A preliminary report on the Ethiopian Airlines plane that crashed last month killing 157 people, says the aircraft persistently nosedived despite pilots following Boeing's emergency procedures, the carrier said on Thursday. In a statement posted on Twitter, Ethiopian Airlines said the investigation into the crash of the Boeing 787 Max 8 could continue for up to a year, as more authorities, including the UAE, joined the FAA investigation board. The report piles pressure on Boeing whose most popular aircraft is grounded worldwide, facing scrutiny over optional safety features and their relationship with the US regulator. Ethiopian authorities stopped short of attributing blame for the crash but suggested Boeing reviews the software suspected of causing the incident. "Despite their hard work and full compliance with the emergency procedures, it was very unfortunate that they could not recover the airplane from the persistence of nosediving," the airline said. The team investigating the crash is continuing to grow, drawing in expertise from across the world. On Thursday, Ismael al Blooshi, assistant director, safety affairs at the General Civil Aviation Authority – the UAE's aviation authority – said they were joining the FAA's investigation board. The preliminary report has not yet been made public, but will be published by Friday, the chief investigator said. On March 10 Ethiopian Airlines flight 302 crashed into arid farmland near Addis Ababa, killing 149 passengers and eight crew from 33 countries. Many of them were travelling to a UN conference in Nairobi. Ethiopian Transport Minister Dagmawit Moges said it was too soon to say if there was a problem with the aircraft, but urged Boeing to review its advice. "Since repetitive uncommanded aircraft nose down conditions are noticed ... it is recommended that the aircraft control system shall be reviewed by the manufacturer," Ms Dagmawit said. The preliminary report appears to keep an open mind about the cause of the crash, Saj Ahmad, chief analyst at StrategicAero Research, told <em>The National.</em> "It seems evident that the report is keen to avoid direct finger-pointing at a singularity and instead keep the focus on a broader approach," he said. "This allows the ongoing investigation to explore any other factors in the crash, as well as the key focus of the MCAS system," he said, referring to the anti-stall feature. The 737 Max 8 – Boeing's most successful model – faced intense scrutiny following the crash, leading to a worldwide grounding of the plane, given the similarities between two deadly crashes using the same model. Last year, a 737 Max 8 operated by Indonesia's Lion Air crashed, killing 189 people. The preliminary report into the Lion Air crash found that an anti-stall feature activated when it was not supposed to. Both pilots in the Ethiopian Airlines crash underwent additional training following the Lion Air incident. Ethiopian Airlines Group chief executive Tewolde GebreMariam said: "We are very proud of our pilots' compliances to follow the emergency procedures and high level of professional performances in such extremely difficult situations." Boeing and the FAA both said they continue to study the reports. Boeing has since stopped deliveries of the 737 Max, and has mandated a previously optional cockpit warning light, but maintains their new fuel-efficient aircraft is safe. The FAA launched a review of the 737 Max. The investigation into the Boeing crash is expected to take up to a year, but the worldwide grounding of planes is a costly exercise for both the manufacturer and the airlines. "The longer the grounding goes on, the bigger the financial fallout," Mr Ahmad said. The US said the grounding could last for weeks and there are no signs of a 737 Max taking to the skies any time soon, taking up precious parking spaces. "Boeing is still maintaining current production rates, this may have to be tempered back if the eventual green light to resume flights slides into June," Mr Ahmad said. But a fix to the Boeing 737 Max will have to win the confidence of not only the FAA, which regulates the plane, but also of the local aviation authorities who have grounded the aircraft model and banned it flying over their airspace, he said.