Using the Autopilot - Version 8



Caution! Version 8.60 introduced a number of subtle but significant changes in how the autopilot works. If you are using X-Plane 8.60 or later, use the V8.60 autopilot page, not this one! Attempting to fly in V8.60 or later using the documentation on this page will cause you a variety of problems.

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...

Austin posted the following description of how to use the autopilot recently. I've added the graphics of the autopilot controls (from the heavy metal airliner control set) and additional explanation here and there. This description applies to X-Plane versions 8.10 and later.

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.
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 or pre-selected 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)
PTCH: Pitch-Sync: Use this to cause the plane to hold it's 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: Level-Change: This will use THROTTLE to go to a new altitude, just adding or taking a way a bit of power to change altitude. Commonly used by airliners to change from one hi altitude to another in cruise.
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.
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.)

OK, now you know what the various options are... how do you use them?


WING-LEVELER and PITCH SYNC:
Just hit them and they hold wings level and pitch-attitude at the current pitch.

HEADING, VERTICAL SPEED, SPEED-HOLD, AUTO-THROTTLE:
Just hit them and they will hold whatever values are entered into the selectors.

Note! When using VERTICAL SPEED, keep an eye on your airspeed and adjust throttle as necessary (or use ATHR)! VERTICAL SPEED will try to maintain your set vertical speed and will stall the plane trying.





ALTITUDE HOLD:
Just hit it and it will climb or descend to and hold whatever value is entered into the selector. Note: If you do not have an altitude SELECTOR instrument, then the autopilot will simply hold the CURRENT altitude.

Note: You must FIRST ENTER THE ALTITUDE IN THE SELECTOR, THEN HIT THE ALTITUDE HOLD BUTTON. Order matters. Why? Because ATC will often tell you to expect a new altitude in 10 minutes, so you want to be able to 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 ALT button to go to that altitude. Then enter a NEW altitude... the plane will not go there until you toggle the ALT button again!

Once you've engaged ALT HOLD, dial in the vertical speed you want to reach the selected altitude at. Keep an eye on your airspeed and adjust throttle as necessary (or use ATHR)! ALT HOLD will try to maintain your set vertical speed and will stall the plane trying.

FLIGHT-LEVEL-CHANGE:
You must be established with ALTITUDE HOLD and AUTOTHROTTLE, holding a constant altitude and constant speed. Dial in the new altitude and hit the FLCH button. Then add or subtract power to climb or descend.

When do you use ALT HOLD 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 ALT 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.

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


LOC and G/S:
These are the ones nobody can figure out. Here is how they work: They must obviously be able to fly either NAV-signal 1, NAV-signal-2, or GPS. But how do they know which of those 3 signals to use? The answer is the switch labelled "NAV-1 NAV-2 FMC/CDU", (with filename "but_HSI_12GPS" in the HSI folder). This switch, based on its position, will cause THE HSI AND THE AUTOPILOT to be based on 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 FMC/CDU, 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:

LOC: Lateral navigation will immediately start going to a GPS destination or VOR radial, and will track an ILS localizer needle ONLY AFTER THE NEEDLE HAS COME OFF OF FULL-SCALE DEFLECTION! This means that if you have a full-scale ILS needle deflection (simply because you have not yet gotten to the localizer) the LOC mode will simply go into ARMED (orange) mode, and NOT DO ANYTHING AT ALL WITH THE PLANE! Your current HEADING or WING-LEVEL mode (if engaged) will remain in force (or you can hand-fly) UNTIL THE LOCALIZER NEEDLE STARTS TO MOVE IN TO THE CENTER. Once that happens, the LOC will suddenly go from ARMED (orange) to ACTIVE, and start actually flying the plane for you, dis-engaging any previous modes.

Why is this? Because you will typically fly HEADING mode until you GET TO THE LOCALIZER, and as soon as the localizer needle comes in, you want the autopilot to forget about heading and start flying the localizer down to the runway. Or you simply hand-fly the plane to the localizer, with no autopilot mode on at all, and you want the autopilot to take over once the ILS needle starts to come in, indicating you are entering the localizer.

G/S: Just like the lateral nav, the vertical nav WILL NOT DO ANYTHING UNTIL THE GLIDESLOPE NEEDLE starts to move... though unlike with LOC, the G/S needle won't do anything until the glideslope needle goes ALL THE WAY THRU THE CENTER POSITION. Why? Because you typically have the airplane on ALTITUDE HOLD until you intercept the glideslope, at which point the plane should stop holding altitude altitude and start flying down to the runway. In other words, the G/S mode will automatically go from ARMED to ACTIVE once the plane hits the CENTER of the glideslope.

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

While still far away from the ILS, and BELOW glideslope:


Enter an ALTITUDE in the ALTITUDE window to hold until you intercept the ILS.


Hit the altitude HOLD button to hold it.

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, as I have found out many times in X-Plane, and several times in my Cirrus.

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 15 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!

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