So this lines of code estimate are spread across all the systems on the aircraft. One reason for all of the lines is that everything is done in C.
Then really the more exact reason is that every command on an aircraft is monitored. You have complex control laws to control a surface and then probably 4 or 5 different monitors that make sure the surface isn't oscillating or the actuators are not fighting each other and all kinds of stuff. It is because the mentality that not only do you design something to not go wrong, you monitor for failures when it does and handles them accordingly.
Finally you have maintenance where an technician can run a test to determine any issue on the aircraft. So there has to be tests and monitors built into the software to determine root cause of an issue down to a single box/actuator/sensor/or wiring problem.
Then really the more exact reason is that every command on an aircraft is monitored. You have complex control laws to control a surface and then probably 4 or 5 different monitors that make sure the surface isn't oscillating or the actuators are not fighting each other and all kinds of stuff. It is because the mentality that not only do you design something to not go wrong, you monitor for failures when it does and handles them accordingly.
Finally you have maintenance where an technician can run a test to determine any issue on the aircraft. So there has to be tests and monitors built into the software to determine root cause of an issue down to a single box/actuator/sensor/or wiring problem.