Using the Autopilot - Version 8.60+


Note! X-Plane 8.60 introduced a number of subtle but significant changes in how the autopilot works. Read the description carefully! If you're used to older X-Plane versions, the Autopilot Problems FAQ page will explain the major differences.

How do I use the autopilot?

There is no simple answer to this question. X-Plane offers a lot of flexibility in the autopilot controls and you'll find different configurations on different planes. Not only that, but aircraft designers can provide custom graphics, so there's no telling what things will look like. However, the standard heavy metal airliner control set is pretty commonly used, so we'll go with that here...

First of all, here are the autopilot functions available in X-Plane. (All of these can be chosen for your panel in the Panel-Editor in Plane-Maker... they are all in the "autopilot" folder of instruments.)

WLV: WING-LEVELER: This will simply hold the wings level while you figure out what to do next. (To be precise, WLV is actually attitude hold. When you engage it the plane will hold its current roll angle.)
HDG: HEADING HOLD. This will simply follow the heading bug on the HSI or Direction Gyro.
V/S: This will hold a constant VERTICAL SPEED by pitching the aircraft nose up or down.
HOLD: This will hold the current ALTITUDE by pitching the nose up or down.
SPD: This will hold the pre-selected AIRSPEED by pitching the nose up or down (leaving throttle alone). You control the plane's climb or descent rate indirectly with the throttle.
PTCH: Pitch-Sync: Use this to cause the plane to hold its nose at a constant pitch attitude. Commonly used in King-Airs to just hold the nose somewhere until the pilot decides what to do next.
ATHR: This will hold the pre-selected AIRSPEED by adding or taking away engine thrust.
FLCH: Flight Level Change: Same as SPD, only if ATHR is on, automatically selects either full or idle power, depending on whether you intend to climb or descend.
LOC: Localizer. This will fly a VOR radial or ILS localizer, or to a GPS destination.
G/S: Glideslope: This will fly the glideslope portion of the ILS.

APP: Approach: This is simply the combination of the LOC and G/S buttons. It will fly the localizer and glideslope components of the ILS.

TERR: Terrain Following: This will fly the aircraft at a low altitude, barely clearing the ground.
VNAV: Vertical Navigation: This will fly the altitude commanded by the FMS if you have an FMS in your airplane.
BC: 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 it's advantage (it's cheaper) but a drawback: The needle deflection on your instruments is BACKWARDS when going the WRONG WAY ON THE ILS! Hit the BC (back coourse) 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: HSI's do NOT reverse the visible needle deflection in the back-course because you turn the housing that the deflection needle is mounted on around 180 degrees to fly the opposite direction... thus reversing the reversal!) (NOTE: The glideslope is NOT available on the back-course, so you have to use the localizer part of the procedure only.)

CWS: Control Wheel Steering. Temporarily disconnects the autopilot and allows you to fly the plane under manual control. There is no panel button for this function; to use it you must assign a joystick button.

OK, now you know what the various options are... how do you use them? First, note that the autopilot always controls both the pitch and roll axes - you cannot engage them separately. When you engage a pitch axis (like VS) the AP automatically engages WLV. Conversely, when you engage any roll mode (like HDG) the AP automatically engages PTCH.


 The flight director switch, if your aircraft has one, controls the operation of the autopilot. If the flight director switch is OFF, your autopilot is OFF and does not fly the plane. If you have engaged an autopilot mode and it doesn't appear to be working, check the flight director switch! If the plane does not have a flight director switch, engaging any autopilot mode automatically turns it on.

When the flight director switch is in the ON position, the autopilot is still not controlling the plane. Instead, it is controlling an attitude cue in your artificial horizon. You fly the plane so that the attitude cue is level and centered in the artificial horizon to follow the autopilot course.

When the flight director switch is in the AUTO position, the autopilot controls the plane. This is the switch position you must set to have the autopilot actually fly the plane.

WING-LEVELER and PITCH SYNC:
Just hit them and they hold the current roll and pitch attitudes.

