Flight Director Systems In Aircraft
A flight director system (FDS) in aircraft combines many instruments to provide an easily interpreted display of the aircraft’s flight path. The pre-programmed path, automatically computed, furnishes the steering commands necessary to obtain and hold a desired path.
The major components of a flight director system in modern aircraft are the flight director indicator (FDI), a horizontal situation indicator (HSI), a mode selector and a flight director computer. The following paragraphs describe a common type of FDS.
FLIGHT DIRECTOR INDICATOR
Elements of a flight director indicator (see figure) are:
- Attitude indicator;
- A fixed aircraft symbol;
- Pitch and bank command bars;
- Glide slope indicator;
- Localiser deviation indicator;
- Slip indicator;
- Warning flag for gyro, computer and glide slope.
1. FIXED AIRCRAFT SYMBOL: The aircraft’s attitude relative to the natural horizon is shown by the aircraft symbol and flight command bars. The pilot can adjust the symbol to one of three flight modes. To fly the aircraft with the command bars armed, the pilot simply inserts the aircraft symbol between the command bars.
2. COMMAND BARS: The command bars move up for a climb or down for descent, and roll left or right to provide lateral guidance. They display the computed angle of bank for standard-rate turns to enable the pilot to reach and fly a selected heading or track. The bars also show pitch commands that allow the pilot to capture and fly an ILS glide slope, a preselected pitch attitude, or maintain a selected barometric altitude. To comply with the directions indicated by the command bars, the pilot maneuvers the aircraft to align the fixed symbol with the command bars. When not using the bars, the pilot can move them out of view.
3. GLIDE SLOPE INDICATOR: The glide slope deviation pointer represents the center of the instrument landing system (ILS) glide slope and displays vertical deviation of the aircraft from the glide slope center. The glide slope scale centerline shows aircraft position in relation to the glide slope.
4. LOCALIZER DEVIATION POINTER: The deviation pointer, a symbolic runway, represents the center of the ILS localizer, and comes into view when the pilot has acquired the glide slope. The expanded scale movement shows lateral deviation from the localizer and is approximately twice as sensitive as the lateral deviation bar in the horizontal situation indicator.
5. SLIP INDICATOR: This provides prompt slip or skid indications.
6. FLIGHT DIRECTOR CONTROL PANEL: The mode selector switch and control panel provides the input information used by the FDS to compute the command and display required for the FDI.
The pitch command control pre-sets the desired pitch angle of the aircraft for climb or descent. The command bars on the FDS then display the computed attitude to maintain the pre-selected pitch angle. The pilot may choose from among many modes including the HDG (heading) mode, the VOR/LOC (localizer tracking) mode, or the AUTO APP or G/S (automatic capture and tracking of ILS localizers and glide path) mode. The auto mode has a fully automatic pitch selection computer that takes into account aircraft performance and wind conditions, and operates once the pilot has reached the ILS glide slope. More sophisticated systems allow more flight director modes.
Turning the control clockwise commands a climb, and counter-clockwise, a descent. The GS (manual glide slope) mode allows the pilot to manually reach and maintain the glide slope through pitch command indications. The GA (go around) mode provides climb command information. The pilot places the command bars in a climb pitch, which is pre-set based on the aircraft performance and remains constant. The pilot may use the GA mode in conjunction with automatic throttle/speed control.
NOTE: The manual glide slope selection normally is used when the pilot intercepts the slope from above.
The ALT HOLD (altitude hold) switch may be operated in the HDG and VOR/LOC modes. Before the aircraft reaches the glide path, the pilot can also operate the switch in the AUTO APP mode. When engaged, pitch commands are referenced to the current barometric altitude indicated on the altimeter. The command bars on the FDI provide the climb or descent information required to maintain the altitude.
HORIZONTAL SITUATION INDICATOR
The horizontal situation indicator (HSI) was developed to assist pilots to interpret and use aircraft navigational aids. The HSI displays information obtained from combinations of the heading indicator, radio magnetic indicator (RMI), track indicator and range indicator. It may also display VOR, DME, ILS or ADF information.
