Chapter 6: Navigation, Autopilots, and Flying on Instruments
From X-Plane Wiki
People often call customer support asking about some of the more advanced things that pilots do in the real world—how to navigate, use an autopilot, or fly on instruments. This chapter will cover these areas in a fair amount of detail, but it is recommended that if users are really serious about mastering these facets of aviation they head down to a local general aviation airport and hire a CFI (Certified Flight Instructor) for an hour or two. Users with a laptop can by all means bring it along and have the instructor detail many of these things in practice. There is much more to review here than this manual could ever cover, so a quick search for information on the Internet will also be of assistance.
Navigating over the Earth's surface is as easy as knowing where your aircraft is and how to get to where you want to go. This isn’t quite as easy as it sounds. Imagine that you're flying IMC conditions (Instrument Meteorological Conditions—that is, in the clouds). You have no reference to the ground and are flying over St. Louis in the middle of an overcast layer. As you might guess, this looks pretty much identical to the view you would have flying over Moscow on instruments. The only way to know that you’re over St. Louis and not over Moscow is to be able to navigate. Navigation is the art of being able to tell where your aircraft is and how to make it go where you'd like.
For the first 30 years or so the best pilots could do was to fly around using what is known as dead reckoning—that is, by confirming their position on a map as they flew, then looking ahead on the map to see when they should be crossing some known landmark, like a road, railroad, town, or lake. Then, the pilots periodically compared their progress over the real ground with the anticipated progress over the map to see how things were going. This really is as simple as it sounds. The biggest trick is to always know where you are and what to be looking for next.
Dead reckoning isn’t too difficult to get down. Shortly after college, Austin Meyer (the author of X-Plane) and I, Randy Whitt, once piloted a Cessna 172 from Kansas City to Chicago after our second (of two) navigation radios gave up and died in mid-flight. No, this is not a typical experience in the aviation world, but it demonstrates that a pilot always needs to be thinking ahead and be prepared for contingencies. That particular aircraft was a well-used rental and Nav 1 was dead from the time we signed it out. When Nav 2 died, we had no operable navigation radios at all and used dead reckoning to fly the last 300 or so miles of our trip, which was most of the journey. We would never have allowed ourselves to get into that position had the weather been poor or had we been flying on instruments—we would have refused to take off into such conditions given the failure in the first radio. But since the weather was nice, we took off with only one navigation radio and were soon flying along on none. X-Plane allows you to practice this all you like.
During the heyday of dead reckoning, the US Mail pilots that were flying on overnight mail routes actually flew from bonfire to bonfire that had been set up along their route, using the light to guide their progress. Just imagine what this must have been like—flying in the mid 1920s in an open cockpit biplane (a Curtis Jenny, perhaps) trying to keep your goggles clean (the engines of the day routinely sprayed oil) and to stay out of the clouds on a cold winter night, flying along a chain of bon-fires to your next destination. Keep in mind these were not closed-cockpit aircraft and the pilot continually had the outside air blowing all around. Wow! I hope you dressed warm and that you are good at folding maps in 80 MPH slipstreams of below-freezing air.
In the mid 1930s or so a system was devised where pilots would fly using aural navigation—that is, they would tune into a new radio system such that if they were to the left of their course they would hear a series of dashes (long radio tones, as in Morse code), and if they were to the right of their course they would hear a series of dots (short tones). If on course, they would hear nothing as the signals containing the dashes and dots canceled each other out. The closer the pilot was to the transmitter the smaller the "Cone of Silence," as it was known, was and the more defined the boundaries between the dashes, dots, and silence. As the aircraft's range from the station increased, the central target (where no signals were heard) was much wider and weaker. Imagine sitting in a cold, dark cockpit listening intently to try and hear over the drone of the engine and whistle of the wind on your wires to see which side of the cone you were on. Airline pilots used this system for years to successfully carry passengers all around the world. This type of navigation is not modeled within X-Plane.
We now come into the area of “modern” navigation based on ground-based transmitters. You'll need a good set of charts if you'd like to actually fly in X-Plane using any of these methods, but the software does contain a full set of (mostly) current charts as well. To see them go to the Location menu, click Local Map, and select one of the five map types that are available in the tabs on top of the window. They are:
- High-Speed—used as high altitude charts by jet and turbo-prop pilots.
- Low Enroute—used as low altitude IFR navigation charts by piston (propeller) aircraft pilots. One of the most important aspects of this chart is the addition of Vector Airways that are virtual highways in the sky that connect different VOR transmitters. These vector airways are given names (for example, V503) and are used by air traffic controls to assign clearances.
