The evolution of cockpit instruments stems from the desire to make flying simpler and easier; a flight with a less taxing task load for the pilot is a safer flight. So what is this instrument called a flight director, and how does it ease the pilot's workload?
Why use the flight director?
The essential task of the flight director is to direct the pilot with visual cues. The pilot follows the visual cues with control inputs to take the aircraft to the destination set in the autopilot. When the auto pilot is not engaged, the pilot uses the flight director to manually fly the aircraft. When the autopilot is engaged, the flight director shows the pilot what the autopilot is doing. Whether the autopilot is engaged or disengaged, the flight director is driven by the same values that drive the autopilot.
The flight director reduces the work load by displaying directions from a horizontal situation indicator (HSI), located on the attitude indicator. The HSI allows the pilot to get course, glide slope, and attitude information from just one instrument. Without a flight director, a pilot is required to include the attitude indicator (AI) and a navigation (nav) instrument in their normal instrument scan. The nav instruments provide course and glide slope information and the AI provides information about changes in turns and pitch.
The flight director computes how much bank and pitch are required to maintain the course set by the pilot. It takes the raw data provided by the HSI, which calculates whether the aircraft is right or left of course, and above or below glide slope. The flight director transposes this data onto the AI to indicate the correct pitch and bank that will keep the aircraft on its programmed course.
Types of flight directors
The various types of flight directors all have some kind of representation on the AI (like the inverted "V" shown below). All three images below show the AI with the flight director engaged. When the flight director is engaged, yellow or magenta command bars appear to indicate the correct pitch or bank that will align the aircraft with its programmed path.
|Standard flight director (Beechcraft Baron).||Glass cockpit flight director (Beechcraft King Air 350).|
|Glass cockpit flight director (Bombardier Learjet 45).|
Engaging the flight director
The flight director is engaged by pressing the flight director button on the panel or radio stack (use the pointer to click the button in Flight Simulator).
Flight director/autopilot panel (Bombardier Learjet 45).
When the flight director button is pressed, the command bars will appear on the face of the attitude indicator.
The magenta, flight director command bars indicate the aircraft is off course.
In the example immediately above, the flight director in the Bombardier Learjet 45 is engaged and the heading bug is set to 318 degrees. The pilot has also set a desired vertical speed of 2,000 feet per minute (fpm). The aircraft representation on the AI indicates the aircraft is in a slight right turn. The magenta flight director command bars indicate that the pilot should turn left to intercept the correct heading. The command bars also indicate that the pitch is correct for the programmed vertical speed. When the pilot acquires the correct pitch and bank, the flight director display indicates that the aircraft is on course, as shown in the image below:
The command bars indicate the aircraft is on course.
Autopilot and the flight director
With the autopilot engaged and the flight director on, the flight director will display how the autopilot is flying the airplane. The flight director's indications can deviate from this when the autopilot is intercepting a new course or if it is compensating for wind drift.
With this in mind, remember that while the flight director is most useful when manually flying an aircraft, it can also be a good cross-reference to ensure the autopilot is tracking correctly.
Flight Simulator aircraft with flight directors: