BANGKOK — Boeing faced new scrutiny on Tuesday over the crash of one of its planes into the sea off Indonesia last month, as airlines, pilots and regulators sought to determine whether the company had underplayed the complexity of a new emergency system suspected of having malfunctioned on the doomed jetliner.
Investigators have been focused on whether the plane, Lion Air Flight 610, crashed because the system, which is designed to pull the plane out of a dangerous stall, activated based on inaccurate data transmitted or processed from sensors on the fuselage.
The plane plunged nose down into the sea, killing all 189 people on board. The precise cause or causes of the crash remain unclear.
Boeing has been selling the model that crashed, the new 737 Max 8, as requiring little additional pilot training for airlines that already use the previous version of the plane. The 737 Max 8 is in a ferocious competitive battle with an update of the Airbus A320, and minimizing the costs of upgrading to the new model is one of the keys to winning orders from airlines.
The pilots’ union for American Airlines, which also flies the Max 8, said Tuesday that the emergency system in question had not been included by Boeing in the standard operating manual. In addition, the flight checklist — which contains information for manually overriding the emergency system — was incorrect, the union said.
The emergency system is intended to maneuver the plane out of a stall, when its nose is often angled too sharply upward. The system automatically pushes the nose down. If activated incorrectly, it could have sent the plane into its fatal dive, especially if the pilots were not properly trained on how to deal with such a situation.
Boeing has delivered 200 of the planes to airlines around the world, with many more in the pipeline. It is already in use in the United States by American Airlines and Southwest, and other customers include Air Canada, Norwegian Air and Icelandair.
Boeing said in a statement that it was assisting in investigations into the crash, but did not directly address questions about why it did not do more to emphasize the changes in the anti-stall system.
“The investigation into Lion Air flight 610 is ongoing and Boeing continues to cooperate fully and provide technical assistance at the request and under the direction of government authorities investigating the accident,” the company said.
The Federal Aviation Administration said Tuesday evening that it had received a letter from Boeing requesting permission to update the 737 Max 8’s flight manual. The letter is not an unusual response after the agency issues airworthiness directives, and it is not known what specific changes Boeing was requesting.
The Wall Street Journal first reported about concerns that Boeing had not adequately informed airlines about the changes to its emergency system.
The question of whether cost pressures contributed to any decision by Boeing to fully brief airlines on how the new emergency system works and how pilots need to respond differently to it in case of a malfunction injected a new element into the investigation.
“We’ve just been informed that there’s an entire new system on the Max,” said Capt. Dennis Tajer, spokesman for the Allied Pilots Association, the union for pilots at American Airlines, and a 737 pilot.
He said he was referring to what is known on the new version of the plane as the M.C.A.S., or maneuver characteristics augmentation system. The previous system, and the one in the standard manual, goes by a different shorthand, E.F.S., for elevator feel shift.
The union is still studying the similarities and differences between the two systems “so that our pilots can be trained up and be able to recognize while they’re actually flying the aircraft,” Captain Tajer said.
In addition, he said, the onboard checklist that pilots had been carrying gave what now appears to be incorrect instructions for pulling out of the emergency condition that apparently confronted the Lion Air pilots.
Boeing had told airlines that pilots qualified to fly the earlier version of the plane, the 737NG, would need to do just 16 hours of training on a computer to be ready to fly the new Max version.
Boeing may have found it particularly important to stress the ease of shifting from the older to the newer version of the plane because the rival Airbus A320neo involves fewer updates from its predecessor. What the industry calls commonality between older and newer versions is important to airlines in keeping costs down.
“Boeing definitely marketed it that way — the idea that it’s a straightforward crossover process,” said Richard Aboulafia, an aerospace analyst and vice president of the Teal Group in Washington. He said commonality would have been important to Boeing.
The company stressed that point in sales presentations. “As you build your 737 MAX fleet, millions of dollars will be saved because of its commonality,” Boeing says in marketing materials aimed at potential customers.
American Airlines said in a statement that it had not had any similar problems with the 16 Max 8 jetliners it is currently flying, but that it had been unaware of the issues with the way the emergency anti-stall system works.
“The work with the FAA and Boeing is ongoing, and we will continue to keep pilots informed of any updates,” the airline said.
Indonesian transportation officials have said repeatedly that the Max 8 manual being used in the country did not contain crucial information about the new anti-stall mechanism.
Soerjanto Tjahjono, the head of Indonesia’s National Transportation Safety Committee, said Boeing’s manual did not adequately describe how this automatic anti-stall system worked and what to do if it was set off incorrectly.
Pilots would only have a few seconds to respond to this situation, aviation experts said, especially if a plane was flying at low altitude, as was the case with Lion Air flight, which plummeted into the Java Sea on Oct. 29.
“The manual has how to handle issues but not that specific combination,” Mr. Soerjanto said. “I don’t know why it was not in the manual. Maybe Boeing never thought that this kind of problem would occur.”
Last week, Boeing released a statement acknowledging that Indonesian officials had told it about repeated errant data readings experienced by the plane. Incorrect data readings can set off the automated anti-stall system to force the plane into a nose-dive, even if the plane is not on autopilot.
Stalls are one of the most common causes of airplane crashes. But if pilots did not know exactly what system was in place, or that the data being fed into the system was wrong, their reactions could be fatally muddled, aviation experts said.
Pilots should follow Boeing’s manual when confronted with this anomalous situation, the company initially wrote in its advisory last week.
But Mr. Soerjanto’s deputy, Haryo Satmiko, said in an interview last Wednesday that Boeing’s manual was “incomplete” when it came to advising pilots on how to handle a situation in which false data is fed into the automatic anti-stall system.
On Monday, the Federal Aviation Administration issued a follow-up directive confirming that the manual for the Max 8 needed to be updated to address this rare but dangerous sequence of events.
The Lion Air plane had experienced faulty data readings on earlier flights, according to Indonesian investigators. At three different airports, Lion Air maintenance crews tried to fix various problems concerning erroneous airspeed and other data, declaring the plane fit to fly.
But the problems continued. On the plane’s next-to-last flight, the crew received errant measurements of the plane’s angle of attack, a data point used to help understand how sharply a plane is ascending or descending, Indonesian investigators said.
Mr. Soerjanto said on Tuesday that the pilot of that flight reacted quickly to the data discrepancy and switched off the tail trim mechanism that might have caused the plane to nose-dive.
“The pilot thought about it and improvised, though it was not in the manual,” Mr. Soerjanto said. “And it worked.”
Because the Max 8 is a reworking of the 737, pilots do not need to undergo extensive specialized training in order to fly the latest model. At Lion Air, pilots who have recorded at least 500 flying hours on the previous 737 model, the NG, only need to undergo a three-hour computerized instruction on the differences between the two models, said Captain Dibyo Soesilo, the head of a Lion Air training center and one of the carrier’s Max 8 instructors.
Once pilots pass an exam on the latest modifications, they take two familiarization flights with an instructor, Mr. Dibyo said.
Indonesian investigators have also focused on Lion Air’s many safety lapses, which have led to at least 15 major incidents since the carrier began operations at the turn of the century.