- High Enroute—very similar to Low Enroute but only showing the information of interest to pilots flying above 18,000 feet and making use of vector airways that are much longer, based on larger VORs with longer ranges.
- Sectional—the standard chart that VFR pilots are familiar with. This map has ground elevation data superimposed via a shaded background and information about the airports that are local to that area.
- Textured—a nice map that is not used in pilot circles. This overlays the X-Plane terrain images on top of the navigation charts to give the user a good bird's eye view of the area he or she is flying over.
Note that more information on the Local Map screen can be found in Chapter 5.
Non-directional beacons were invented in the late 1940s and consisted of a ground-based transmitter that broadcast a homing signal. A receiver in the aircraft could be tuned to one of about 300 discrete frequencies in order to tune to a particular transmitter. With that done, an instrument in the panel, called the NDB (or, interchangeably, the ADF, or Automatic Direction Finder), would point to the station. This system was a large technological leap forward over the older aural-based system and was actually quite easy to use, provided that the wind was perfectly calm or blowing in a direction that was exactly parallel to the direction of flight. Of course, that pretty much never happened, resulting in the aircraft always being blown off course. As a result, the pilots had to watch the trend of movement in the needle over a relatively long period of time (e.g., five to eight minutes) to see if the angle to the station that was depicted stayed constant or was changing. If it was changing, it indicated that the aircraft was being blown off course and the pilot had to turn in the opposite direction by half of the deviation. After holding that heading for another five minutes or so the pilot would again observe the relative trend of the needle and correct again.
The trick was to fly as straight as possible from one station to another. Although nearly abandoned in the United States, NDBs are still used in many countries around the world. It is for this reason that they are modeled in X-Plane. They appear as in the following image.
For example, in the image above, the Innsbruck NDB (whose identifier is INN) transmits at a frequency of 420 kHz.
An ADF is located in the instrument panel for the Cessna 172S that comes with X-Plane. It is located above the mixture knob and trim wheel, below the dual VOR CDIs, and is shown in the following image.
Very High Frequency Omni-Range navigation (or VOR) was introduced in the mid-1950s and represented a large improvement in navigation accuracy. Instead of an NDB that a pilot could home in on, the VOR sends a series of 360 discrete little carrier tones on a main frequency. Each of these carriers is oriented along a different radial from the station, one of 360 just like a compass rose. Thus, when you are flying along and tune in the main VOR frequency, you then fine tune your navigation display to tell you which of the 360 radials you are flying and also whether the transmitter station is in front of or behind you. Impressive! This finally gave pilots a means of telling exactly where they were in relation to a fixed spot on earth, and this system “automatically” adjusted for any winds aloft as the system would quickly display any error in track that the plane was making. This error could only be due to two factors—either the pilot was not flying along the radial or the wind blew the airplane slightly off of course. VORs are modeled in X-Plane.
VOR stations appear in the X-Plane maps as relatively large circles with notches around the edges, similar to a clock face. In the hi-speed and enroute maps, they are black, while in the sectional map, they are blue, as seen in the image below.
They are tagged with boxes that have their name and identifier on the left side and their VOR frequency on the right. For instance, in the image above, the Kempton VOR, whose identifier is KPT, transmits at a frequency of 109.60 MHz.
A specific type of VOR, a VOR-DME, combines the lateral guidance (that is, guidance left and right) of a VOR with the distance guidance of a DME (distance measuring equipment). In X-Plane, this is labeled as in the image on the following page.
Another type of VOR beacon, a VORTAC, is also found throughout the X-Plane maps. This is a transmitter that combines both VOR and TACAN features. TACAN (or tactical air navigation) provides special information to military pilots similar to a civilian VOR. However, for our purposes, this is functionally identical to a VOR-DME. A VORTAC in X-Plane is labeled as in the following image.
To use a VOR, first look on either the sectional or low enroute map to find a VOR station that is fairly close the location of the aircraft. Tune this station’s frequency into your VOR radio (in the Cessna 172SP, the NAV 1 radio is found on the far right of the cockpit, beneath the GPS). The little red 'nav1' or 'nav2' flags on your CDI (Course Deviation Indicator) should disappear (keep in mind that you may have to hit the flip-flop switch to bring the frequency you just tuned into the active window). Now rotate the OBS (Omni Bearing Selector) knob so that the vertical white indicator is perfectly centered in the little white circle in the middle of the instrument. At this point the vertical white line should be truly vertical and your aircraft is either on the radial from the station indicated by the arrow at the top or at the bottom of the instrument, labeled TO or FR. Now fly that exact heading and you will be flying directly towards or away from the station, as shown by the little white up or down (to or from, respectively) arrow that will be on the right side of the CDI, either above or below the white horizontal glide slope indicator.
Note that the vertical reference line indicates how far you are from your desired radial. To the left and right of the center target (the little white circle) the instrument displays five dots or short lines on each side. Each of these dots indicates that you are two degrees off of course. Thus, a full scale left deflection of the vertical reference indicates that the aircraft is 10 degrees right of the desired radial if the station is in front of you. Of course, if the station is behind you then the instrument is reverse sensing and that means that a left deflection indicates that the plane is to the left of your desired radial—yes, it can be a bit confusing. Just remember that as long as you are flying towards the VOR, the line on the CDI indicates the location of the desired course. If the reference line is on your left that means that your target radial is on your left.
With only one VOR you really don't know where you are along a given radial, only that you are in front of or behind a station and what radial you're on. You have no way of telling if you are 15 miles from the station or 45 miles away. The solution is to use two VOR radios so that you can plot your location from two different VORs. If you can determine that you're on the 67th radial from the OJC VOR and on the 117th radial from the MKC VOR then you can pinpoint your location on a sectional chart. Don't forget that you'll have to work fast as your position will be continually changing.
An ILS (or instrument landing system) differs from a VOR in that it provides both lateral guidance (left and right, as given by a VOR) and vertical guidance (up and down). An ILS is therefore made up of two transmitters, a localizer and a glideslope—one for each component of the navigation. Both these components of the ILS are tuned together; tuning an ILS is just like tuning in to a VOR.
A localizer (LOC) transmitter provides lateral guidance to the centerline of a runway. It works by sending out two signals on the same channel, one of which modulates at 90 Hz and the other of which modulates at 150 Hz. One of these signals is sent out slightly to the left of the runway, while the other sent out slightly to the right of it. If an aircraft is picking up more of the tone modulated at 150 Hz, it is off to the left. If it is picking up more of the tone modulated at 90 Hz, it is off to the right. The course deviation indicator (or CDI) in the instrument panel then indicates this so that the pilot can correct it. When both tones are being received in equal amounts, the craft is lined up with the physical centerline of the runway. These LOC transmitters do not necessarily have to be paired with a glideslope (thus making them an ILS). In X-Plane, a standalone LOC transmitter is marked as in the following image.
In the example above, the LOWI runway 26 localizer transmits at a frequency of 111.10.
An ILS combines the functionality of a localizer, which provides lateral guidance, with a glideslope transmitter, which provides vertical guidance to the runway. The glideslope beacon functions similarly to the localizer, sending out two tones that have the same frequency, but different modulations. The difference is that the glideslope tells the plane that it is either too high or too low for its distance from the runway. The pilot uses this information to push the craft's nose up or down as needed. The ILS will allow a pilot to fly on instruments only to a point that is a half mile from the end of the runway at 200 feet (depending on the category of the ILS) above the ground. If the runway cannot be clearly seen at that point the pilot is prevented from executing a normal landing. If this happens, the pilot in real life is required to fly a "missed approach" and climb back to altitude in order to try again or go somewhere else.
In X-Plane, an ILS transmitter is marked as in the following screenshot.
Global Positioning Systems were first invented for the US military and introduced to the public in the early 1990s. This system consists of a series of satellites orbiting the Earth which continuously send out signals telling their orbital location and the time the signal was sent. A GPS receiver can tune into the signals they send out and note the time it took for the signal to travel from the satellite to the receiver for several different satellites at once. Since the speed at which the signals travel is known, it is a simple matter of arithmetic to determine how far from each satellite the receiver is. Triangulation (or, rather, quadrangulation) is than used to determine exactly where the receiver is with respect to the surface of the Earth. In an aircraft, this information is compared with the onboard database to determine how far it is to the next airport, navigational aid (NAVAID), waypoint, or whatever. The concept is simple, but the math is not. GPS systems have turned the world of aviation on its head, allowing everyday pilots to navigate around with levels of accuracy that were unimaginable 20 years ago.
There are several types of GPS radios available, and about 11 of these have been modeled in X-Plane. While the intricate workings of the various GPS radios are complex, the basic principals are pretty consistent. If you want to navigate from one location to another just launch X-Plane, open the aircraft of your choice, then press the “Direct To” key on the GPS radio (sometimes shown as a 'D' with an arrow through it, from left to right) and enter the airport ID you'd like to navigate to. On the Garmin 430, entry is performed using the control knob on the bottom right of the unit. Use the outer knob to select which character of the identifier to modify, the use the inner knob to scroll through the characters (see the Note on Radio Tuning for more info on using the knobs). Also, keep in mind the ID conventions discussed in the Airport IDs section of Chapter 4 and enter the beginning 'K' as appropriate.
