Southwest Airlines and the Federal Aviation Administration (FAA) officials who monitor the carrier were unaware that a standard safety feature, designed to warn pilots about malfunctioning sensors, on Boeing 737 Max jets was turned off when Southwest began flying the model in 2017, reported Andy Pastzor of the Wall Street Journal.

In earlier 737 models, the safety feature alerted pilots when a sensor called the "angle-of-attack vane" incorrectly conveyed the pitch of the plane's nose, according to Pastzor. In the Max, it functions as such while also signaling when the Maneuvering Characteristics Augmentation System (MCAS) — a new automated system linked to both October's Lion Air crash and March's Ethiopian Airlines crash — could misfire; but these alerts were only enabled if carriers purchased additional safety features, Pastzor wrote.