General

·        Each item of installed equipment must be labeled as to its identification, function, or operating limitations, or any applicable combination of these factors. [14 CFR 23.1301(b); 25.1301(b); 27.1301(b); 29.1301(b)]

See also: TSO-C165/RTCA DO-257A, 2.1.5.1 which is worded slightly differently.

·        The function and direction of motion of each command reference control, such as heading select or vertical speed, must be plainly indicated on, or adjacent to, each control if necessary to prevent inappropriate use or confusion. [14 CFR 25.1329(f)]

·        The applicant should show display text and auditory messages are distinct and meaningful for the information presented. [AC 25.1302-1, 5-5.b(4)(e)]

See also: AMC 25.1302, 5.4.2.e which is worded slightly differently.

·        Text and icons should be shown to be distinct and meaningful for the function(s) they label. [AC 25-11A, 31.c(2)(a)]

        See also: AMC 25.1302, 5.3.3(b) which is worded slightly differently.

·        Text should be shown to be distinct and meaningful for the information presented. Messages should convey the meaning intended. [AC 25-11A, 31.c(1)]

·        Labels are oriented to facilitate readability. [AC 25-11A, 31.c(2)(d)]

·        Regardless of the font type, size, color, and background, text should be readable in all foreseeable lighting and operating conditions from the flightcrew station (§ 25.1321(a)). [AC 25-11A, 31.c(1)(a)]

·        All displayed information such as symbols, graphics, and alphanumeric characters should be clearly differentiated from one another and legible under all ambient illumination conditions. [AC 23.1311-1C, 16.1]

See also: Chapter 3.3 Symbols

Control Labels

See also: Chapter 6.1 Controls: General

·        Each cockpit control, other than primary flight controls and controls whose function is obvious, must be plainly marked as to its function and method of operation. [14 CFR 25.1555(a), 27.1555(a), 29.1555(a)]

        See also: 14 CFR 23.1555(a) which is worded slightly differently.

·        Each secondary control must be suitably marked. [14 CFR 23.1555(b)]

·        Labels shall be used to identify the functions of all controls used to manipulate the information content and operating characteristics of the display. [TSO-C165/RTCA DO-257A, 2.1.5.1]

        Note: This requirement applies to standard mechanical controls (e.g., buttons, knobs, etc.). [TSO-C165/RTCA DO-257A, 2.1.5.1]

        See also: RTCA DO-256, 2.1.3.5 which is worded slightly differently.

·        Controls whose functions are not obvious should be marked or identified so that a flightcrew member with little or no familiarity with the airplane is able to rapidly, accurately, and consistently identify their functions. [AC 25-11A, 31.c(2)]

·        If a control performs more than one function the labels should include all intended functions, unless the function of the control is obvious. [AC 25-11A, 31.c(2)(b)]

        See also: Chapter 8 Intended Function

·        Labels of graphical controls accessed via a cursor control device should be included on the graphical display. [AC 25-11A, 31.c(2)(b)]

·        On multi-function displays, a label should be used to indicate the active function(s), unless its function is obvious. When the function is no longer active or being displayed, the label should be removed unless another means of showing availability of that function is used. [AC 25-11A, 31.c(2)(d)]

·        Terminology for labeling should describe the function of the control in meaningful terms. Terms should be consistent with those on the display of the function or mode selected and spelled out whenever possible. [TSO-C146c/RTCA DO-229D, 2.2.1.1.1.2]

·        Labels are the most common means used to identify and describe controls and other devices in the flight deck. They can be full text (e.g., “Standby”), abbreviated text (e.g., “STBY”), acronyms (e.g., “AGL” for “Above Ground Level”), as well as icons (e.g., for “On/Off”). [AC 20-175, 2-8.a]

        Note: While a limited number of control functions might have icons associated with them that pilots would likely know, most functions have no universally accepted icons. [AC 20-175, 2-8.a]

·        Control labels must be visible, legible, and understandable for the population of pilots that will use the controls, per § 2X.1555(a). [AC 20-175, 2-8.b]

·        Unless the control function and method of operation are obvious or indicated through other means (e.g., form, location), the control labeling scheme should clearly and unambiguously convey: [AC 20-175, 2-8.c]

        -   The current function performed by each control,

        -   The method for actuating the control when performing the current function.

