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Chapter 2. General Controls Guidance
2-1. General Controls Design Philosophy .
a. If you are an applicant, document and follow a design philosophy for controls, which supports the intended functions (14 CFR 2x.1301). The documented design philosophy may be included as part of a system description, certification plan, or other document that is submitted to the FAA during a certification project. The design philosophy should include a high level description of controls features, such as labeling, feedback, automated behavior, and error recovery. Also include a high level description of human performance considerations, such as flightcrew workload, error potential, and expected training requirements.
b. Apply a particular design philosophy consistently throughout the flight deck to the greatest extent practicable.
2-2. Environment and Use Conditions
a. Consider a variety of environments, use conditions, and other factors that can impact flightcrew interaction with controls during aircraft operations that can be reasonably expected in service, including:
Appropriate representation of pilot population;
Bright and dark lighting conditions;
Use of gloves;
Turbulence and other vibrations;
Interruptions and delays in tasks;
Objects that may physically interfere with the motion of a control;
Incapacitation of one pilot (multi-crew aircraft);
Use of the non-dominant hand; and
Excessive ambient noise.
b. Since all possible environment and use conditions cannot be specifically addressed, develop a representative set that includes nominal and worst cases. These cases should cover the full environment in which the system is assumed to operate, given its intended function. This includes operating in normal, non-normal, and emergency conditions. The following paragraphs describe the above list of environment and use conditions in more detail.
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c. Appropriate Representation of Pilot Population. Controls are designed with an assumption of a certain range of pilot attributes. These assumptions may include physical attributes, such as body size and proportion, and non-physical attributes, such as experience with a given type of controls.
(1) Design the controls to provide acceptable performance for a broad range of pilot physical attributes. The appropriate pilot representation is key in demonstrating compliance to the applicable regulations. For example, buttons that are too small for a given finger size can be prone to usability problems, such as finger positioning errors, finger slippage, inadequate feedback, insufficient label size, and inadvertent operation.
(2) In some cases, certain attributes of the pilot population are addressed within controls-related regulations. For example, 14 CFR 2x.777 requires controls to be located and arranged in a manner that provides full and unrestricted movement of controls.
Title 14 CFR 25.777 specifies a pilot population that ranges in height from 5’2” to 6’3”. Title 14 CFR 27.777 and 29.777 specify a pilot population that ranges in height from 5’2” to 6’0”. (Part 23 does not specify a pilot population.) To show compliance with this aspect of the regulation, show that all positions of the control fall within the reach envelopes of the intended pilot population.
(3) Since human dimensions can vary greatly, the environment and use conditions should account for such variations. Some acceptable means of accounting for human size variations include:
Selecting individuals for testing based on reference to an anthropometric database. Anthropometric databases contain information collected from comparative studies of human body measurements and properties;
Supplementing physical mock-ups with computer anthropometrically-based models; or Comparing physical measurements of control positioning relative to physical measurements in anthropometric databases.
(4) In addition to considerations for unrestricted movement of controls, anthropometric data is important for many other performance considerations, including inadvertent activation and physical workload. Controls-relevant anthropometric data also includes hand and finger size.
(5) Consider other pilot characteristics that are relevant to controls, beyond anthropometrics. Other important physiological pilot characteristics can include physical strength, visual acuity, and color perception. Pilot cultural characteristics are also relevant in the selection of graphical elements, word choice, and certain control features (e.g., menu structure).
It is generally appropriate to assume minimal prior pilot experience with a given control.
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(6) Environment and use conditions should cover a range of pilot characteristics relevant to controls in order to represent the intended pilot population. Show not only that the controls are acceptable over this range, but also provide data describing the range of pilot characteristics, and how this range represents the intended pilot population.
d. Bright and Dark Lighting Conditions.
(1) Controls should be operable under foreseeable lighting conditions. 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:
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.
(2) 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.
e. Use of Gloves. Pilots might wear gloves during operations, such as in cold weather.
Design assumptions regarding skin contact (e.g., tactile feedback, system capacitive sensing), finger size (e.g., button spacing), and other finger characteristics alone might not adequately cover situations in which pilots wear gloves. Therefore, include gloved pilot operations in environment and use conditions. In cases where controls cannot be operated with gloves, clearly describe any limitations or methods for determining limitations, in the aircraft flight manual or flight manual supplement, as appropriate.
f. Turbulence and Other Vibrations.
