Boeing did not tell airline pilots about features of a new flight control system in its 737 MAX plane that reportedly is a focus of the investigation into last month's deadly crash in Indonesia, according to pilots who fly the jet in the US.

Key points: Lion Air's flight JT610 from Jakarta to Pangkal Pinang crashed on October 29 killing all 189 on board

Lion Air's flight JT610 from Jakarta to Pangkal Pinang crashed on October 29 killing all 189 on board Investigators' attention was initially on potential maintenance problems, but now procedures and pilot training is the focus

Investigators' attention was initially on potential maintenance problems, but now procedures and pilot training is the focus US Federal Aviation Administration (FAA) issued an emergency airworthiness directive urging airlines to update their flight manuals

Pilots said they were not trained in new features of an anti-stall system in the aircraft that differ from previous models of the popular 737.

The comments shed further light on the areas under scrutiny as investigators prepare to publish their preliminary report on November 28 or 29, one month after the Lion Air Boeing 737 MAX dived into the Java Sea, killing all 189 people on board.

The automated system is designed to help pilots avoid raising the plane's nose too high, which can cause the plane to stall, or lose the aerodynamic lift needed to keep flying.

The system automatically pushes the nose of the plane down.

But if that nose-down command is triggered by faulty sensor readings — as suspected in the Lion Air crash — pilots can struggle to control the plane, which can go into a dive and perhaps crash, according to a Boeing safety bulletin and safety regulators.

The bulletin included new details on how to stop a runaway series of events from leading to a crash, pilots say.

"It is something we did not have before in any of our training," said Dennis Tajer, a 737 pilot and spokesman for the pilots' union at American Airlines.

"It wasn't in our books. American didn't have it. Now I have to wonder what else is there?"

Jon Weaks, a 737 captain and president of the pilots' union at Southwest, said he could not recall a similar omission in a Boeing operating manual.

"I was not pleased. How could something like this happen?" he said.

"We want to be given the information to keep our pilots, our passengers and our families safe."

But Mr Weaks said he was satisfied that "we have been given, finally, the correct information".

Soerjanto Tjahjono, head of Indonesia's transportation safety committee of crash investigators (KNKT), said on Monday that Indonesian regulators would tighten training requirements as a result of the findings of the investigation so far.

"We know, because this incident happened, we know we need additional training," he said.

Investigator are expected to deliver a report into the Lion Air crash on November 28 or 29. ( Reuters: Beawiharta )

Airlines unaware of new automated system

The MAX is the newest version of the twin-engine Boeing 737.

More than 200 have been delivered to airlines worldwide, including American, Southwest and United.

Boeing Chairman and CEO Dennis Muilenburg said on Tuesday that the Chicago-based company remained confident the MAX was a safe airplane.

He said Boeing did not withhold operating details from airlines and flight crews.

"We ensure that we provide all of the information that is needed to safely fly our airplanes," Mr Muilenburg told Fox Business Network.

He said Boeing bulletins to airlines and pilots "point them back to existing flight procedures" to handle the kind of sensor problem suspected in last month's crash.

A Southwest spokeswoman said the new automated manoeuvring system was not included in the operating manual for MAX models.

An American Airlines spokesman said the airline was unaware of some new automated functions in the MAX but has not experienced nose-direction errors.

A United spokesman said Boeing and the Federal Aviation Administration (FAA) do not believe additional pilot training is needed.

Crash investigation sparked warning to US airlines

The FAA issued an emergency directive last week to airlines, telling them to update flight manuals to include instructions for how pilots can adjust flight controls under certain conditions.

Indonesian navy frogmen try to retrieve debris from the water during a search operation. ( AP: Tatan Syuflana )

"The FAA will take further action if findings from the accident investigation warrant," the agency said in a statement on Tuesday.

John Cox, a former 737 pilot and now a safety consultant to airlines, said Boeing's steps since the crash "have been exactly correct".

"They have increased pilot awareness, they have reminded them of the proper procedure to disable (the automatic nose-down action), which stops the problem," he said.

Last week, Boeing said the fix for this type of event — known as a runaway stabiliser — was covered by existing procedures.

Even though this problem was, according to investigators, not covered in the flight manual, pilots did have access to a checklist designed to turn off errant systems when the plane started nosing downwards at the wrong time, said Soejono, a Lion Air instructor who like many Indonesians goes by one name.

Experts said investigators would be examining whether the crew examined the checklist and if so whether they had time to cut off the automated nose-down system while flying at a relatively low altitude of about 1,500 metres.

Sorry, this video has expired Indonesian divers are reported to have retrieved one of two black boxes but is still searching for the voice recorder.

To answer that question fully, investigators may need access to cockpit voice recordings thought to be hidden in the seabed.

A search for the plane's missing cockpit voice recorder was continuing and could provide important information about human factors relating to the crash, Mr Soerjanto said.

AP/Reuters