The aircraft heading is displayed on a rotating compass card under the heading lubber line. The card is calibrated in 5° increments. The heading pointer provides magnetic bearing information from the aircraft to the selected ground station (VOR or ADF). The fixed aircraft symbol and floating track bar display the aircraft’s position relative to the selected track (VOR or ILS localizer).
When a VOR station is selected, the inner dot on the track bar azimuth scale indicates approximately 5° and the outer dot approximately 10° (the aircraft’s operating manual should give details). In ILS applications the inner dot indicates approximately 1 1/4° and the outer dot approximately 2 ½°, depending on the actual width of the localizer. The distance measuring equipment (DME) displays slant ranges in nautical miles to the selected DME station and, depending on the installation, may operate in the ILS Mode.
The pilot may adjust the track selector to indicate any of 360° tracks. To select a desired track, the pilot rotates the head of the track arrow by turning the track selector knob to the desired track on the compass card, and then checks the track selector window for precise setting. When the TO-FROM indicator points to the head of the track arrow, it indicates that the selected track, if intercepted and flown, will lead the aircraft to the station. This may be reversed by selecting the reciprocal track on the compass card.
To intercept the inbound track, the pilot sets the desired track in the selector window and crosschecks the TO-FROM indicator to make sure that it points to the head of the track arrow. The pilot turns the aircraft in the shortest direction to an interception heading (normally 30°-45°). The pilot then flies the intercept angle, ensuring that the head of the track arrow is in the top half of the HSI with an adequate interception angle. The bearing pointer should be between the heading lubber line and the head of the track arrow. The angle should not exceed 90° from the selected track.
For outbound tracking, the pilot selects the desired track in the selector window and ensures that the TO-FROM indicator points toward the tail of the track arrow. The pilot then turns the aircraft in the shortest direction to an interception track that places the head of the track arrow in the upper half of the HSI with a suitable interception angle (normally 45°).
Immediately after passing the station, the pilot intercepts the outbound track by turning the aircraft to parallel the track. The pilot sets the outbound track in the selector window. When the track bar and bearing pointer stabilize, the pilot notes the degrees off track and turns towards the track by this amount, allowing for wind drift. The intercept angle should not exceed 45°.
FLIGHT DIRECTOR COMPUTER
The basic flight director computer receives information from the:
- VOR/localizer/glide slope receiver;
- Attitude gyro;
- Radar altimeter;
- Compass system;
- Barometric sensors.
The computer uses this data to provide steering command information that enables the pilot to:
- Fly a selected heading;
- Fly a predetermined pitch attitude;
- Maintain altitude;
- Intercept a selected VOR or localiser track, and maintain that track;
- Fly an ILS glide slope.
OTHER TYPES OF FLIGHT DIRECTOR SYSTEMS
Flight director systems vary greatly. In aircraft equipped with Flight Management Systems (FMS), the flight director is much more sophisticated and receives input from various sensors and one or more air data computers. Therefore, the pilot must consult the operating instructions for the particular aircraft model for specific information.
ELECTRONIC FLIGHT INSTRUMENT SYSTEM (EFIS)
EFIS refers to a system where conventional electro-mechanical flight instruments have been replaced by cathode ray tubes (CRT). These CRTs electronically display flight information in much the same presentation as electro-mechanical instruments bur they also have the flexibility for selecting additional information to be added to the display and for altering the presentation. integrated into various combinations of displays depending on the equipment installed.
The two most commonly used EFIS instruments are the electronic horizontal situation indicator (EHSI) and the electronic attitude director indicator(EADI). These can also be called an ND (Navigation Display) or a PFD (Primary Flight Display). The system may also include a multifunctional display (MFD) on a larger CRT which can provide expanded displays of HSI, radar, and navigation data from flight instruments and can include other data such as checklists, emergency procedures, etc. Data from various sources can be integrated into various combinations of displays depending on the equipment installed.
