Chapter

14

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Introduction						273
The Conceptual Model . . . . . . . . . . . . . . . . . .	. .	. .	. .	.	.	273
Hardware - Design of Flight Decks						274
Hardware - Displays						275
Hardware - Engine Instruments . . . . . . . . . . . . . .	. .	. .	. .	. .		.277
Hardware - Controls						279
Software						281
Hardware and Automation						282
Intelligent Flight Decks . . . . . . . . . . . . . . . . . .	. .	. .	. .	.	.	287
Colour Displays . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .		.287
System Active and Latent Failures/Errors . . . . . . . . . . .	. .	. .	. .	.	.	288
System Tolerance . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	.	.	288
Design-induced Errors . . . . . . . . . . . . . . . . . .	. .	. .	. .	.	.	289
Questions . . . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .		.290
Answers . . . . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	.	.	294

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Introduction

The design of the human body is for life on the ground, but now aviation has transported mankind into the air. Our survival in this new dimension depends upon the effectiveness of the design and manufacture of, not only the aircraft in which we travel, but of the systems by which they are flown and controlled. It is therefore important that the relationship of this dichotomy is considered and studied.

The Conceptual Model

It is useful to construct a simple model to show the relationship of man to all the factors with which he has to relate. The SHELL concept, shown in Figure 14.1, is named after the initial letters of its components and was proposed in 1972 by a psychologist named Edwards:

L =	Liveware	At the centre of the model is the pilot. Man - the most valuable and
		flexible component of the system. The second ‘L’ represents other
		humans whether inside or outside the aircraft.
S =	Software	Procedures
		Manuals
		Checklist layouts
		Symbology
		Computer programmes
		Maps and charts
H =	Hardware	Design of flight decks
		The physical structure of the aircraft
		Presentation of instruments
		Positioning and operating sense of controls
E =	Environment	The conditions both inside and outside the cockpit.

Figure 14.1

The edges of this block are not simply straight lines and so the other components of the system must be carefully matched to them if stress in the system is to be avoided and eventual breakdown prevented.

Liveware and Environment

The interrelationship of the pilot with these two aspects have already been covered in earlier chapters. The pilot’s interaction with other people (Liveware - Liveware) is subject, as we have seen, to many variations such as personality, behaviour, ability and performance.

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Man has adapted the environment to match human requirements in the air through systems such as pressurization, soundproofing and air conditioning to control temperature and humidity (Liveware - Environment).

The two remaining interfaces to be considered are those of Hardware and Software.

Hardware - Design of Flight Decks

Eye Datum

A basic feature of a cockpit design is that the pilot should be able to view all important displays within the aircraft and maintain an adequate view of the world outside without the need to make more than the minimum of head movements. It follows that the cockpit space must be designed around a defined position of the pilot’s eye. This position is the Eye Datum, Design Eye Position, or Reference Eye Point and is often indicated in the cockpit by the provision of an indicator on the central windscreen pillar which only appears aligned when the pilot’s eye is at the designed point.

As the external view is of particular importance, the pilot must, without strain be able to look over the top of the instrument panel and see sufficient of the ground ahead to enable him to land the aircraft.

If the pilot should be sitting below the eye datum then the undershoot will be obscured, if sitting higher than the datum, the overshoot area may not be visible. In the latter case the aircraft instruments may also be difficult to read accurately.

Once the design eye position has been set, and the anthropometric range of pilots has been determined, the size of the cockpit work space and the amount of adjustment to seat, rudder pedals, etc. can be established. The designers will be constrained by the fact that the cockpit, for aerodynamic reasons, is placed in the narrowest section of the aircraft.

AircraftWindows

External vision is of great importance but the size and shapes of windows will be determined by aerodynamic and weight restrictions. Large windows will need to be of thicker glass and require stronger and thicker frames; a compromise must be reached whereby reasonable external vision is obtained without too great a weight penalty.

Design of Cockpit Seats

Each year the duration of flights continues to increase. Pilots will be spending a longer time in the seat and it is of the utmost importance that the seating is comfortable and adjustable to the individual pilot’s size and shape. Flight deck seating must have a lumbar support to maintain the natural spine shape and thereby reduce the chances of lower back pain caused by a failure of the shock absorbing discs between the vertebrae. Additionally the seat should, if possible, be isolated from vibration of the airframe.

Restraint must be provided by a 5 point harness with a negative g strap to hold the harness in position during negative g manoeuvres and prevent ‘submarining’ under the lap strap during rapid deceleration.

