.S. and European models on display as Britain ponders whether to replace its recently scrapped Nimrod aircraft.
The aerospace industry is itching for Britain to open bids for a replacement after cancelling the delayed BAE Systems Nimrod MRA4 in 2010, but Defence Minister Philip Hammond has suggested it may study a more complex solution including unmanned vehicles rather than a quick plane order. (Additional reporting by Karen Jacobs; Editing by Mark Potter and Jason Neely)