(Reuters) - Boeing Co won approval on Wednesday from the U.S. Federal Aviation Administration to fly its 737 MAX jet again after two fatal crashes that triggered two years of regulatory scrutiny and corporate upheaval. The FAA also detailed some changes that Boeing must make on the jet. In accidents in Ethiopia in 2019 and Indonesia in 2018, a stall-prevention system known as MCAS, triggered by faulty data from a single airflow sensor, repeatedly and forcefully shoved down the jet’s nose as the pilots struggled to regain control. After a series of investigations and regulatory reviews, here are the major changes happening before the 737 MAX returns to the sky. SOFTWARE Boeing’s Maneuvering Characteristics Augmentation System, known as MCAS, will now rely on readings from two Angle of Attack (AoA) sensors that measure the angle at which the wing slices through the air, instead of just one. MCAS will activate only if both sensors are in agreement, and it will activate only one time, Boeing has said. Separately, Boeing is making standard an alarm alerting pilots to a mismatch of flight data from the sensors. Boeing did not tell U.S. regulators for more than a year that it inadvertently made the so-called AoA Disagree alert optional on the 737 MAX, instead of standard as on earlier 737s. Boeing has said the missing display represented no safety risk, an FAA official said in May 2019. PILOT TRAINING Pilots must undergo new simulator training, including training on multiple flight-deck alerts during unusual conditions, along with how to respond to a situation known as runaway stabilizer with timely pilot actions required. Pilots must also get training for erroneous, high Angle of Attack malfunctions. American Airlines expects to train some 1,700 of its 4,000 737 pilots in December with a one hour and 40 minute iPad course and a two-hour simulator session that will follow a one-hour briefing, the union representing its pilots said. WIRING BUNDLES Boeing said it will separate 737 MAX wiring bundles, flagged by regulators as potentially dangerous, before the jet returns to service. The jet’s wiring raised the potential of a short circuit that, in certain remote circumstances, could lead to a crash if pilots did not react in time. One industry source said the wiring separation was “still continuing” this week. FOREIGN OBJECT DEBRIS In February, Boeing said it found “foreign object debris” - an industrial term for rags, tools, metal shavings and other materials left behind by workers during production - in the fuel tanks of dozens of undelivered 737 MAX jets. Boeing said it has inspected all of the stored aircraft and “shared inspection recommendations and detailed instructions with customers storing their own airplanes.”