        See also: AC 25.1302-1, 5-4.c(2)(a) which is worded slightly differently.

·        Labels and other information related to a control’s function and method of operation should be readable over a wide range of ambient illumination, including, but not limited to: [AC 20-175, 2-2.d]

        -   Direct sunlight on the controls;

        -   Indirect sunlight through a front window illuminating white clothing (reflections);

        -   Sun above the forward horizon and above a cloud deck in a flightcrew member’s eyes; and

        -   Night and/or dark environment.

        Consider the above conditions when evaluating controls, and show that the controls are acceptable. Compensating factors such as tactile characteristics, can also be included as part of the environment and use conditions. Special consideration is needed for controls whose function is affected by illuminated information (see paragraph 2-9 in this chapter), such as lighted switches and soft keys on displays. [AC 20-175, 2-2.d]

·        If a control can be used for multiple functions, the current function shall be indicated either on the display or on the control. [TSO-C165/RTCA DO-257A, 2.1.5.1]

        See also: AMC 25.1302, 5.3.3; TSO-C146c/RTCA DO-229D, 2.2.1.1.4.1; RTCA DO-256, 3.2.1.2.2 which are worded slightly differently.

·        Size control labels to be easily legible from the pilot’s normally seated position. [AC 20-175, 2-8.d]

·        Multi-function controls should be labeled such that the pilot is able to: [AC 25-11A, 41.a]

        -- Rapidly, accurately, and consistently identify and select all functions of the control device.

        -- Quickly and reliably identify what item on the display is “active” as a result of cursor positioning, as well as what function will be performed if the item is selected using the selector buttons and/or changed using the multi-function control.

        -- Determine quickly and accurately the function of the control without extensive training or experience.

·        Labels of graphical controls accessed by a cursor device such as a trackball should be included on the graphical display. When menus lead to additional choices such as submenus, the menu label should provide a reasonable description of the next submenu. [AC 25.1302-1, 5-4.c(2)(a)]

        See also: AMC 25.1302, 5.3.3.b which is worded slightly differently; Chapter 6.4.5 Cursor Control Devices (CCDs)

·        Use terms, icons, or abbreviations recommended in applicable FAA policy and other standards (e.g., International Civil Aviation Organization (ICAO), Document 8400, ICAO Abbreviations and Codes, Sixth Edition, date 2004 or SAE ARP 4105B*, Abbreviations and Acronyms for Use on the Flight Deck, reaffirmed June 2004), for labels, when available. Otherwise, use labels that are in general use in aviation. [AC 20-175, 2-8.e]

        See also: AMC 25.1302, 5.3.3.b which is worded slightly differently; TSO-C165/RTCA DO-257A, 2.2.2

·        For controls using icons in lieu of text labeling, substantiate that pilots, with the minimum expected training program, can adequately perform their duties at an acceptable level of workload, as required by normal, non-normal, and emergency situations. If appropriate, consider incorporating icons in controls to complement rather than replace text labels (e.g., continuous text display, temporary “mouseover” display). [AC 20-175, 2-8.f]

·        If multiple controls exist for the same function, clearly label all such controls. Exceptions can include alternate controls that provide flexibility to accommodate a wide range of pilots. For example, experienced users might choose less-intuitive methods in order to gain a performance advantage such as speed. Double-clicking or push-and-hold are examples that are generally not recommended as a sole method of operation, but may be acceptable as a secondary method (e.g., for advanced users). Show that multiple controls for the same function are acceptable, and do not result in confusion or inadvertent operation. [AC 20-175, 2-8.g]

·        If multiple controls exist (multi-crew aircraft) for the same function, show that there is sufficient information or other means available to make each crewmember aware of which control is currently functioning. [AC 20-175, 2-8.h]

See also: AC 25.1302-1, 5-4.c(3)(a) and AMC 25.1302, 5.3.3.c which are worded slightly differently.

