X-Plane auto pilot explained

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AUTOPILOT HOW-TO

One of the most-often asked questions in X-Plane is the same as one of the most often-asked questions in real planes: HOW DO I WORK THE AUTOPILOT? This is no surprise, as pilots don't know how to work their autopilots all the time. I have even been on AIRLINER where the plane was jerking abruptly left and light for 5 minutes or so as the flight crew was clearly trying to figure out how to properly program and engage their autopilot.

Well, for X-Plane, here are the autopilot functions available: (all of these can be chosen for your panel in the Panel-Editor in Plane-Maker... they are all in the "autopilot" instrument folder). Each of these is a mode you can put your plane in simply by hitting that button on the panel with the mouse.

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.

LOC: Localizer. This will fly a VOR or ILS radial, or to a GPS destination... AND THE GPS CAN GET DATA FROM THE FMS IF THE GPS IS SET TO LISTEN TO THE FMS (explained soon).

HOLD: This will hold the current or pre-selected ALTITUDE by pitching the nose up or down.

V/S: This will hold a constant VERTICAL SPEED by pitching the aircraft nose up or down.

SPD: This will hold the pre-selected AIRSPEED by pitching the nose up or down. (leaving throttle alone)

FLCH: (Flight-Level Change)This will hold the pre-selected AIRSPEED by pitching the nose up or down. (leaving throttle alone) This is commonly used to change altitude in Airliners by simply letting the pilot add or take away power, while the airplane pitches the nose to hold the most efficient speed. If the pilot adds power, the plane climbs. If he takes it away, the plane descends. SPD and FLCH are currently 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 or descents.

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.

G/S: Glideslope: This will fly the glideslope portion of the ILS.

VNAV: Vertical Navigation: This will fly automatically load altitudes from the FMS (Flight Management System) into the autopilot for you, to follow route altitudes. (explained soon).

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 course) 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 glide-slope 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?

Well, first of all, you need to turn the autopilot ON!

The autopilot power is disguised as a 'Flight Director Mode' switch, which has modes OFF, FDIR, AUTO.

What this means is:

If the Flight director is OFF, then NOTHING will happen when you try to use the autopilot.

If the Flight director is ON, then the autopilot will not physically move the airplane controls, but it will move little target wings on your artificial horizon that you can try to mimic as you fly. If you do this, then you will be following the guidance that the autopilot is giving you, even though you are the one actually flying. The flight director is, at that point, following whatever autopilot modes you have selected, and you are following the flight director as you 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, if you have a flight director switch, then make sure it is in the right mode for the type of autopilot guidance you want! (None, flight-director only, or actual servos driving the controls).

Now that you have set the flight director to the right mode, let's look at the various modes you can use to command that flight director and possibly autopilot servos!

WING-LEVELER AND PITCH SYNC:

Just hit them and they hold wings level and pitch-attitude at the current pitch.

HEADING, ALTITUDE , VERTICAL SPEED, SPEED-HOLD, FLIGHT-LEVEL-CHANGE, AUTO-THROTTLE:

Just hit them and they will hold whatever values are entered into the selectors, with most values auto-set to your current speed or altitude at the moment they are hit for smooth transitions. Now, this makes perfect sense at first: Simply hit the VVI button and the autopilot will grab and hold your current VVI. Same with airspeed. Same with altitude. BUT WHAT IF YOU WANT THE PLANE TO CLIMB TO A NEW ALTITUDE YOU HAVE NOT REACHED YET? Well, at that point, you have to ask yourself: Do you want the airplane to hold a constant VERTICAL speed to that new altitude, or a constant AIRSPEED to that new altitude? Since airplanes are most efficient at some constant indicate AIRSPEED, climbing by holding a constant airspeed is usually most efficient.

Let's start with the vertical speed case though.

Let's say you are flying along at 5,000 feet and you hit ALT. That grabs your CURRENT altitude of 5,000 feet.

Now let's say you want to climb to 9,000 feet.

First, dial 9,000 into the altitude window. Note that the plane does NOT go there yet!

The NEXT step is to decide HOW you want to get to 9,000 feet!

Hit the VVI button and the plane will capture your current VVI (maybe 0) and simply dial the VVI up or down to get to 9,000 feet more or less quickly.

