The top US air transport regulator on Wednesday doused Boeing's hopes that its 737 MAX will return to the skies this year, but the agency's chief still faces tough questions over its response to two deadly crashes.

In an interview just ahead of an expected grilling in the US Congress, Federal Aviation Administration chief Steve Dickson told CNBC the aircraft will not be cleared to fly before 2020.

The process for approving the MAX's return to the skies still has 10 or 11 milestones left to complete, including a certification flight and a public comment period on pilot training requirements, he said.

"If you just do the math, it's going to extend into 2020," he said.

The MAX has been grounded since March following the second of two crashes that killed a total of 346 people.

Boeing has been aiming to win regulatory approval this month, with flights projected to resume in January.

But Dickson said, "I've made it very clear Boeing's plan is not the FAA's plan." He added that "we're going to keep our heads down and support the team in getting this report done right."

Boeing and the FAA have been under intense scrutiny following the crashes, for their response to issues with the aircraft, including the flight-handling system involved in both accidents.

Lawmakers have questioned whether FAA officials were too cozy with Boeing, leading to lax oversight during the original certification process for the aircraft.

The delay in allowing the MAX to resume flights have prompted Boeing to cut production of the top-selling jet while new plane deliveries are suspended.

- Fresh questions for FAA -

A Wall Street Journal story Wednesday said an FAA analysis following the October 2018 Lion Air crash estimated that unless fixes were made to a key flight handling system, the MAX was prone to as many as 15 similar catastrophic accidents over its decades-long lifespan, a much higher rate than other planes.

The Journal article quoted a former FAA safety official who said the elevated rate "would be an unacceptable number in the modern aviation-safety world."

Rather than grounding the plane at that point, the FAA determined that it would require Boeing to revise the MCAS flight handling system in a process overseen the FAA.

The FAA also issued guidelines to flight crews worldwide on how the respond to a problem with the Characteristics Augmentation System, an automated system that pilots were unable to control in the Lion Air crash.

Those moves proved insufficient to prevent the Ethiopian Airlines crash in March which MCAS was again implicated as a major factor.

Dickson told CNBC that "grounding an airplane is really an unprecedented decision, it's really only happened three times in our history."

The FAA's response after the Lion Air crash resulted from a "data driven" risk management process, he said. "Remember, MCAS didn't bring down this airplane by itself.

"We had maintenance issues with the aircraft. There were issues with how the aircraft was operated. All of these things acting together create a certain level of risk that we need to manage and bring down to an appropriate level."

The House Transportation Committee also will hear from Edward Pierson, a former senior manager at Boeing, who told company brass he feared production problems put plane safety at risk; and Michael Collins, a former FAA safety engineer who has criticized the agency's move to delegate some decisions to Boeing.

Families of crash victims, who have been a major presence at earlier sessions on Capitol Hill, also are expected to attend the hearing.