flight hazard . The interfaces with other aircraft equipment must be designed such that normal or abnormal positioning/navigation equipment operation must not adversely affect the operation of other equipment nor must operation of other equipment adversely affect the positioning/navigation equipment operation.
11-3. Software Considerations.
a. AC 20-115B Radio Technical Commission for Aeronautics, Inc. Document RTCA/DO-1 78B, defines an acceptable means of qualifying software. In December 2011, RTCA published RTCA/DO-178C, Software Considerations in Airborne Systems and Equipment Certification. The FAA plans to update AC 20-115B to recognize RTCA/DO-178C as an acceptable means of qualifying software, but there currently is no AC or TSO recognition for RTCA/DO-178C. In the interim, applicants may choose to use RTCA/DO-178C instead of RTCA/DO-178B provided they declare their intention to the applicable aircraft certification office for the project. However, doing so will most likely require submitting a deviation request, particularly if a TSO that invokes RTCA/DO-178B is involved.
b. The applicant is encouraged to submit the Plan for Software Aspects of Certification (PSAC) early in the software development process. Early submittal will allow timely resolution of issues such as partitioning and determining software levels. Software certification is normally addressed during the design approval for positioning/navigation equipment, but should be confirmed at installation to ensure the certification level is appropriate for the installation intended function.
Note: It may be possible to obtain approval for navigation functions that have hazardous level effects with FMS software developed to DO-178B level C criteria. AC 20- 174, Development of Civil Aircraft and Systems provides an acceptable method for establishing design assurance levels.
This guidance may be helpful for navigation architectures containing independent functions.
11-4. Hardware Considerations.
AC 20-152 (latest revision) RTCA, Inc., Document RTCA/DO- 254, Design Assurance Guidance for Airborne Electronic Hardware, defines an acceptable means of qualifying complex electronic hardware devices. The applicant is encouraged to submit the Plan for Hardware Aspects of Certification (PHAC) early in the development process. Hardware certification is normally addressed during the design approval for positioning/navigation equipment; but, should be confirmed at installation to ensure the certification level is appropriate for the installation intended function.
11-5. Installed Equipment Compatibility.
There is no one, single, standard ized interface for positioning and navigation equipment to other equipment on-board the aircraft such as displays, autopilots, terrain warning, etc.
Positioning and navigation equipment manufacturer’s installation manuals/instructions should define what other equipment is compatible with the positioning and navigation system.
Equipm ent manufacturers are encouraged to develop compatible equipment lists and/or detailed interface requirements for their installation manuals/instructions. The installer must ensure that other on-board equipment interfaced with the positioning and navigation systems is compatible, and the airworthiness approval holder is ultimately responsible for defining the installation. But the installation burden is greatly eased when equipment manufacturers define compatible systems in their installation manual/instructions, including software version and/or part number if necessary.
11-6. Installation Instructions.
a. The equipment m ust be installed in accordance with the instructions and limitations provided by the positioning and navigation equipment manufacturer. The airworthiness approval will address any additional installation specifics.
b. Equipment should be installed with separation between wires of redundant equipment (see AC 43.13-1 latest revision). Wire separation will mitigate the possibility of loss of navigation due to a single event. During installation approval of the system, when wire separation cannot be achieved, the following questions must be evaluated and the potential for single event upsets minimized:
(1) Is it possible for a cable harness to be exposed to wire chafing in a manner that both channels fail simultaneously?
(2) Is the cable harness located near flight control cables, high electrical capacity lines or fuel lines?
(3) Is the cable harness located in a protected area of the aircraft (isolated from engine rotor burst)?
(4) Is any electromagnetic interference (EMI) between systems caused by cable routing?
