An official from the Indonesian National Transportation Safety Commission

examines an engine from the downed Lion Air Flight 610 in Jakarta on Nov.

4. PHOTO: BEAWIHARTA/REUTERS

By  Andy Pasztor

Updated Nov. 28, 2018 7:33 a.m. ET

LOS ANGELES—Investigators are looking at potential maintenance

mistakes that are suspected of touching off the rapid sequence of cockpit

events leading up to last month’s fatal Lion Air jet crash, according to

people briefed on the accident probe.

Possible errors or lapses by mechanics who worked on a malfunctioning

sensor just before the flight, these people said, are now among the top

issues for Indonesian investigators delving into why the Boeing Co. 

BA 0.53% 737 MAX jetliner plunged into the Java Sea at a steep angle

and high speed shortly after taking off from Jakarta, killing all 189 people

on board.

On Wednesday, Indonesian crash investigators released an interim report

highlighting problems with altitude and airspeed displays during the four

flights prior to the accident. They said they were comparing maintenance

performed on the jet throughout that period with procedures listed in a

troubleshooting manual.

“At this stage, we cannot determine if [the actions were] correct or not,”

said Nurcahyo Utomo, an investigator with Indonesia’s National Safety

Transportation Committee, which is leading the crash probe.

But the preliminary report disclosed that in the wake of the accident,

Lion Air took steps intended to ensure that its maintenance records provide

“adequate alert on repetitive problems,” and instructed mechanics to

make sure “repetitive problems were addressed.”

According to the report, about two weeks after the Oct. 29 crash, the

airline performed special inspections across its entire 737 MAX fleet to

determine whether an important sensor had been installed properly.

The Indonesian-led probe is looking at everything from aircraft design,

to part replacements in the days before the crash, to pilot training and

possible pilot confusion stemming from the operation of certain

flight-control systems. From the beginning, maintenance has been

mentioned as one of several avenues of investigation.

But as the probe expands and accelerates, greater attention is being paid

to Lion Air’s maintenance practices. Specifically, people briefed on the

probe said investigators are delving into the reasons why the twin-engine

aircraft, which experienced problems with essential flight-control systems

on trips before the crash, was dispatched without first undergoing a test

flight without passengers.

The report didn’t draw any conclusions about what caused the accident.

A final report is expected next year.

The interim document, however, provides the clearest signals yet that

investigators have found a range of safety challenges and shortcomings

at the fast-growing budget carrier.

The report suggested Lion Air needs to “improve safety culture” by

training pilots to better understand when to land a plane that is experiencing

problems. On the jet’s second-to-last trip, a crew continued flying after

experiencing serious cockpit display problems and receiving emergency

stall warnings shortly after taking off from Bali’s airport.

Edward Sirait, a Lion Air director, said the airline “will surely follow up

on all recommendations.”

During a press conference Wednesday, investigators also questioned

whether Boeing’s recommended training process for MAX pilots was

adequate and wondered why the company hadn’t included details of a

new anti-stall system for the jet in its pilot manuals.

Boeing reiterated that it is “taking every measure to fully understand all

aspects of this accident.” It said the 737 MAX is as safe as any airplane

that has ever flown.

The company also noted the maintenance performed in the days before

the fatal flight failed to fix problems with the jet. It noted that the new

report didn’t include records related to the installation or calibration of

a new angle-of-attack sensor, which investigators say malfunctioned on

the jet’s final two flights. The sensor tells the plane’s computers if the

nose is tilted at too high an angle and the aircraft is in danger of losing lift.

Faulty signals from an angle-of-attack sensor occurred on both trips,

according to the report, but pilots on the accident flight weren’t aware

of cockpit actions by the previous crew to deal with the problem.

Technicians who serviced the jetliner immediately before its final flight

worked on several elements of the flight control system, but not the

malfunctioning angle-of-attack sensor itself.

The interim report used information downloaded from the jet’s flight-data

recorder to highlight how the pilots of Lion Air Flight 610 struggled with

a flurry of warnings and sought to use manual commands to override an

automated system that repeatedly pushed down the jet’s nose.

Investigators have already tentatively concluded that erroneous signals

from an angle-of-attack system activated the automated nose-down

commands.

Indonesian authorities are still searching for the aircraft’s cockpit-voice

recorder to better understand what happened.

In recent days, safety experts from Boeing and the U.S. Federal Aviation

Administration have discussed whether revised maintenance procedures

or safety bulletins—or possibly new provisions in maintenance manuals

covering the latest 737 models—are necessary in light of lessons learned

from the crash. Such changes aren’t imminent, according to two people

familiar with the matter, though they could come within weeks.

One potential change under consideration, according to one of these people,

is a broader maintenance test to ensure correct operation and coordination

of all the subsystems that feed into what is called the overall MCAS system,

the automated flight-control feature that pushes a plane’s nose down under

certain unusual conditions.

Separately, Boeing is devising and testing a software fix that would prevent

false angle-of-attack signals from activating the automated flight-control

feature that confronted the Lion Air pilots, according to industry and

government officials.

The FAA said it “continues to participate in the Indonesian government’s

investigation,” and weeks ago issued an emergency safety directive

advising crews how to counter certain automated nose-down commands.

Since the accident, some pilot-union leaders and others have publicly

criticized Boeing for failing to highlight the operating principles—and

potential hazards—of the MCAS system. Boeing Chief Executive Dennis

Muilenburg has denied the company intentionally withheld information

about the stall-prevention feature and said Boeing had described its relevant

function in its manual.

As the crew of the next-to-last flight was fighting the MCAS system and

scrambling to raise the plane’s nose, according to the report, the co-pilot

saidthe control yoke was too heavy to pull back. The issue was resolved

when pilots turned off the automated system pushing the nose down.

—Andrew Tangel, Ben Otto, I Made Sentana and Robert Wall contributed

to this article.

Write to Andy Pasztor at andy.pasztor@wsj.com