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CHAPTER 9. Display Systems
9.1. Situational Awareness
The knowledge and understanding of all factors which affect or will affect the function one is responsible for performing.
In aircraft operation this includes:
a) Attitude
b) Altitude
c) Airspeed
d) Power setting
e) Position
f) Nearby traffic etc.etc.
In the early days of aviation, most of this was done, literally, “by the seat of the pants”
a) Attitude was determined by reference to the visible horizon
b) Airspeed could be estimated by the sound of the airflow through the aircraft structure
c) Power could be estimated by the sound of the engine
d) Navigation was done by reference to maps and visual observation of the ground
The development of enclosed cockpits and “blind flying” led to the requirement for instruments which could provide the information in visual form.
9.2. Purpose and Characteristics
Cockpit displays are the basic interface between the pilot and the many aircraft systems. A good display must
be:readable, unambiguous and easily understood.
The functions of the display system are:
a) to provide aircraft attitude information
b) to provide navigation information
c) To help the crew monitor the functioning of the aircraft systems
In the cockpit of a fast moving aircraft the pilot is usually so busy that there is no time to spend on interpretation
of displays. This is especially true in an emergency.
9.3. Information to be presented
The information to be presented can be divided into three classes; primary, secondary and tertiary
9.3.1 Primary
The primary displays relate directly to the control and navigation of the aircraft. and include attitude, position,
and deviation from nominal
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The basic flight instruments are located directly in front of the pilot in an internationally standardized (ICAO)
arrangement called the “basic T”.
Airspeed
Attitude
Altimeter
Horizontal
Situation
Rate
of
Climb
Figure 50:
Basic T Instrument Configuration
The attitude instrument was originally just the artificial horizon, but as systems grew more complex it became
the ADI (Attitude Director Indicator)
An electromechanical version of this instrument is shown in the Figure 49. To relieve the pilot of the necessity
to scan several instruments during an approach, several functions have been added to the artificial horizon.
These include: Glide slope deviation indicator, speed deviation indicator, horizontal or localizer deviation indicator and a radar altimeter/decision height indicator.
In addition, there are so-called “Command bars” which indicate the difference between the current attitude and
the attitude computed by the flight director required to put the aircraft back on course. Note: if the flight director
is coupled to the autopilot, this is the error information being fed to the autopilot.
In the “split cue” design coomands for the two axes are displayed separately.
The ADI is probably the most complex electromechanical aircraft display instrument ever developed.
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Figure 51:
Electromechanical ADI with
“split cue” Command Bars
Another version of the ADI is shown in Figure 50. This is a simplified ADI but it shows the “single cue” or
“V bar” command bars. These two bars move together. They move up or down to indicate the correct pitch
angle and they rotate to indicate the correct bank angle. The pilot simply banks and or pitches the aircraft to
keep the yellow triangle “aircraft” in the triangle formed by the lower edges of the command bars as shown by
the hatched triangle. In the figure, the flight director is commanding a climbing turn to the right.
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V bar
Command bars
Aircraft
Figure 52:
Simple ADI with Combined
Command Bars
The Horizontal Situation Indicator (HSI) started as two separate instruments. One was a radiomagnetic indicator (RMI) which combined a compass with a couple of needles which showed the relative bearing to the VOR
or ADF stations which were being used. The other was a course deviation indicator (CDI) which indicated the
deviation from the desired course.
The HSI combines these two instruments and usually adds a display of the selected VOR radial and the distance
from the selected DME.
The altimeter and airspeed indicator are simple pointer on dial instruments.
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9.3.1.1 Secondary
Examples of secondary instruments are
a) engine status (turbine temperature, turbine/fan speed, oil temperature and pressure)
b) fuel status (quantity, rate of consumption)
c) electrical system status (voltage, current, frequency for AC)
These displays are necessary but are used for only a short time on each flight, for example, the engine oil pressure and turbine temperature are needed only during engine start or when there is an abnormal condition involving these quantities.
9.3.1.2 Tertiary
a) outside temperature, check lists
9.3.2 Disadvantages of electromechanical instruments
a) although an instrument may be required for only a very small percentage of time, it still occupies
space on the instrument panel for the whole time
b) with instruments such as the ADI, the limit of electromechanical complexity had been reached
c) there is little flexibility e.g. capability to change display colours depending on the situation
9.3.2.1 Pressure to reduce space and reduce crew
With the multiplicity of displays, most multiengine aircraft (3+) required a third crew member (second
officer) to monitor the engine and fuel systems.
To reduce personnel costs there was a great deal of pressure to eliminate the necessity for the third crew
member.