(1) Ensure that controls are operable during vibrations. Vibrations affect not only the ability of pilots to intentionally activate a control, but also can affect inadvertent activation and awareness of activation. Vibrations can be caused by turbulence, propulsion systems, or other means.
(2) Title 14 CFR 25.771(e), 27.771(c), and 29.771(c) require that vibration and noise characteristics of cockpit equipment not interfere with the safe operation of the aircraft.
Theoretical analysis alone is insufficient for demonstrating compliance, but it can be complementary. Therefore, also show through other means, such as test or demonstration, that 6
the control is acceptable over a range of vibration environments for the intended aircraft and operations. Multifunction controls tend to be particularly susceptible to vibrations (see chapter
3). For functions with multiple means of control access, ensure that at least one of the controls is operable during vibrations.
g. Interruptions and Delays in Tasks. Some control operations involve multiple steps such that interruptions and delays might affect successful completion. For example, pilots might forget to complete a task they started (e.g., air traffic control calls), or they might not understand how the system behavior accommodates unfinished tasks (e.g., data entry timeouts). In environment and use conditions, include interruptions and delays during pilot-system interaction tasks to understand if the controls’ behavior results in any safety-critical consequences.
h. Objects that Can Physically Interfere with the Motion of a Control. Ensure the control motion is not obstructed by other objects in the flight deck. Title 14 CFR 2x.777 addresses unrestricted movement of each control without interference. Interference can potentially arise not only from other components of the flight deck and the flightcrew's clothing, but also from objects that are brought on board. Such objects include electronic flight bags (EFBs), headsets, pilot kneeboards, flashlights, and portable electronic devices (PEDs).
Consider objects that might reasonably be in the flight deck for physical interference, state the objects considered, and show that the control is acceptable in the presence of these objects.
i. Incapacitation of One Pilot (Multi-Crew Aircraft). For aircraft that are designed for multi-crew operation, the incapacitation of one pilot must be considered in the determination of minimum flightcrew, per § 2X.1523. Any control required for flight crewmember operation in the event of incapacitation of other flight crew member(s), in both normal and non-normal conditions, must be viewable, reachable, and operable by flightcrew members, from the seated position (§ 23.777(b), § 25.777(c), § 27.777(b), and § 29.777(b)).
j. Use of the Non-dominant Hand. Controls should be useable by both left-handed and right-handed pilots. Give special consideration to controls that require speed or precision in force or motion (e.g., cursor control devices) and controls designed to be operated with one specific hand (e.g., controls that can only be reached with the right hand).
k. Excessive Ambient Noise. Controls should be operable during conditions of excessively high ambient noise (e.g., from engines, airflow). Make noise conditions represent what can reasonably be expected for the intended aircraft and operation. Excessive ambient noise interferes with the ability of pilots to hear aural feedback and other sounds important to control functions. In addition, excessive ambient noise can sometimes interfere with non-aural sensory modes. When aural feedback is a control feature, it may be necessary to incorporate other sensory feedback as well (e.g., visual, tactile).
2-3. Controls Layout and Organization. Use the following guidance to the extent practicable (considering system tradeoffs) when developing a layout and organization scheme that will support pilot performance of system functions:
a. Group and arrange controls logically, such as by function or by sequence of use.
b. For functions that are frequently used by the flightcrew, controls should be readily accessible.
c. Position controls to allow a clear view of their related elements (e.g., displays, indications, labels) when in operation. In general, placing controls below the display or to one side of these elements will minimize visual obstruction during control operation.
d. Arrange controls to be easily associated with their related elements (e.g., displays, indications, labels). Ensure the association is readily apparent, understandable, and logical (e.g., line select function keys that align with adjacent text on a display). Give special consideration to large spatial separations between a control and its indication or display.
e. Position controls that are common to multiple tasks (e.g., numerical entry controls, “enter” or “execute” keys) to be easily accessible to the pilot and easily associated with their function(s). Position controls to prevent substitution errors, which can arise from inconsistent control placements.
f. Ensure that hand-operated controls are operable with a single hand. The remaining hand will then be free to operate the primary flight controls.
g. Ensure that maintenance functions or other functions not intended for pilot use are not readily accessible by pilots during operations.