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Thus the main considerations in the design of cockpit seats are:

•Lumbar Support

•Vibration absorption

•Long-term comfort

•Anthropometric data

•G-force protection

•Prevention of ‘Submarining’

•Securing the pilot

Hardware - Displays

Presentation Requirements

When deciding on the best way to display information we have the basic choice of a digital or analogue display. Even when using a cathode ray tube to show information we have the choice of a digital or an analogue display. Experiments have shown that for the display of purely quantitative information, amounts of fuel in a tank for example, then digital displays give the better results. For displaying qualitative or comparison information then an analogue display provides more easily assessed information. If the end point of a display, such as an altimeter, is important then moving tape displays should not be used.

Standardization

The most important requirement in display and control design is that of standardization. This should allow the pilot to make an easy transfer from one aircraft type to another with minimum training time and expense. Standardization can also prevent accidents due to the transfer of procedures between aircraft types and models. Total standardization is, however, not possible and would inhibit new design technology, but it should certainly be the goal for all similar types within an operating fleet.

Conventional Analogue Standard “T” Display

An aircraft using conventional displays will usually have a standard ‘T’ lay out in which the most important instrument, the artificial horizon or attitude indicator, is at the centre. The other primary flight instruments, altimeter, airspeed indicator and direction indicator, are grouped around it.

Figure 14.2 The standard ‘T’

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Digital Display and the Compass

The conventional compass card gives a better picture of the aircraft orientation. A digital readout for heading makes it more difficult to determine such factors as the shortest way to turn onto a new heading however it is ideal for the display of quantitative information.

Combination of Analogue and Digital Displays

It is practicable to combine both digital information and analogue information in a single instrument, as seen in Figure 14.3, in which the thousands and hundreds of feet are displayed digitally. The hundreds of feet are also shown by a single pointer. The use of a single moving pointer against a fixed scale will give a much better mental picture to the pilot when approaching the end of the scale, i.e. approaching the ground. This form of display is also excellent for showing small changes such as when levelling off or departing inadvertently from the selected altitude.

Glass Cockpit Display

Basic presentation is maintained to some extent in the modern ‘glass cockpit’, in which the instruments are displayed on a Cathode Ray Tube (CRT). The attitude may be presented in the traditional way but other items, such as speed and altitude, may be displayed on moving tape displays, with a conventional compass card, or as a digital display readout. (Figure 14.4).

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Figure 14.3 Combination of analogue and digital

Figure 14.4 A ‘Glass Cockpit’ flight display using tapes

Tapes and a Perception of Rolling

The use of tapes does present some problems in the climb and descent. If we maintain the convention of having the high figures at the top of the presentation then during a climb, with a decreasing speed and an increasing altitude, there may be a perception of the aircraft rolling.

Other displays have the higher values at the bottom of the displays. As there is no industrywide standard for presentation of information it is possible, on transfer of aircraft type, to have presentations working in opposite senses to which the pilot has become familiar. Such a state of affairs is plainly undesirable and a recipe for possible errors.

Head Up Displays

A promising development for future displays is the Head Up Display (HUD). In this system the information required by the pilot is projected on a translucent screen between the pilots and the forward flightscreen window. As the display is projected at infinity it enables the pilot to view the outside world through the display. It has been in use for many years in military aircraft and is now being incorporated into commercial aircraft.

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The greatest success is in the presentation of ILS information on the windscreen, when information from the ILS equipment is processed by a computer to show a constantly changing picture of runway data as the procedure is flown. Although there is no requirement for a change of eye focus for the pilot, there is still the need for the transfer of attention.

Voice Presentation

An alternative method of presenting information is the use of a recorded voice message. This technique has been developed in some experimental aircraft but has not been adopted for normal commercial aircraft. The voice has been found distracting and after a time tends to be ignored. Voice information has only found a major use in the Ground Proximity Warning System (GPWS) and Traffic Collision Avoidance System (TCAS) systems.

Hardware - Engine Instruments

General

As the information from the engine instruments is as relevant to the pilot as that from the flight instruments, it is important to ensure that these instruments are not only easy to read but as far as possible unambiguous. The instruments in each column should all relate to only one engine, and the instruments in each row should show the same information (RPM, TIT, torque) as shown in Figure 14.5.

This enables the operator to spot immediately any discrepancy on any instrument and identify the engine concerned in the minimum time. In an ideal layout the columns of instruments will be aligned with the appropriate power lever, all No 1 engine instruments being above No 1 power lever and so on. Another aid to rapid identification of a problem is to rotate the instruments so that all needles are aligned, vertically or horizontally, in normal cruise flight.

Primary and Secondary Instruments

As well as the primary engine instrument a number of instruments are required to display secondary information. There are a number of different possible configurations, two of which are shown in Figure 14.6.

