As the Lion Air crew fought to control their diving Boeing Co. 737 Max 8, they got help from an unexpected source: an off-duty pilot who happened to be riding in the cockpit.

That extra pilot, who was seated in the cockpit jumpseat, correctly diagnosed the problem and told the crew how to disable a malfunctioning flight-control system and save the plane, according to two people familiar with Indonesia’s investigation.

The next day, under command of a different crew facing what investigators said was an identical malfunction, the jetliner crashed into the Java Sea killing all 189 aboard.

The previously undisclosed detail on the earlier Lion Air flight represents a new clue in the mystery of how some 737 Max pilots faced with the malfunction have been able to avert disaster while the others lost control of their planes and crashed. The presence of a third pilot in the cockpit wasn’t contained in Indonesia’s National Transportation Safety Committee’s Nov. 28 report on the crash and hasn’t previously been reported.

The so-called dead-head pilot on the flight from Bali to Jakarta told the crew to cut power to the motor in the trim system that was driving the nose down, according to the people familiar, part of a checklist that all pilots are required to memorize.

By contrast, the crew on the flight that crashed the next day didn’t know how to respond to the malfunction, said one of the people familiar with the plane’s cockpit voice recorder recovered as part of the investigation. They can be heard checking their quick reference handbook, a summary of how to handle unusual or emergency situations, in the minutes before they crashed, Reuters reported, citing people it didn’t name.

Lion Air spokesman Danang Prihantoro declined to comment on the role of a third pilot, saying, “All the data and information that we have on the flight and the aircraft have been submitted to the Indonesian NTSC. We can’t provide additional comment at this stage due the ongoing investigation on the accident.”

The Indonesia safety committee report said the plane had had multiple failures on previous flights and hadn’t been properly repaired.

Airline mechanics tried four times to fix related issues on the plane starting Oct. 26, according to the Indonesia preliminary report. After pilots reported issues with incorrect display of speeds and altitude in the two prior flights, workers in Denspasar, Bali, replaced a key sensor that is used by the Boeing plane to drive down its nose if it senses an emergency.

Flight data shows the sensor, called the “angle of attack” vane, which measures whether air is flowing parallel to the length of the fuselage or at an angle, was providing inaccurate readings after that.

However, the pilots on the harrowing Oct. 28 flight from Bali to Jakarta didn’t mention key issues with the flight after they landed, according to the report.

Their request for maintenance didn’t mention they had been getting a stall warning since about 400 feet after takeoff as a result of the faulty angle-of-attack sensor. It was still giving false readings the next morning on the flight that crashed, according to flight data.

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After the Lion Air crash, two U.S. pilots’ unions said the potential risks of the system, known as the Maneuvering Characteristics Augmentation System, or MCAS, hadn’t been sufficiently spelled out in their manuals or training. None of the documentation for the Max aircraft included an explanation, the union leaders said.

“We don’t like that we weren’t notified,’’ Jon Weaks, president of the Southwest Airlines Pilots Association, said in November. “It makes us question, ‘Is that everything, guys?’ I would hope there are no more surprises out there.’’

The Allied Pilots Association union at American Airlines Group Inc. also said details about the system weren’t included in the documentation about the plane.

Following the Lion Air crash, the FAA required Boeing to notify airlines about the system and Boeing sent a bulletin to all customers flying the Max reminding them how to disable it in an emergency.

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Authorities have released few details about Ethiopian Airlines Flight 302 other than it flew a “very similar” track as the Lion Air planes and then dove sharply into the ground. There have been no reports of maintenance issues with the Ethiopian Airlines plane before its crash.

If the same issue is also found to have helped bring down Ethiopian Airlines Flight 302, one of the most vexing questions crash investigators and aviation safety consultants are asking is why the pilots on that flight didn’t perform the checklist that disables the system.

“After this horrific Lion Air accident, you’d think that everyone flying this airplane would know that’s how you turn this off,” said Steve Wallace, the former director of the U.S. Federal Aviation Administration’s accident investigation branch.

The combination of factors required to bring down a plane in these circumstances suggests other issues may also have occurred in the Ethiopia crash, said Jeffrey Guzzetti, who also directed accident investigations at FAA and is now a consultant.

“It’s simply implausible that this MCAS deficiency by itself can down a modern jetliner with a trained crew,” Guzzetti said.

MCAS is driven by a single angle-of-attack sensor near the nose even though there are two of the sensors on the plane. Boeing is planning to alter the system to rely on both sensors to reduce the chances of a malfunction.