While it’s still not clear what caused the Lion Air flight 610 and Ethiopian Airlines flight 302 to crash, it is now clear that Boeing reportedly sold the 737 planes without two safety features that are offered to airlines at an additional cost. Aircraft manufacturers generally offer upgrades to standard planes. The upgrades are typically related to aesthetic features, not safety. Airlines are not legally required to buy optional extras, so many low-cost carriers choose not to.
The safety features are the “angle of attack indicator” and the “angle of attack disagree light.” The angle of attack indicator displays readings for sensors that determine where the nose of the plane is pointing. The disagree light feature alerts the pilot when those sensors don’t agree.