The databases in these radios are not limited simply to the identifiers of the airports you may wish to fly to. You can enter the IDs for any VOR or NDB station you'd like, or the name of any waypoint or fix you'd like to go to.
One of the most frequently asked questions from X-Plane users is the same as one of the most frequently asked questions from real-world pilots—how do I work the autopilot? Many pilots have never taken the time to learn—Randy Witt has even been on airliner where the plane was jerking left and right for five minutes or so as the flight crew tried to figure out how to program and engage their autopilot.
The following autopilot functions are available in X-Plane. Each of these can be chosen for an aircraft’s panel in the Panel-Editor of Plane-Maker. They are located in the "autopilot" instrument folder. Each of these is a mode that the aircraft can be put into simply by clicking that button on the panel with the mouse.
Descriptions of Autopilot Functions
The wing leveler button. This will simply hold the wings level while the pilot figures out what to do next.
The heading hold button. This will simply follow the heading bug on the HSI or direction gyro.
The localizer button. This will fly a VOR or ILS radial, or to a GPS destination. Note that the GPS may be programmed by the FMS (explained in the following sections).
The hold button. This will hold the current or pre-selected altitude by pitching the nose up or down.
The vertical speed button. This will hold a constant vertical speed by pitching the aircraft nose up or down.
The speed button. This will hold the pre-selected airspeed by pitching the nose up or down, leaving the throttle alone.
The flight-level change button. 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 by simply letting 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 s/he takes it away, the plane descends. SPD and FLCH are currently 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 or descents. The difference is, if you have auto-throttle on the airplane, FLCH will automatically add or take away power for you to start the climb or descent. SPD, on the other hand, will not.
The pitch sync button. 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 glideslope button. This will fly the glideslope portion of the ILS.
The vertical navigation button. This will automatically load altitudes from the FMS (Flight Management System) into the autopilot for you in order to follow route altitudes (explained in following sections).
The back course button. 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 (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: HSIs 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 (it would be 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.
Using the Autopilot
Turning It On
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 is, at that point, following whatever autopilot modes are selected, and you, in turn, 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, 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 to AUTO (as shown in the following image) 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.
When you first turn the flight director to ON or AUTO, it will automatically engage in the pitch sync and wing leveler modes, which will simply hold the craft’s current pitch and roll until some other mode is selected.
Note: If the system is turned on with less than 7 degrees of bank, then the system will assume you want the wings level, thus leveling the wings for you.
Now that you have set the flight director to the right mode, let's look at the various modes you can use to command the flight director and autopilot servos.
Using the Controls
Wing Leveler and Pitch Sync
Hit either of these and they hold wings at the current bank (or level the wings if you engage it with less than 7 degrees of bank) and pitch-attitude at the current pitch.
Heading, Altitude, Vertical Speed, Speed Hold, Flight Level Change, Auto-Throttle
Hit these buttons 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 (vertical velocity indicator) button and the autopilot will grab and hold your current VVI. The same goes for airspeed and altitude.
If you want the autopilot to guide the aircraft to a new altitude that not yet been reached, 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 indicate airspeed, climbing by holding a constant airspeed is usually most efficient.
Regardless, we’ll start with the vertical speed case.
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. Imagine, though, that you want to climb to 9,000 feet. You would first dial 9,000 into the altitude window. The plane, though, does not go there yet. The next step requires you to choose how you want to get to 9,000 feet.
If you decide to get there via a vertical velocity, hit the V/S button and the plane will capture your current vertical velocity (possibly 0). Then, simply dial the VVI (vertical velocity indicator) up or down to get to 9,000 feet more or less quickly. When you get to 9,000 feet, the autopilot will automatically disengage the vertical speed mode and drop right back into altitude mode at your new altitude.
Now, to get there 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 make the plane pitch the nose up or down to maintain your current indicated airspeed. Now, simply add a dose of power (if needed), causing the nose of the plane to rise 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.
As you can see, both the airspeed and vertical speed modes will be maintained until they reach the specified altitude, at which point they will abandon that mode and go into altitude-hold mode. The same thing will happen with the glideslope control. If the glideslope is armed (that is, lit up after you pushed the button), then the autopilot will abandon your vertical mode when the glideslope 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 glideslope.
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, glideslope, and localizer are all armed, and sit there in standby (armed) until one of the modes above intercepts the altitude, glideslope, 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 s/he gets there (including the initial altitude before s/he takes 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):
- 1. While on the ground, short of the runway, the pilot is told to maintain 3,000 feet. S/he is give a runway heading and is cleared for takeoff.
- 2. The pilot enters 3,000 feet into the ALTITUDE window and a runway heading (for instance, 290) into the HEADING window.
