* Sunday's flight had unusual altitude, airspeed variations
* Jet experienced "technical problem" during that flight - CEO
* Plane crashed into sea with nose slightly down - fishermen
By Jamie Freed and Tom Allard
SINGAPORE/JAKARTA Oct 30 (Reuters) - The Lion Air Boeing 737 MAX jet that crashed in Indonesia on Monday morning flew erratically during a flight the previous evening when it experienced a "technical problem", according to data from flight tracking website FlightRadar24.
After taking off from Denpasar on the holiday island of Bali on Sunday evening, the jet reported unusual variations in altitude and airspeed in the first several minutes of flight - including an 875-foot drop over 27 seconds when it would normally be ascending - before stabilising and flying on to Jakarta.
However, the pilots kept the plane at a maximum altitude of 28,000 feet compared with 36,000 feet on the same route earlier in the week.
Lion Air CEO Edward Sirait told reporters on Monday a technical problem had occurred on the Denpasar-Jakarta flight but it had been resolved "according to procedure".
Two of that flight's passengers posted on Instagram, reporting that they had been concerned about problems with the air conditioning system and cabin lighting before the plane departed nearly three hours late.
"I was angry because as a passenger who had paid her ticket, we have every right to question the aircraft's safety," said one of them, TV presenter Conchita Caroline. She added there was a "weird" engine noise upon take-off that continued during flight.
It was not clear if the cabin problems were in any way related to the technical trouble mentioned by the airline's CEO.
The Denpasar-Jakarta flight landed at 10:55 p.m. local time on Sunday, giving engineers six-and-a-half hours at most for checks before it was dispatched for the fatal Jakarta-Pangkal Pinang flight at 6:20 a.m. on Monday.
The plane plunged into the sea minutes after taking off from Jakarta and all 189 people on board are believed dead.
FlightRadar24 also reported unusual air speeds and altitudes in the few minutes that Flight JT610 was in the air on Monday.
Photos of alleged technical and maintenance logs following the Sunday flight have been circulating online, but to date they have not been verified as accurate by the airline or investigators.
Sirait declined to detail the maintenance procedures taken, and on Tuesday he told Reuters the airline had provided the relevant aircraft flight and maintenance logs to Indonesia's National Transporation Safety Committee.
NTSC Chairman Soerjanto Tjahjono said there was a similarity between the maintenance log circulating online and the one received from Lion Air but he had not checked the exact details.
Safety experts say the crash investigation is at a very preliminary stage and it is too early to speculate about the cause.
But pilot and engineering sources told Reuters the FlightRadar24 data for both flights, while not conclusive, could be a potential indicator of something wrong with the pitot static systems. Those are pressure-sensitive instruments that feed airspeed and altitude information to an avionics computer.
"Certainly the big changes in climb/descent very early on in the flight is what makes me think unreliable speed," said a pilot at another airline, who declined to be named because he was not authorised to speak to media.
The data from FlightRadar24 shows the first sign that something was amiss on Monday's fated flight came about two minutes after take-off when the plane had reached 2,000 feet.
At that point, it descended more than 500 feet and veered to the left before climbing again to 5,000 feet, where it stayed during most of the rest of the flight.
It began gaining speed in the final moments and reached 345 knots (397 mph) before data was lost when it was at 3,650 feet.
"They were going way faster than you would normally expect," said a second pilot at another airline.
Two fishermen who saw the crash from their boat out at sea told Reuters that the plane swayed slightly but made no noise as it fell, almost horizontal with its nose slightly down. There was an explosive sound as it plunged nose-first into the sea, and then there was a column of smoke.
The weather was clear at the time of the crash at 6:33 a.m. on Monday, according to the head of Indonesia's transport safety committee, who said the pilots had requested a turnback to Jakarta.
A safe visual landing should have been possible even with faulty indicators and a lack of autopilot systems, the pilot sources said.
In July, a Malaysia Airlines flight took off from Brisbane, Australia without removing covers on the pitot tubes.
The pilots landed the Airbus SE A330 safely after obtaining groundspeed information from air traffic control and using the jet's radar altimeter, according to a preliminary report from the Australian Transport Safety Bureau. There were no reported injuries.
In 2009, Air France flight 447 crashed into the Atlantic Ocean after icing that blocked the pitot tubes caused unreliable airspeed data and the confused pilots entered a high-altitude stall and ignored cockpit alarms, according to a report from France's BEA. All 228 people on board died. (Reporting by Jamie Freed and Tom Allard; additional reporting by Fergus Jensen in Pakisjaya, Cindy Silviana in Jakarta and Tim Hepher in Hong Kong Editing by John Chalmers and Raju Gopalakrishnan)