The EFIS uses input data from several sources including:
- VOR/localizer/glideslope/TACAN/microwave landing system (MLS) receiver;
- Pitch, roll, and heading rate, and acceleration data from an Attitude Heading System (AHS) or conventional vertical gyro, compass system, and longitudinal accelerometer;
- Radar altimeter;
- Air data system;
- Area navigation system (RNAV) (i.e., ONS, INS, VLF, LORAN, GPS, etc.);
- Vertical navigation system;
- Weather radar system; and
A typical EFIS is composed of a Primary Flight Display. a Navigation Display, a Display Select Panel, a Display Processor Unit, a Weather Radar Panel, a Multifunction Display, and a Multifunction Processor Unit.
1. PRIMARY FLIGHT DISPLAY (PFD):
A typical EFIS is composed of a Primary Flight Display. a Navigation Display, a Display Select Panel, a Display Processor Unit, a Weather Radar Panel, a Multifunction Display, and a Multifunction Processor Unit.
1. PRIMARY FLIGHT DISPLAY (PFD): The typical PFD is a multicolor CRT or LCD display unit that presents a display of aircraft attitude and flight control system steering commands including VOR, localizer, TACAN, or RNAV deviation; and glide slope or pre-selected altitude deviation. Flight control system mode annunciation, auto-pilot engage annunciation, attitude source annunciation, marker beacon annunciation, radar altitude, decision height set and annunciation, fast-slow deviation or angle-altitude alert, and excessive ILS deviation (when Category II configured) can also be displayed. The PFD displays all information critical to flight, including CAS, altitude, heading, attitude, VSI and Yaw. It is designed to improve the pilots situational awareness by integrating this information into a single display instead of six different analog instrument including time.PFD’s also increase the Situational Awareness by alerting the aircrew to unusual or potentially hazardous condition ( eg low air speeds, low height, high rate of descend.
2. NAVIGATION DISPLAY (ND): The typical ND is a multicolor CRT or LCD display unit that presents a plan view of the aircraft horizontal navigation situation. Information displayed includes compass heading, selected heading, selected VOR, localizer, or RNAV course and deviation (including annunciation or deviation type), navigation source annunciation, digital selected course/desired track readout, excessive ILS deviation (when Category II configured), to/from information, back course localizer annunciation, distance to station/waypoint, glideslope MGP, or VNAV deviation ground speed, time-to-go, elapsed time or wind, course information and source annunciation from a second navigation source, weather radar target alert, waypoint alert when RNAV is the navigation source, and a bearing pointer that can be driven by VOR, RNAV or ADF sources as selected on the display select panel. The ND can also be operated in an approach format or an en route format with or without weather radar information included in the display
Colour Schemes used in EFIS Display
|Caution and Non Normal Sources||Yellow/Amber|
|Scales, Dials, Tapes and Associated Information Elements||White|
|Engaged Modes/ Normal Conditions||Green|
|ILS/ ILS Dev Pointer||Magenta|
|Deviation Lines, Limits and Labels for Inactive Soft Buttons||Light Grey|
3. DISPLAY SELECT PANEL (DSP): The display select panel provides navigation sensor selection, bearing pointer selection, format selection, navigation data selection (ground speed, time-to-go, time, and wind direction/speed), and the selection of VNAV (if the airplane has a VNAV system), weather, or second navigation source on the ND. A DH SET knob that allows decision height to be set on the PFD is also provided. Additionally, course, course direct to, heading, and heading sync are selected from the DSP. Each Pilot has his own DSP and control of both pilots screen may be delegated to one DSP in the event of failure. DSP are designed with blind operation in mind and each button therefore, is designed for recognition by touch alone.
4. DISPLAY PROCESSOR UNIT (DPU): The display processor unit provides sensor input processing and switching, the necessary deflection and video signals, and power for the electronic flight displays. The DPU is capable of driving two electronic flight displays with different deflection and video, signals. For example, a PFD on one display and an ND on the other.
5. WEATHER RADAR PANEL (WXP): The weather radar panel provides MODE control (OFF, STBY, TEST, NORM, WX, and MAP), RANGE selection (10, 25, 50, 100, 200 and 300 nm), and system operating controls for the display of weather radar information on the MFD and the ND’s when RDR is selected on the MFD and/or the display select panel.
