On Autopilot

Autopilot Directing

September 1, 2000

Who's flying—you or the autopilot?

Who's flying? You or the autopilot? The question may sound a mite daft to any pilot who has spent any time at all flying with a basic autopilot. Why, of course you know who's flying. If you've turned on the unit and given it a task—say, follow a heading or track a navigation course—then the autopilot's doing the flying. But when it comes to autopilots with flight directors, neophytes to flight-director flying can have a tough time understanding this fancy new set of equipment. Based on my experience, most problems come from the neophyte's believing that the autopilot must be engaged in order for the flight director to work. And it just ain't so.

Flight-director flying

Simply put, flight directors tell you where to go. They take the commands you've selected on your autopilot or flight control mode selector panel, then translate them into a graphic display for you to use. They do this by means of command bars or cross-pointers. Command bars are flat, inverted-V-shaped guidance cues that appear on the attitude indicator. On larger airplanes, some avionics manufacturers offer a choice between V-bars or cross pointers, which look rather like the crosshairs of a telescopic gunsight. This kind of setup has a switch that lets you select either the V-bars or the cross-pointers. It's strictly a personal choice as to which type of guidance symbology you prefer.

Hand-flying the flight director

Yes, you can hand-fly a flight director's V-bar or cross-pointer guidance cues. In the installation I'm most familiar with—Honeywell's Bendix/King KFC 200—you simply push the FD (flight director), HDG (heading), NAV (navigation), APCH (approach), or any other mode selector button on the autopilot control panel and the command bars (in this case, V-bars only) pop up, ready for action. The autopilot's On-Off switch remains in the Off position.

Now you'll be getting cues from the flight director, but you—not the autopilot—will have to do the flying.

Let's say you want to follow a heading. Push the HDG button, watch the HDG light illuminate on the autopilot annunciator panel to confirm its selection, then rotate the heading bug on the heading indicator to the heading you want. You'll see the V-bars bank off in the direction you should fly.

How do you follow the command bars' dictates? By manipulating the flight controls so as to make the attitude indicator's symbolic airplane snug up into the notch in that inverted V. The symbolic airplane's "nose" should be right in the apex of the V. With cross-pointers, you put the "nose" of the symbolic airplane right on the intersection of the crosshairs.

The same methods hold true for flying modes other than HDG. In the NAV mode, the command bars will steer you so as to maintain the course you've selected. If your airplane is equipped with altitude preselect, then the command bars will move up or down to climb or descend you to the altitude you've selected—at the rate of climb or descent you've selected. All you have to do is keep the symbolic airplane in the command bars' notch (or cross-pointers' crosshairs) and you'll fly like the smoothest pro around. In the altitude hold (ALT) mode, the command bars will rise and fall in turbulence, telling you what pitch attitude you need to maintain the altitude you've selected. In moderate to severe turbulence, it's a good idea to disengage the altitude hold mode and ride out the bumps. If on an IFR flight plan, asking air traffic control for an altitude block will let you remain legal as you rise and fall with the punches.

With ILS approaches, command bars will take you unerringly down the localizer and glideslope. Just follow your cues, and the needles on your HSI or separate nav head needles will stay bolted to dead center. As for nonprecision approaches, there will not be any vertical guidance. For that, you can calculate and set up your own descent rate.

CWS/TCS

Those acronyms stand for Control Wheel Steering or Touch Control Steering, as it's called by some avionics manufacturers. When you're hand-flying the flight director, you can momentarily push the CWS/TCS button (usually located on the top of the left horn of the control yoke) to reinitialize the command bars to your current attitude—as represented by the attitude indicator's symbolic airplane.

Let's say you're hand-flying and are given a clearance to descend. You push on the control yoke, the symbolic airplane in the attitude indicator moves down several degrees, but the command bars remain up where they used to be. To synch up the command bars with your new descent attitude, you simply push once on the CWS/TCS button and the command bars drop down to match your descent angle. This feature can also be used for crude descent guidance during nonprecision approaches.

Any other selected flight guidance commands—NAV, HDG, and so on—will continue to be issued by the command bars. But now the vertical guidance is up to you.

