Using the Autopilot

The autopilot works by implementing a number of different functions. These include, among other things, the ability to automatically hold a certain pitch, altitude, heading, or speed, or to fly to a commanded altitude.
The following autopilot functions are available in X-Plane. Each of these is a mode that the aircraft can be put into simply by clicking that button on the panel with the mouse. Not all aircraft have autopilot, and some of the simpler craft, such as the Cessna 172, may have fewer modes than those listed below. The actual use of these autopilot functions will be discussed in the following sections.
The WLV button is the wing leveler. This will simply hold the wings level while the pilot figures out what to do next.
The HDG button controls the heading hold function. This will simply follow the heading bug on the HSI or direction gyro.
The LOC button controls the localizer flight function. This will fly a VOR or ILS radial, or to a GPS destination. Note that the GPS may be programmed by the FMS.
The HOLD button controls the altitude hold function. This will hold the current or pre-selected altitude by pitching the nose up or down.
The V/S button controls the vertical speed function. This will hold a constant vertical speed by pitching the aircraft’s nose up or down.
The SPD button controls the airspeed function. This will hold the pre-selected airspeed by pitching the nose up or down, leaving the throttle alone.
The ATHR button also controls airspeed, but in the reverse of the SPD button. It will maintain the craft’s current airspeed by increasing or decreasing the throttle, while leaving the pitch alone.
The FLCH button controls the flight-level change function. This will hold the pre-selected airspeed by pitching the nose up or down, adding or taking away power automatically. This is commonly used to change altitude in airliners, as it allows the pilot add or take away power while the airplane pitches the nose to hold the most efficient airspeed. If the pilot adds power, the plane climbs. If they take it away, the plane descends.
SPD and FLCH are almost identical functions in X-Plane—they both pitch the nose up or down to maintain a desired aircraft speed, so adding or taking away power results in climbs and descents, respectively. The difference is that if you have auto-throttle on the airplane, FLCH will automatically add or take away power for you to start the climb or descent, whereas SPD will not.
The PTCH button controls the pitch sync function. Use this to hold the plane’s nose at a constant pitch attitude. This is commonly used to just hold the nose somewhere until the pilot decides what to do next.
The G/S button controls the glide slope flight function. This will fly the glide slope portion of an ILS.
The VNAV button controls the vertical navigation function. This will automatically load altitudes from the FMS (Flight Management System) into the autopilot for you in order to follow route altitudes.
The BC button controls the back course function. Every ILS on the planet has a little-known second localizer that goes in the opposite direction as the inbound localizer. This is used for the missed approach, allowing you to continue flying along the extended centerline of the runway, even after passing over and beyond the runway. To save money, some airports will not bother to install a new ILS at the airport to land on the same runway going the other direction, but instead let you fly this second localizer backwards to come into the runway from the opposite direction of the regular ILS! This is called a back course ILS.
Using the same ILS in both directions has its advantages (e.g., it’s cheaper), but there’s a drawback: the needle deflection on your instruments is backwards when going the wrong way on the ILS. Hit the BC autopilot button if you are doing this. It causes the autopilot to realize that the needle deflection is backwards and still fly the approach.
Note that HSIs do not reverse the visible needle deflection in the back-course; you must turn the housing that the deflection needle is mounted on around 180° to fly the opposite direction.
Note also that the glide slope is not available on the back course, so you have to use the localizer part of the procedure only.
Turning It On and Off
Before using the autopilot, it needs to be turned on. The autopilot power switch is labeled “Flight Director Mode,” or simply “FLIGHT DIR.” It has OFF, ON, and AUTO modes.
If the flight director is OFF, nothing will happen when you try to use the autopilot. If it is ON, then the autopilot will not physically move the airplane controls, but will rather move little target wings on your artificial horizon that you can try to mimic as you fly. If you do this, you will be following the guidance that the autopilot is giving you, even though you are the one actually flying. The flight director, then, is following whatever autopilot mode you selected, and you, in turn, are following the flight director to actually fly the plane. If the flight director is set to AUTO, then the autopilot servos will actually fly the airplane according to the autopilot mode you have selected.
In other words, turning the flight director ON turns on the brains of the autopilot, displaying the commands from the modes above on the horizon as little magenta wings you can follow. Turning the Flight Director switch to AUTO turns on the servos of the autopilot, so the plane follows the little magenta wings for you without you touching the stick.
Therefore, if you have a flight director switch, make sure it is in the right mode for the type of autopilot guidance you want—-none, flight director only, or servo-driven controls.
