If you’re an airline passenger, automation is your friend — setting aside the fears over its role in the crashes of two Boeing 737 Max planes in the past five months. The gradual spread of automation through the civil aircraft fleet is a primary reason the accident rate worldwide has fallen from about four accidents per million flights in 1977 to less than 0.4 today. Pilots today, as one former pilot puts it, are less stick-and-rudder movers than they are overseers of systems.

Automation is not without its own hazards, though. As it has become ubiquitous in cockpits, automation-related accidents have come to make up a significant proportion of all air disasters. In 2013, the Federal Aviation Administration published a study of accidents over the previous two decades and found that reports of “unexpected or unexplained” behavior of automated systems were present in 46 percent of the accident reports and 60 percent of the major incident reports collected by the researchers.

There are two main ways in which automation can lead to catastrophe. Sometimes a malfunction causes the autopilot to run haywire and put the plane into a dangerous state. This is what seems to have happened to the Lion Air 737 Max that crashed last October shortly after takeoff from Jakarta, Indonesia. (It may have occurred in the case of Sunday’s Ethiopian Airlines crash as well; on Wednesday, the F.A.A. said that newly available satellite data “indicates some similarities” between the two accidents.)

Because the 737 Max had been outfitted with larger new engines that could cause its nose to pitch dangerously skyward, Boeing had added a maneuvering characteristics augmentation system, or M.C.A.S., that would kick in and push the nose down if necessary. But a faulty sensor fed incorrect information to the Lion Air flight’s M.C.A.S., causing it to put the plane into a steep dive. Pilots on at least two flights in the United States reported similar problems, but in those cases they were able to disengage the autopilot system and recover control of the plane.