·        Use only one abbreviation and/or one icon for labeling a function. This is to prevent confusion when a label appears in multiple locations. [AC 20-175, 2-8.i]

·        Ensure that the labels resist scratching, hazing, erasure, disfigurement, and other legibility degradation that might result from normal use. [AC 20-175, 2-8.j]

·        Indicate a control’s function in a manner that is readily discernable from the current state. For example, a button labeled “Track Up” should not represent the current display orientation of “Heading Up,” but should instead change the display orientation to “Track Up” when selected. [AC 20-175, 3-3.a]

·        Ensure that pop-up text that describes a control’s function does not result in unacceptable distractions, interference, or clutter. [AC 20-175, 3-3.b]

·        If a control activates several different functions based on sequential commands or selections, clearly label each of the functions. [AC 20-175, 3-3.c]

·        There should be a clear indication when any control is in an altered state and not the default (e.g., if a knob is pulled out and functions differently). [TSO-C146c/RTCA DO-229D, 2.2.1.1.4.1]

See also: Chapter 6.3 Operation of Controls

·        Control markings should be evaluated to ensure that a logical and consistent labeling convention has been applied throughout the cockpit. The evaluation should also consider electronic control labeling, particularly as applied across all display pages. It is important that the terminology chosen for that control function is immediately and clearly understood by the expected pilot population. The evaluation should verify that the terms chosen conform to standardized aviation conventions. [PS-ACE100-2001-004, Appendix A]

·        Pilots must be able to quickly and reliably identify the function being controlled by these software labels. The standard that should be applied is that pilots must be capable of performing control-related tasks to the same performance standards as would result from the use of conventional controls unless the decrement is inconsequential and the design enables other significant performance gains or design simplifications. [PS-ACE100-2001-004, Appendix A]

·        All control markings should be evaluated to ensure they are visible and evenly illuminated during both night and day operations. It should also be noted that font size (variations, e.g., character stroke size, width and height) of the illuminated displays can affect readability and perceived brightness. Variations in font size may create perceived lighting imbalances. (Reference ARP4103 for recommendations.) [PS-ACE100-2001-004, Appendix A]

Data Field Labels

·        Fields that are editable, selectable, or require operator entry should be clearly denoted. [TSO-C146c/RTCA DO-229D, 2.2.1.1.4.1]

·        Data fields should be uniquely identified either with the unit of measurement or a descriptive label. However, some basic “T” instruments have been found to be acceptable without units of measurement. [AC 25-11A, 31.c(2)(c)]

        See also: TSO-C165/RTCA DO-257A, 2.2.4 which is worded slightly differently.

Function Labels

·        RTCA DO-229D, Table 2-6 lists potential functions and indications, and provides the associated label or message.  Not all of these functions are required.  If a function is implemented as a discrete action, the equipment shall use the labels or messages in the Table.  If several of the following functions are accomplished as a discrete action, one of the applicable labels in the table shall be used (e.g., suspend automatic sequencing and accessing the ability to select a course to or from a waypoint would be labeled “DCRS”). Except for waypoint identifiers, these abbreviations shall not be used to represent a different term. [TSO-C146c/RTCA DO-229D, 2.2.1.1.6]

(RTCA DO-229D, Table 2-6 is included in Examples)

Labels for Fixes, Waypoints, and other Symbols

·        Labels shall be used to identify fixes, other symbols, and other information, depicted on the display, where appropriate. [TSO-C165/RTCA DO-257A, 2.2.2]

·        Fix labels shall be oriented to facilitate readability. [TSO-C165/RTCA DO-257A, 2.2.2]

        Note: One method of compliance is to continuously maintain an upright orientation. [TSO-C165/RTCA DO-257A, 2.2.2]

·        If named automatically, the equipment should label the flight plans with the departure and arrival airports when any flight plan is presented for review, edit, activation, or deletion.  If no departure or arrival airport is identified, the flight plan should be labeled with the first and last waypoints as appropriate. [TSO-C146c/RTCA DO-229D, 2.2.1.2.1]

·        Waypoint names shall be consistent with published names. [TSO-C146c/RTCA DO-229D, 2.2.1.2.1(b)]

·        Airport identifiers shall be accessible using standard ICAO nomenclature when available (e.g., KJFK). [TSO-C146c/RTCA DO-229D, 2.2.1.2.1(b)]

Bearing Labels

·        All bearing data fields shall be labeled as “°” to the right of the bearing value.  All true bearing data fields shall be labeled as “°T” to the right of the bearing value. The “°T” label could be indicated with a one or two characters. (This applies to all courses, tracks, and bearings). [TSO-C146c/RTCA DO-229D, 2.2.1.1.4.8]

        See also: TSO-C165/RTCA DO-257A, 2.2.4 which is worded slightly differently.

