~~ ~ David W. Hempe Advisory U.S. Department of Transportation Federal Aviation Circular Administration Subject: Controls for Flight Deck Systems Date: 12/08/2011 AC No: 20-175 Initiated by: AIR-120 This advisory circular (AC) provides guidance for the installation and airworthiness approval of flight deck system control devices, from primarily a human factors perspective. It does not address primary flight controls, secondary flight controls, or controls that are not located in the flight deck. This AC addresses traditional dedicated controls such as physical switches and knobs, as well as multifunction controls such as touch screens and cursor control devices.
Manager, Aircraft Engineering Division Aircraft Certification Service
Table of Contents Paragraph Page
Chapter 1. General Information1
1-1. Purpose1
1-2. Audience1
1-3. Scope1
1-4. Regulatory Requirements2
1-5. Background3
Chapter 2. General Controls Guidance4
2-1. General Controls Design Philosophy4
2-2. Environment and Use Conditions4
2-3. Controls Layout and Organization7
2-4. Movement of Controls8
Table 1. Examples of Conventional Relationships between Control Functions and
Movements9
2-5. Sensitivity and Gain of Controls9
2-6. Feedback from Controls9
2-7. Identifiable and Predictable Controls10
2-8. Labeling of Controls11
2-9. Controls Lighting12
2-10. Preventing Inadvertent Operation of Controls13
2-11. Controls for Data Entry15
2-12. Continued Airworthiness16
Chapter 3. Additional Guidance for Multifunction Controls17
3-1. Multifunction Controls—General17
3-2. Multifunction Controls—Inadvertent Operation18
3-3. Multifunction Controls—Labeling18
3-4. Multifunction Controls—Cursor Control Devices (CCDs)19
3-5. Multifunction Controls—Touch Screens19
3-6. Multifunction Controls—Menus and Navigation20
3-7. Multifunction Controls—Voice Recognition and Voice Activated20
Appendix A. Related Documents and AcronymsA-1
1. ReferencesA-1
2. How to Get Referenced DocumentsA-1
Appendix B. Control-Function MatrixB-1
1. IntroductionB-1
2. Table B-1B-1
Table B-1. Example Template for Control-Function MatrixB-3
Appendix C. RegulationsC-1
Table C-1. Quick Reference to Regulations for Controls Related to Human Factors.C-1
Table C-2. Additional Cited RegulationsC-2
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Chapter 1. General Information
1-1. Purpose.
a. We, the Federal Aviation Administration (FAA), wrote this AC for aircraft and avionics manufacturers and designers of systems that are installed on the flight decks of any aircraft type certificated per Title 14 of the Code of Federal Regulations (14 CFR) part 23, 25, 27, or 29. This AC provides general guidance for the design, installation, integration, and approval of controls for flight deck systems.
b. This AC is not mandatory and does not constitute a regulation. This AC describes an acceptable means, but not the only means, to demonstrate compliance to controls-related requirements. If you use the means described in this AC, you must follow it entirely. The word “must” is used to express a regulatory (mandatory) requirement.
c. This material does not change, create any additional, authorize changes in, or permit deviations from, regulatory material.
1-2. Audience. This AC is for applicants seeking a type certificate (TC), supplemental type
certificate (STC), amended type certificate (ATC), and amended supplemental type certificate (ASTC) for the installation of flight deck system controls.
1-3. Scope .
a. Controls are the components of a flight deck interface that allow pilots to provide information to an aircraft system. Example control functions are to operate, configure, and manage systems. This document addresses controls on civil aircraft flight decks from primarily
a human factors perspective.
b. For the purposes of this AC, "controls" covers both the hardware and software
components of the input device (e.g., button) and its identifier (e.g., label), as well as any other components related to its intended function. This AC also covers components of other systems that affect controls and control-related information.
c. The guidance in this document applies to the approval of:
Controls for avionic systems in the flight deck;
Dedicated controls (e.g., physical switches, knobs); and Multifunction controls (e.g., cursor control devices, touch screens).
d. The guidance in this document does not address:
(1) Primary and secondary flight controls, including:
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Yoke,
Side-stick, Rudder pedals, Throttle levers and related powerplant controls, and
Flaps.
(2) Controls located outside the flight deck;
(3) Controls installed in the flight deck that are not intended for use by the flightcrew
(e.g., controls supporting airplane maintenance activities);
(4) Aircraft performance responses to manipulation of the “in scope” controls;
(5) Control failure analysis or control behavior in the presence of other system failures (system safety analysis);
(6) Detailed design guidance (e.g., knob resistance); and
(7) Controls guidance not related to usability or human performance.
e. If there is guidance in other ACs for specific aircraft categories (parts 23, 25, etc.), equipment, and systems, that specific guidance has precedence if a conflict exists with the guidance provided in this AC.
1-4. Regulatory Requirements.
Note: In the following sections, the notation "2x" is used for brevity to indicate multiple regulations, as applicable, in 14 CFR parts 23, 25, 27, and 29.
a. A system must comply with the full text of all applicable regulations in 14 CFR.
When the system includes flight deck controls, include special consideration of the following regulations: 14 CFR 2x.671, 2x.771, 2x.777, 2x.1301, 2x.1322, 2X.1381, 2x.1523, and 2x.1555.
These are the regulations that are related to human factors. They address separate but related topics, including the following human factors topics associated with controls:
Minimum crew;
Accessibility;
Concentration and fatigue;
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Convenience and ease of operation;
Identification and marking;
Confusion and inadvertent operation;
Unrestricted movement; and Intended function.
b. This AC provides compliance guidance related to human factors topics concerning controls in the flight deck. Appendix B, in this AC, provides a tool that applicants can use to organize their design and compliance effort. Appendix C, table C-1, provides a quick reference that maps these topics to their associated regulations.
1-5. Background. Flight deck controls can present unique opportunities and challenges to the design and certification process. In particular, showing compliance with regulatory requirements related to the latest flight deck controls has been subject to various degrees of interpretation by applicants and the FAA. Whereas traditional controls are typically dedicated hardware components, such as single-function knobs, modern controls are often highly software-based, multifunctional, and integrated with displays and other systems. A key challenge is to interpret and comply with the regulations consistently and appropriately, across the design spectrum of flight deck controls.
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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.