The final report is due only in 10 months' time but aviation experts are already questioning the credibility and independence of the investigation into the Lion Air crash near Jakarta in October which killed all 189 people on board.
Aviation pundits told The Straits Times there could be possible breaches of guidelines set out by International Civil Aviation Organisation, after vague statements from Indonesia's National Transport Safety Committee (KNKT) on the airworthiness of the Boeing 737 MAX aircraft, and in its preliminary investigation report.
Meanwhile, Lion Air's decision to fund the search for the plane's wreckage and second black box - the cockpit voice recorder - goes against global norms which recommend that governments foot the bill to preserve independence.
The flight went down minutes after it left Jakarta for Pangkal Pinang on Oct 29. The same Boeing plane had encountered a technical issue hours earlier when it flew from Bali to Jakarta, but the pilot resolved the issue during the flight.
On Nov 28, Mr Nurcahyo Utomo, KNKT's department head for air accidents, was cited by various media as saying the plane was not airworthy, referring to the Denpasar-Jakarta flight. His statement triggered a protest from Lion Air.
Lion Air's CEO Edward Sirait refuted KNKT's preliminary finding, flatly saying it was "not true". He said the plane for the Denpasar-Jakarta flight was declared airworthy according to pre-flight documents and a certified technician who checked the plane.
KNKT then issued a statement to clarify that the plane was airworthy for the Denpasar-Jakarta and Jakarta-Pangkal Pinang flights.
When contacted by ST, Mr Nurcahyo said the plane was airworthy before take-off on both flights but ran into problems after that.
In its preliminary investigation report, published on Nov 28, KNKT said: "The flight from Denpasar to Jakarta experienced stick shaker activation during the take-off rotation and remained active throughout the flight. This condition is considered as un-airworthy and the flight shall not be continued."
It continued: "KNKT recommends… in order to improve the safety culture and to enable the pilot to make proper decision to continue the flight."
An aviation expert who spoke on condition of anonymity criticised the inconsistent wording of the report, and said the recommendation should include an option for the pilot to discontinue the flight.
"In one part, it says the flight should not be continued, but in another it implies it is OK to continue the flight. That gives the impression that continuing the flight was not a problem," he said. He noted that the pilot on the Denpasar-Jakarta flight had to take the extraordinary step of switching off the automatic system and using manual controls to continue flying.
"KNKT must... consider why the Denpasar-Jakarta flight continued. In the preliminary investigation report, they use vague language which does not firmly say that under such conditions the flight should not continue," he said.
Annex 13 of the Convention on International Civil Aviation says the causes of an aircraft accident must be identified in order to prevent it from occurring again, and this is best done through a "properly conducted investigation".
Aviation analyst Gerry Soejatman said the larger issue was the pilot's apparent lack of knowledge regarding the plane's systems.
"They (the pilot and co-pilot) should have discontinued (the flight). Had they known at that time the severity of the problem? No, they didn't. Nobody knew about the MCAS," he told ST, referring to the Manoeuvring Characteristics Augmentation System, the new anti-stall flight-control system for the Boeing 737 MAX.
Lion Air's decision to fund the search for the plane's wreckage and second black box goes against global norms which recommend that governments foot the bill to preserve independence.
The system pushes the plane's nose down if it detects that the plane's angle of attack is too high and could stall the engines. It has been reported that the system sensors on the Lion Air plane may have erroneously read the angle of attack and repeatedly pushed the jet's nose down.
After the crash, pilots who fly the same jet in the US said they were not trained in the new features of the anti-stall system. Boeing has said it provided all the information needed to safely fly their planes.
KNKT chief Soerjanto Tjahjono did not respond to queries from ST.
Experts have also questioned the independence of the probe, after Lion Air decided to finance the search for the second black box. Indonesian investigators told Reuters that bureaucratic wrangling and funding problems had hampered the search for the recorder and they had turned to the airline for help.
Lion Air is owned by Indonesian tycoon Rusdi Kirana. He is the Indonesian ambassador to Malaysia and deputy chairman of PKB (National Awakening Party), which is a member of the ruling coalition and has MPs on the parliamentary transportation committee.
Retired Air Vice-Marshal Tatang Kurniadi, KNKT chief from 2007 to 2015, told ST he believes that any investigation team, including KNKT, would do its utmost be independent and professional.