Flight Directors

FLIGHT DIRECTORS

Purpose of this tutorial

To give the computer flight simmer a general idea of what Flight Directors are, what they look like, and how they operate.

So, just what is a Flight Director?

Well, it won't fly your airplane for you- that's the job of the autopilot. It won't plan your trip for you- that's your job. It won't navigate for you- once again, that's the job of the autopilot.

So just what does it do? Watch this. Basically a Flight Director gives visual cues to the pilot, so that the pilot can follow these cues with his inputs to the aircraft's controls and therefore take the airplane to the programmed destination.

Huh?

Let me try that again. The Flight Director displays visual cues on the Attitude Indicator, the AI. If you, the pilot, follow those visual cues by turning or pitching the aircraft as "directed", you will end up at your destination, be that the end of a runway, a VOR, a waypoint etc. In other words, the Flight Director directs.

Aha! Now the name Flight Director starts to make some sense.

The benefits of this type display are multiple. In a conventional panel layout, without a Flight Director, the pilot must refer back and forth between the Attitude Indicator (AI), and the Horizontal Situation Indicator (HSI). You, the pilot, watch the AI for the status of your airplane, turning, climbing, descending etc. In addition to that, you have to refer to the HSI for your navigational status, left or right of course, high or low on the Glideslope. The Flight Director resolves this referring back and forth between the two instruments by displaying the navigational information from the HSI on the AI in the form of visual cues.

In addition, the visual cues that are displayed give computed information from the autopilot, rather than raw data.

What do Flight Directors look like?

As with so many things in aviation, the Flight Director display varies from one manufacturer to another. You would think that something that is possibly this critical would have a standard. Alas, this is not the case.

First off, here's a standard Attitude Indicator:

The Flight Director "cross hair" display

This type of display uses the conventional vertical and horizontal indicators to show the commands. Just as when flying an ILS approach, if the "needle" goes up, the pilot raises the nose of the aircraft until the "miniature airplane" on the Attitude Indicator rests on top of the horizontal "needle". If the "needle" goes left or right, the pilot turns the aircraft until the "miniature airplane" on the Attitude Indicator rests on top of the vertical "needle".

In the above display the pilot has satisfied the horizontal cue, but must turn left until the vertical cue centers on the Attitude Indicator.

In the above display the pilot must pitch up and turn right until both the horizontal and vertical cues center on the Attitude Indicator.

You can see that familiarity with the conventional ILS display would make this type of Flight Director display attractive to some operators.

The "inverted V" display

The black "inverted V" represents the visual cue from the Flight Director. The orange "V" represents the "miniature airplane" on the Attitude Indicator. In this situation the Flight Director cues have been satisfied. If you, as the pilot, keep the orange "V" tucked up into the black "V" you will always satisfy the cues from the Flight Director.

In the above display the pilot must turn left until the orange "V" tucks up into the black "V" on the Attitude Indicator. There is no need to go "up/down" as the black command bar is centered. Were it required for the pilot to go "up/down" the black command bar would be displaced appropriately.

In the above display the pilot has turned the aircraft left and has satisfied the visual cue from the Flight Director. As the course, be it a Localizer, VOR radial, GPS track etc. is intercepted the Flight Director will give visual cues to slowly start rolling out of the left bank and return to level flight. If the pilot keeps his orange "miniature airplane" tucked up into the black Flight Director "V" he will roll out on course.

CRT, or "Glass Cockpit" displays

Once again, these can be displayed in a variety of ways. Unfortunately, I have no experience with them, so I cannot address the issue at this time. As X-Plane evolves with CRT Flight Directors I plan to come back to this tutorial and expand this coverage.

OK, that's how they look. How do they function?

Look at it this way: when you are flying, say an ILS, the information that is being displayed on your HSI is considered to be "raw data". If, for instance, the ILS needle goes to the left, you know that you have to turn left to re intercept the course. But, how much of a turn left? Ten degrees? Twenty? You really have no idea. The Flight Director displays "computed data" from the autopilot on your primary situational instrument: the Attitude Indicator. It "knows" that, for example, a 15 degree bank to the left is the correct angle to re intercept the course. As the needle starts to return to the center, the Flight Director "senses" this, and will give the correct visual cue to roll out to the correct heading to maintain course. The same applies to the vertical component, the glideslope.

Some Flight Directors give other visual cues, such as a "rising runway" that will appear from the bottom of the Attitude Indicator as the ILS approach nears its conclusion. Some even show a visual passage of a VOR station.

For all intents and purposes the Flight Director gives a visual presentation of what the autopilot is doing in the background.

The Flight Director may be used in the "hand flying" mode, in which you follow the visual cues as appropriate to the function selected in the autopilot, I.E. "Altitude Hold", or "Nav.", or "Heading".

If the Flight Director is turned on, it will give cues based on the mode that the autopilot is in.

When you are running with the autopilot engaged, the Flight Director is just along for the ride. The display will almost always show that the cues are satisfied. The exceptions usually being during an initial course intercept, or while the autopilot is seeking the correct heading to compensate for wind drift.

When in the autopilot mode, the Flight Director may be turned "On" or "Off". When turned "OFF", the Flight Director display will drop out of view on the Attitude Indicator. It's your call.

The combination of an autopilot and a Flight Director is usually referred to as an "IFCS", an Integrated Flight Control System.

Conclusion

The only Flight Directors that I have personally used were in the Cessna 421B's that I operated while I was a corporate pilot. I tried them out a couple of times, but as a matter of personal preference, I usually ended up turning them off and using the raw data displays. They worked just fine, it's just that my comfort level was in using "raw data" from the HSI display. As I said above, it's your call.

I hope that this tutorial has helped you understand how the Flight Director functions.

Have fun flying.

Thank you.

This narrative, along with aditional content, is available as a CD or an eBook.

For CD information click here. For eBook information click here.

Hal Stoen

revised: 1/06/2006

12/28/2002: Correction made in graphic reference. Thanks to Fabrice Stijnen for pointing this error out.

1/18/06: Correction made in graphic. Thanks to Barry Troy for pointing out this error.

4/05/2011: Correction made in graphic. Thanks to Enis Mabrouk for pointing out this error.

click to return to the index