11-7. Environmental Considerations.
a. Most equip ment will have been tested by the equipment manufacturer to the environmental categories described in RTCA/DO-160 (applicable revision), with results described in an environmental qualification form. The airworthiness approval applicant must ensure the environmental categories (or criteria) to which the equipment has been tested are compatible with the aircraft environment in which the positioning and navigation equipment is installed. See AC 21-16F (or later revision) RTCA Document DO-160 versions D, E, and F, Environmental Conditions and Test Procedures for Airborne Equipment, for guidance on differences among RTCA/DO-160 versions D, E, and F.
b. For equipment that has not been tested to the environmental categories described in RTCA/DO-160 (applicable revision), either the equipment manufacturer or airworthiness applicant must thoroughly define the intended aircraft environment and conduct comparable normal and abnormal environment testing.
11-8. General Human Factors Considerations.
a. Control Accessibility.
(1) Controls installed for in-flight operation should be readily accessible from the flight crew’s seated position. Only single-hand operation should be required, the controls should be readily identifiable, and the use of controls should not obscure pertinent displays.
(2) The equipment should be designed so that controls intended for use during flight cannot be operated in any position, com bination, or sequence that would result in a condition detrimental to the operation of the aircraft or the reliability of the equipment.
(3) Controls should provide feedback (e.g., tactile, visual) when operated.
(4) Control operating force should be appropriate for its intended function.
Note: Low forces might provide insufficient feedback, while high forces might impede intentional rapid use.
(5) Controls should be resistant to inadvertent activation. Common and acceptable means of reducing the likelihood of inadvertent operation through design include the following:
(a) Placing fences between closely spaced adjacent controls.
(b) Concave upper surface of keys to reduce slippage.
(c) Size of control surface sufficient to provide for accurate selection.
(d) Placing guards over controls.
(e) Providing appropriate sensory feedback, or explicit confirmation of activation.
(6) The function for each control should be indicated, unless obvious. Pilots should be able to quickly and accurately determine the function of the control with minimal experience or training. If a control can be used for multiple functions, the current and inactive functions should be distinguishable.
(7) Controls/input devices should be organized according to the following principles, when feasible:
(a) Controls should be organized in logical groups according to function and frequency of use.
(b) Controls should be easily associated with the related elements (e.g., displays and labels) that identify their function. For example, line select function keys should align with adjacent text.
(c) Controls used m ost frequently should be the most accessible.
(d) Dedicated controls should be used for frequently used functions.
(e) Line select function keys should align with adjacent text.
(8) Use of two or more controls simultaneously (e.g., pushing two buttons at once) in flight should not be required to perform a function.
(9) Controls that do not require adjustment by the flight crew (e.g., maintenance functions) should not be readily accessible to the flight crew.
(10) Controls and their labels should be identifiable during all normally expected ambient light conditions, including direct sunlight and nighttime.
(11) Control identifiers and other information should not be obstructed by the control input devices.
b. Display Visibility.
(1) Each display element, used as a primary flight instrument in the guidance and control of the aircraft, should be located where it is clearly visible to the pilot with the least practicable deviation from the pilot’s normal position and line of vision when looking forward along the flight path.
Note 1: CDI displays contained in the Control Display Unit (CDU) will most likely not be acceptable for IFR operations.
Note 2: FTE can be reduced when numeric display information is integrated with the non-numeric display or is located within the pilot’s primary field of view. Both digital cross-track and track angle error have been shown to reduce FTE. This information should be displayed together (either within the CDU or remotely displayed near the non-numeric display) for better tracking performance.
Using non-numeric cross track data integrated with nonnumeric track angle error data into one display may provide the optimum situation and control information for the best overall tracking performance.
(2) The appropriate flight crew member(s) should have an unobstructed view of displayed data when in the seated position.
(3) Connection to external displays (CDI, horizontal situation indicator (HSI), moving map, annunciator panel, etc.) should be consistent with the equipment installation instructions.
(4) The horizontal (and vertical) deviation(s), display(s), and failure annunciations should be located within the pilot's primary field of view, as should any indication requiring immediate aircrew action.
(5) Compliance with limitations in the equipment installation instructions for display readability should be verified.
c. Lighting Conditions.