Terrain Following: Just hit the button to engage, and be prepared for a wild ride! A plane with fast pitch response is highly recommended!
AUTO-THROTTLE: Engage AUTO-THROTTLE and the speed setting is set to your current speed. The autopilot will adjust the throttle to maintain the set speed. Change the setting to adjust your speed.
HEADING HOLD:
Just hit it and the autopilot will fly the heading you've entered in the heading selector (shown) or the heading bug on the HSI. You can change heading while in the HDG mode and the autopilot will follow. The outer dial on the Heavy Metal heading selector allows you to set the maximum roll angle the plane will use in turns.
ALTITUDE HOLD:
Just hit it and the autopilot will simply hold the CURRENT altitude. The altitude selector is not strictly necessary; if present it will indicate the altitude being held.


VERTICAL SPEED:
First select your target altitude. Then engage VERTICAL SPEED and the autopilot will climb or descend to the selected target altitude. When it arrives at the target altitude the autopilot switches over to ALTITUDE HOLD mode. When you engage VS, your vertical speed is initially set to your current vertical speed. Once engaged, use the vertical speed selector to dial in your desired vertical speed.

ATC will often tell you to expect a new altitude in 10 minutes, so you can dial the expected altitude into the autopilot in advance, even though you aren't allowed to actually climb or descend yet. Thus, FIRST enter the desired altitude. Then, whenever you like, hit the VS button to go to that altitude. Then enter a NEW altitude... the plane will not go there until you press the VS button again!

Keep an eye on your airspeed and adjust throttle and/or vertical speed as necessary (or use ATHR)! VS will try to maintain your set vertical speed and will stall the plane trying.

FLIGHT-LEVEL-CHANGE or SPD:
You must be established with ALTITUDE HOLD, holding a constant altitude and constant speed. Dial in the new altitude and hit the FLCH or SPD button. If you have AUTOTHROTTLE engaged, FLCH causes the autopilot to automatically set engine power to maximum or idle, depending on whether your target  altitude is higher or lower than your current altitude. SPD does not alter your throttle setting. Once engaged, you can adjust your climb or descent rate by changing your throttle setting.

When do you use VS and when do you use FLCH? FLCH is an effective mode for climb-out in heavies. It allows you to set the engines for climb power, and then let the plane climb to cruise altitude at whatever rate the engine power will allow. Remember that the engines become less powerful at higher altitudes, so your climb rate must be less as you approach cruise level. If you try to climb out using VS mode with a set vertical speed and ATHR, you'll periodically have to reduce your climb rate to avoid overstressing the engines, or, in the extreme case, running out of power, losing airspeed, and stalling the plane.

VS is the better choice for descent, giving you a predictable descent rate, and with ATHR,  predictable airspeed, getting you to target altitude at the right time and place.

CWS: Control Wheel Steering. Press and hold the joystick button that you've assigned to engage CWS. This will temporarily disconnect the autopilot and allow you to fly the plane under manual control. When you release the button you disengage CWS and revert to the previous autopilot mode, but with the settings changed to match the plane's current situation. For example, if you were in ALT mode, you revert to ALT mode, but at your new altitude. If you were in VS mode, you revert to VS with your new vertical speed.



LOC, G/S, and APP:
These modes follow your selected navaid signal. LOC flies a VOR radial, an ILS runway localizer, or a GPS leg. GS flies an ILS glideslope. You have potentially three nav sources: the NAV1 receiver, the NAC 2 receiver, or the GPS. But how do the autopilot know which of those 3 signals to use? The answer is the switch labelled "NAV-1 NAV-2 GPS". This switch causes the HSI and the autopilot to use either Nav-1, Nav-2, or the Flight Management Computer (which gets its signal from the GPS).

If you set this switch to Nav-1, then the the HSI will show deflections from the Nav-1 radio, and the autopilot will fly VOR or ILS signals from the Nav-1 radio if you hit the LOC or G/S buttons.

If you set this switch to Nav-2, then the the HSI will show deflections from the Nav-2 radio, and the autopilot will fly VOR or ILS signals from the Nav-2 radio if you hit the LOC or G/S buttons.

If you set this switch to GPS, then the the HSI will show deflections from the GPS, and the autopilot will fly to the GPS destination if you hit the LOC button. Remember that if you enter destinations into the FMS, they will automatically feed into the GPS, so the autopilot will follow them if you select LOC

So now that you know how to send the right signal (Nav-1, Nav-2, or GPS) to the autopilot for LOC and G/S (lateral and vertical navigation), how do you use those modes?

