WASHINGTON, Aug 7 (Reuters) -

The head of the National Transportation Safety Board said on

Wednesday the Alaska Airlines Boeing 737 MAX 9

mid-air emergency

was entirely avoidable because

the planemaker

should have addressed unauthorized production work long

ago.

"This accident should have never happened. This should

have been caught years before," NTSB Chair Jennifer Homendy told

reporters on the second day of a hearing into the Jan. 5

incident.

"There have been numerous, numerous Boeing ( BA ) audits, FAA

audits, compliance reviews, compliance actions plans, noting a

history of an unauthorized work, unauthorized removals," she

added.

She said added there was no guarantee the issue would

not occur again.

Boeing ( BA )

created no paperwork for the removal of 737 MAX 9 door plug

- a piece of metal shaped like a door covering an unused

emergency exit - or its re-installation during production, and

still does not know what employees were involved. The plug was

missing four key bolts when it was delivered to Alaska Airlines,

NTSB has said.

Boeing ( BA ) did not immediately comment.

If Boeing ( BA ) had learned from prior unauthorized work,

"then this would have been caught and this would have been

prevented,"

Homendy said, adding the board is also scrutinizing the Federal

Aviation Administration's oversight of Boeing ( BA ).

"We have a lot of questions -- there was information known,"

Homendy said about FAA oversight of Boeing ( BA ), citing defects,

missing and incorrect documents, as well as incorrect policies

that "have been issues for years. This is not new."

Homendy has questions about FAA audit procedures and whether

Boeing ( BA ) previously received advance notice of reviews and asked

if they were too focused on reviewing paperwork.

After the incident, the FAA barred Boeing ( BA ) from expanding

production beyond 38 planes per month and announced a 90-day

review of the planemaker and has required significant quality

and manufacturing improvements before it will allow the

planemaker to hike production.

FAA Administrator Mike Whitaker said in June the agency was

"too hands off" in Boeing ( BA ) oversight. The FAA's approach before

the mid-air accident was "too focused on paperwork audits and

not focused enough on inspections," Whitaker added. The FAA has

also boosted the number of inspectors at Boeing ( BA ) and Spirit

factories.

"We will continue our aggressive oversight of the company

and ensure it fixes its systemic production-quality issues," the

FAA said Wednesday.

Last week, Senate Commerce Committee chair Maria Cantwell

and Senator Tammy Duckworth introduced legislation to review and

strengthen safety management systems at the FAA.

Homendy said the NTSB plans to conduct a safety culture

survey of employees at Boeing's ( BA ) Renton factory that builds the

737 MAX.