(1) All displays, controls, and annunciators must be easily readable under all normal cockpit conditions and expected ambient light conditions (total darkness to bright, reflected sunlight).
(2) Night lighting provisions must be compatible with other cockpit lighting, particularly if night vision goggle compatibility is desired.
11-9. Magnetic Variation Data and Onboard Database.
a. Avionics typically convert true heading references to m agnetic heading references by using worldwide magnetic variation data contained in an on-board magnetic variation database and a set of software algorithms. The algorithms convert the data into a specific magnetic heading reference for a specific geographic reference point. However, since the Earth’s magnetic fields constantly change, magnetic variation databases need periodic updates to provide accurate magnetic heading references. The National Oceanic and Atmospheric Administration (NOAA) offers a World Magnetic Model available at http://www.ngdc.noaa.gov/geomag/geomag.shtml, and this product is an acceptable source for updating an aircraft magnetic variation database. NOAA updates the model on a five-year cycle, and magnetic variation databases should be updated consistent with this cycle.
Note: If the magnetic variation database is not kept current, the navigation systems no longer support their intended functions for use during flights conducted under IFR. Failure to maintain and update the magnetic variation database can result in presentation of misleading magnetic heading references that can result in placing the aircraft outside the terrain and obstacle protection provided in instrument procedure and route designs.
b. Equipment design approval holders should identify any operating limitations in their equipment associated with application of the magnetic variation data (e.g., operations using magnetic heading references may only be valid between 82° north latitude and 82° south latitude) to ensure accurate magnetic heading references are presented and used in the aircraft.
The equipment design approval holders should also define a continuing airworthiness requirement to update the magnetic variation database and, if applicable, conversion algorithms or hardware when the periodic updates become available. This requirement should also define the maintenance procedures necessary to update the on-board magnetic variation database for their avionics.
Chapter 12. General In stallation Considerations - Antennas.
12-1. Antenna placement.
Paragraphs (a) through (f) below are specific to GNSS antennas.
a. Typically, a GNSS antenna is lo cated forward or aft of the wings on the top of the fuselage.
b. GNSS antennas must be installed to provide the widest, unobstructed field of view to the satellites. The antenna installation instructions should provide information on how to determine a location that minimizes the potential for signal blockage by any portion of the aircraft. Shadowing by aircraft structure (and in some cases by rotorcraft blades) can adversely affect the operation of the GNSS equipment.
c. The antenna placement on the airframe should be optimized to ensure the receiver can take full advantage of a 5 degree mask angle (or lower if the receiver installation instructions identify the equipment is certified to a lower mask angle). This includes consideration of antenna location with respect to blockage from airframe components, airframe structures, engine components, or other antennas with the aircraft at typical level cruising attitude. The antenna should also be located to minimize the effects of aircraft shadowing during typical maneuvers.
d. The antenna should be separated as much as practical from transmitting antennas
(e.g., VHF and high frequency radio, satellite communications (SATCOM), DME, direction finder, emergency locator transmitter, and traffic and collision alerting system). For small aircraft, the antenna should also be separated as much as practical from the windscreen to prevent case-to-antenna coupling.
e. The VDB antenna should be installed so that the maximum received power from any on-board transmitter does not exceed the desensitization levels of the VDB receiver specified in RTCA/DO-253C, paragraph 2.2.9.1.
f. The GNSS multi-path error model specified in RTCA/DO-229D and RTCA/DO-253C was developed and validated using flight test data collected on a variety of large, fixed-wing aircraft. Further validation was done using computer modeling of large, fixed-wing aircraft. For installations on rotorcraft, antenna location should consider the compatibility of the resulting multi-path errors with this model. This may require additional equipment functional flight evaluation or analytical evaluation to confirm the antenna location is suitable.
g. For installations on rotorcraft, the effects of the rotor blades on antenna performance should be considered. This may require additional equipment functional flight evaluation or analytical evaluation to confirm the antenna location is suitable.
h. For multiple-sensor installations, the installer should reduce the likelihood that a single lightning strike affects all the sensors (i.e., do not place them in a straight line from the front to the rear of the fuselage).
