Electronic cockpit displays
As in all new aircraft, helicopter cockpits are increasingly incorporating electronic displays in place of mechanical, analogue dials. These can range from large multifunction displays (MFD) used to present flight information to small light emitting diode (LED) displays for an individual parameter. The electronic display offers a number of advantages over the mechanical dial, such as reduced maintenance costs, flexibility, clarity, and significantly smaller space requirements. The considerations for the display of information already stated in this section apply equally to electronic displays, however, there are a number of specific considerations that must be included in any assessment of these more modern instruments.
As the designer can place information on an MFD very easily there are sometimes problems with size and clutter. Each individual number, letter and symbol must be large enough to be distinguished easily and quickly. In addition the pilot must be able to locate and identify the parameter required with rapidity and with precision. Some MFDs provide a great deal of information in a very small space but do not reduce the pilot’s workload. When assessing these types of displays an important element is determining if the size and number of display elements helps or hinders the presentation of essential information for each phase of the mission.
There are a number of steps that the designer can take to improve this situation. Firstly the crew can be given some control over the displays to select the items of information they would like presented. Clearly this brings its own problems and will require the test pilot to check that essential information is never missing. Another approach commonly used in modern aircraft is the ‘black cockpit’. In this the pilot is only given information on items such as system status when he or she selects it or if the system detects an unusual change in a parameter. For example, engine oil pressure would not be displayed if the value lay within a pre-determined band and the rate of change was below a certain value. Thus by reducing the total amount of information presented and automating the systems monitoring task the designer can reduce the crew’s workload. Many pilots have resisted this approach, however, preferring instead to decide for themselves what to monitor and believing that they are better at detecting subtle changes in system status than an automated system. Another solution to a surfeit of information is to automatically de-clutter a display in certain circumstances. For instance heading and navigational information can be removed from a primary flight display if the helicopter attitude exceeds a certain value. This allows the pilot to concentrate on attitude information to effect a recovery to a normal flight condition.
The presentation format also requires careful assessment. Unlike conventional instruments the electronic display gives the manufacturer an almost infinite number of ways to present information. It is important that the manufacturer chooses the optimum format and it is often in this area that the test pilot with an extensive knowledge of the role requirements can make a significant contribution. The display format should not merely reproduce conventional instruments in electronic form but should also employ the greater flexibility offered by this technology. For example, airliners such as the A320 provide trend arrows showing the predicted airspeed in 10 seconds in addition to airspeed limit markings that change with aircraft configuration. The provision of flight path vector information is another common example. Many electronic displays use digital formats to present numerical information. This type of presentation has the advantage of providing information to a high degree of accuracy and does not require the pilot to interpret a pointer position. However, these displays are very poor at presenting trends as the pilot has to interpret rapidly changing numbers. Data sampling rates can also pose problems. If this is set too high the presentation will be changing constantly, if it is set too low then it will introduce significant lag. Sometimes cockpit designers will combine digital displays with other
displays to attempt to get the advantages of greater accuracy but without the disadvantages of poor trend information. One example of this is the VIDS display in the Sikorsky Hawk (UH/SH-60) family of helicopters which uses strips of coloured LEDs together with digital displays at the top of each strip. Unfortunately strip instruments themselves are poor at displaying rate of change information. When assessing any display format, but especially digital or strip formats, conducting tasks that result in rapidly changing parameters is always important.
Any evaluation of electronic displays also includes an assessment of the appropriateness of the information displayed. The greater flexibility offered by these displays should be utilized to reduce the pilot’s workload by displaying information that is appropriate to the flight phase or mission task that the aircraft is in. Perhaps the best example of this approach again comes from the civil airliner world in the form of the Airbus aircraft. The A320 for instance automatically displays different systems pages depending on cabin door position, engine condition, airspeed, etc. System malfunctions result in the display of the appropriate systems page and automated checklists. A rotary example comes from the Merlin helicopter where raising the sonar body changes the cable angle display to a Doppler hover presentation. In all cases the test pilot uses his or her experience of the role to decide if the designer has produced the most appropriate display. Where displayed limitations change with aircraft state it is important that the change happens quickly. On some helicopters it has been found that the more restrictive Power-On rotor limits do not change quickly enough to the Power-Off limits when entering FIG leading to spurious NR overspeed warnings.
Arguably the most important consideration when dealing with electronic displays is the ease with which the pilot can see the information presented. This will be affected by a number of factors such as viewing angle, colours used, contrast ratio, refresh rate, and display brightness. With the earlier type of LCD the viewing angle could present problems for centrally mounted displays in aircraft with side-by-side seating. This has become less of a problem with active matrix LCDs. Colour is used extensively in cockpit displays both to present information and to warn or advise of changes in state such as entering a 5-minute rating band or arming of a missile. During an evaluation the appropriateness of the colours used is determined. Using colours to alert the pilot to approaching limits may not always be enough. On the EHI Merlin, for example, it was found that despite changing the torque display to red at high torque values a master caution caption had to be added to provide greater attention-getting qualities. All civil airworthiness authorities dictate which colours are permitted on flight displays and define the appropriate purpose for each colour. This ensures commonality on civil aircraft. Night operations can be a critical area for colour displays as, with the brightness dimmed down, a display that is satisfactory during daylight may be unsatisfactory at night. This is due to the colours becoming indistinct at low intensity and therefore any night evaluation concentrates on areas where colour is used as a primary source of information. The brightness of a display is assessed in the full range of lighting conditions including direct sunlight. In addition the control of brightness is checked. Associated with the overall display brightness is the contrast ratio, this is the ratio between the luminance of any display element and the background. The contrast ratio should be such that throughout the range of available screen brightness it is possible to identify the individual elements quickly and easily.