Label Placement/Location

·        The spatial relationships between labels and the objects that they reference should be clear, logical, and, where possible, consistent. [TSO-C165/RTCA DO-257A, 2.2.2]

        See also: AC 25-11A, 31.c(2)(d) which is worded slightly differently; Chandra et al., 2003; RTCA DO-256, 2.1.3.5

·        Labels should be placed such that: [AC 25-11A, 31.c(2)(d)]

        •  The spatial relationships between labels and the objects they reference are clear.

        •  Labels for display controls are on or adjacent to the controls they identify.

        •  Labels for display controls are not obstructed by the associated controls.

        •  Labels are oriented to facilitate readability. For example, the labels continuously maintain an upright orientation or align with an associated symbol such as a runway or airway.

·        Labels should be unobstructed by controls when viewed within the angle of regard and located next to or on the controls that they reference. [TSO-C146c/RTCA DO-229D, 2.2.1.1.1.2]

        See also: AC 25-11A, 31.c(2)(d); AC 20-138, 11-8.a(11); McAnulty, 1995; RTCA DO-256, 2.1.2.2

·        Label placement should follow a consistent logic. [TSO-C146c/RTCA DO-229D, 2.2.1.1.1.2]

·        Markings and labels should be positioned such that their meaning is clear yet they do not hinder interpretation. [AC 25-11A, 31.c(4)(b).3]

        See also: Chapter 3.4 Markings, Dials, Tapes, and Numeric Readouts

Viewing Distance

·        All labels shall be readable at a viewing distance of 30 inches under the full range of normally expected flight deck illumination conditions (Reference MIL STD 1472D and SAE AIR 1093**). [TSO-C165/RTCA DO-257A, 2.2.2]

        See also: TSO-C146c/RTCA DO-229D, 2.2.1.1.1.2 which is worded slightly differently.

        Note: The size of numbers and letters required to achieve acceptable readability may depend on the display technology used. [TSO-C165/RTCA DO-257A, 2.2.2]

·        Display of letters and numbers depicting primary data shall be readable from viewing distances of 30 inches under anticipated lighting conditions. [TSO-C146c/RTCA DO-229D, 2.2.1.1.4.5]

Consistency of Labels

·        Labels should be consistent with related labels located elsewhere in the flight deck. [AC 25-11A, 31.c(2)(c)]

·        Label terminology and abbreviations used for describing control functions and identifying controls should be consistent with RTCA DO-257A, Appendix A. [TSO-C165/RTCA DO-257A, 2.2.2. See Examples)

·        Labels for mode and source selection annunciators should be compatible throughout the cockpit. [AC 23.1311-1C, 18.2]

·        Acronyms and labels should be used consistently, and messages/annunciations should contain text in a consistent way. Inconsistencies should be evaluated to ensure that they are not susceptible to confusion or errors, and do not adversely impact the intended function of the system(s) involved. [AC 25-11A, 31.b]

        See also: Wright and Barlow, 1995; Chapter 3.1 Electronic Display Information Elements and Features: General; Chapter 8 Intended Function

·        Annunciations/labels on electronic displays should be identical to labels on related switches and buttons located elsewhere on the flightdeck. If display labels are not identical to related controls, the applicant should show flightcrew members can quickly, easily, and accurately identify associated controls so they can safely perform all the tasks associated with the systems’ and equipment’s’ intended function (§ 25.1302). [AC 25.1302-1, 5-4.e(2)(e)]

See also: AMC 25.1302, 5.3.5(b), which is worded slightly differently; Chapter 8 Intended Function

·        When a control or indication occurs in multiple places (for example, a “Return” control on multiple pages of a flight management function), the label should be consistent across all occurrences. [AC 25-11A, 31.c(2)(c)]

·        It is also important to have consistent function labels; a function should have the same name regardless of the display page on which it appears. Evaluations should be performed to determine that labels are consistently placed on the same key on the display when pages are changed. The evaluator should ensure that all identical functions that are available across multiple screens or pages are consistently mapped to the same control to the maximum extent possible. One must also assess whether frequently used functions are readily accessible. [PS-ACE100-2001-004, Appendix A]

Icons

·        When using icons instead of text labels, only brief exposure to the icon should be needed in order for the flightcrew to determine the function and method of operation of a control. The use of icons should not cause flightcrew confusion. [AC 25-11A, 31.c(2)(e)]

        See also: AMC 25.1302, 5.3.3(b) which is worded slightly differently.