With the flight director set to the right mode, you can engage the autopilot functions by simply pressing the desired button in the instrument panel. To turn off an autopilot function, simply hit its button once again. When all other autopilot functions are turned off, the autopilot will revert to the default functions.
To turn the autopilot off altogether, simply turn the FLIGHT DIR switch to OFF. Alternatively, assign a key or joystick button to turn it off in the Joystick & Equipment dialog box of X-Plane.
Using the Controls
With the autopilot turned on (either to the flight director-only mode or the servo-driven control mode), you are ready to use the autopilot functions. We will discuss when it would be appropriate to use some of the most common functions.
Wing Leveler and Pitch Sync
Hit either the wing leveler (WLV) or the pitch sync (PTCH) to hold the current roll and pitch attitude, respectively. This is useful when switching between autopilot functions.
Heading, Altitude, Vertical Speed, Speed Hold, Flight Level Change, and Auto-Throttle
Hit the heading hold (HDG), altitude hold (ALT), vertical speed (V/S), speed hold (SPD), flight level change (FLCH), or auto-throttle (ATHR) buttons and the autopilot will maintain whatever values are entered into their respective selectors. For the sake of smooth transitions, many of these values will be set by default to your current speed or altitude at the moment the autopilot function buttons are hit.
If you want the autopilot to guide the aircraft to a new altitude, you have to ask yourself: Do you want the airplane to hold a constant vertical speed to reach that new altitude, or a constant airspeed to reach it? Since airplanes are most efficient at some constant indicated airspeed, climbing by holding a constant airspeed is usually most efficient.
Imagine you are flying along at 5,000 feet and you hit ALT, causing the autopilot to store your current altitude of 5,000 feet. Now, though, you want to climb to 9,000 feet. You would first dial 9,000 into the altitude window. The plane will not go there yet; before it will, you must choose how you want to get to this new altitude.
To get to your new altitude via a given airspeed (as airliners do), after dialing in 9,000 feet in the altitude window, you would hit the FLCH or SPD buttons. This will cause the plane to pitch the nose up or down to maintain your current indicated airspeed. Now, simply add a dose of power (if needed) to cause the nose of the plane to rise (which the autopilot will command in order to keep the speed from increasing). When you reach 9,000 feet, the autopilot will leave speed-hold mode and go into altitude-hold mode, holding 9,000 feet until further notice.
Both the airspeed and vertical speed modes will be maintained until you reach the new specified altitude, at which point the autopilot will abandon that mode and go into altitude-hold mode. The same thing will happen with the glide slope control. If the glide slope is armed (that is, lit up after you pushed the button), then the autopilot will abandon your vertical mode when the glide slope engages. This will also happen with the localizer control. If the localizer is armed, the autopilot will abandon your heading mode when the localizer engages. This is referred to as “capturing” the localizer or glide slope.
The key thing to realize is that the vertical speed, flight level change, and heading modes are all modes that command the plane the moment they are engaged. Altitude, glide slope, and localizer, on the other hand, are all armed (in standby) until one of the modes above intercepts the altitude, glide slope, localizer, or GPS course.
An exception to the above rule is altitude. If you hit the altitude button, the autopilot will be set to the current altitude. This is not the way a smart pilot flies, though. A smart pilot with a good airplane, a good autopilot, and good planning will dial in the assigned altitude long before he or she gets there (including the initial altitude before take off) and then use vertical speed, flight level change, or even pitch sync to reach that altitude.
Here is how the system in a real plane would be used (and thus how the system in X-Plane is best used):
While on the ground, short of the runway, you are told to maintain, say, 3,000 feet. You are given a runway heading and cleared for takeoff.
You enter 3,000 feet into the ALTITUDE window and a runway heading (for instance, 290) into the HEADING window.
You take off.
In the initial climb, around maybe 500 feet, you set the flight director to AUTO. The autopilot notes the plane’s current pitch and roll and holds the plane steady.
You hit the HDG button, and the plane follows or turns to the heading. Once there, click the HDG button again and the plane will maintain its course.
You hit either the V/S, FLCH, or SPD button. The autopilot automatically notes the current vertical velocity or airspeed, and the plane flies at that airspeed or vertical velocity until it gets to 3,000 feet, where it levels off.
You are given a new heading and altitude by ATC, and dial these into their respective windows.
You click the HDG button and the aircraft turns to the new heading, then hit V/S, FLCH, or SPD to let the plane zoom to the new altitude.