Flight Director System

in progress (with fancy photos)


The flight director system is part of the navigation and control system. The navigation system consists of the flight director system for manual control of the airplane and the autopilot for automatic control of the airplane. The flight director can be used also to monitor correct maneuvers of the autopilot. The instruments utilized in the flight director system are two horizon flight director indicators (HDI’s) and two course deviation indicators (CDI’s). The components used to organize and control the navigation and steering command information are a flight director roll computer and a flight director pitch computer, two integrated instrument system racks (IIS), a flight director altitude control (altitude sensor) the rate of turn rack, transfer relays, control switches and two flight director mode annunciator panels, one on the captain’s and the other on the copilot’s panel.

Course Deviation Indicator (CDI)

The course deviation indicator (COI) is a panel mounted instrument which presents a plan view or map-like display of the navigation situation of the airplane position relative to the compass
heading and radio beams (VOR, LOC, GS). The COI contains the following components: A course select knob on the lower left corner of the instrument is used for selection of a desired
radial under VOR-operation and to provide the flight director system and the autopilot system with heading displacement signals, radial heading under VOR operation and runway heading under LOC operation. This knob locates the selected course cursor on the compass card, providing a visual indication for selecting course, rotates the course deviation bar and course deviation scale which are perpendicular at all times, rotates the to-from flag in line with the course deviation bar sets the selected course in the selected course window to provide a digital read-out of the course and emits a course signal to the flight director computer. These units rotate with the compass card as the airplane heading changes. If the course select knob is in the emergency position (knob pulled out from case front) the course deviation components of the system are usable in the event of compass system malfunction by this way, that the vertical deviation bar can be positioned
parallel to the instrument vertical axis without disturbing selected course information. When the course knob is in the emergency position, a warning flag will appear in the 7 o’clock position.
A heading knob marked HDG on the lower right corner of the CDI is provided for selection of desired compass heading. Difference between the selected heading value and actual heading is  given as a displacement signal in the flight director computer and autopilot computer for heading mode operation. Rotating the heading knob moves the heading cursor, a triangular shaped marker around the compass card to the desired heading. Once set, the heading cursor moves with the compass card and provides a continuous display of the selected heading.

NOTE: FD cannot be used in any mode, if the course selector knob is in the emergency
position or the compass card operation is defective. AP can be used only in MAN
mode if selected compass system is inoperative.

A compass card, receiving information from the selected compass system, is in the share of a ring on the instrument face. This  card has a 20 graduation from 0° to 360° with longer 100 graduation marks on the inner edge of the dial. The airplane compass heading can be read on this card, with respect to the lubber line. A compass power failure warning flag (1 o’clock position) bearing the word COMPASS will appear if compass system power fails o’r if the servo amplifier power supply is defective or if the instrument transformer (TRANS NO. 1 or NO.2, P5) fails.  A course deviation bar in angular direction aligned with the course cursor will indicate airplane deviation from radio beam center against the course deviation pointer scale which is always perpendicular to the bar. Each dot on the scale represents a course deviation of app. 5° on VOR and 1,25° on LOC. The zero position of the course deviation bar (the point at which the course deviation bar aligns with the selected course cursor) indicates, that the airplane has no lateral displacement from radio beam (radial) center. A L-C warning flag appears, when the
VOR or LOC displacement signal becomes unreliable or fails. A fixed airplane reference is in the center of the CDI. The position of the fixed airplane reference with respect to the course deviation bar displays the position of the actual airplane with respect to the selected course. A lubber line is a pointer at the top of the CDI face. The compass card is read against the lubber line to determine airplane heading. A to-from flag is a triangular shaped flag which appears at either the head end or the tail end of the selected course cursor to indicate whether the VOR station is ahead or behind the airplane. This flag is not displayed when operating on LOC.

A glide slope bar and scale is provided to indicate airplane deviation from glide slope beam center. Each do. of the glide slope scale represents app. 0,25° beam displacement. If no LOC-frequency is selected, the glide slope bar is retracted to the upper part of the CDI by a fixed DC voltage. A glide slope warning flag (9 0′ clock position), bearing the letters GS, will appear if a LOC frequency is selected and the GS displacement signal becomes unreliable (too weak) or fails.

The attitude indication includes the horizon sphere, the horizon line, the pitch scale and the roll scale. The horizon sphere is a spherical surface in the center of the HDI, divided by the horizon line in a blue upper part and a black lower part. The sphere has internal light. A pitch scale engraved in the horizon sphere has a 5°, 10° graduation and is marked each 20°. The roll scale on
the upper part of the instrument is graduated in 10°, 20°, 30° and 60° marks in each direction. So  pitch and roll angle of the airplane can be read directly in degrees beside general attitude displacement indication of the sphere. Attitude information signal will be received from the vertical gyros. When the vertical gyro switch is in the No.1 position, captains HDI will receive attitude signals from VG No.1. When the switch is in the No.2 position, captains HDI will be slaved to copilots HDI which will receive its signals always from VG No.2. The HDI sphere is driven by follow-up motors in roll and pitch controlled by servo amplifiers, located in the IIS rack. A pitch trim knob on the lower right-hand corner of the indicator provides for adjustment of the horizon line in the pitch axis only to compensate parallax error and/or basic fuselage angle of the airplane during level flight. A test button, marked· TEST, is on the lower left-hand part of the indicator. With this button depressed, the horizon sphere will indicate actual attitude plus 20° right roll and plus 10° pitch up.

Note: During acceleration respective deceleration may be an erratic pitch attitude
indication. The max. error is about 4° pitch up respective pitch

Two command bars, roll and pitch, indicate the flight director steering commands. Displacement of the vertical bar (roll) to the right or left indicates that airplane must be rolled right or left to center the bar for approaching or tracking a desired flight path. Displacement of the horizontal bar pitch up or below the fixed airplane symbol indicates that airplane must be pitched up or down to center the bar.