When you get to 9,000 feet, the autopilot will automatically DIS-ENGAGE the vertical speed mode and drop right back into altitude mode at your new altitude.

Now let's do this the way airliners do:

You are at 5,000 feet in altitude-hold, flying at a constant speed.

You dial in 9,000 feet because you want to climb.

You hit FLCH or SPD!

This make the plane pitch the nose up or down to maintain your current indicated SPEED!

Now, simply add a dose of power and the nose of the plane will raise up to keep the speed from increasing, and up you go!

When you get to 9,000 feet, the pane autopilot will LEAVE speed-hold mode and go into altitude-hold mode, holding 9,000 feet until further notice.

So, as you see, the speed and vertical speed modes will be held just fine... UNTIL THEY GET TO THE ALTITUDE YOU HAVE DIALED IN, A WHICH POIT THEY WILL ABANDON THAT MODE AND GRAB ALTITUDE HOLD MODE. The same thing will happen with GLIDESLOPE! If the glideslope is armed (lit up because you pushed the button!) then the autopilot will abandon your vertical mode when the glideslope engages. The same thing will happen with the LOCALIZER! If the Localizer is armed (lit up because you pushed the button!) then the autopilot will abandon your heading mode when the localizer engages!

This is called 'CAPTURING' the localizer or glideslope.

PITCH SYNC WITH PITCH-SYNC JOYSTICK BUTTON: You can assign a joystick button to be 'Pitch Sync', in which case the autopilot will match the autopilot settings to whatever you are doing as you fly the plane... then, when you RELEASE the pitch-sync joystick button, the autopilot will GRAB HOLD of the yoke (engage servos) and maintain the vertical speed, altitude, airspeed, or pitch that you were just flying! How does this work? Here is an example:

Let's say you are at 3,000 feet, and you are in ALTITUDE mode, the autopilot holding 3,000 feet for you. You hit the PITCH SYNC joystick button. When you do this, the autopilot servos turn the yoke loose and let you fly... you fly to 3,500 ft (autopilot still in altitude mode!) and let go of the PITCH SYNC joystick button. At that point, the autopilot will try to hold 3,500 ft, since you were in altitude mode at 3,500 feet at the moment you let go of the pitch-sync button.

Now let's say you are in VVI mode.. then the autopilot will try to maintain the vertical speed that you had at the moment you released the pitch-sync button.

Now let's say you are in SPEED or LEVEL-CHANGE mode.. then the autopilot will try to maintain the airspeed (by pitching nose up or down!) the airspeed that you had at the moment you released the pitch-sync button.

So, when you HIT the pitch-sync joystick button, the autopilot turns OFF the servos and lets you fly, but when you RELEASE the button, the servos take hold and try to maintain the speed, altitude, or vertical speed that you had at the moment you released the pitch-sync joystick button. The same applies to bank angle: If if you are in wing-level or heading mode when you hit pitch-sync, then the plane will try to maintain the bank-angle you had at the moment you released the button one you release it. (Note: if the bank angle is less than 6 degrees, then the plane will just level the wings, assuming that you want nose-level).

LOC and G/S:

These are the ones nobody can figure out, partially because the right frequencies and HSI mode must be selected to use them, and partially because they WILL NOT DO A THING until they CAPTURE the approach path they are looking for... and some OTHER MODE (any of the ones discussed above) must be engaged to do that.

So, here is how these modes work:

These modes capture an ILS or VOR or GPS course, so they must obviously be able to fly either NAV-1, NAV-2, or GPS.

But how do these know which of those 3 signals to use?

The answer is the button labelled "NAV-1 NAV-2 FMC/CDU", (with filename "but_HSI_12GPS" in the HSI folder), which is the HSI source selector.

Here is why: The AUTOPILOT will fly whatever is THE HSI IS SHOWING (if you have one), so you need to decide what you want the HSI to show: Nav-1, Nav-2, or GPS (labeled FMC/CDU, for Flight Management Computer, which gets it's signal from the GPS). Once you decide what you want the HSI to display with this button, that is what the autopilot will fly.

If you put this button 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 put this button 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 put this button to FMC/CDU, then the the HSI will show deflections from the GPS, which can be set manually or by the FMS, and the autopilot

will fly to the GPS destination if you hit the LOC button. Rememebr 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 once engaged.

But, it will only track a VOR radial or ILS localizer 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 (yellow) 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 (yellow) 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. Interestingly, this is much the same as the altitude modes! Just as the localizer is ARMED by hitting the LOC button, and you can do anything until the localizer arms and then takes over lateral control, the altitude is also ARMED (always, and automatically) and you can fly any vertical speed or airspeed or pitch (manually or on autopilot) until the altitude is reached, at which point the autopilot will go into altitude-hold mode.

G/S: Just like the lateral nav, the vertical nav WILL NOT DO ANYTHING UNTIL THE GLIDELSOP NEEDLE starts to move... though unlike with the localizer, the G/S mode won't do anything until the glidelsope 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:

->Hit the altitude ALTITUDE button to hold current ALTITUDE.

->Enter an HEADING in the HEADING window to follow until you intercept the ILS.

->Hit the HEADING button to hold it.

->Hit the LOC button. It will ARM (yellow)

->Hit the G/S button. It will ARM (yellow)

Now, as soon as you intercept the localizer:

->the LOC will go from yellow to green, abandoning the HEADING mode and fliying the localizer.

Now, as soon as you intercept the CENTER of the glideslope:

->the G/S will go from yellow to green, abandoning the ALTITUDE HOLD mode and fliying the glidelsope.

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 loclalizer 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.

OK, you should know how to fly with the autopilot now.

Now let's see how you can fly an FMS PLAN.

A few things must happen:

-You must enter all your flight plan into the FMS

-you have to have the HSI set to GPS, NOT nav 1 or nav 2 (because remember, the autopilot will fly whatever it sees on the HSI!)

-you must have the LOC button selected ON since that button makes the autopilot follow the localizer (or whatever is on the HSI)

-the FLIGHT DIR button must be set to AUTO, so the servos are running.

-the VNAV button should be hit IF you want the FMS to also load ALTITUDES into the altitude window

Do all these things, and the plane will follow any FMS plan, assuming, of course, the plane you are flying HAS all this equipment, which of course some do not.

Now, the next question a lot of people ask is: HOW DO I USE THE FMS????????

Well, it's pretty darn easy!

Here's how:

Open of the Boeing 777 for this one... hit the INIT button on the FMS: this gets the FMS inited to receive a flight plan.

Now hit the AIRP button: this tells the FMS that you are about to go to an AIRPORT.

Now enter the ID of whatever airport you want to by hitting the keypad keys with the mouse.

Now, if you like, hit the line-select button on the left side of the FMS next to the text "FLY AT ______ FT"... and enter the altitude you want to fly at with the keypad again.

Now, if you want to do more than just fly to an airport, hit the NEXT button on the FMS and repeat the steps above for the next waypoint.

There is a back-arrow to erase mistakes, VOR, NDB, FIX, and LAT/LON buttons to enter those types of destinations, and PREV/NEXT buttons to cycle thru the various waypoints in your plan, as well as a LD and SA button load and save flight plans if you want to use them again.

Now, once you have entered the plan into the FMS, take off and set the "SOURCE" button for the HSI to "GPS" so the HSI is getting data from the GPS (not the nav-1 or nav-2 radios) and move the "FLIGHT DIR" button to "AUTO" so the autopilot servos are actually running, and hit the "LOC" autopilot button to follow the HSI lateral guidance.... which you just set to get data from the GPS, with the servos on to actively command the plane. (And, if you bothered to enter an altitude into the FMS, which is totally optional, then hit the VNAV autopilot button to track the entered altitude").

So, it is pretty easy once you just get the basics.

Now, using the autopilot is only one basic step... the next level is to use the FMS! (flight management system). To do this, a few things must happen: You must enter all your flight plan into the FMS, AND you have to have the HSI set to GPS, NOT nav 1 or nav 2 (because remember, the autopilot will fly whatever it sees on the HSI, so you must get the HSI to show you what is being generated by the GPS, so you have to set the HSI to GPS) AND you must have the LOC and VNAV buttons selected ON, AND the FLIGHT DIR button must be set to AUTO, so the flight director is NOT OFF, NOT just ON, but actualy DRIVIGN THER AUTOPILOT. Do all these things, and the plane will follow any FMS plan, assuming, of course, the plane you are flying HAS all this equipment, which of course most do not.