U.S. aviation regulators plan to order airlines to follow Boeing Co.’s advisory on how pilots should handle false readings from a plane sensor that authorities linked to last week’s deadly 737 Max jet crash off the coast of Indonesia.

The Boeing bulletin combined with statements by Indonesian investigators suggest that the pilots on the Lion Air 737 Max 8 were battling the plane as its computers commanded a steep dive. The issue is easily solved — Boeing’s notice said crews should follow an existing procedure to combat it — but can be difficult to address if pilots become confused.

The so called angle-of-attack sensor failed on Lion Air Flight 610 and had been replaced the previous day after earlier faults, the Indonesia National Transportation Safety Committee said in a briefing. The malfunction can cause the plane’s computers to erroneously register an aerodynamic stall, causing the aircraft to abruptly dive to regain the airspeed it needs to keep flying.

The U.S. Federal Aviation Administration on Wednesday said it would issue an airworthiness directive on the issue and “will take further appropriate actions depending on the results of the investigation.” The FAA also notified regulatory counterparts around the world, which typically follow the U.S. agency’s lead on safety matters.

No Service Disruption

The Boeing bulletin only reminds operators of the plane to follow existing procedures and doesn’t require any physical fixes that could disrupt service. It’s still possible the FAA may order the Chicago-based planemaker to redesign the Max’s flight computers in the wake of the Oct. 29 accident, which left 189 people dead.

The Lion Air jetliner plunged into the Java Sea minutes after takeoff from Jakarta airport, nosing downward so suddenly that it may have hit speeds of 600 miles an hour before slamming into the water.

Moments earlier, the pilots radioed a request to return to Jakarta to land, but never turned back toward the airport, according to Indonesia’s National Transportation Safety Committee and flight-tracking data. The committee said the pilots were dealing with an erroneous airspeed indication.

Boeing said it is cooperating fully and providing technical assistance as the investigation continues.

The company’s shares fell less than 1 percent to $363.85 at 9:43 a.m. in New York. The stock had climbed 24 percent this year through Tuesday.

An erroneous angle-of-attack reading while pilots are flying manually can cause the plane’s flight computers to command the Max models to dive, Boeing said in the bulletin to airlines. While planes like the Max fly mostly on autopilot, pilots can fly manually if they’re dealing with unusual situations like the malfunctions that occurred on the Lion Air flight.

Pilots can override the nose-down movement by pushing a switch on their control yoke, but the plane’s computers will resume trying to dive as soon as they release the switch, the manufacturer said.

Pilots should follow a separate procedure to halt the potentially dangerous action by the plane, the bulletin said. Flight crews are taught to handle “uncommanded nose-down stabilizer trim” by memorizing a procedure to disengage the angle-of-attack inputs to the plane’s computer system.

That angle-of-attack sensor is intended to measure the direction of air flow over wings so that they maintain lift. If the flow is disrupted by a plane going too slow or climbing too steeply, that can cause an aerodynamic stall and a plane will plummet. However, if the sensor malfunctions, it can cause the plane’s computers to erroneously think it is in a stall — which can then command the aircraft to abruptly dive.

The jet reported a discrepancy in its angle of attack sensor during a flight from Bali to Jakarta the day before it crashed. The device was replaced after pilots reported a problem with the airspeed reading, the Indonesian transportation safety regulator said Wednesday.

Boeing has delivered 219 Max planes — the latest and most advanced 737 jets — since the models made their commercial debut last year with a Lion Air subsidiary. Boeing has more than 4,500 orders for the airliners, which feature larger engines, more aerodynamic wings and an upgraded cockpit with larger glass displays. The single-aisle family is Boeing’s biggest source of profit.

Aircraft and engine manufacturers routinely send bulletins to air carriers noting safety measures and maintenance actions they should take, most of them relatively routine. But the urgency of a fatal accident can trigger a flurry of such notices.

Alan Levin, Julie Johnsson and Harry Suhartono, ©2018 Bloomberg L.P.

Accident Response

After an engine on a Southwest Airlines Co. plane fractured earlier this year over Pennsylvania, killing a passenger, CFM International Inc. issued multiple bulletins to operators of its CFM56-7B power plants.

Aviation regulators such as the U.S. FAA and the European Aviation Safety Agency often follow such actions by mandating that carriers follow the bulletins.

Pilots raise and lower the nose of Boeing jetliners by pushing and pulling on a yoke in the cockpit, which controls panels at the tail known as elevators. In addition, a system known as pitch trim can be changed to prompt nose-up or nose-down movement.

The angle of attack readings are fed into a computer that in some cases will attempt to push down the nose using the pitch trim system. In the early days of the jet age, the pitch trim system was linked to several accidents. If pilots aren’t careful, they can cause severe nose-down trim settings that make it impossible to level a plane.

Such an issue arose in 2016 at Rostov-on-Don Airport in Russia when a FlyDubai 737-800 nosed over and slammed into the runway at a steep angle, according to an interim report by Russian investigators. That case didn’t involve the angle-of-attack system. One of the pilots had trimmed the plane to push the nose down while trying to climb after aborting a landing, the report said. All 62 people on board died.

–With assistance from Kyunghee Park and Anurag Kotoky.

©2018 Bloomberg L.P.

This article was written by Julie Johnsson, Harry Suhartono and Alan Levin from Bloomberg and was legally licensed through the NewsCred publisher network. Please direct all licensing questions to legal@newscred.com.

Photo Credit: Southwest has more Boeing 737 Max jets than any other U.S. airline. The U.S. Federal Aviation Administration plans to issue a safety warning about an issue that may have caused a crash last week. Southwest Airlines