12-2. Anti-Ice Protection.
If the aircraf t is approved for flight into known icing conditions, the antenna must not be susceptible to ice buildup (i.e., is installed in a non-icing location on the aircraft, or is of a sufficiently low profile that ice does not accumulate on the antenna). Alternatively, the equipment can be shown to operate satisfactorily when the antenna is subject to icing if there are no harmful effects of the ice build-up, such as possibly ingesting accumulated ice into the engine or degradation in aerodynamic performance. The effects of ice accumulation on the antenna, if any, can be found in the equipment manufacturer's installation manual/instructions.
See the latest revisions of ACs 23.1419-2 and AC 25.1419-1 for additional fixed-wing aircraft guidance. For additional rotorcraft guidance, AC 27.1419 is contained in the latest revision of AC 27-1 and AC 29.1419 is contained in the latest revision of AC 29-2.
12-3. Antenna Cables.
Double shielded cables should be used to prevent interference coupling into the cable.
12-4. Structure.
Any m odifications to the aircraft to install a new antenna need to be evaluated as part of the airworthiness approval for their impact on aircraft structure. Examples are evaluating and testing antenna installations for buffet and vibration (compliance with 14 CFR part XX.251), or when modifying pressurized structures (compliance with 14 CFR part XX.841 and XX.843).
For specific guidance relating to modifications in the fuselage structure, see AC 43.13-2 (latest revision). Additional guidance information related to structures can be found in the latest revisions of ACs 20-107, 23-13, and 25.1529-1.
Chapter 13. General In stallation Considerations - Sensors.
13-1. Introduction.
This chapter provides guidance related to positioning/navigation sensor installation. T his chapter also applies to stand-alone navigation equipment installation, since that equipment has an embedded sensor. This guidance applies to all installations, but is only expected to be specifically evaluated as part of the initial airworthiness approval (i.e., initial airworthiness approval may provide sufficient substantiation for follow-on installations).
13-2. Positioning and Navigation Configuration Strapping.
Often, positioning and navigation equipm ent uses software configuration straps or installation set parameters entered either manually by the installer or by using an aircraft personality module. These software configuration straps or parameters disable/enable functionality; configure the input/output formats; determine the displayed units of measure; etc.
The software configuration programming documentation must become a part of the installation package and should be included in the master drawing list. When using an STC-AML (see paragraph 2-3), approved multiple configurations may be specified.
13-3. Navigation Source Selection.
a. For insta llations where positioning and navigation outputs can drive a display that is shared with other navigation equipment (e.g., VOR/DME, ILS, GPS/SBAS, baro-VNAV), the pilot must be able to select the source of guidance, and the selected source must be clearly annunciated to the flightcrew on or near the affected display.
b. There are many possible navigation source and service level combinations and permutations given the variety of sensors and capabilities that can be integrated into the aircraft.
Some of the capabilities are very similar, but have subtle differences that are very significant.
The aircrew must have a clear, unambiguous annunciation of what capability is being provided by the navigation system. This can either be accomplished by multiple annunciation indicators, or by removing some navigation capabilities when others are added to reduce the annunciator complexity.
c. GNSS-based navigation (GPS, GPS/SBAS, GPS/GBAS) might not require differentiation unless integration complexity requires it for clear, unambiguous indication of selected capability to the flightcrew. For example, oceanic, enroute, or terminal RNAV modes or LNAV-only approach capability may not require differentiation.
d. When applicable, annunciation of the selected source should be driven by the same relay switch to preclude erroneous information in case of switch failure.
e. Autopilot navigation modes related to the navigation source should be inhibited if the displayed source is not the same as the source driving the autopilot. This is particularly critical for approach operations (i.e., autopilot in approach mode). One possible method is to consider integrations that do not permit separate display source and autopilot source navigation 76
selection.
f. Navigation source annunciations included in the optim um primary field of view do not also have to be shown on a heads-up display.