9.3.2.2 “The Glass Cockpit”
The answer to the ever more crowded cockpit came with the introduction of microprocessors powerful and
reliable enough to drive CRT displays
Note: CRT displays had been used for many years as part of the weather radar system
With the advent of CRT displays for flight critical data, the same space or display could be used to show
data only when it was needed. In addition, new, more intuitive formats could be developed since the
limitations of the electromechanical devices was no longer a limitation.
9.3.3 Display methodologies, analog vs digital
With the almost unlimited display flexibility now possible, it is necessary to determine the best way to present
the necessary information.
One choice is between analog and digital presentation of data. While the digital form can provide much more
accuracy and better in most cases, the analog form gives a much better idea of the rate of change Some information is best provided in both formats e.g altimeter
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9.3.4 Display technologies
9.3.4.1 Requirements:
9.3.4.1.1 Viewing angle:
Since not all of the displays can be mounted directly in front of the pilot, some displays will necessarily be
viewed from an angle off the perpendicular. There is also may a requirement for the pilot to see the
copilot’s display and vice versa. Thus the greater angles from which the display can be seen clearly the
better. Typically this is specified as ± 60˚ horizontally and ± 35˚ vertically
9.3.4.1.2 Resolution:
One of the main strengths of the computer generated display is the ability to generate symbols and these
symbols may be rotated through any angle. Thus the resolution must be compatible. The CRT standard of
about 0.5 mm line width is a reasonable benchmark
9.3.4.1.3 Resistance to “wash out”:
The ambient light level in the cockpit ranges from extremely bright at high altitudes to dark at night. The
display system must be able to maintain a readable contrast through all of these levels. “Wash out” is the
term used for loss of contrast under high ambient light conditions.
9.3.4.1.4 Ruggedness:
9.3.4.1.5 Low price:
9.3.4.1.6 Ideally the displays should also weigh nothing and consume no power.
9.3.4.2 Candidate Technologies:
9.3.4.2.1 CRT
The cathode ray tube has been around since the 1930s with colour capability being introduced in the
1960s.
In the CRT, a cloud of electrons is generated by heating a tungsten filament. The electrons are accelerated
towards the screen by a high voltage and are shaped into a narrow beam by magnetic or electrostatic
lenses. This is called the electron gun.The screen is coated with a phosphor compound and when the beam
strikes it, a glowing spot results. The beam can be steered by the electromagnetic effect of the deflection
coils.
In a colour CRT, there are three separate electron guns, one for each of the three colours Red Green and
Blue. On the screen there is an array of red, green and blue phosphor dots and just behind the screen is a
mask which is aligned so that the electron beam from the “red” gun can reach only the red phosphor dots
etc.
Because of the need for the electron gun and deflection mechanism, CRTs tend to be “deep” which may
present problems in cockpit installations where space behind the instrument panel can be limited
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Principle of Operation of the Colour CRT
9.3.4.2.2 Liquid crystals (LCD)
Liquid crystals are made from a material which possesses properties between those of a crystal and a
liquid. The molecules are long and thin and tend to orient themselves parallel to one another in a given
plane. In this way they resemble crystals and, like crystals, they tend to polarize light passing through them
in a manner determined by the alignment axis of the molecules.
However, the axis of alignment, unlike that in the crystal can be rotated to align itself with an electrical field.
In the so-called nematic phase the orientation of the molecules is the same. A twisted nematic cell is one in
which the orientation at the bottom boundary is 90˚ to the orientation at the top boundary. This causes the polarization of light passing through the cell to undergo a 90˚ rotation. Applying a voltage in excess of a given
threshold across the cell causes all of the molecules to align in the same direction and thus the polarization of
light traversing the cell is not rotated.
Thus, by placing a polarized plate at the output, the amount of light transmitted can be controlled by the application of the voltage. The colour of the light can be controlled by varying the magnitude of the voltage
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Construction of a Liquid Crystal Cell
9.3.4.2.3 Light Emitting Diodes (LEDs)
LEDs are simply semiconductor diodes which emit light when a voltage is applied. The colour emitted can
be determined by the manufacturing process and can be almost any shade required.
They have been used for several years in linear arrays for engine indicating instruments, but their
application to flat panel displays has been slow to develop. This is mainly because of the complexity and
resulting high cost. Litton Systems Canada produces 64 x 64 element arrays in 1 inch square format.
These can be put together to make larger panels such as an ADI for the F16
.
