Originally Posted by JamesNoBrakes

135 commuter air carrier is only "one level" below 121, in many cases, they run more complex operations with many more airplanes, staff, etc. There are some pretty small 121 operations out there. If you want to be grouping "airlines" though, it's best to specify, since airline and air carrier are words that can easily contain both parts, based on all the requirements they meet and the service they provide. There are lot more of these options (both small 121 and large 135) than many people know about. Yes, many 135 operators with two-crew operations/environments, FOs, etc. For the larger aircraft, many of the requirements are the same or very similar. 121 goes into more depth in most cases, but not all. If one is looking at airliner fatalities, why wouldn't we consider all scheduled operations?