2-4. Movement of Controls. Control devices typically transform their movement and/or force to achieve a control’s function. Ensure that the interaction between a control and its related elements (e.g., aircraft systems, displays, indications, labels) are readily apparent, understandable, logical, and consistent with applicable cultural conventions and with similar controls in the same flight deck. Table 1 provides examples of conventiona l relationships between the movement of a control and its function.
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• 12/08/2011 AC 20-175 Table 1. Examples of Conventional Relationships between Control Functions and
Movements.
Function Direction of Movement Increase Up, Right, Clockwise, Forward, Push-in, Depress Decrease Down, Left, Counter-Clockwise, Rearward, Pull-out, Release On Off Up, Right, Depress, Clockwise, Forward Down, Left, Release, Counter-Clockwise, Rearward Raise Lower Up, Rearward, Pull-out Down, Forward, Push-in Right Left Right, Clockwise Left, Counter-Clockwise
a. Interpretation of the terms used in table 1 may be dependent upon the specific function, or upon the installation location of the control. For example, the interpretation of "increase" is dependent upon the parameter that the control modifies (e.g., magnification and range scale are inversely related, so as one increases, the other decreases). Or, a device with a linear movement might be oriented vertically, horizontally or in between, such that "pull-out" can result in an "up" or "down" component, depending on installation location in the flight deck.
b. The conventions in table 1 are examples only and might be conflicting or ambiguous due to installation location, cultural convention, flight deck consistency, or otherwise.
2-5. Sensitivity and Gain of Controls. Since many controls transform their movement and force to achieve a function, the gain or sensitivity is a key design parameter. In particular, it strongly affects the tradeoff between task speed and error. High gain values tend to favor pilot comfort and rapid inputs, but can also contribute to errors (e.g., overshoot, inadvertent activation). Low gain values tend to favor tasks that require precision, but can also be too slow for the task. Gain and sensitivity of the control typically need to be traded off to support the intended function. Give special consideration to variable-gain controls. Accurately replicate the response lag and control gain characteristics that will be present in the actual airplane, and show that gain and sensitivity of the control are acceptable for the intended function.
2-6. Feedback from Controls.
a. Design the controls to provide feedback to the pilot when operated. Feedback from controls provides pilots with awareness of the effects of their inputs, including the following effects, as applicable:
Physical state of the control device (e.g., position, force);
State of data construction (e.g., text string);
State of activation or data entry (e.g., "enter");
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State of system processing;
State of system acceptance (e.g., error detection); and State of system response (e.g., cursor position, display zoom, autopilot disconnect).
b. Feedback can be visual, aural, and/or tactile. If feedback/awareness is required for safe operation, it should be provided to inform the flightcrew of the following conditions:
State of activation or data entry;
State of system processing (for extended processing times); and State of system response, if different from the commanded state.
c. Provide clear, unambiguous, and positive feedback to indicate the successful or unsuccessful actuation of a control action. Feedback within the control device (such as the tactile snap of a switch) without any other system effect should not be the sole means of detecting the actuation of a control.
d. The type, response time, duration, and appropriateness of feedback will depend upon the pilots’ task and the specific information required for successful operation.
e. The final display response to control input should be fast enough to prevent undue concentration being required when the flightcrew sets values or display parameters (§ 2X.771(a)). The specific acceptable response times depend on the intended function.
f. Once a control device is activated, if processing time is extended it might be appropriate to display progress to provide the pilot with a sense of time remaining for completion.
g. If control device position is the primary means of indicating the status of a function
(e.g., switch in the Up position indicates that the function is On), the control position should be obvious from any pilot seat.
h. When a control is used to move an actuator through its range of travel, the equipment should provide operationally significant feedback of the actuator’s position within its range.
i. Show that feedback is adequate in performance of the tasks associated with the intended function of the equipment.
2-7. Identifiable and Predictable Controls.
a. Pilots must be able to identify and select the current function of the control with speed and accuracy appropriate to the task, per § 2X.777(a). Make the function and method of operation of a control readily apparent (i.e., predictable and obvious), so that little or no familiarization is needed. Show that the intended pilot population can rapidly, accurately and consistently identify and execute all control functions, assuming qualified and trained pilots.