13-4. Dual Positioning and Navigation Installation s.
a. Paragraph 13-4 describes guidelines for synchronizing dual positioning and navigation equipment installations. Synchronization refers to the ability for control inputs to one positioning and navigation equipment set to automatically update the other equipment set.
Dual installations should be synchronized whenever possible to reduce crew workload and prevent confusion over which system/flight plan is driving the navigation data. If the functional hazard assessment requires dual positioning and navigation equipment or is operationally required, then synchronization is required.
b. If the navigation equipment is not synchronized, then the following issues should be evaluated:
(1) The pilot and co-pilot (if applicable) should be able to view and enter data in the offside equipment. This must not interfere with throttle levers, flap handle, etc.
(2) The workload associated with manually updating both equipment sets to maintain consistency between them should not be too burdensome.
(3) There should be no possible confusion as to which equipment is driving the autopilot and navigation displays.
(4) Controls, displays and annunciations must not result in misleading information, pilot confusion or unacceptable workload due to possible inconsistencies from differences in the equipment (e.g., different flight plans making one unit go into approach mode while the other does not). Evaluations for these types of inconsistencies are of particular concern when the cockpit architecture enables cross coupling (e.g., GNSS-2 switched to drive side 1 displays).
13-5. Intermixing RNAV equipment.
a. Where m ultiple RNAV systems are intended to be used simultaneously (typically in two-pilot aircraft), installing area navigation equipment with different crew interfaces can be very confusing and could lead to problems since they may have conflicting methods of operation and conflicting display formats. There can be problems even when intermixing different versions of the same equipment.
b. At a minimum, consideration should be given to the following potential incompatibilities, particularly when the cockpit architecture allows for cross coupling (e.g., RNAV-2 switched to drive side 1 displays):
(1) Data entry.
The two units m ust have consistent methods of data entry and similar pilot procedures for accomplishing common tasks. Workload should be acceptable, and it should be easy for pilots to identify and recover from data entry mistakes.
(2) CDI scaling.
CDI scaling sensitivity must be consistent across different equipment or labeled.
(3) Display symbology and mode annunciations.
There must be no conflicting symbols or annunciations (e.g., a common symbol used for two different purposes), and differences should be specifically highlighted during the evaluation for any potential confusion they may cause.
(4) Mode logic.
The modes internal to the equipment and interfaced to the rest of the aircraft should be consistent across RNAV units.
(5) Equipment failure.
The effect of failure of one RNAV unit must not result in misleading information or failures in the other unit.
(6) Displayed data.
The display of primary navigation parameters must use consistent units and a consistent notation across multiple RNAV units. Industry standard labels and nomenclature should be used.
(7) Database differences.
Intermixed RNAV units must use the same navigation database to prevent conflicting information among the equipment.
13-6. Interface to FGS.
a. Paragraph 13-6 describes guidelines for interfacing the positioning and navigation equipm ent with a FGS. When used in paragraph 13-6, the word “consistent” means the installation does not alter the positioning and navigation equipment’s intended function, nor does it alter the cockpit design philosophy.
b. Connection to the autopilot must be consistent with the autopilot installation instructions and the positioning and navigation installation instructions. The positioning and navigation system manufacturer should provide instructions on connection with specific 78
autopilots to which they have shown compatibility.
c. Operational compatibility should be verified between the positioning and navigation equipment and the FGS. Protection should exist in the positioning and navigation equipment or the FGS to keep the aircraft within the approved operational envelope (e.g., roll, pitch, speed, and altitude). See the relevant guidelines for the particular aircraft (e.g., the latest revision of AC 23-17, AC 25.1329-1, AC 27.1329 contained in AC 27-1, and AC 29.1329 contained in AC 29-2).
d. A continuous and unambiguous indication must be provided of the FGS modes actually in operation, as well as those that are armed for engagement. This includes a clear indication whenever the mode has been armed either manually by the flight crew, or automatically by the system.
