Philipp made a change to the Cessna’s autopilot that makes it more realistic, but also slightly trickier to use.
As of 11.30, the Cessna’s autopilot uses HDG + GPSS for nav course selection. Philipp wrote a developer article explaining what all five of X-Plane’s nav course source modes do, but here are a few notes.
Among autopilots that aren’t totally hopeless (e.g. the Sperry, “none” for its course source) there are two major divisions:
- Autopilots that get the heading for a VOR or ILS from the autopilot heading bug.
- Autopilots that get the heading for a VOR or ILS from an (E)HSI, leaving you to use the heading bug to pick your own heading.
This first category is less high tech and less useful to the pilot, and it’s the bucket the Cessna is in. If you want to fly a localizer, you have to set your AP heading bug to the localizer’s course, or the autopilot is going to lose its mind.
What this means is: if ATC gives you a heading to intercept the localizer, you cannot just engage heading mode, arm localizer mode, and fly the course to intercept – when you intercept the localizer, you won’t track it properly. You will have to adjust your heading bug onto the localizer front course once you start intercepting it.
The GPS is a special case: It works like a VOR/LOC when you are in NAV mode (affected by the heading bug) and like you’d expect when you are in GPSS mode (not affected by the HDG bug). So if you want to fly a GPS flight plan with twists and turns fully automated, you must press the NAV button twice in order to turn the autopilot to GPSS.
Something that can trip people up is that you cannot use GPSS and APR/GS at the same time – if you want to fly a WAAS approach with vertical guidance, you have to step down from NAV GPSS to NAV APR, which means you’re dependent on the pesky heading bug again. Confusing? Yes. Annoying? Certainly. Realistic? Definitely! This is exactly as annoying as the real thing.
The second category gets the localizer heading from either magic (GPS/LOC) or the OBS knob “OBS”) and always gets the VOR radial from the OBS course (which make sense). These setups let you fly a “dual mode” approach – engage HDG, arm LOC, and fly the intercept heading – when the localizer comes alive, the AP can switch modes and know which way to turn to intercept without you having to mess with the HDG dial.
You’ll find GPS/LOC-based systems in our Kingair (which has an EHSI) and Baron (which has a HSI).