- 3. The pilot takes off.
- 4. In the initial climb, around maybe 500 feet, the pilot sets the flight director to AUTO. The autopilot notes the plane’s current pitch and roll and holds the plane steady.
- 5. The pilot hits the HDG button, and the plane follows the initial runway heading.
- 6. The pilot hits either the V/S, FLCH, or SPD buttons. 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.
- 7. The pilot is given a new heading and altitude by ATC.
- 8. The pilot dials the new heading into the window, dials the new altitude into its window, and then hits V/S, FLCH, or SPD to let the plane zoom to the new altitude.
- 9. The pilot is cleared to the plane’s destination or some other fix. Those coordinates are entered into the GPS and the HSI source is set to GPS (as the autopilot follows the HSI). The pilot hits the LOC button. The autopilot will then follow the HSI needle deflection laterally as it climbs to the new altitude.
Do this, and you can get where you are going.
Pitch Sync with the Pitch Sync Joystick Button
You can assign a joystick button to the pitch sync control. When pressed, this button will make the autopilot match its settings to whatever you are doing as you fly the plane. Then, when you release the pitch-sync joystick button, the autopilot servos will take hold of the yoke and maintain the vertical speed, altitude, airspeed, or pitch that you were just flying.
To assign a button to pitch sync, do the following:
- 1. Move the mouse to the top of the screen, causing the menu to appear.
- 2. Click Settings, then click Joystick & Equipment, as seen in the following image.
- 3. Click the Buttons: Adv tab, as seen in the following image. If the Buttons: Adv tab does not exist and there is only a Buttons tab, X-Plane has not been updated to the latest version (see Updating X-Plane in Chapter 2 for information on fixing this).
- 4. Press the button on the joystick that you would like to assign to pitch sync.
- 5. Click the round button next to autopilot, found near the center of the screen.
- 6. Press the round button next to pitch¬_sync (seen in the following screenshot), found about halfway down the first column of the options that appeared in Step 5.
- 7. Exit the Joystick & Equipment window.
Here’s how the pitch sync works. Imagine you are at 3,000 feet. The flight director is in altitude mode, so the autopilot is holding 3,000 feet for you. You hit the pitch sync joystick button. When you do this, the autopilot servos release control of the yoke and let you fly. You fly to 3,500 feet (with the autopilot still in altitude mode) and let go of the pitch sync joystick button. At that point, the autopilot will hold 3,500 feet, since you were in altitude mode at 3,500 feet at the moment you let go of the pitch sync button.
If you are in vertical speed mode, the autopilot will try to maintain the vertical speed that you had at the moment you released the pitch sync button.
If you are in speed or level change mode, the autopilot will try to maintain the airspeed (by pitching nose up or down) that you had at the moment you released the pitch sync button.
So, when you press the pitch sync joystick button, the autopilot turns the servos off 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 when you released the pitch sync joystick button. The same applies to bank angle. 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.
Note: If the bank angle is less than 7 degrees, the autopilot will just level the wings, as it assumes that you want nose level.
LOC and G/S
These are the options that nobody can figure out, partially because the right frequencies and HSI mode must be selected to use them, and partially because they will do nothing until they capture the approach path they are looking for. For that to happen, some other mode (any of the ones discussed above) must be engaged to do that.
These modes capture an ILS, VOR, or GPS course, so they must obviously be able to fly either NAV 1, NAV 2, or GPS. The autopilot only knows which of these three to use when you tell it which one. This is done with the button labeled "NAV-1 NAV-2 FMC/CDU" (with filename "but_HSI_12GPS" in the HSI folder), which is the HSI source selector.
Note: In some aircraft, this is instead a three-position switch labeled SOURCE, as seen below.
The autopilot will fly whatever course 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 its signal from the GPS). Once you decide, use this button to tell the HSI what to display. The autopilot will then fly to that course.
If you set this button to NAV 1, the HSI will show deflections from the NAV 1 radio, and the autopilot will fly VOR or ILS signals from the NAV 1 radio when you hit the LOC or G/S buttons.
Similarly, if you set this to NAV 2, then the HSI will show deflections from the NAV 2 radio, and the autopilot will fly VOR or ILS signals from the NAV 2 radio when you hit the LOC or G/S buttons.
If you set this switch to FMC/CDU, then 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 when 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.
To repeat: be sure to send the right signal (NAV 1, NAV 2, or GPS) to the autopilot when using the LOC and G/S (lateral and vertical navigation) buttons.
Now let’s discuss how to actually use the LOC and G/S buttons.