e. The positioning and navigation equipment must be compatible with the FGS modes of operation. For example, the arm, engage, and disengage sequence of the positioning and navigation equipment should be consistent in the annunciation and engage/disengage timing of the FGS. This is particularly important during GPS/SBAS-augmented GNSS approaches that include different levels of service.
f. If the positioning and navigation equipment is connected to a reduced bank selector, the equipment bank angle, like the FGS, must not be limited by this switch selection during approach. Consistency in the operation must be maintained.
g. Some aircraft operate in areas of the operational flight envelope where buffet onset can occur when normal maneuvering load factors are present (e.g., at higher altitudes). This is a concern since the positioning and navigation equipment can automatically sequence waypoints and initiate turn maneuvers that may or may not have the pilot’s attention. If buffet protection is not automatically provided in the positioning and navigation equipment or FGS
(e.g., bank angle limiting, or cruise altitude limiting based upon buffet margin), an AFMS/RFMS limitation should be added to the performance section, prohibiting cruise operation at weight/altitude combinations that result in buffet margins less than 1.4g.
h. For those aircraft that have incorporated bank angle limiting, consideration should be given to whether the positioning and navigation equipment is defining a path that cannot be flown or does not conform to the published procedure.
i. If the aircraft includes a synthetic vision system (SVS) or enhanced vision system (EVS) displaying flight guidance, the positioning and navigation input should meet the performance criteria in this AC, as appropriate, to meet the SVS or EVS intended function.
13-7. Interface to Magnetic/True Switch.
If a magnetic/true heading reference switch is installed in the aircraft, the positioning and navigation equipment should be driven by the same switch to maintain consistency in the displays and operation for both manual and automatic heading reference changes. Magnetic heading is the primary heading reference.
Note: Mag netic heading is the primary heading reference in the United States. However, aviation authorities in other countries might use true heading as the primary heading reference in areas of extreme latitude.
13-8. Interface to Air Data and Inertial Reference Systems.
a. If there is an air data source select switch, when the pilot de-selects an air data source, the de-selected source must no longer be used by the positioning and navigation equipment. This becomes important for GNSS barometric-aided FDE since erroneous altitude inputs to the FDE algorithm could lead to improper satellite FDE.
b. If there is an inertial source select switch, when the pilot de-selects an inertial source, the de-selected source must no longer be used by the positioning and navigation equipment.
c. If the positioning and navigation equipment requires barometric corrected (or pressure) altitude data for certain operations (identified in the equipment manufacturer’s installation manual/instructions), the installation should provide an automatic altitude input from the air data system to the positioning and navigation equipment such that additional pilot actions are not required.
d. If barometric corrected altitude is required by the positioning and navigation equipment and an automatic input of the barometric corrected altitude is not available, an alert for the pilot during approach must be provided indicating the need to enter the barometric pressure setting in the equipment. If the equipment requires multiple entries to enter the correction, the workload should be evaluated during an approach.
13-9. Antenna/Sensor Compatibility.
a. Using the guidelines in the equipm ent manufacturer’s installation manual/instructions, verify the antenna installation is compatible with the positioning and navigation equipment. This may be accomplished by one of the following:
(1) The antenna (part number) is identified in the positioning/navigation equipment manufacturer’s installation manual/instructions as compatible.
(2) The positioning/navigation equipment manufacturer’s installation manual/instructions state that the equipment is compatible with an appropriate antenna that has a TSOA/LODA (refer to Table 2 for GNSS antennas). Compare the RTCA/DO-160 (appropriate revision) category for lightning-induced transients specified in the antenna installation instructions with the category defined in the receiver installation instructions. Also, for active GNSS antennas, a signal power budget must be defined that takes the output power at the low-noise amplifier of the antenna (as defined in the antenna installation instructions) and matches that to the receiver dynamic range and sensitivity (as defined in the GNSS equipment installation instructions).