US regulators considered grounding some Boeing 737 MAX planes last year after learning belatedly of a problem with a system that is now the main suspect in two deadly crashes, a source close to the matter said.
Investigators in the Lion Air crash in October off the coast of Indonesia and the Ethiopia Airlines disaster in March have zeroed in on the planes' anti-stall system, called the Maneuvering Characteristics Augmentation System, or MCAS.
Last year, before the Lion crash, inspectors with the Federal Aviation Administration discovered that the manufacturer had de-activated a signal designed to advise the cockpit crew of a malfunctioning of the MCAS system, the source said.
These inspectors were in charge of monitoring Southwest Airlines, the biggest user of 737 MAX planes, with a fleet of 34 of them at the time, added the source.
The inspectors came up with this hypothesis -- the signal alert being switched off -- as they tried to determine whether pilots flying these planes needed additional training, said the source.
After some debate, they gave up on this hypothesis but it was never passed on to higher-ranking officials in the FAA, the source said, confirming a story in The Wall Street Journal.
The inspectors learned that Boeing had opted to make the malfunction signal optional and an extra that would cost more money.
This came after Southwest asked Boeing to reactivate the signal after the Lion Air crash, which killed all 189 people on board.
Boeing had deactivated the signal on all 737 MAX delivered to Southwest without telling the carrier.
Neither the airline nor its pilots were aware of these changes when they started flying the planes in 2017, a spokeswoman for Southwest told AFP.
Just like the inspectors, they only found out only after the Lion Air crash.
"Prior to the Lion Air event, the (signals) were depicted as operable by Boeing on all MAX aircraft" regardless of whether the cockpit crew thought they had them turned on or off, said the Southwest spokeswoman.
"After the Lion Air event, Boeing notified Southwest" that the signals were turned off unless they were specifically designated as being turned on, she said.
It was at this point that Southwest chose this option for all its aircraft, the spokeswoman said.
Boeing has not yet answered an AFP's request for comment.
The FAA would not comment on the planes coming close to being grounded last year. But a spokesman for the agency said the signal "is an option for carriers."
The Ethiopia Airlines crash left all 157 people on the plane dead and led to all Boeing 737 Max planes all over the world being grounded. In this case too the MCAS is being looked at as a possible cause of the crash.
In times of mid-air distress, this system is supposed to activate on its own and push the nose of the plane down to keep it from stalling.
Boeing is working on changing the MCAS so it can get the planes back in the air.
The grounding has already cost the carrier a billion dollars, Boeing said last week.
But the bill will probably climb because Boeing is expected to pay money to airlines, which have been forced to cancel thousands of flights and hire more reservations and services staff.
Boeing has suspended deliveries of Boeing 737 Max planes and cut production of them by 20 percent.
Neither of the Boeing 737 Max planes in the Lion crash in Indonesia or the Ethiopian crash were equipped with the signal that is supposed to show a malfunctioning of the MCAS, an industry source told AFP in March.
Called "disagree lights" in Boeing parlance, these lights turn on when faulty information is sent from so-called angle of attack sensors to the MCAS. Those sensors monitor whether the wings have enough lift to keep the plane flying.
In the case of the Lion Air crash, investigators think one of the angle of attack sensors may have failed and sent incorrect data to the MCAS, causing its nose to go down as pilots fought to bring it back up.
The MCAS overrides the pilots manual efforts to point the plane up or down.
With the angle of attack sensor not working properly, the thing to do would have been to turn off the MCAS. But the Lion Air cockpit crew did not know this.