·        When using icons instead of text labeling, the applicant should show the flightcrew requires only brief exposure to the icon to determine the function of a control and how it operates. Based on experience, the following guidelines for icons have been shown to lead to usable designs. [AC 25.1302-1, 5-4.c(2)(d)]

        1  The icon should be analogous to the object it represents.

        2  The icon should be in general use in aviation and well known to flightcrews.

        3  The icon should be based on established standards, when they exist, and conventional meanings.

        See also: AMC 25.1302, 5.3.3(b) which is worded slightly differently.

·        In all cases, the applicant should show the use of icons to be at least equivalent to text labels in terms of flightcrew task accomplishment speed and rate of errors made. Alternatively, the applicant should show that icons not equivalent to text labels have no unacceptable effect on safety or flightcrew workload (e.g. incurred task times) and do not confuse the flightcrew. [AC 25.1302-1, 5-4.c(2)(e)]
See also: AMC 25.1302, 5.3.3(b) which is worded slightly differently.

·        If icons are used, the level of pilot performance should not be reduced when compared with performance obtained using text labeling, and as measured using time to interpret and accuracy of interpretation of that control function. [PS-ACE100-2001-004, Appendix A]

Capitalization

·        Standard grammatical use of upper and lower case letters is recommended for lengthy documentation and lengthy messages. Using this format is also helpful when the structure of the text is in sentence form. [AC 25-11A, 31.c(1)(a)]

·        The use of only upper case letters for text labels is acceptable. [AC 25-11A, 31.c(1)(a)]

Font

·        To facilitate readability, the font chosen should be compatible with the display technology. [AC 25-11A, 31.c(1)(b)]

·        Serif fonts may become distorted on some low pixel resolution displays. However, on displays where serif fonts have been found acceptable, they have been found to be useful for depicting full sentences or larger text strings. Sans serif fonts (for example, Futura or Helvetica) are recommended for displays viewed under extreme lighting conditions. [AC 25-11A, 31.c(1)(b)]

·        Sans serif fonts (for example, Futura or Helvetica) are recommended for displays viewed under extreme lighting conditions. [AC 25-11A, 31.c(1)(b)]

·        Alphanumeric fonts should be simple and without extraneous details (e.g., sans serif) to facilitate readability. [TSO-C165/RTCA DO-257A, 2.2.2]

See also: TSO-C146c/RTCA DO-229D, 2.2.1.1.5.1 which is worded slightly differently.

·        An alphanumeric font should be of a sufficient thickness and size to be readable when the flightcrew are seated at the normal viewing distance from the screen. [AC 25.1322-1, Appendix 1, 2.b(6)]

        Note 1:   Minimum character height of 1/200 of viewing distance is acceptable (for example, a viewing distance of 36 inches requires a 0.18 inch character height on the screen) per DOD-CM-400-18-05, Department of Defense User Interface Specifications for the Defense Information Infrastructure, Defense Information Systems Agency, February 1998, p 12-1). [AC 25.1322-1, Appendix 1, 2.b(6)]

        Note 2:   Arial and sans serif fonts are acceptable for visual alert text. The size of numbers and letters required to achieve acceptable readability depends on the display technology used. Stroke width between 10% and 15% of character height appears to be best for word recognition on text displays. Extensions of descending letters and ascending letters should be about 40% of letter height. [AC 25.1322-1, Appendix 1, 2.b(6)]

        Note 3:   Different fonts can be used to differentiate between new and previously acknowledged visual alert information. [AC 25.1322-1, Appendix 1, 2.b(6)]