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b. Controls can be made distinguishable or predictable by differences in attributes such as form, color, location, and labeling. For example, buttons, which are pushed, should be readily discernable from knobs, which are rotated. Control shapes that are easily determined with tactile senses can improve ease of operation, particularly during periods when pilot tasks require significant visual attention.
c. If color is used for coding task-essential information, use at least one other distinctive coding parameter (e.g., size, shape, label). Whenever possible, color coding should be consistent across all controls and displays. Consider the effect of flight deck lighting on the appearance of the label, and the use of colors throughout the flight deck (i.e., color philosophy).
2-8. Labeling of Controls.
a. 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”).
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.
b. Control labels must be visible, legible, and understandable for the population of pilots that will use the controls, per § 2X.1555(a).
c. 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:
The current function performed by each control, The method for actuating the control when performing the current function.
d. Size control labels to be easily legible from the pilot’s normally seated position. SAE International, (SAE) Aerospace Standard (ARP) 4102/7, Electronic Displays, dated July 2007 provides guidelines on font sizes that are generally acceptable.
e. 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.
f. 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 11
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).
g. 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.
h. 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.
i. Use only one abbreviation and/or one icon for labeling a function. This is to prevent confusion when a label appears in multiple locations.
j. Ensure that the labels resist scratching, hazing, erasure, disfigurement, and other legibility degradation that might result from normal use.
2-9. Controls Lighting.
a. For controls with visual markings that are intended for use in low-light conditions, the markings must be lighted in some way that allows them to be easily read, for compliance with § 2X.1555(a) and § 2X.1381(a). This lighting might be from an external source (e.g., cockpit) or from an internal source (e.g., backlighting or luminous controls). Controls should be visible over a wide range of ambient light conditions (see paragraph 2- 2d).
b. Ensure that lighting of controls is consistent with flightcrew alerting such as warning, caution, and advisory lights (§ 2X.1322).
c. For low-light conditions, make lighted controls dimmable to brightness levels commensurate with other flight deck instrument lighting. This allows for the flightcrew’s adaptation to the dark, so controls are legible, and outside vision is maintained.
d. Ensure that lighting of controls from an internal source is not dimmable to brightness levels so low that the controls appear inactive.
e. Ensure that lighting of controls from an internal source does not produce light leaks, bright spots, or reflections from the windshield that can interfere with pilot vision or performance.
f. Automatic adjustment of lighted controls may be employed. Consider preference differences in multi-crew operations.
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g. Ensure that lighted controls intended for operation in a night vision imaging system (NVIS) lighting-modified cockpit meets (a) through (e) above, and are compatible with night vision goggles (NVG). For additional guidance see:
AC 27.1, Certification of Normal Category Rotorcraft, refer to miscellaneous guidance (MG) 16, AC 29.2, Certification of Transport Category Rotorcraft, MG 16, or RTCA, Inc. (RTCA) document (DO) RTCA/DO-275, Minimum Operating Performance Standards for Integrated Night Imaging System Equipment, dated October 12, 2001.
Note: NVIS lighting must allow color transmission to meet aircraft certification regulations (e.g., §§ 2X.1381, 2X.1555). For controls that do not need color discrimination, NVIS A lighting can be used. For controls where color discrimination is needed, then NVIS lighting must (§ 2X.1555) allow the pilot to easily discern the required colors (typically accomplished using NVIS white).
Make perceived color for reds, yellows (or ambers), and greens the same across the cockpit.
2-10. Preventing Inadvertent Operation of Controls.
a. Protect controls against inadvertent operation. This type of error can occur for various reasons, such as when a pilot accidentally bumps a control, or accidentally actuates one control when intending to actuate a different control.
b. Provide mitigation for inadvertent operation as appropriate. Consider these questions when designing and installing the control:
Are there any safety-critical consequences if the pilot is not aware of the inadvertent operation?
What will the pilot need to do to correct an inadvertent operation?
Is the control designed to support “eyes free" use (i.e., when the pilot is not looking at the control)?
Are there aspects of the design that will decrease the likelihood of inadvertent operation?
Are there aspects of the design that will increase the likelihood of the pilot detecting an inadvertent operation?
c. The following paragraphs provide multiple methods that reduce the likelihood of inadvertent operation of controls.