There are advantages and disadvantages to each layout. The ideal layout could have a bank of instruments below the primary instruments but cockpit space may not allow this. In Figure 14.6 the layout at A. might be preferable but this type of layout could only be used with an even engined aircraft and could not be used with a three engined aeroplane. As the purpose of these instruments is to warn of possible problems, and guide the pilot to the correct identification of the engine concerned, there is a great deal of research needed to identify the best layout for each aircraft type.

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Figure 14.5 An ideal engine instrument layout

Figure 14.6 Possible engine instrument layouts

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Cockpit Lighting

All instruments need a lighting system to enable readings to be noted in all light conditions. On conventional dials there is a choice of internal lighting on each instrument or external lights to illuminate a group. In most cockpits there is a mixture of both internal and external lights. In the glass cockpit display the brilliance control will act as an adjusting mechanism to cater for varying light conditions. Once set, the screen brilliance may be automatically retained by an ambient light sensor fitted in the cockpit.

What is essential is an adjustment system that allows for both the state of natural light and individual preference. All lighting systems should avoid harsh shadows and reflected glare.

There has been a tendency in modern civil flying to use higher brightness levels on the flight deck. Research has indicated that on long night flights, fatigue and drowsiness seem to be less with higher brightness levels. With age visual acuity decreases, and older pilots require a higher brightness level.

Should there be a possibility of thunderstorms or lightning, cockpit lights should be turned fully up to reduce, as far as possible, the ‘blinding’ effect of flashes.

Hardware - Controls

Basic Considerations

Displays enable information to be passed from the aircraft to the pilot - controls enable instructions to be passed from the pilot to the aircraft. There are certain basic considerations which govern the way controls should be designed and arranged.

•Standardization

Most importantly, controls should be standardized in their location and sense of use from one aircraft to another, and between different aircraft types. For example, to operate a manual valve, rotation should be:

•Clockwise to close

•Anticlockwise to open

•Frequency of use

Controls should be located such that they are within an easy reach envelope of all designed users of the aircraft. Controls that are used frequently or for protracted periods should be located so that they do not require an awkward or fatiguing posture of the pilot.

•Sequence of use

Controls that are frequently used in a given order should be laid out so that the sequence of use is represented in the layout of the controls. As well as convenience, the layout itself acts as a prompt for the pilot.

•Importance

Important controls must be located in easily reached and unobstructed positions.

•Visual/Tactile dissimilarity

Switches and knobs that control different functions should not look or feel the same thus reducing the chances of inadvertent operation.

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• Symbolism

Controls, if possible, should be designed to contain some reference to their function. Thus undercarriage levers can be shaped like a wheel and flap levers can resemble a cross section of a flap.

• Control/Display compatibility

Controls should be located such that they maintain some spatial logic with the display that they are associated with. For example the columns of engine instruments should be aligned with their relevant power levers. (See Figure 14.5 and Figure 14.6).

• Control loading

The force required to operate any control should not only be within that which can be exerted by the target population of pilots but should be harmonized with the forces required by other related controls. For example, a control column will be difficult to use if it requires a large force to control roll but only light force to control pitch.

• Prevention of inadvertent use

		Controls should be designed to minimize the chances of inadvertent operation. Where this
		could be dangerous, the control should be fitted with a guard.
		• Control position and present demand
		The position of the control should indicate the selected function. In some modern cockpits
	14	the conventional column has been replaced by a sidestick. Both pilots’ sidesticks should
		move in unison so that, on change of operator the pilot taking control will know the already
	and Man	selected position.
		• Simultaneous use
	Machine
		located to enable this to take place.
		Those controls requiring simultaneous use, such as the throttle and trim controls, should be
		Great progress is being made to meet all of the above requirements but even today there
		are problems with some aircraft designs. Some are merely a nuisance but others should not
		be tolerated. Some examples of problems will be found in Chapter 16 dealing with reports
		submitted by pilots.
		Warnings
		It is essential that all warnings should be ‘attention getting’ without being startling. As well
		as attracting attention the warning should inform the pilot of what is wrong and if possible;
		guide the pilot to the correct actions. The alerting function for all important failures should
		be fulfilled by an audio warning. This is mandatory if the pilot is required to assume control.
		Even the most conspicuous visual warnings rely on head and gaze orientation. In a more
		extreme example, the use of any visual warning is rendered useless if the pilots should be
		asleep. The ideal warning system is to have a single audio warning to alert the pilot to a failure
		and to direct his attention to a single central warning panel that announces the nature of the
		problem with a suitable illuminated caption.
		It is vital that warning systems be reliable, that is they respond to all genuine problems, but
		do not generate false alarms. Early GPWS systems were well known for generating spurious
		warnings and it has been suggested that CFIT accidents have been caused by pilots (used to
		hearing spurious warnings) ignoring genuine alarms.

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