The U.S. Federal Aviation Administration issued an emergency airworthiness directive Wednesday giving pilots of Boeing 737 Max 8s and Max 9s procedures to follow in the event of runaway horizontal trim caused by faulty angle-of-attack inputs to the airplane’s flight control system. The AD comes a day after Boeing issued an operations manual bulletin in response to investigators’ findings that the Lion Air 737 Max 8 that crashed into the Java Sea on October 29 experienced erroneous input from one of the sensors.
According to the AD, analysis performed by Boeing showed the defect could lead to repeated nose-down trim commands of the horizontal stabilizer, thereby compromising aircraft controllability and leading to excessive nose-down attitude, “significant” altitude loss, and, ultimately, a crash.
The directive requires a revision to the chapters in the airplane flight manual dedicated to certificate limitations and operating procedures for addressing runaway stabilizer.
The AD comes nine days into the crash investigation and about a week after search crews found the stricken airplane’s flight data recorder in the wreckage at the bottom of the Java Sea. Readouts from the FDR showed that the Boeing 737 Max 8 experienced faulty airspeed readings during its last four flights, but investigators until Tuesday had yet to determine any connection between the instrument anomalies and the October 29 crash that killed all 189 onboard.
Search-and-rescue teams have yet to recover the airplane’s cockpit voice recorder (CVR), believed embedded in the muddy seabed, northeast of Jakarta, where the 737 Max 8 crashed 13 minutes into its flight. On November 5, navy divers recovered the emergency locator transmitter from a depth of 98 feet. Searchers have also retrieved parts of the airplane’s landing gear and fuselage from the Java Sea.