* Preliminary report on crash does not give cause
* Pilots on final flight reported 'control problem'
* Crashed jet had experience maintenance problems for days
* 'Too early' to say if new Boeing system contributed (Adds investigator quote, analysts comment, shares, background)
By Cindy Silviana and Fergus Jensen
JAKARTA, Nov 28 (Reuters) - A Lion Air jet that crashed into the sea off Indonesia last month was not in a safe condition on its second-to-last flight, when pilots experienced similar problems to those on its doomed last journey, Indonesian investigators said on Wednesday.
A preliminary report unveiled fresh details of efforts by pilots to steady the jet as they reported a "flight control problem", including the captain's last words to air traffic control asking to be cleared to "five thou" or 5,000 feet.
Indonesia's transport safety committee (KNKT) focused on airline maintenance and training and the response of a Boeing anti-stall system to a recently replaced sensor but did not give a cause for the crash that killed all 189 people on board.
"At this stage I do not have the answer," KNKT investigator Nurcahyo Utomo told reporters.
Contact with the Boeing 737 MAX jet was lost 13 minutes after it took off on Oct 29 from the capital, Jakarta, heading north to the tin-mining town of Pangkal Pinang.
The brand-new airplane had suffered a sequence of problems in cockpit readings since Oct 26, culminating in a decision to change a key sensor known as the angle-of-attack probe before the penultimate flight from Denpasar to Jakarta.
During the fatal night-time flight, a "stick shaker" was vibrating the captain's controls, warning of a stall throughout most of the 13 minutes aloft based on what investigators believe to have been erroneous data on its angle to the oncoming air.
That angle is a key flight parameter that must remain narrow enough to preserve lift and avoid an aerodynamic stall.
The airplane's anti-stall system repeatedly pushed the nose down, which is how pilots usually get air under the wings.
But the system experienced an over-reaction known as a runaway, prompting an eight-minute battle between the captain pulling the nose back up and computers holding it down.
Pilots flying the same plane on its penultimate flight a day earlier had experienced a similar problem, en route from Denpasar, Bali to Jakarta. But they used switches to shut off the system and then flew on manual controls, KNKT said.
Although that flight landed safely, the continued shaking of the control column meant the plane was considered "un-airworthy" and the flight should have been discontinued, the report said.
Lion Air CEO Edward Sirait rejected reports that the Boeing 737 MAX had not been airworthy since its previous flight, telling reporters it had been cleared by engineers.
"I think pilots can judge for themselves whether to continue," Lion Air Managing Director Daniel Putut, a former pilot, added.
The report set out a sequence of glitches and incomplete debriefings as well as a mix-up over paperwork over the number of cabin crew on board when Flight JT610 plunged into the sea.
Utomo said the agency had not determined if a new version of the anti-stall system, which was not explained to pilots in manuals, was a contributing factor.
"We still don't know yet if it contributed or not," he said when asked about the new software patch on the Boeing 737 MAX. "It is too early to conclude."
Shares in Boeing, which had come under pressure when word of the undocumented new system first emerged, rose 4.5 percent.
"While the flight control problem appears important, crews have been reminded how to address it, and the fix may well be a software change," said Bernstein analyst Douglas Harned.
"There appears no reason to expect a MAX fleet grounding, impact to demand, or other significant programme implications."
The world's largest planemaker has 4,542 of the upgraded 737 jets on order from airlines, worth over half a trillion dollars at list prices, or about half that after typical discounts.
In a statement, Boeing threw the focus on a list of airline maintenance actions set out in the report but stopped short of blaming ground workers or pilots for the accident.
It did not make any reference to the revised anti-stall system introduced on the 737 MAX, which investigators and U.S. pilots say was missing from the operating manual.
Boeing says the procedure for dealing with a so-called runaway remains unchanged from previous 737 models.
Pilots however say the control column behaves differently in certain conditions, which could confuse pilots who have flown the earlier model.
Indonesia's director general of aviation, Polana Pramesti, told Reuters the agency planned to require pilots in Indonesia to be trained on simulators for the MAX series rather than the solely computer-based conversion course for 737 pilots.
Wednesday's report provided new recommendations to Lion Air on safety on top of earlier recommendations about the flight manual that have already been implemented by Boeing.
Authorities are meanwhile still looking for the cockpit voice recorder (CVR) believed to be lying among oil pipelines.
Indonesia plans to deploy a ship from Singapore capable of staying in position without dropping anchor in the risky zone.
Asked what was needed from the CVR, Utomo said: "A lot. Discussions between the left and right pilots were about what? What procedures did they carry out. Were there any strange noises?"
Without it, he added, there would be "a lot of guessing". (Reporting by Cindy Silviana and Fergus Jensen; additional reporting by Tim Hepher in Paris, David Shepardson in Washington, Tracy Rucinski in Chicago, Eric M Johnson in Seattle and Gayatri Suroyo in Jakarta; Writing by Ed Davies, Jamie Freed, Tim Hepher; Editing by Darren Schuettler, Nick Macfie and David Evans)