6. MULTIFUNCTION DISPLAY (MFD): The multifunction display is a multicolor CRT or LCD display unit that mounts in the instrument panel in the space normally provided for the weather radar indicator. Standard functions displayed by the unit include weather radar, pictorial navigation map, and in some systems, check list and other operating data. Additionally, the MFD can display flight data or navigation data in case of the malfunction in either of the PFD’s or ND’s
7. MULTIFUNCTION PROCESSOR UNIT (MPU): The multifunction processor unit provides sensor input processing and switching and the necessary deflection and video signals for the multifunction display. The MPU can provide the deflection and video signals to the PFD and ND displays in the event of failures to either or both display processor units.
EFIS furnishes the pilot with the following common features:
- large easy to interpret 5 in. by 5 in. or 5 in. by 6 in. displays for PFD and ND’s;
- displays that have superior readability even under full sunlight cockpit lighting conditions;
- full screen earth/sky representation on the PFD adds to the realism of the attitude display;
- display only the data needed at the time it is needed; for example, GS, LOC and radar altitude can be shown during approach and removed en route to decrease display clutter;
- strapping options allow selecting V bar or cross-pointer presentations on the PFD, and the addition of a speed command display;
- multifunction, pilot selectable ND formats; for example, full compass rose or sectored rose (approach or en route modes) with or without weather radar; and
- superior auto-pilot/flight director mode and NAV source annunciation.
Horizontal Situation Indicator or Electronic Horizontal Situation Indicator (EHSI)
An HSI, or Horizontal Situation Indicator, is a combination of two familiar cockpit instruments: the directional gyro with a heading bug and a VOR/ILS indicator.
Combining the directional gyro and the NAV indicator into one instrument reduces pilot workload by providing heading, course reference, course deviation and glide slope information – all in one visual aid. In addition, an HSI makes it easier to visualize the aircraft’s position with reference to the selected course or holding patterns. The “split needle” presentation made up of the course and reciprocal pointers and the VOR/LOC deviation indicators, clearly shows both selected course and course deviation.
It also gives standard sensing and course deviation indication on back course ILS approaches provided the front course heading is set under the head of the course pointer and you fly toward the course deviation indicator. Provides convenient 45° tic marks to help visualize procedure turns and reciprocals so that pilots need not memorize outbound/inbound headings or add/subtract 45° for intercepts or offsets. The HSI provides a heading bug for autopilot coupling or as a heading reminder in aircraft not equipped with autopilots. HSI basically gives
- Selected Radial
- Present Radial
- Lateral Deviation from Selected Radial
- Sector: To/ From
- DME Distance
Heading Select Knob
Rotating this knob sets the heading bug and will also align a heading transformer for coupled autopilot use, to the selected heading. Pulling this knob out will cage the gyro.
This red warning flag indicates loss of electrical power to the gyro. Heading information is then unusable but all course information (comparable to a standard VOR/ILS) remains valid.
Course Select Knob
Rotating this knob sets the course pointer to a selected course, and if so equipped, a course transformer for coupled autopilot use.
This pointer indicates the selected course. Turning the course select knob will rotate the course pointer, VOR/LOC deviation indicator, and course reciprocal around the compass card. As the aircraft’s heading changes, the course pointer will rotate with the compass card to indicate the difference between the course, under the course pointer, and the actual aircraft heading, under the lubber line. The course selector may also be coupled to an autopilot or flight director. When coupled, “off course” signals will be generated which direct the autopilot to maintain or acquire the selected course.
VOR/LOC Deviation Indicator
The centre portion of the course pointer needle moves to indicate deviation from selected course. A series of “dots” provides a linear indication of how far the aircraft is “off course.” In VOR use, each dot represents 5 degrees; when being used to fly the localizer, it shows 1 1/4 degrees per dot; for RNAV “APPR” mode, 0.625 nm per dot; and for RNAV “Enroute” mode it indicates 1.25 nm per dot. An “on course” condition is indicated when the course pointer, the course deviation bar, and the course reciprocal are all “in line.”
