All F-35 grounded

WASHINGTON, Aug 3 (Reuters) - The U.S. Defense Department suspended fleetwide ground and flight test operations on Wednesday of Lockheed Martin Corp's F-35 Joint Strike Fighter aircraft, its costliest arms purchase and largest international cooperative program.

The department acted after the failure on Tuesday of one aircraft's 'integrated power package', a turbo-machine that starts the engine and cools the plane, the Pentagon's F-35 Joint Program Office said.

The office said in a statement that the suspension was a precautionary measure until experts understand the root causes of the failure aboard an F-35 conventional takeoff and landing variant at Edwards Air Force Base, California.

There are three F-35 models, or variants: A conventional takeoff-and-landing type for the Air Force; a short takeoff-vertical landing model for the Marine Corps and a carrier takeoff-and-landing variant for the Navy.

'Once the facts are understood, a determination will be made when to lift the suspension and begin ground and flight operations of the 20 F-35s currently in flying status,' the statement said.

These aircraft are part of the system development and demonstration and low-rate initial production fleet.

The United States is developing the family of radar-evading F-35s with eight international partners -- Britain, Italy, the Netherlands, Turkey, Canada, Australia, Denmark and Norway.

It is currently projected to cost the United States more than $382 billion to buy a total of 2,443 F-35 models over the next two decades. Other countries, including the co-development partners, are expected to buy roughly another 750 aircraft.

The F-35 is projected by Lockheed Martin, the Pentagon's No. 1 supplier by sales, to account for more than 20 percent of its revenues once the Pentagon starts full production runs, likely in another few years.

The program had built enough 'margin' into the test schedule to accommodate 'these kinds of incidents that occur in a development effort,' the Pentagon statement said.

Periodic updates on the matter will be released as warranted, it said.

Lockheed Martin is supporting the review efforts 100 percent, said Michael Rein, a company spokesman.

'We are working very hard with all involved to resolve this issue so we can begin flight operations again which is everyone's goal,' he said.

F-35 competitors for international sales include Boeing Co's F/A-18E/F Super Hornet, Saab's Gripen, Dassault's Rafale, Russia's MiG-35 and Sukhoi Su-35 and the Eurofighter Typhoon made by a consortium of British, German, Italian and Spanish companies.

Lockheed's chief F-35 subcontractors are Northrop Grumman Corp and BAE Systems Plc . United Technologies Corp's Pratt & Whitney unit is building the engine.

(Reporting by Jim Wolf; editing by Carol Bishopric )