Originally Posted by
berkinan
When pressing the VNAV button on the AFCS panel in AW139, AW169, and AW189 helicopters, it shows "VNAV is not installed" or "function unavailable."
Why is VNAV not engaged on these helicopters?
If NAV SOURCE is set to FMS, pressing the APP button activates a VGP approach. If NAV SOURCE is set to ILS, pressing the APP button activates an ILS approach.
When and how does the AFCS use the VPATH mode of VNAV submode?
First Q, because, as indicated by the aircraft, it's not installed. It has never been installed but it may yet be in the future, so the button is present.
Second Q. It doesn't except to generate annoying messages on the 139 about resetting the ALT select. The AFCS doesn't do anything useful with VNAV info, because, well, see answer 1....