(Adds comment from Marine Corps general)
By Andrea Shalal
RAF FAIRFORD, England, July 12 (Reuters) - The U.S. Navy maintained a grounding order for F-35 B-model and C-model fighter jets built by Lockheed Martin Corp, saying it was still not clear what caused a massive engine failure on an Air Force F-35 jet last month.
The fleetwide grounding order kept three Marine Corps F-35 B-model jets and one British F-35B from traveling to Britain for widely publicized appearances at two air shows.
"At this time, I do not have sufficient information to return the F-35B and F-35C fleet to flight," Vice Admiral David Dunaway, who heads the Navy's Air Systems Command, said in an update to a grounding order issued by U.S. officials on July 3. A copy of the document was obtained by Reuters.
In the document, Dunaway said he was committed to returning the F-35 fleet to flight as soon as possible, but there was still "no discernible event that represents a root cause."
The memo emerged after a lengthy meeting on Friday of the officials responsible for determining the "airworthiness" of the F-35, according to sources familiar with the matter.
In the incident on June 23 at a Florida air base, the Pratt & Whitney engine on an Air Force F-35 A-model jet broke apart, pushing through the top of the airplane and catching fire while a pilot was preparing to take off.
Marine Lieutenant General Jon Davis, deputy commandant for aviation, told Reuters late on Friday that the jets might not appear at the Farnborough air show, which begins next week. But he did not see the engine issue as a long-term setback.
"I think we're being very, very conservative in our approach to this, which is the right thing to do," he said. "This is a fantastic program and we don't want to do anything to screw it up ... No one wants to do anything that isn't prudent."
Until the grounding is lifted, the U.S. Marine Corps and Britain will not be able to ferry the F-35B aircraft to Britain for the jet's planned international debut at two air shows there this month - the Royal International Air Tattoo, the world's largest military air show that began on Friday, and the Farnborough air show, which starts on Monday and runs until July 20.
It would be a embarrassment to the United States if the jets miss both shows and could weaken the plane's export prospects just as two initial partners in the program - Canada and Denmark - are weighing fresh orders.
The engine issue also prevented the F-35 from appearing at the July 4 naming of Britain's new aircraft carrier.
Dunaway said the Navy and other services are continuing to investigate the incident, and planned to update the flight directive no later than July 16 - two days after the start of the Farnborough air show outside London.
Lockheed spokesman Michael Rein said the company still hoped the jets could be cleared to fly in time to make the show.
Dunaway's memorandum did not rule out the possibility that the most advanced U.S. fighter jet could still appear toward the end of the Farnborough air show, but the prospects of that occurring appeared to be dimming fast.
"There are specific additional evaluation conditions required to support the Farnborough air show in the UK, including the ferry flight across the Atlantic and performance in the air show itself. Additional work is required in order to understand and mitigate air show unique risks," it said.
The three Marine Corps F-35B jets are waiting at a Maryland air base to fly across the Atlantic, and the UK jet that had been due to join them remains at Eglin Air Force Base in Florida. A fourth Marine Corps jet is also waiting at the Maryland base in case the British jet cannot make the trip.
The U.S. jets were ready to fly as soon as the grounding order was lifted, said one person familiar with the situation.
Dunaway said the current analysis would have to be refocused to allow the F-35's participation in the Farnborough air show. The jets need a full day in Britain to allow for any maintenance or repairs before they participate in flying demonstrations.
Dunaway said officials were looking at possible operational restrictions for the jets while the issue was being investigated, and would also likely implement repeated engine inspections to monitor for any indication of a future problem. But those measures would take several more days to complete.
It remains unclear what caused the third stage of the Pratt engine to break apart. The incident severed a fuel line, which then caused a fire.
Frank Kendall, the Pentagon's chief weapons buyer, told U.S. lawmakers on Thursday there was "growing evidence" that the incident was an individual event and had not been caused by a systemic issue. He said all existing engines had been inspected and that no similar issues had been found, but he said safety was the Defense Department's top priority. (Editing by Sandra Maler, Ken Wills, Pravin Char and Dan Grebler)