Display of information
Having looked at the factors concerning the way the pilot is able to control the aircraft systems the next major area to consider is the way that information is displayed to the aircrew. Although the human being is extremely adept at processing large amounts of information quickly, pilots often reach saturation point in flight where the amount of information and the way it is presented make it difficult for him or her to analyze it. The type of information the pilot is required to receive and process is extremely varied. He or she must constantly monitor the aircraft’s state in relation to the external environment encompassing such factors as velocities in all axes, altitude or height, separation from obstacles, heading and attitudes, etc. This may need to be achieved using external cues, instrument displays or most commonly a combination of both. In addition all the aircraft systems need to be monitored, some, such as the transmission torque and rotor speed, may require careful attention during manoeuvring. On top of all this there may be a requirement to take in and act upon the information presented on tactical and navigation displays.
There are a number of factors that affect the precision and speed with which the pilot is able to obtain and process the information presented on the aircraft displays. Instruments need to be positioned within the pilot’s normal field of regard (the area of the cockpit that the pilot is normally viewing) so that they can be seen easily when operating the aircraft. In addition, the position of the instruments in relation to each other is an important factor. The pilot will often have to obtain information from several systems in a short space of time to perform a task such as starting an engine or performing an autorotation; a poor instrument layout will add significantly to the difficulty of the task. Associated with the position of instruments is the ease with which the correct instrument can be located and identified. If all instruments are of the same size and general appearance then it can be difficult to find the correct instrument quickly, particularly if there is no logical structure to the layout. Even worse, the pilot may misidentify the instrument during the stress of an emergency and shut down a serviceable system. Thus instruments must be easy to view and identify while conducting role tasks.
The instruments used for controlling the flight path of the aircraft are the most important instruments in the cockpit and therefore their layout has come in for particular study. Because of their importance they are always given the most prominent position on the instrument panel directly in front of the pilot. Following the example of their fixed wing predecessors the flight instruments of nearly all helicopters are arranged in the classic ‘T’ layout which puts the artificial horizon or attitude indicator (AI) in the centre with the directional indicator below. The airspeed indicator is located to the left of the AI and the altimeter to the right. This arrangement is usually laid down in specification documents, such as the Ministry of Defence Standard [7.2] and Joint Airworthiness Requirements [7.6]. Other instruments such as the vertical speed indicator and the radar altimeter are located below the ‘arms’ of the ‘T’. This layout allows the pilot to build a viewing strategy known as a ‘scan’ that is centred on the AI; each instrument is scanned in turn with a scan of the AI in between. Thus when flying solely by reference to instruments the pilot is able to monitor the most important of all parameters, the aircraft attitude, frequently enough to prevent minor deviations from becoming larger errors.
The suitability of the layout of the flight instruments will naturally be affected by the requirements of the role. For example, an aircraft that is required to operate at night, low level over the sea will need radar height displayed more prominently than barometric height. For this reason some naval helicopters, such as the Westland Sea King, have the radar altimeter positioned in the place normally reserved for the barometric altimeter.
The position and distinctiveness of instruments is only part of the story. The pilot must also be able to interpret the information displayed easily and for this the size, markings and scales used must all be correct. The ease with which the pilot is able to interpret the information displayed by an instrument is often referred to as ‘readability’. The effect that size has on readability needs little amplification but when assessing the size of an instrument consideration must be given to the distance it is from the pilot’s eye, the frequency with which the pilot will need to interrogate it, and the precision with which information will need to be gathered. During an instrument approach, for example, the pilot will need to monitor the direction indicator frequently and to an accuracy of one or two degrees. This is in contrast to an oil temperature gauge that will need only infrequent scanning with far less precision.
The markings and scales used on instruments can be a more complex aspect of assessment as they are dependent on the way in which the pilot is required to use the information presented. For example, when considering a gauge displaying rotor speed (NR) it is clear that the pilot will be interested primarily in that portion of the scale which relates to rotor speeds that will be seen in flight. If the full range of rotor speed is shown on the gauge from zero to the maximum permitted, the size of the portion corresponding to flight values is likely to be very small. This problem is sometimes tackled by using scales with expanded portions as in the case of radar altimeters where the scale is larger at low altitudes where greater precision is required. The test pilot must make the decision as to whether or not the choice of scale provided is the optimum for all mission tasks. Care must also be taken with non-uniform scales to ensure that rate of change information on the parameter displayed does not lead to confusion as a constant rate of change will not lead to a uniform rate of pointer movement.