The Wonder of a Flight Director

One of the least appreciated or commented on features of the new glass primary flight displays (PFD) that are the norm in all new airplanes is the flight director. The vertical tape displays of altitude and airspeed, the full width artificial horizon line and other new information on thePFD have been talked and written about endlessly, but the flight director is largely ignored. I guess the reason the flight director gets so little attention or respect is that the light airplane instructor corps never really embraced the technology when it became available in piston airplanes about 30 years ago. Because the instructors didn’t understand or appreciate the utility of the flight director, new instrument pilots didn’t learn to use it. Thousands of piston airplanes are flying with a flight director-and nearly all new airplanes will have them as standard-but few pilots are actively using them. As the name implies, the flight director guides you along a selected path through the sky. By observing this single instrument you can maintain a precise path through space with a greatly reduced need to scan the other instruments. The roots of the flight director go back at least 50 years, when the leading avionics and instrument companies worked to develop technology that would allow pilots to fly very accurate ILS approaches every time. The twin goals were to improve safety by giving pilots a tool to precisely follow the ILS signal, and then to use that extra precision and predictability to reduce ILS approach minimums. Early attempts at flight directors concentrated on the navigation display. One of the first was the Zero Reader from Sperry. It was a navigation display where, as the name implies, the needle stayed centered – zeroed, in other words – if the pilot was on center line or correcting to the center line. The Zero Reader was a big improvement over the display of raw localizer and glideslope data because the Zero Reader showed the pilot that his path was correct, or correcting, instead of simply showing a deviation from center line. The Zero Reader laid the foundation for all flight directors in that it commanded only change to correct for an error, and issued no commands when the flight path was on target. I don’t know what company was first to move the flight director commands from the navigation indicator to the attitude indicator, but Collins and Sperry were certainly among the leaders. And overlaying the commands on the attitude indicator was the crucial step. With the flight director commands superimposed over the attitude indicator, a pilot can look at a single instrument and see present attitude plus commands of how to maneuver to fly the desired flight path. You could call it a video game – and it is now with a glass PFD – but for decades flight directors were entirely mechanical, though the effect is the same as playing a video game. Early flight directors had split command bars with a horizontal indicator moving up and down to command pitch, and a vertical needle moving left and right to command turns. With this split-cue flight director you pitched the nose up or down until the horizontal needle centered, and you banked left or right until the vertical needle centered. I find it easy to fly a split-cue flight director once established on an ILS approach when attitude changes are small, but it is not entirely intuitive when making large heading changes, for example, to intercept the course. The people at Collins Radio recognized this shortcoming in the split-cue flight director and, in a stroke of brilliance and excellent electrical and mechanical engineering, created the single-cue, or V-bar, flight director. On the Collins flight director, V-shaped command bars moved up and down and banked right and left to command maneuvers in a totally natural and intuitive way. To follow the commands you simply maneuver the airplane so that the little delta-shaped airplane symbol on the attitude indicator snuggles into its place in the command bars. Any teenage gamer could do it perfectly the first time. The V-bar presentation is so favored by pilots that you almost never see a split-cue flight director anymore. Some PFDs have the ability to allow pilots to switch between the two formats, and a few pilots, particularly those with military backgrounds from the 1960s and ’70s, still prefer the split-cue. But the discussion is really over and the V-bar has won. The reliability and precision of the flight director allowed the FAA to approve Category II ILS approach minimums. The flight director computer had the precision to follow the ILS perfectly, and pilots quickly had the ability to follow the command bars with great reliability, so that decision height on the ILS could be brought down to as low as 100 feet above the runway. The industry and FAA had more confidence in the reliability of a human pilot following flight director commands than it did in the dependability of autopilots, so early Cat II approaches were approved for hand flying following flight director commands. As electronic technology improved the flight director and autopilot were integrated into a single system, so approaches could be hand flown following the flight director or the autopilot could manipulate the controls. It is this integrated system that first appeared in piston airplanes in the 1970s. Several companies, including King Radio, Bendix and Mitchell, offered integrated flight director autopilot systems for piston airplanes, but the King KFC 200 quickly dominated the market. The KFC 200, with its companion HSI compass system, was priced attractively for high-performance singles and piston twins and brought nearly all the flight guidance capability of jets down to pistons. But the flight director portion of the KFC 200 was little used. Pilots either turned on the autopilot – which automatically brought the flight director into view – or they punched off both the flight director and autopilot. The potential of the flight director was largely squandered. So how can the piston pilot put his flight director to work? By flying like jet pilots do.

In a jet, the flight director is used in every phase of flight. It starts with takeoff where go-around mode is selected to show you a target pitch attitude for rotation. After takeoff the flight director guides you on the assigned heading. As you near the assigned altitude, the flight director commands a level off. And on approach the flight director guides you through the final course intercept and commands corrections to stay on the center line. Of course, the autopilot can do all of that, and it should be used much of the time, particularly by single pilots flying IFR. But even when the autopilot is engaged the flight director is important because it shows you what the system is thinking. For example, if the flight director command bars show an attitude change that the autopilot doesn’t follow, you know there is a problem in the system. The flight director is the brains of the system, and the autopilot is the muscle. If the muscles are not following the brain’s commands you need to punch off the autopilot and figure out what’s wrong. I look at the flight director all of the time in any airplane I fly that has one. Even in good VFR conditions a quick glance at the flight director tells me I’m on assigned heading and altitude without looking at either the compass or altimeter. And on approach I usually click off the autopilot while still above minimums and follow the flight director commands for the final several hundred feet of descent. In this way the airplane is trimmed exactly as I want it and I’m ready to complete the landing without giving up any precision in following the flight director approach guidance. I hope that with the FAA’s emphasis on technically advanced aircraft (TAA) the flight director and its value will not be forgotten, and piston pilots will be taught to use it as thoroughly as pilots do in jets. The flight director in the complete Garmin G-1000 system is at the jet level of capability and is available in Bonanzas, Barons and Columbias, and soon will be in Cessna singles and Mooneys. And the combined S-Tec and Avidyne Entegra flight director system in Pipers and Cirrus airplanes is worthy of regular use. When you buy a glass system, the flight director will be there, and you should learn to use what you have already paid for.


New to Flying?


Already have an account?