Planemakers are riding high on surging demand that has pushed orders well over $100 billion a year, but cracks are beginning to show and the sprawling Paris air show which opens on Monday could reveal a slowdown looms.

Surprisingly weak recent airline traffic figures and rising calls for stricter environmental controls pose threats to an airliner sector coming off its strongest two years in history.

Southwest, Continental and other carriers have suggested they might defer new orders, while leading aircraft leasing company ILFC has warned that the cycle could cool after 2008.

Brokerage analysts say the world's oldest and largest air show will likely see deals for more than 300 planes worth $30 billion, but that is down sharply from $50 billion at the last Paris show.

Boeing heads into the show with 175 planes already booked for unidentified customers, many of whom should step forth at Paris.

Analysts say Emirates, Qantas subsidiary Jetstar and India's Kingfisher are among the airlines expected to announce deals.

U.S. Airways had indicated it wanted the Airbus A350, but is tipped by some analysts to be ready to switch to Boeing's hot-selling 787 instead.

Singapore Airlines was rolled out as the launch customer for the revamped Airbus A350 XWB (Extra Wide Body) a year ago but has yet to firm up the deal.

If such deals do not come through, Paris will go down as the biggest marker yet of a looming industry slowdown.

That would be especially painful for France-based Airbus, which enters the air show playing catch-up as U.S. rival Boeing Co. has stormed ahead in new orders this year.

Airbus' A350 XWB, a $10 billion project that has seen fitful starts and stops, has fallen five years behind the Boeing 787 due next year.

BIGGER AND BOLDER

Airbus will be under pressure to serve up some major A350 orders at the air show and to assure customers it can stick to its 2013 target for delivery.

The urgency contrasts with the jubilation at Paris for Airbus just four years ago, when its sales had topped Boeing after 30 years of battle and it dreamed of bigger and bolder airliners to transform the industry.

The toothy grin of chief executive Noel Forgeard was the expression of a man at the top of his game and Airbus looked set for a long reign.

By the Paris show two years ago, Forgeard had gone and the company's fortunes were turning, symbolised by the A380 superjumbo which for all its innovativeness was proving a nightmare to assemble.

Its massive wings were not the trouble, nor the hulking fuselage, but rather a trail of skinny wires no passenger will ever see which refused to squeeze into place.

At Paris this time, Airbus is expected to confirm Singapore Airlines will get the first A380 on time by the end of the year.

Boeing will also be asked for assurances that the 787 remains on time, especially as its factory roll-out is set for next month.

The half a million visitors expected at the event, which runs from June 18 to 24, will see 140 planes on show and flying displays involving about 40 aircraft, including fighter jets and attack helicopters.

An increasingly international set of aerospace players vie for attention at Paris, with both Russia and China less a year from first flights for new regional jets that are seen as major advances for their industries -- Russia's Superjet 100 and China's ARJ21.

Embraer enters the show with an equally bold idea -- that Brazil, not the United States or Europe, will build the world's next major military transport plane.

The proposed Embraer C-390 typifies the growing globalisation in aerospace which Paris will also reflect, with some 150 countries represented including debuts from Australia, Libya and Mexico.

India