X-Plane V6 allows the aircraft designer complete freedom in the choice and placement of the autopilot controls. So it's impossible to describe what the autopilot panel on any given plane will look like. I've chosen the 777 as a reasonably comprehensive example.
Here's a screenshot of the of the 6.70 777's autopilot panel. The 777 has a default panel, so the appearance of the AP controls comes from X-Plane's heavy metal control selection. There are sets of controls for other classes of aircraft with different appearances. Let's take it from left to right and top to bottom.
IAS/MACH: Airspeed setting for auto throttle.
HDG: Magnetic heading selection for heading hold. (Also displays as the heading bug on the HSI and directional gyro.)
ALT: Altitude target for altitude hold.
VERT SPEED: Vertical speed in feet per minute, used for both altitude hold and vertical speed mode.
ATHR - auto throttle or SPD HLD (speed hold). Holds the indicated speed dialed into the autopilot.
HDG - heading hold using the heading bug.
LNAV - NAV receiver heading hold - holds the selected bearing on the VOR the NAV receiver is tuned to, or holds the selected localizer course if the receiver is tuned to a localizer.
GPS - GPS heading hold. Hold the heading direct to the currently active GPS airport or navaid displayed.
HOLD - altitude hold (if you're not at that altitude, the vertical speed selection will be used as the descend or climb rate to reach that altitude)
V/S - vertical speed hold - fly at the selelected vertical speed.
VNAV - NAV receiver vertical hold - holds the glideslope the NAV receiver is tuned to.
AP1 - autopilot 1 select - selects the NAV1 receiver for LNAV and VNAV.
AP2 - autopilot 2 select - selects the NAV2 receiver for LNAV and VNAV.
BC - back course - selects localizer back course mode.
Note! As of V7.10, the GPS, AP1 and AP2 buttons no longer exist. Instead, you select your autopilot navigation source with the HSI source selector switch.
Several changes have been made in the autopilot controls and behavior since V7.0:
- The GPS, AP1 and AP2 buttons no longer exist. Instead, you select your autopilot navigation source with the HSI source selector switch.
- New autopilot modes:WLV: Wing Level
PTCH: Pitch Attitude Hold
FLCH: Flight Level Change - trims for speed, use manual throttle to climb or descend as required, automatically changes to ALT HOLD plus ATHR when correct altitude reached. Pressing this button automatically disengages ALT HOLD, ATHR and VVI HOLD if in use.
ASI: trims for speed (using climb rate), rather than adjusting throttle
ATHR: Auto-Throttle, used to be labelled ASI
- Changes in behavior:ALT HOLD: To change selected altitude, you first enter the new altitude and then either disengage and re-engage ALT HOLD or select FLCH.
VNAV: Only intercepts glideslope from below. You must be flying using another AP mode (ALT HOLD or V/S), be receiving a valid glideslope signal, and be below glideslope. Pressing VNAV arms the AP for glideslope intercept while is continues to fly in the current mode. When the plane intercepts the glideslope the AP switches to VNAV mode and follows the glideslope signal.
LNAV: When used for VORs and localizers, arms and engages in a manner similar to VNAV. You must be flying under AP control in HDG mode and be receiving a valid VOR or localizer signal. (Check your HSI or VOR head for an indication that makes sense.) Pressing LNAV arms the AP for VOR or localizer intercept while it continues to fly the current heading. Then the plane intercepts the localizer signal or selected VOR radial and AP switches to LNAV mode and follows the localizer or VOR radial.
Note that as of V7.41, X-Plane models the published VOR, localizer, and glideslope signal ranges. Attempting to engage VNAV or LNAV modes when the plane is not already under AP control or there is no valid nav signal will result in unpredictable and potentially fatal behavior. Everything I've read and been told says that real autopilots have safer behavior and will not engage LNAV or VNAV if there is no valid signal, but Austin says he likes it this way because it keeps people on their toes.
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