The device linked to the Boeing 737 Max software that has been scrutinized after two deadly crashes was previously flagged in more than 200 incident reports submitted to the Federal Aviation Administration, but Boeing did not flight test a scenario in which it malfunctioned, CNN has learned.
The angle-of-attack (AOA) sensor, as it’s known, sends data to a 737 Max software system that pushes the nose of the aircraft down if it senses an imminent stall. That software, triggered by erroneous data from AOA sensors, is believed to have played a role in crashes of Lion Air and Ethiopian Airlines jets.
Former Boeing engineers and aviation analysts interviewed by CNN have criticized Boeing’s original software design for relying on data from a single AOA sensor, claiming that those devices are vulnerable to defects.
FAA data analyzed by CNN supports that assessment.
The FAA has received at least 216 reports of AOA sensors failing or having to be repaired, replaced or adjusted since 2004, according to data from the FAA’s Service Difficulty Reporting website.
Those reports, about one-fifth of which involve Boeing planes, include incidents in which AOA sensors were frozen, improperly installed, struck by lightning or even hit by flying birds. In some cases, faulty sensors led to stall warnings, forcing pilots to abort takeoffs or perform emergency landings.
In one 2011 case, the flight crew on a Boeing 737-800 reported that the “angle of attack and airspeed failed” and declared an emergency. An AOA sensor was then replaced. The FAA also issued two directives for various Boeing aircraft models before the 737 MAX was released, indicating that Boeing was aware of the potential for the sensors to cause problems in its planes.
A 2013 directive mandated inspections of certain AOA sensors to prevent possible problems that included “obstacles after takeoff, or reduced controllability of the airplane.”