CWS/TCS can also used when the autopilot is flying. In this application, holding down the CWS/TCS button lets you momentarily disengage autopilot commands while you manually steer around that buildup, for example. Release the button, and you're right back to autopilot flying—without the need to go through the whole rigamarole of turning off the autopilot, resetting the flight control modes, and re-engaging the autopilot.

Go-around (GA)

Airplanes with flight directors are also equipped with go-around (GA) buttons. This button is usually on the left side of the leftmost throttle or thrust lever handle. It's intended for use during go-arounds or missed approaches, as the name suggests.

Push the go-around button and the command bars will rise to the pitch attitude the airplane's manufacturer has calculated is necessary for a best rate of climb at gross weight. Gone down to decision height and still no field in sight? Apply full power, hit the GA button and climb away at the attitude indicated by the V-bars. This feature makes a critical maneuver a little less harrowing.

The GA feature also comes into play during takeoffs. After lining up for takeoff, hit the GA button and you instantly have a pitch cue for a straight-ahead climb. If your airplane has a flight director, you should use this capability on every takeoff—especially those that involve launching into instrument meteorological conditions.

I don't want command bars!

Command bars—be they V-bars or cross-pointers—can be confusing at times. Sometimes pilots want a simple look at the attitude indicator, one not cluttered up with command bars. Hitting the FD button ought to do the trick. In the KFC 200, this switch toggles the command bars on and off.

Testing, testing

Just as you perform a preflight inspection of other parts of the airplane, a preflight test of the flight director also is a must. Hit the FD button to call up the command bars, synchronize your heading bug with your current heading, and then hit HDG. Now rotate the heading bug left or right. The command bars should move in the correct direction. In addition, make sure that no heading flags are in the HSI, that the compass card turns in the correct direction during taxi turns, and that your compass slaving system indicator is giving the correct indications (a deflection during a turn, centering of the needle once the turn is completed).

Autopilot flying

Now for the easy part. Let's say you're tired of hand-flying the flight director and want George to do the work. Make sure you've selected the functions you want, then turn on the autopilot switch. Now you're off the hook and your autopilot is following the flight director's commands.

In the navigation and approach modes, it's important to maneuver the airplane close to the radials, localizers, courses, or bearings you're interested in intercepting. Your intercept heading should be around 30 degrees to the course. This lets the autopilot do a smooth capture of the track you're interested in following. Before locking on to the desired course, you'll see an "Armed" annunciation on the annunciator panel. After the course is captured and your autopilot starts tracking, you'll see a "Captured" annunciation.

A common routine

Most pilots who do a lot of instrument flying with their flight directors use the following routine:

  • Use the GA function for takeoff
  • Fly the flight director until reaching 1,000 to 2,000 feet agl
  • Call up the modes required for the departure procedure
  • Engage the autopilot for the rest of the departure procedure and the en route segments of the flight
  • Fly the flight director on approach or, if the weather is low IFR, let the autopilot fly the command bars to just above minimums (let's face it, in the approach mode autopilots can fly better than most of us).

I don't want the autopilot!

Turning off the autopilot was an earlier topic in this "On Autopilot" series, and I'd refer you to that article ("On Autopilot: Autopilot Turnoffs," June Pilot) for the many ways you can shut down the autopilot. The simplest way, however, is to push the red autopilot disconnect button, also located at the top of the leftmost horn of the control yoke. This shuts off the autopilot and stows the command bars in an instant. Now you really are hand-flying.

Ups and downs

The upside of having a flight director is that it can make your flying more precise and reduce your workload at busy times. The downside is that misunderstood, the flight director can lead to confusion and a lot of head-down time trying to learn the system on your own.

Here's where you can benefit from some quality instruction from an instructor experienced in autopilots, flight directors, and flight control systems (which is what the autopilot/flight director combination really is). Unfortunately, there are few of those. Perhaps this article will encourage those of you thinking of moving up to a flight control system to seek out more detailed advice from pilots who regularly use them—and hopefully an in-flight demonstration.


Links to additional information on autopilots can be found on AOPA Online ( www.aopa.org/pilot/links/links0009.shtml). E-mail the author at tom.horne@aopa.org.