Is Chelton a common FMS ?
Depends on what the meaning of "common" and "FMS" is. The Chelton (now Cobham) Flight Logic system is
AFAIK the only FMS STC'd (and appropriate) for installation in light helicopters. However, one doesnt find all that many installed. There are other glass panel displays installed even more rarely (Sagem now and Garmin G500H once certified) but these dont fit the common understanding of what constitutes an FMS as these require a separate (not integrated) navigation system, usually a GPS such the Garmin 430/530. Some may consider the 430/530 a reduced function FMS, but these are more properly called GPS navigators IMO.