See also: Chapter 4 Considerations for Alerting

Character Size

·        The required size may depend upon the display technology used.  Initial guidelines for symbol sizes for the indicated categories of information are: [TSO-C146c/RTCA DO-229D, 2.2.1.1.4.5]

                0.18” for primary data

                0.12” for secondary data

        0.09” for legends

        Note:  Installation restrictions may be used to compensate for limitations of display designs. [TSO-C146c/RTCA DO-229D, 2.2.1.1.4.5]

Character Spacing

·        Break lines of text only at spaces or other natural delimiters. [AC 25-11A, 31.c(1)(a)]

Terminology

·        Equipment should display standard and/or non-ambiguous abbreviations and nomenclature, which should be consistent within a function and across the flightdeck. [AC 25.1302-1, 5-5.b(4)(e); AMC 25.1302, 5.4.2.e]

        See also: AC 25-11A, 31.c(2)(a) which is worded slightly differently.

·        Abbreviations and acronyms should be clear and consistent with established standards. [AC 25-11A, 31.c(1)]

·        Avoid abbreviations and acronyms where practical. [AC 25-11A, 31.c(1)(a)]

·        Avoid contractions, such as “can’t” instead of “cannot.” [AC 25-11A, 31.c(1)(a)]

·        Another way is to standardize aspects of the design by using accepted, published industry standards such as the labels and abbreviations recommended in ICAO 8400/5. [AC 25.1302-1, 5-8.b(2); AMC 25.1302, 5.7.2]

General

·        Letters and numbers that are easily confusable should be easily distinguished. (McAnulty, 1995)

·        The format or position of the label should be distinctive so that it can be easily distinguished from other display features. (Smith and Mosier, 1986)

Control Labels

·        Soft control labels (e.g., response options associated with line select keys that may change depending upon what page is displayed) should be displayed in a consistent location on all display screens. [RTCA DO-256, 2.1.3.5]

·        Soft control labels shall be unambiguously associated with the control they label (e.g., either through location or through an indicator of which control is associated with the label). [RTCA DO-256, 2.1.3.5]

·        Labels should not be placed directly on rotating controls because the orientation of the control will change when it is manipulated. (McAnulty, 1995)

·        Lines should be used to connect soft labels to the controls they identify to minimize parallax issues. (Yeh, 2004)

·        Soft function key labels should be drawn in a reserved space outside of the main content area. (Chandra et al., 2003)

·        When used for function selection, the selected function shall be clearly indicated and the selector position shall be identified by a distinguishable detent. [SAE ARP4102, 5.3.1.3]

·        The face of the rotary control shall be clearly marked to enhance identification of the control position. [SAE ARP4102, 5.3.1.4]

Data Field Labels

·        If symbols are used to label data, the system shall use appropriate symbols for degrees, minutes, and seconds to improve readability (DOD-CM-400-18-05, p 12-2). [RTCA DO-256, 2.1.3.5]

·        Numeric message fields shall include a display of labels or units of measure for altitude, heading, and speed. [RTCA DO-256, 2.1.3.5]

·        Data field labels shall be located sufficiently close to, but separated by at least one space from, the associated data field (Smith and Mosier, 1986). [RTCA DO-256, 2.1.3.5]

·        The location of data field labels should be consistent. (McAnulty, 1995)

·        Prompts for acceptable data formats and values should be provided. (McAnulty, 1995)

Labels for Fixes, Waypoints, and other Symbols

·        Significant features on a moving map should be labeled if doing so does not create additional clutter. The labels should be placed in a consistent location in relation to the features designated. (McAnulty, 1995)

·        Alphanumeric data and legends on map displays should not obscure moving symbols or tracks. (McAnulty, 1995)

·        Except for numbers on the heading scale, alphanumeric data and legends should remain upright with map rotation. (McAnulty, 1995)

Text Labels

·        Labels should be concise with minimal redundancy. (McAnulty, 1995)

·        All abbreviations and acronyms used should be listed and defined in user documentation.

