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Airline industry rushes to understand nuances in 737 Max systems

Airlines, unions and regulators are working to understand the 737 Max's flight systems while unions uncover seeming errors in emergency "runaway stabiliser" checklists.

The work comes amid reports that the US industry was unaware Boeing had equipped the 737 Max with a new system that has been linked to the 29 October crash of a Lion Air 737 Max 8.

"We are working at an extraordinarily positive pace to share information," says a spokesperson for the Allied Pilots Association (APA), which represents pilots at American Airlines, a 737 Max operator.

"We are looking at differences between the Max and the non-Max aircraft" in an effort to understand nuances in stall prevention and notification systems, the union adds.

The issue apparently rests with the 737 Max's Manoeuvring Characteristics Augmentation System (MCAS), which is not on earlier-generation 737s. The technology automatically trims the aircraft's stabiliser, dropping the nose, to avoid stalls, according to American Airlines memo to pilots.

However, the system can also cause aircraft to dive if computers receive incorrect angle-of-attack data – a scenario called a "runaway stabiliser". Investigators have suggested faulty angle-of-attack information played a role in the Lion Air crash.

The APA says inclusion of the MCAS systems seems to have been accompanied by slight changes in how pilots should respond to runaway stabilisers.

Pilots have long been taught that pulling back on a 737's control column can arrest that condition – a fix pilots call a "breakaway", says the APA.

Indeed, American's runaway stabiliser checklist, dated 10 July, says, "stabiliser trim commands are interrupted when the control column is displaced in the opposite direction".

But the APA has now learned that the 737 Max is apparently different.

The APA cites a 7 November memo from American to pilots, in which the airline says that pulling the control column on a 737 Max will not arrest stabiliser movement if the dive was caused by faulty angle-of-attack data. "Control column force will not stop electric trimming", the memo says.

"On the 737NG, they had this breakaway system," says the APA. "On the Max, the company note says [that] pulling up on the stick doesn't work."

The union stresses that American's checklists also tell pilots to switch off the stabiliser system, which should fix the problem.

It adds that differences between 737 Max and 737NGs are not a problem – so long as pilots know about the changes.

The APA was not the only organisation caught unaware.

Also in the dark were American, Southwest Airlines, and, reportedly, Southwest's pilot association.

"We value our partnership with Boeing, but were unaware of some of the functionality of the Manoeuvring Characteristics Augmentation System (MCAS) installed on the Max 8," American tells FlightGlobal. "The work with the FAA and Boeing is ongoing, and we will continue to keep pilots informed of any updates."

"The MCAS was not mentioned in the manuals that Boeing provided to Southwest," Southwest says in a statement. "Therefore, MCAS is not mentioned in the Southwest Max 8 manuals."

The Dallas-based carrier's pilot union did not respond to requests for comment.

Boeing declines to comment about the MCAS, saying it is working to understand circumstances related to the Lion Air crash, which killed 189 people.

"We are taking every measure to fully understand all aspects of this incident, working closely with the investigating team and all regulatory authorities involved," it says. "We are confident in the safety of the 737 Max."

Following the crash, Boeing issued a service alert and the Federal Aviation Administration issued an emergency airworthiness directive requiring airlines to revise flight manuals to specify how pilots should response to runaway stabilisers.

American and Southwest have complied with the order, they say.

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