The LOC button will immediately begin lateral navigation (navigating to a GPS destination) once engaged. It will, however, 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 will not do anything yet to the plane. Your current heading or wing level mode (if engaged) will remain in force (or you can fly by hand) until the localizer needle starts to move in towards the center. Once that happens, the LOC will suddenly go from armed mode (yellow) to active mode. This causes the autopilot to start flying the plane for you, disengaging any previous modes.
The reason that LOC mode disengages previous modes is that 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. Alternatively, you may simply fly the plane by hand to the localizer (with no autopilot mode on at all) and have the autopilot 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 take over lateral control, the altitude is also armed (always and automatically) and you can fly any vertical speed, airspeed, or pitch (manually or on autopilot) until the altitude is reached, at which point the autopilot will go into altitude hold mode.
Just like the lateral nav (that is, LOC mode), the vertical nav ('G/S mode) will not do anything until the glideslope needle starts to move. Unlike with the localizer, though, the G/S mode won't do anything until the glideslope needle goes all the way through the center position. It does this because you typically have the airplane on altitude hold until you intercept the glideslope, at which point the plane should stop holding altitude and start descending 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.
Let’s now put the LOC and G/S settings into use to fly an ILS.
Flying an ILS Using LOC and G/S
To fly an ILS, do the following while still far away from the ILS and below glideslope:
- 1. Hit the altitude button to hold the current altitude.
- 2. Enter a heading in the heading window to be followed until you intercept the ILS.
- 3. Hit the heading button to hold that heading.
- 4. Hit the LOC button. It will go to “armed” (yellow).
- 5. Hit the G/S button. It will also go to “armed” (yellow).
- 6. As soon as you intercept the localizer, the LOC button will go from yellow to green, abandoning the heading mode to instead fly the localizer.
- 7. As soon as you intercept the center of the glideslope, the G/S button will go from yellow to green, abandoning the altitude hold mode to instead fly the glideslope.
- 8. The autopilot will track you right down to the runway, and even flare at the end, cutting power if auto-throttle is engaged.
Just as in a real airplane, these things only work well if you:
- intercept the localizer far away (outside of the outer marker) and below the glideslope,
- intercept the localizer at less than a 30° angle, and
- hold altitude when you intercept the glideslope.
If you come in above the glideslope, cross the localizer at a wide angle, or intercept the localizer too close to the airport, the autopilot will not be able to maneuver the airplane for landing (again, just as in a real plane).
Now that we’ve detailed flying with the autopilot, let's talk about flying an FMS (flight management system) plan.
Flying an FMS Plan
To fly a flight management system plan, a few things must happen:
- a) You must enter your entire flight plan into the FMS.
- b) You have to have the HSI set to GPS, not NAV 1 or NAV 2 (because the autopilot will fly whatever it sees on the HSI).
- c) You must have the LOC button selected ON since that button causes the autopilot to follow the localizer (or whatever is on the HSI).
- d) You must have the FLIGHT DIR switch set to AUTO, so that the servos are running.
- e) You must hit the VNAV button 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, that the plane you are flying has all this equipment (which of course some do not).
To demonstrate the use of an FMS, we’ll go through the procedure in a typical aircraft (a Boeing 777). The steps will be similar in any aircraft.
- 1. Open up the Boeing 777 using the Open Aircraft dialog box (see Chapter 4). It is found in the Heavy Metal aircraft folder.
- 2. The FMS is found on the right half of the screen, near the middle of the panel (it should be displaying the text “PLAN SEGMENT 01”). Hit the INIT button (as shown in the following image) on the FMS. This gets the FMS ready to receive a flight plan.
- 3. Now hit the AIRP button (shown in the following screenshot), telling the FMS that you are about to go to an airport.
- 4. Now enter the ID of the destination airport by hitting the keypad keys with the mouse. For instance, in the screenshot below, we’re starting at San Diego International Airport (KSAN) and we’re flying to San Bernardino International (KSBD).
- Remember that more information on airport IDs can be found in Chapter 4.
- 5. If you like, hit the line-select button on the left side of the FMS next to the text "FLY AT ______ FT" (seen in the following screenshot) and enter the altitude you want to fly at using the keypad.
- 6. 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, as well as VOR, NDB, FIX, and LAT/LON buttons to enter those types of destinations. The PREV and NEXT buttons will cycle through the various waypoints in your plan, and the LD and SA buttons will load or save flight plans if you would like to use them again.
- 7. Once you have entered the plan into the FMS, take off and set the SOURCE switch for the HSI (found near the left edge of the panel) to GPS so that the HSI is getting data from the GPS (rather than the NAV 1 or NAV 2 radios).
