Which post-crash warning from Boeing? About 737 Max means: QuickTake

Minutes after takeoff on 29 October, a Boeing 737 Max belonging to the budget, the Indonesian courier Lion Air, entered the Java Sea with more than 600 miles per hour, crashed into pieces and killed all 189 passengers and crew. Based on preliminary findings from an investigation into the crash, Boeing Co. has Air carriers operating the new 737 Max warned that erroneous measurements from a flight monitoring system could cause the planes to do exactly that – abrupt diving.

1. What caused the plane to crash?

Researchers are still trying to find this out. But the Boeing warning and a related report from the US Federal Aviation Administration indicate that they are focusing on a possible failure of sensors that may have registered an aerodynamic stall, causing the aircraft's computers to dive to recover the flight speed it needs to keep flying. That would explain why it collapsed into the sea with so much speed. Another indication: the so-called incident angle sensor had failed on an earlier flight and was replaced the previous day.

2. What does an angle sensor do?

The system measures the direction of the air flow over wings. If the flow is disrupted by an aircraft that goes too slow or climbs too steeply, this may cause the aircraft to lose the lift needed for flying and lowering. When the sensors detect such an aerodynamic box, they send a command to the computers to point the nose of the aircraft down to restore flight speed.

3. What did Boeing's warning say?

Pilots are trained to disable the incident angle sensors from the aircraft's computers when they receive false measurements. The system warns pilots if it detects a possible fault. But switching off the sensors in the heat of the moment, especially if equipment is malfunctioning and alarms sound, can be difficult. Boeing's bulletin only reminds operators of the aircraft to follow the procedure and does not require physical repairs that can shut down the aircraft.

4. Are such announcements customary?

Aviation and engine manufacturers routinely send bulletins to airlines, where they are aware of safety measures and maintenance measures they have to take. Most are relatively routine. But the urgency of a fatal accident can trigger a flurry of such messages. Aviation regulators such as the FAA and the European Aviation Safety Agency often follow such actions by stipulating that carriers follow the bulletins – something the FAA says it will do in this case.

5. Why did Boeing not ground the planes?

There are no reports so far suggesting that the problem has been detected in other aircraft. It is still possible that the FAA can order the Chicago-based planner to redesign the flight computers of the Max. The agency says it will work with Boeing and the accident investigators and "take further appropriate actions, depending on the results of the investigation."

6. How common is the 737 Max?

Boeing has delivered 219 Max aircraft – 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 aircraft.

Contact the reporters about this story: Alan Levin in Washington, alevin24@bloomberg.net; Julie Johnsson in Chicago, jjohnsson@bloomberg.net

Contact the editors responsible for this story: Jon Morgan at jmorgan97@bloomberg.net ,; Brendan Case on bcase4@bloomberg.net, Lisa Beyer

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