Here is the answer:

First off, the autopilot will only respond to the nav signal if it's getting a valid CDI reading. CDI stands for Course Deviation Indicator. It's the needle in the VOR head or HSI that indicates how far off course you are. If you are way off course the needle will be pinned to its extreme deflection. As you approach the correct course, the needle will come off its extreme position and move toward the center. The autopilot will only fly the navaid course when the CDI is not pinned.

What this means is that the plane will not fly an ILS approach or VOR radial or GPS course if you are off in some random location and just set up the nav receiver. You must first set up the nav receiver and then fly the plane, either manually or with the autopilot HEADING HOLD mode, so that your flight path intercepts the navaid course. So when you first press the LOC or GS button, the mode is armed but not active. What this means is your previous mode, like HEADING HOLD, is still active. The Armed mode (LOC) will become active as the plane approaches the nav course and the CDI comes off the pin.

Glideslope works the same way. You would typically approach an airport using ALT HOLD, flying at a low enough altitude that you are below the glide slope. You press GS to arm glideslope capture and remain in ALT HOLD mode. When the glideslope CDI crosses the centerline, GS mode activates and the plane flies down the glideslope. You must approach glideslope from below; the autopilot will only lock on from below. The reason for this is that in real life, there are false glideslope signals caused by reflections from the ground that appear above the true glideslope signal. Follow one of the false signals and you will fly into the ground in front of the runway! By approaching the glideslope from below you are assured of capturing the lowest, true glideslope signal.

So how do you use these systems to fly an ILS?

While still far away from the ILS, and BELOW glideslope:
Hit the altitude HOLD button to hold your current altitude (if you haven't already used VS or FLCH to fly to and hold an approach altitude).

Enter a HEADING in the HEADING window to hold until you intercept the ILS. Check your map to make sure the heading will cross the localizer centerline an adequate distance away from the airport.


Hit the HEADING HOLD button to hold it.

Hit the LOC button. It will ARM (orange)

Hit the G/S button. It will ARM (orange)


Now, as soon as you intercept the localizer:
the LOC will go from orange to yellow, abandoning the HEADING mode and flying the localizer.


Now, as soon as you intercept the CENTER of the glideslope:
the G/S will go from orange to yellow, abandoning the ALTITUDE HOLD mode and flying the glideslope. The autopilot will then track you right down to the runway, and even  flare at the end, cutting power if autothrottle is engaged.

Just as in a real airplane, these things only work well if you intercept the localizer far away (OUTSIDE the Outer Marker) and BELOW the glideslope, intercepting the localizer at less than a 30-degree angle, and holding altitude when you intercept the glideslope. If you are above the glideslope, or crossing the localizer at a wide angle, or intercept the localizer too close in to the airport, the autopilot will not be able to manuever the airplane for landing.

One other thing: make sure you are in range of the localizer and glideslope signals before arming LOC and G/S. Check the audio from the ILS transmitter and the readings on your HSI or VOR head to make sure that the signals are valid and stable. Localizers are good to about 18-20 miles out and glideslope signals are good to about 10 miles, but ranges vary from airport to airport. Arming LOC or G/S when you don't have valid and stable signals can have unpredictable and potentially fatal results!

How do you use the autopilot to intercept a VOR radial? It's the same general idea as the ILS approach: tune the nav receiver to the VOR and select your VOR bearing on the VOR head or HSI. Then fly a course using HEADING HOLD that will intercept the VOR radial and press LOC to arm it. It will capture and switch to LOC mode as you approach the selected VOR radial.

How do you use the autopilot to fly a GPS course? With the basic GPS unit, all you can do is dial in a destination. The GPS can select any type of navaid or identified location: airport, VOR, NDB, or fix. First press the correct button to select which type of destination you want. Then use the arrow keys to toggle in the identifier of the destination. The left and right arrows position you to the different letters of the identifier; the up and down arrows select different letters or digits. Once you've selected your destination, the GPS establishes a course from your location at that instant to the destination. If you do not immediately engage the autopilot or manually fly in the direction of the GPS course, you will get far enough off course that the autopilot will not lock on. If this happens you must fly an intercept course manually or with HDG mode, just as you would with a VOR radial.

How do you use the autopilot to fly a multi-segment route with the FMS?  See the FMS page for details.

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