- 8. Move the FLIGHT DIR switch to AUTO so the autopilot servos are actually running, and hit the LOC autopilot button (at the top of the panel, shown in the following screenshot) to follow the HSI lateral guidance (which was just set to get data from the GPS), with the servos actively flying the plane.
- If you entered an altitude into the FMS, you’ll also need to hit the VNAV autopilot button to track the entered altitude.
- 9. Sit back and let the autopilot take you to your destination.
Turning the Autopilot Off
Now, to turn off an autopilot mode, simply hit the currently selected mode button once again. When that mode is turned off, the autopilot will revert to the default mode that was selected when the autopilot is first turned on—pitch and roll hold modes.
To turn the autopilot off altogether, simply turn the FLIGHT DIR switch to OFF. Alternatively, hit the ‘!’ key on the keyboard or assign a joystick button to turn it off in the Joystick & Equipment screen of X-Plane.
Flying on Instruments
Though for a long time considered impossible in aviation circles, the ability to fly an aircraft through a large cloud or fog bank relying completely on the aircraft’s instruments was made possible in the 1920s. Before then, nearly everyone that attempted this had become just another part of the wreckage, smoldering in a field. Now it is commonplace for even relatively inexperienced pilots to fly long distances in clouds. An instrument rating only requires 125 hours total flight time—although it would certainly not be wise for a 130- or 140-hour pilot to attempt an instrument approach in a 200 ft overcast with 1/2 mile visibility or to take off on a foggy day. Modern gyroscope-based instrumentation and continual training make it possible to safely fly with reference to only the instrument panel.
The Inner Ear and Your Sense of Balance
To begin a discussion on instrument flight, we must first discuss why it is so difficult. It isn’t that the principles behind flying on instruments are so difficult or that interpreting what the instruments are telling you is that difficult. Rather, the difficulty lies in believing what the instruments are saying. Your body had developed a system of balance and equilibrium that has evolved in humans over millions of years, and forcing your brain to ignore these signals and to believe what the instruments are telling you is very difficult. To put it bluntly, in a real aircraft, your life depends on ignoring your feelings and senses and flying based solely on the information in front of you.
This is why it’s so difficult. Your sense of balance comes from three sources within your body. These are, in order of prerogative, your inner ear, your eyes, and your sense of touch and even hearing. You should remember from high school that your inner ear is a series of semi-circular canals that are filled with fluid. They are positioned in your head in different planes and each is lined with thousands of small hairs. The root of each hair is connected to your nervous system. As your body changes position in space, the fluid is moved due to momentum. The resulting bending of these hairs feeds your brain signals that indicate the orientation of your head in space. This information is continually updated and corrected by what your eyes are sending your brain as well as by your sense of touch. While standing stationary on the ground, your ears tell you that your head is positioned vertically and not moving, your eyes tell you that the ground is stationary beneath your feet, and the skin on the bottom of your feet tells you that it is standing on the ground. All of these inputs align to say the same thing—that you're standing on the ground.
One limitation to your sense of balance is seen when you are accelerating very slowly, or when you accelerated briefly and have now stopped. Think of a post on a playground that stands vertically in the sand with a seat affixed to it a couple feet from the ground. It can be extremely disorienting to sit on the seat, close your eyes, and then have someone spin you at a constant rate. It doesn't matter if you're being spun to the left or the right—what is critical is that you are quickly accelerated and then kept at a constant angular velocity. When you first begin to spin, your inner ear will detect that you are accelerating and spinning. Before long, however, the fluid in your ears will stop moving, since you are no longer accelerating but rather just spinning. Stay like this for a few seconds and it will fell like you're just sitting stationary. You may still feel a breeze on your face or hear sounds “spinning” about you, but your inner ear will be telling your brain that you're sitting stationary and your brain will believe it. Now if you’re suddenly stopped, you will instantly feel an incredible sense of angular acceleration in the opposite direction, like you are being spun wildly the other way. Open your eyes and they will tell your brain that you are stationary, but the feeling within your head (a primal, driving sensation) is that you have just started to spin. In scientific circles, this is called “vertigo,” but the sensation is commonly referred to as being dizzy.
The same thing can happen in a cockpit pretty quickly. Imagine for a moment that there is a large bank of clouds in front of you on a calm day. With a few passengers on board you can enter the cloud in a left bank of, say, 20 degrees. Then, after entering the cloud very slowly and very smoothly, you start to bank the aircraft to the right. If you do this slowly and smoothly enough, no one on board will notice. Before you come out of the cloud, you get to a substantially different attitude (perhaps banked 30 degrees right). The unsuspecting passengers may feel the very beginning of the change in bank, but they will probably suspect you're banked to the left. When you suddenly fly through the other end of the cloud, BAM! They're in a right hand turn. While this was fun and harmless to do to unsuspecting friends in college, it underlines the difficulty that unsuspecting pilots can find themselves in if they are not careful.