(1) Location & Orientation. Title 14 CFR 2x.777 requires controls to be located to prevent inadvertent operation. Locate, space, and orient controls so that the operator is not likely to strike or move them accidentally in the normal sequence of control movements. For example, switches located close to a frequently-used lever could be oriented so the axis of rotation for the switches is perpendicular to the axis of rotation for the lever.
(2) Physical Protection. Physical obstructions can be built into the design of a control to prevent accidental actuation of the control. Examples include: recessed controls, shielded controls, flip-covers, and guards. Make physical protections so they do not interfere with the visibility or operation of the protected device or adjacent controls. Physical protections should be appropriately durable to ensure continued airworthiness.
(3) Slippage Resistance. The physical design and materials used for controls can reduce the likelihood of finger and hand slippage (especially in the presence of vibration). For example, buttons can be designed with concave, textured, or tacky upper surfaces to prevent finger slippage.
(4) Hand Stabilization. Provide hand rests, armrests, or other physical structures as a stabilization point for the pilot’s hands and fingers when they are operating a control. This can be particularly useful for controls used in the presence of turbulence and other vibration, helping the pilot make more precise inputs.
(5) Logical Protection. Software-based controls and software-related controls may be disabled at times when actuation of the control would be considered inappropriate, based on logic within the software. Make disabled (inactive) controls clearly discernable from active controls.
(6) Complex Movements. The method of operation for a control can be designed so that complex movement is required to actuate it. For example, a rotary knob can be designed so that it can only be turned when it is also being pulled out. Double-click or push-and-hold methods are not recommended methods of protection.
(7) Tactile Cues. The surfaces of different controls can have different shapes and textures, supporting the pilot in distinguishing different controls when operating in a dark or otherwise “eyes free” environment. For example, most keyboards have a small ridge on the “J” and “F” keys, cuing the user to the proper placement of their index fingers. Similarly, 14 CFR 25.781 and 23.781 requires specific shapes for certain cockpit controls.
(8) Locked/Interlocked Controls. Locking mechanisms, interlocks, or the prior operation of a related control can prevent inadvertent operation. For example, a separate on/off control can activate/deactivate a critical control, or physically lock it in place.
(9) Sequential Movements. Controls can be designed with locks, detents, or other mechanisms to prevent the control from passing directly through a sequence of movements. This method is useful when strict sequential actuation is necessary.
(10) Motion Resistance. Controls can be designed with resistance (e.g., friction, spring, inertia) so that deliberate effort is required for actuation. When this method is employed, the level of resistance cannot exceed the minimum physical strength capabilities for the intended pilot population.
d. Any method of protecting a control from inadvertent operation should not preclude operation within the required pilot task time, or interfere with the normal operation of the system. If a control is inadvertently operated, multisensory information can assist pilots in detecting the error. Feedback can include one or more auditory cues, tactile cues, or visual cues.
As a general rule, the greater the consequence of an unintended operation, the greater the prevention method needed, and the more salient the cues that should be provided for detection.
2-11. Controls for Data Entry.
a. Numerous flight deck tasks require some form of data entry, such as entering the airport code or radio frequency. Some controls lead pilots to enter data through compact devices
(e.g., knobs that scroll through the alphabet), which might result in increased workload and headdown time. Controls for data entry must support the pilot when entering required data to support the intended function, per § 2X.1301. Show that the controls are acceptable for data entry speed, accuracy, error rates, and workload.
b. If data entry involves multiple steps, make sure that each step is clearly discernable.
For example, pilots should be aware that the first step is to construct a string of multiple characters and the second step is to enter the string into the system.
c. During data construction, ensure that automatically constructed data is clearly discernable from manually constructed data. Regardless of how the data was constructed, the system should allow pilots to readily determine the data that is entered into the system.
d. Data entry controls should allow pilots to easily recover from typical inputs errors, such as a simple keyboarding error or an incorrect auto fill.
e. Previously approved controls for data entry have used physical configurations and design features based on the following:
(1) Letter keys that are arranged in a QWERTY format (preferred) or alphabetically.
(2) Numeric keypads that are arranged in a 3x3 matrix with zero (0) at the bottom.
(3) Concentric knob assemblies that contain no more than two knobs per assembly.
(4) Cursors that are automatically placed in the first data entry field.