Boeing and the FAA decided that the AoA display and an AoA disagree light, which signals if the sensors give different readings, were not critical features for safe operation.[80] Boeing charged extra for the addition of the AoA indicator to the primary display.[81][82] In November 2017, Boeing engineers discovered that the standard AoA disagree light cannot independently function without the optional AoA indicator software, a problem affecting 80% of the global fleet that had not ordered the option.[83][84] The software remedy was scheduled to coincide with the roll out of the elongated 737 MAX 10 in 2020, only to be accelerated by the Lion Air accident. Furthermore, the problem had not been disclosed to the FAA until 13 months after the fact. Although it is unclear whether the indicator could have changed the outcome for the ill-fated flights, American Airlines said the disagree indicator provided the assurance in continued operations of the airplane.