Table 4:
Technology
Advantages
Disadvantages
Cathode Ray Tube
Well proven technology
High resolution and contrast
Wide range of colours
High brightness
Wide viewing angle
Low cost
Potential for “thin” display units
Heavy and bulky
High voltage
High power consumption
High temperature
Sensitive to external electromagnetic
effects
Affected by bright light
Vulnerable to shock, vibration and catastrophic failure
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Table 4:
Technology
Advantages
Disadvantages
Light Emitting Diodes
(LED)
High display brightness
Wide viewing angle
Well proven technology
Extensive application
Low voltage operation
High power demand
Low resolution
Not yet available for large arrays
Liquid Crystal
Displays (LCD)
Low voltage
Low power
High resolution
High contrast in bright light
Non catastrophic failure
Slow response in low temperature
Has to be backlit in low ambient light
Narrow viewing angle(??)
Limited display size(??)
High cost and circuit complexity for full
colour
Electroluminescent
(EL)
Rugged, lightweight and reliable
Long life
High contrast and good resolution
High brightness
Wide viewing angle
Extensive application
Low power consumption
Brightness reduces with increased area
Complex circuits required to drive display
Full colour range yet to be perfected
Plasma
Rugged
High reliability
Suitable for large displays
High resolution and contrast
Wide viewing angle
Expensive to produce
“Wash out” in high ambient lighting
Affected by low pressure
Restricted control of brightness
Complex circuits needed
Limited application in industry.
Note: the (??) indicates that these factors have been improved a great deal since the table was drawn up, primarily
due to developments for the laptop computer business.
9.3.5 Electronic Library Systems
One of the capabilities which the new, flexible displays have made possible is the electronic library.
It is now possible to store such things as the aircraft technical manuals, the operating manuals and emergency procedures on a CD ROM or other storage device and to be able to display them on request to the pilot or maintenance
technician.
9.3.6 Head up displays HUD)
In the last couple of years the HUD has moved from the military domain to the civil airliner.
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9.3.6.1 Disadvantages of existing systems:
In the existing (head down) display arrangement, in instrument flying conditions, the pilot flies the aircraft
by reference to the panel instruments while the first officer monitors the view out the cockpit window.
When the first officer can see enough of the runway approach environment (approach lights, runway lights,
terrain) the captain is notified and makes the transition from instruments to visual cues to continue the
approach to land. This transition can take several seconds before the captain is comfortable with the
change and the delay, especially in Cat II conditions may be such as to lead to an overshoot or missed
approach. (in Cat II the time from breakout to touchdown is about 12 seconds)
9.3.6.2 Solution
A solution to this is the head up display in which the pertinent data are projected on to a transparent screen
between the pilot and the windshield. Thus the pilot has all of the information needed to fly the approach
while looking out of the windshield.
Such a system presents a challenge to the optical designer since the image projected must be at infinity,
and because of the limited space available. Another requirement is for a relatively wide field of view so
that the pilot can move to see objects not directly ahead and still see the display. It also must be transparent
over most of the spectrum.
To accomplish these objectives devices called holographic combiners are used. These combine the high
reflectance required for a bright display with high transmissivity for a clear view outside the cockpit. The
holographic combiner is shaped to focus the image and is also (through the holographic process) a
diffraction grating which is “tuned” to the wavelength of the CRT phosphor light output. Thus it is highly
reflective over a narrow band of wavelengths and transparent over the rest of the spectrum.
To overcome the physical constraints, the optical path is arranged as shown below:
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9.3.7 Input Devices
Given that present day displays can present an almost unlimited array of data, some attention must be paid to
the methods by which the pilot controls what is displayed when. In historical order these are switches, touch
screen and direct voice input
9.3.7.1 Switches
These are typically of the push button type. To provide feedback they usually incorporate some sort of
tactile response such as variable resistance. Some systems provide an audible feedback capability. The
layout and spacing of the switches especially in keypads must be done carefully so as to reduce the
possibility of pressing the wrong switch or key or hitting two keys at a time
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9.3.7.2 Touch screen
With the increasing number of functions, the switch/keyboard area in the cockpit is increasing so it seems to
make sense to take the same approach with switches as was done with displays i.e. make the display also the
input device. touch screens use three major techniques: infrared scan, resistive overlay and capacitive overlay.
a) In the infrared scan, an infrared beam is scanned parallel to the surface of the display. When the operator’s
finger interrupts the beam, its position can be determined and passed to the computer.
b) With the resistive overlay a glass substrate is installed over the screen. Touching the substrate alters its resistance in such a way that the point of contact can be determined
c) The capacitive overlay is similar except that the capacitance is altered by contact.
9.3.7.3 Direct Voice Input
Advances in the digital processing of sound and especially voice have made it possible to develop voice
and command recognition systems. These systems require some “training” in that they need a sample
(called a template) of the operators voice. This template allows the system to respond to the particular
characteristics of a pilot’s voice.