SEOUL/PARIS (Reuters) - Next month marks three years since a China Eastern jet plunged into a hillside killing 132 people, with relatives still waiting to learn what caused China's deadliest air disaster in three decades.

It is one of dozens of accidents worldwide in which investigators have yet to issue a final report designed to help prevent new accidents, despite a target of one year.

While recent deadly crashes in Kazakhstan, South Korea and the United States and the non-fatal flip of a crash-landed jet in Canada have thrust safety into the spotlight, the industry is worried that too many past accidents remain unresolved.

According to airlines, almost half of 268 accidents involving fatalities or major damage between 2018 and the end of 2023 lacked a final report.

"That's a really big concern," said Mark Searle, head of safety at the International Air Transport Association (IATA).

"The lessons that we learn from accident reports are absolutely critical to avoid future events."

Aviation safety has improved markedly over decades based on open sharing of information, with investigations intended to draw lessons rather than assign blame.

The "brace position" for emergency landings, for example, was refined over years thanks to such investigations. By pure chance, the least injured person in a fatal 1976 crash in New Jersey had his head between his knees due to air sickness.

Technology to avoid collisions, the importance of not inflating life jackets inside planes and improved seat design are all lessons learned from past crashes.

Coordinated by the U.N.'s International Civil Aviation Organization (ICAO), global guidelines call for an initial report in 30 days and a final one ideally within a year. Failing that, investigators should issue statements on each anniversary.

But in a recent paper, IATA and six other aviation bodies raised the alarm over delayed or non-existent final reports.

"I think a number are being held up at the political government level because they are narratives that perhaps they are not too keen to hit the public eye," Searle said.

Others blame judicial interference or a shortage of resources for independent investigations in many countries.

The warning comes as social media has transformed how the public interact with air disasters.

Fuelled by statements from U.S. President Donald Trump that January's mid-air collision in Washington stemmed from diversity policies and a helicopter flying too high, a flurry of rumours circulated online about pilot identities and accident causes.

It is often only when publicity fades that forensic work can unlock the multiple factors in an accident, experts say.

The resulting final reports carry particular weight because they are based on validated data.

Three years on, however, the China Eastern crash remains shrouded in speculation and no final report has been published - only a preliminary one and two anniversary updates.

At three and seven paragraphs long, these contain scant data compared to other major accidents, such as the 150-page interim statement Japan published on a fatal collision in January 2024.

Following the China Eastern crash, investigators examined crew actions after finding no malfunctions, two people briefed on the matter said at the time.

In May 2022, the Wall Street Journal reported that black box data indicated someone had intentionally crashed the Boeing 737, citing a preliminary assessment from U.S. officials.

China's regulator CAAC has said speculation "gravely misled the public" and interfered with accident investigation work.

Chinese lawmakers will now consider significant changes to the country's civil aviation law, including measures against spreading rumours about aviation safety, CAAC said this week.

CAAC did not respond when asked if it would issue a final report for the March 21 anniversary, or if the proposed law revisions were linked to the China Eastern crash.

ICAO, without commenting on any specific probe, said there can be challenges in releasing reports "particularly in cases involving security sensitivities or political considerations".

The guidelines have recently been amended to allow more information to be published earlier to curb speculation.

UNEVEN REPORTING

Even when reports are published, recommendations are sometimes ignored or findings disputed, experts say.

A pair of hotly debated Egyptian accidents almost a decade ago illustrates the uneven reporting practices.

In 2016, EgyptAir 804 plunged into the Mediterranean, killing 66 people. After eight years, a final report identified explosives as the probable cause.

But France's BEA agency said the report's accuracy was "questionable" and a cockpit fire likely initiated events.

Even so, the eventual publication of the report - which allowed those dissenting comments to be published - was seen as a victory for the diplomatic machinery behind safety reporting.

By contrast the Sinai crash of an Airbus operated by Russian airline Metrojet in 2015, which Moscow blamed on a bomb while Egypt doubted a terrorist link, has not produced a final report but suspicions of a criminal act were reported to prosecutors.

The International Society of Air Safety Investigators is worried that cases of "unlawful interference" are not always investigated from a safety perspective.

Even when a crime is suspected, civil investigations have a place alongside police probes, it said in a recent paper.

French investigators issued a report urging better mental health checks following the pilot-suicide crash of a Germanwings jet, whose 10-year anniversary also falls next month.

Despite the gaps, investigators argue the reporting system remains robust with tangible gains in safety.

ICAO data indicates reporting rates for fatal accidents rose to 76% between 1990 and 2022 from 41% between 1990 and 2016.

But the issue remains a "significant focus area," it said.