WASHINGTON (BLOOMBERG) - Two US pilots' unions say the potential risks of a safety feature on Boeing Co's 737 Max aircraft that has been linked to a deadly crash in Indonesia weren't sufficiently spelled out in their manuals or training.

Boeing and the Federal Aviation Administration issued directives last week telling flight crews about the system, which is designed to provide extra protection against pilots losing control.

That prompted aviators, unions and training departments to realise that none of the documentation for the Max aircraft included an explanation of the system, the union leaders said.

"We don't like that we weren't notified," said Captain Jon Weaks, president of the Southwest Airlines Pilots Association.

Captain Dennis Tajer, a 737 captain and spokesman for the Allied Pilots Association at American Airlines Group Inc, said his union's members were also concerned.

The complaints from pilot union leaders at Southwest Airlines Co and American are significant because of the size of those carriers' 737 fleets and their Max purchases.

Southwest is the largest operator of the 737 Max and has the most on order, with 257 of the jets yet to be delivered. American, the world's largest airline, has outstanding orders for 85 of the planes.

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"This is not about silos and layers of bureaucracy, this is about knowing your airplane," Capt Tajer said. "We will always be eager and aggressive in gaining any knowledge of new aircraft."

A bulletin from APA to American's pilots said details about the system weren't included in the documentation about the plane.

"This is the first description you, as 737 pilots, have seen," it said.

"The companies and the pilots should have been informed," Capt Weaks said. "It makes us question, 'Is that everything, guys?' I would hope there are no more surprises out there."

Boeing said it is confident in the safety of the 737 Max family of jets.

"We are taking every measure to fully understand all aspects of this incident, working closely with the investigating team and all regulatory authorities involved," the company said in a statement by e-mail.

"Safety remains our top priority and is a core value for everyone at Boeing."

Few details have been released about the underlying causes of the Lion Air crash on Oct 29 in the sea near Jakarta, but Indonesian investigators say that an erroneous sensor prompted the plane's computers to push the aircraft into a steep dive.

A new safety measure added on the Max models to prevent pilots from losing control is what caused the plane to point downward, according to the FAA and Boeing.

A long-standing procedure taught to pilots could have halted the dive, according to the regulator and the manufacturer. The FAA ordered airlines to add an explanation into flight manuals.

Indonesia's National Transport Safety Committee is continuing to search for the plane's crash-proof cockpit voice recorder under the sea, it said on Monday.

The investigative agency plans to release a preliminary report between Nov 28 and 29, a month after the crash, as mandated by international treaty.

The FAA, which certified the plane, said in a statement it couldn't comment on the matter while the investigation in Indonesia remains open.

The FAA's emergency directive required that US carriers revise flight manuals and said the agency "will take further action if findings from the accident investigation warrant".

While the design of the Max has been under a spotlight since the accident, other factors in the crash could eclipse it in importance.

They include questions about how maintenance was performed after problems arose on at least three prior flights of the Lion Air jet and the actions of the pilots on its last flight.

When Boeing designed its latest version of the 737, it added the new safety feature to combat a loss of lift, which is a leading contributor to the loss-of-control accidents that by far cause the most crash deaths around the world.

Known as the Manoeuvring Characteristics Augmentation System, it was added "to compensate for some unique aircraft handling characteristics", according to a bulletin sent by Southwest's flight operations division to its pilots on Nov 10.

SHARP DIVE

When the system senses the plane is close to losing lift on the wings, it automatically commands a lowering of the nose to counteract the risk. However, the chief sensor used to predict a loss of lift - known as an angle-of-attack vane - was malfunctioning on the Lion Air flight. It essentially tricked the system into ordering a sharp dive.

Pilots are drilled on how to cut power to the so-called trim system if the plane starts to dive or climb on its own, but that procedure was never linked directly to a malfunctioning angle-of-attack sensor in training or the documentation.

"At the present time, we have found no instances of AOA anomalies with our 737 Max 8 aircraft," the APA bulletin said, referring to angle of attack as AOA.

"That is positive news, but it is no assurance that the system will not fail."

Because the system is only designed to operate in rare conditions while pilots are manually flying, "pilots should never see" the system in operation, according to the Southwest memo.

As a result, Boeing chose not to include a description of it in the extensive manuals it prepared for the Max models, said the memo.

That reasoning doesn't make sense, said Mr Roger Cox, a retired investigator with the US National Transportation Safety Board and a former airline pilot.

Flight crews have a right to be concerned that details about the new system weren't included in manuals and the short training courses they were required to take before flying the upgraded 737, Mr Cox said.

"I would be pretty p***ed" about not being told, he said. "This is important systems information that pilots should know about."