·        Abbreviations should not be used for words that have four letters or less, unless the abbreviation and the word are synonymous. (McAnulty, 1995)

Label Placement/Location

·        If the system dynamically labels controls to indicate message response options, then response options that accept the message (e.g., WILCO, AFFIRMATIVE) on one screen shall not be presented in the same location as responses that reject the message (e.g., UNABLE, NEGATIVE) on another screen. [RTCA DO-256, 2.1.3.5]

·        Labels should be oriented horizontally. (McAnulty, 1995; Chandra et al., 2003)

·        Labels should not obscure other needed information. (McAnulty, 1995)

Consistency of Labels

·        Wording, abbreviations, and acronyms for different displays or controls should be distinct. The same word or words with similar meanings should not be used in different labels as the similarity may be confusing. (McAnulty, 1995)

Capitalization

·        Capitalization should be used sparingly. It is appropriate for single words, acronyms, or labels. (Ahlstrom and Longo, 2003)

·        Mixed case should be used for continuous text, messages, menu descriptions, button descriptions, or screen identification. (Ahlstrom and Longo, 2003)

Font

·        The display shall use an alphanumeric font of a sufficient thickness and size to be readable when users are seated at the normal viewing distance from the screen. [RTCA DO-256, 2.1.3.5 and 3.1.1.3]

·        Serif fonts are recommended if the display resolution is high or the typeface is large so that the serifs are not distorted. (Ahlstrom and Longo, 2003)

·        Sans serif fonts are recommended for low-resolution displays or small text. (Ahlstrom and Longo, 2003)

Character Size

·        At a minimum, character height and symbol size should be 1/200 of viewing distance (e.g., a viewing distance of 36 inches requires a .18 inch character height on the screen) (DOD-CM-400-18-05, p.12-1). [RTCA DO-256, 2.1.3.5 and 3.1.1.3]

·        The minimum visual angle for characters should be 16 minutes of arc. However, a minimum visual angle of 24 minutes of arc is preferred for characters that must be read under aircraft environmental conditions. (Ahlstrom and Longo, 2003; McAnulty, 1995)

        Note: 1 degree = 60 minutes of arc (Ahlstrom and Longo, 2003; McAnulty, 1995)

·        The ratio of character height to width should be: (Ahlstrom and Longo, 2003; McAnulty, 1995)

        a)     At least 1:0.7 to 1:0.9 for equally spaced characters and lines of 80 or fewer characters

        b)    At least 1:0.5 if it is necessary to have more than 80 characters per line, or

        c)     As much as 1:1 for characters such as “M” and “W” for proportionally spaced characters.

·        The stroke width for black characters on a white background should be 0.1667 to 0.1429 of the height of the character. If legibility at night is important, then the stroke width for black characters on a white background should be from 0.1429 to 0.125 of the height of the character. (MIL-STD-1472G, 5.4.6.3.7.a-b)

·        Text that is in color should be presented at a larger size than black text (on a white background) to provide similar legibility. (Cardosi and Murphy, 1995)

Character Spacing

·        The minimum spacing between characters or scale graduation marks should be one stroke width or 10% of the character height, whichever is greater. (McAnulty, 1995; Ahlstrom and Longo, 2003)

·        The minimum spacing between words should be one character width for equally spaced characters or the width of the capital letter “N” for proportionally spaced characters. (Ahlstrom and Longo, 2003)

·        The minimum spacing between lines in a display should be two stroke widths or 15% of character height, whichever is greater. (McAnulty, 1995; Ahlstrom and Longo, 2003)

Contrast Ratio

·        The contrast of all symbols and text shall be no less than 3:1. [RTCA DO-256, 2.1.3.2 and 3.1.1.3]

        Note:  The American National Standards Institute (ANSI) recommends a contrast ratio of 7:1 for alphanumeric characters, and cites 3:1 as a minimum (ANSI, 1988). The International Civil Aviation Organization (ICAO,1993) recommends a contrast ratio of 8:1 for items (such as data blocks) that need to be read.  For details that do not need to be read, such as maps and range rings, a contrast ratio of 3:1 is acceptable. [RTCA DO-256, 2.1.3.2 and 3.1.1.3]

·        For optimum legibility, character contrast should be between 6:1 and 10:1. (Ahlstrom and Longo, 2003)

Terminology

·        Familiar terms and symbols should be used. (McAnulty, 1995)

·        Cultural conventions should be considered in the selection of graphical elements, display of text, and word choice.