Gyroscopes and Their Application to Flight
The gyroscope was invented many decades before aircraft, but its tremendous implications for flying were not realized until the mid- to late 1920s. The basic principal that they work on is that if you take a relatively heavy object and rotate it at a high rotational velocity it will hold its position in space. You can then mount this stable, rigid gyroscope in an instrument that is fixed to your aircraft and measure the relative motion of the instrument case (and thus the airplane) about the fixed gyro. The gyroscope is physically attached to an indicator of some sort, and these indicators then relay critical information to the pilot concerning the aircraft’s attitude (that is, its orientation relative to the horizon). There are three primary gyroscopic instruments in the panel. They are:
- the attitude indicator (or AI—normally driven by a vacuum pump on the engine),
- the turn coordinator (or TC—typically electronically driven), and
- the directional gyro (or DG—typically vacuum powered, though possibly electric).
The AI indicates what attitude the aircraft is flying at—how far the nose is above or below the horizon and simultaneously how far the wings are banked and in which direction. The TC indicates the rate of turn—that is, how steep or shallow your bank is in relation to a standard 2 minute turn rate, and the DG is nothing more than a gyroscopically driven compass that is more stable and accurate than the old standby, the magnetic (or “whisky”) compass.
The Six Primary Flight Instruments
There are six primary instruments that have become standard in any instrument panel. Since the early 1970s, these have been arranged in a standard layout referred to as “the six pack.” They are laid out in two rows of three instruments each. The top row, from left to right, contains the airspeed indicator (ASI), the attitude indicator (AI) and the altimeter (ALT). The bottom row contains the turn coordinator (TC) the directional gyro (DG) and the vertical speed indicator (VSI). A summary of these instruments follows.
The “standard six” are shown in the following image, taken from the Cessna 172 cockpit.
The Airspeed Indicator (ASI)
The airspeed indicator (labeled 1 in the image above) shows the speed at which the aircraft is traveling through the air. In its simplest form, it is nothing more than a spring which opposes the force of the air blowing in the front of a tube attached under the wing or to the nose of the aircraft. The faster the airplane is moving the stronger the air pressure is that acts to oppose the spring and the larger the deflection of the needle from which the pilot reads the craft’s speed. Obviously, it's quite a bit more complicated than this, as the pressure exerted by the stream of air varies with the local air density (which continually changes as the airplane climbs or descends), and the ASI must account for this.
The Attitude Indicator (AI)
The attitude indicator (labeled 2 in the previous image) informs the pilot of his or her position in space relative to the horizon. This is accomplished by fixing the case of the instrument to the aircraft and measuring the displacement of the case with reference to a fixed gyroscope inside.
The Altimeter (ALT)
The altimeter (labeled 3 in the previous screenshot) looks somewhat like the face of a clock and serves to display altitude. This is measured by the expansion or contraction of a fixed amount of air acting on a set of springs. As the airplane climbs or descends, the relative air pressure outside the aircraft changes and the altimeter reports the difference between the outside air pressure and a reference, contained in a set of airtight bellows.
The Turn Coordinator (TC)
The turn coordinator (labeled 4 in the previous screenshot) measures the rate of turn for the aircraft. The instrument is only accurate when the turn is coordinated—that is, when the airplane is not skidding or slipping through the turn. A skid is the aeronautical equivalent to a car that is understeering, where the front wheels do not have enough traction to overcome the car's momentum and the front of the car is thus plowing through the turn. In a car, this results in a turn radius that is larger than that commanded by the driver. A slip is a bit more difficult to imagine unless you're a pilot already. It results from an aircraft that is banked too steeply for the rate of turn selected. To correct the slip, all the pilot has to do is increase back pressure on the yoke, pulling the airplane 'up' into a tighter turn, such that the turn rate is in equilibrium with the bank angle.
The Directional Gyro (DG)
The directional gyro (labeled 5 in the screenshot on the previous page) is a simple instrument that points north and thus allows the pilot to tell which way she or he is flying.
The Vertical Speed Indicator (VSI)
The vertical speed indicator (labeled 6 in the previous image) reports the craft’s climb or descent rate in feet per minute. Typically, non-pressurized airplanes will climb comfortably at about 700 fpm (if the plane is capable) and descend at about 500 fpm. Descent rates faster than this cause discomfort on the occupants which is felt in passengers’ ears. Pressurized airplanes can climb and descend much more rapidly and still maintain the cabin rate of change at about these levels, since the cabin altitude is not related to the ambient altitude unless the pressurization system fails.