The landing gear of a Boeing Co. 737 Max 9 jetliner is seen at the company’s manufacturing facility in Renton, Washington, U.S. (Photographer: David Ryder/Bloomberg)

What Boeing's Post-Crash Warning About 737 Max Means: QuickTake

(Bloomberg) -- Minutes after takeoff on Oct. 29, a Boeing 737 Max belonging to the budget Indonesian carrier Lion Air dove into the Java Sea at more than 600 miles an hour, shattering into pieces and killing all 189 passengers and crew. Based on preliminary findings from an investigation of the crash, Boeing Co. has warned airlines operating the new 737 Max that erroneous readings from a flight-monitoring system can cause the planes to do just that -- abruptly dive.

What Boeing's Post-Crash Warning About 737 Max Means: QuickTake
What Boeing's Post-Crash Warning About 737 Max Means: QuickTake

1. What caused the plane to crash?

Investigators are still trying to figure that out. But the Boeing alert and a related one from the U.S. Federal Aviation Administration indicate that they are focusing on a potential malfunction of sensors that may have erroneously registered an aerodynamic stall, causing the aircraft’s computers to command a dive to regain the airspeed it needs to keep flying. That would explain why it plunged into the sea at such high velocity. Another clue: the so-called angle-of-attack sensor had failed on an earlier flight and had been replaced the previous day.

2. What does an angle-of-attack sensor do?

The system measures the direction of air flow over wings. If the flow is disrupted by a plane going too slowly or climbing too steeply, that can cause the plane to lose the lift required for flight and plummet. When the sensors detect such an aerodynamic stall, they send a command to the computers to point the aircraft’s nose down to regain flight speed. United Technologies Corp. supplies the angle-of-attack sensors and indicator for the 737 Max, according to Airframers.net. The data are fed into computers made by Honeywell International Inc. for the plane.

3. What did Boeing’s warning say?

Pilots are trained to disengage the angle-of-attack sensors from the plane’s computers when they get false readings. The system alerts pilots if it senses a possible malfunction. But disengaging the sensors in the heat of the moment, particularly if equipment is malfunctioning and alarms are sounding, can be difficult. The Boeing bulletin only reminds operators of the plane to follow the procedure and doesn’t require any physical fixes that could take the aircraft out of service.

4. Are such notices common?

Aircraft and engine manufacturers routinely send bulletins to air carriers noting safety measures and maintenance actions they should take. Most of them are relatively routine. But the urgency of a fatal accident can trigger a flurry of such notices. Aviation regulators such as the FAA and the European Aviation Safety Agency often follow such actions by mandating that carriers follow the bulletins -- something the FAA says it will do in this case.

What Boeing's Post-Crash Warning About 737 Max Means: QuickTake

5. Why didn’t Boeing ground the planes?

There have been no reports so far suggesting the problem has been detected in other planes. It’s still possible the FAA may order the Chicago-based planemaker to redesign the Max’s flight computers. The agency says it is working with Boeing and the accident investigators and “will take further appropriate actions depending on the results of the investigation.”

6. How common is the 737 Max?

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.

What Boeing's Post-Crash Warning About 737 Max Means: QuickTake