This indicator is a white triangle and appears underneath the VOR/ILS deviation indicator. It shows whether the selected course will take the aircraft either TO or FROM the VOR station.
Representing the actual aircraft, this symbol is fixed and is located “in line” with the lubber line.
This orange line, located at the top of the display, indicates the aircraft’s magnetic heading on the compass card. The lubber line is “in line” with the reference symbol to reinforce this association.
This card, located beneath the lubber line, indicates the aircraft’s current heading. The card is mechanically coupled to the compass card set knob and, at the start of each flight, must be set by the pilot to agree with the magnetic compass heading. As the flight progresses and headings change, the directional gyro rotates the card to indicate the current heading. As with any standard unslaved DG, some gyro precession will occur. Therefore, it is necessary to check and reset the compass card at periodic intervals.
The selected heading is marked by an orange heading bug which can be moved to any point about the perimeter of the compass card. As the aircraft’s heading changes, the bug rotates with the compass card, thus alerting the pilot to the difference between the selected heading (located under the bug) and the actual aircraft heading. The heading bug may also be coupled to an autopilot or flight director system. When coupled, “off heading” signals will be generated causing the autopilot to fly the aircraft so as to maintain the selected heading.
This red warning flag indicates inadequate VOR or LOC signal, or loss of power to meter circuits. Under these conditions, course information is unusable, however, all heading information remains valid.
Glide Slope Deviation Indicator
This yellow wedge relates the vertical glide path centreline to the aircraft’s position. The aircraft is “on glide slope”, when the wedge covers the horizontal index. Each dot on this vertical scale represents approximately four-tenths degrees vertical deviation from the centreline.
Glide Slope Mask
This mask will cover the glide slope deviation indicator in the absence of a usable signal or when a VOR frequency is selected.
The legend “RNAV” will appear in the lower right corner of the instrument face when the HSI is part of an area navigation system and that mode is engaged.
A slaving meter is located in the upper right hand corner of the instrument face on slaved models. If equipped with a slaving feature, an HSI will automatically “update” its compass card with the magnetic compass to correct for precession. The meter needle will oscillate slowly when the compass card is properly aligned with the magnetic compass.
* To function properly, an HSI’s compass card must be properly set, and selected course and heading information applied.
* Because the HSI is a gyroscopic device, it must be set to the magnetic heading and checked periodically. Even in a slaved system, the action of the slaving meter and compass card correspondence with the magnetic compass should be checked periodically.
* For back course approaches, set in the ILS front course heading and then fly “the picture” using normal response to needle movements (i.e., needle left, correct left; needle right, correct right).
* Don’t confuse a radial (which radiates outward from a VOR) with an inbound course (i.e., the reciprocal).
* HSIs are equipped with “TO-FROM” NAV function, heading and glide slope warning flags. Improper performance of each function is indicated by its respective warning flag.
The Electronic Horizontal Situation Indicator is an electronic display of Horizontal Situation Indicator. The Electronic Horizontal Situation Indicator (EHSI), often referred to as the Navigation Display (ND), replaces a number of different instruments found on a conventional aircraft instrument panel, and may be used to depict some or all of the following information:
- heading flown;
- heading or track selected;
- bearing to or from a navigation beacon (VOR, DME);
- lateral deviation from a selected track;
- ground speed, distance and time to go;
- aeronautical map;
- weather information;
plus much more information according to design. It is a part of EFIS and features in modern glass cockpit. It looks and acts like a mechanical HSI, but has the capability of multiple configuration. A number of configurations are available to the pilot depending on preference and phase of flight. It is capable of showing the Navigation information in a traditional form or as a moving map display to improve situational awareness. Additional information such as weather radar and TCAS can be superimposed to further reduce the scan.
EICAS (Engine Indication and Crew Alerting System)
Improves situational awareness by allowing the aircrew to view complex information in graphical format and also help by alerting crew if there is any failure in any system. Unlike traditional round gauges, many levels of warning and alarms can be set.