Chapter 13. General Installation Considerations - Antennas.
13-1. Antenna Placement.
13-1.1 GNSS-Specific Considerations.
a. Typically, a GNSS antenna is located forward or aft of the wings on the top of the fuselage. 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. Consideration should be given to the following items when choosing an antenna location on the aircraft.
(1) The GNSS 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).
(2) Shadowing by aircraft structure (and in some cases by rotorcraft blades) can adversely affect the operation of the GNSS equipment. Antenna location should minimize blockage from airframe components, airframe structures, engine components, or other antennas with the aircraft at typical level cruising attitude.
(3) The antenna should also be located to minimize the effects of aircraft shadowing during typical maneuvers.
(4) Avoid locations that could subject the antenna to engine exhaust contamination.
(5) Antenna shadowing and engine exhaust can present unique problems on rotorcraft. One possible ground-based method to aid in selecting an optimum location is to temporarily attach the antenna at various places and compare the signal strength. If using this method, it is important to perform the check with the rotors turning since they can be a source of shadowing. But, this method does not remove the need to do functional flight checks to ensure the antenna location is suitable (see paragraphs 13-1.1.c and 13-1.2.a).
(6) It is desirable to minimize the cable length between the antenna and receiver to reduce signal loss. In general, a shorter cable length improves signal strength to the GNSS receiver.
(7) Attempt to place the antenna on the aircraft’s centerline (i.e., symmetrically on the airframe). This is particularly important on metal aircraft as symmetry can potentially affect antenna gain patterns.
b. 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.
c. 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.
13-1.2 General Considerations.
a. In general, the effects of the rotor blades on antenna performance should be considered for any antenna installations on rotorcraft. This may require additional equipment functional flight evaluation or analytical evaluation to confirm the antenna location is suitable.
b. 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).
13-2. Anti-Ice Protection.
If the aircraft 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 instructions/ manual.
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.
13-3. Antenna Cables.
Double shielded cables should be used to prevent interference coupling into the cable.
GNSS equipment manufacturers should indicate the type of cable to use in their installation instructions/manual to minimize insertion losses that could result in reduced performance.
GNSS installations should use the equipment manufacturer-specified cable. However, if the equipment manufacturer did not specify a particular cable, the installation should use high quality coax and connectors because of the high frequency signals and sensitivity to cable losses.
13-4. Structure.
Any modifications 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 105
revision). Additional guidance information related to structures can be found in the latest revisions of ACs 20-107, 23-13, and 25.1529-1.
13-5. Additional Guidance.
Further information on antenna installation considerations, and good avionics installation practices in general, can be found in AC 43.13-1 (latest revision), Acceptable Methods, Techniques and Practices – Aircraft Inspection and Repair and AC 43.13-2 (latest revision), Acceptable Methods, Techniques and Practices – Aircraft Alterations.
-- Chapter 14. General Installation Considerations - Sensors.
14-1. Introduction.
This chapter provides guidance related to positioning/navigation sensor installation. This 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).
14-2. Positioning and Navigation Configuration Strapping.
Often, positioning and navigation equipment 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.
14-3. Navigat ion Source Selection.
a. For installations 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 flight crew 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 flight crew. 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 107
selection.
f. Navigation source annunciations included in the optimum primary field of view do not also have to be shown on a heads-up display.
14-4. Dual Positioning and Navigation Installations.
a. Paragraph 14-4 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 pilot workload associated with manually updating both equipment sets to maintain consistency between the equipment sets.
(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).
14-5. Intermixing RNAV equipment.
a. Where multiple 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 must 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.
14-6. Interface to FGS.
a. Paragraph 14-6 describes guidelines for interfacing the positioning and navigation equipment with a FGS. When used in paragraph 14-6, 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 autopilots to which they have shown compatibility. Interfacing digital positioning and 109
navigation systems with an analog FGS (for example, autopilots) may present potential hazards and is not recommended. However, applicants that desire to integrate these systems in legacy aircraft will need to perform a thorough engineering evaluation to ensure compatibility. The AFM(S)/RFM(S) must reflect any limitations resulting from the installation.
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 or enabled. 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 output to the SVS or EVS function should meet the appropriate system performance criteria in this AC. The latest revision of AC 20-167, Airworthiness Approval of Enhanced Vision System, Synthetic Vision System, Combined Vision System, and Enhanced Flight Vision System Equipment provides additional guidance for these systems.
14-7. 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: Magnetic heading is the primary heading reference in the U.S. However, aviation authorities in other countries do use true heading as the primary heading reference in areas of extreme latitude.
14-8. 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.
14-9. Antenna/Sensor Compatibility.
a. Using the guidelines in the equipment 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 111
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 (pre-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).
b. Use the guidelines in the equipment manufacturer’s installation manual/instructions when determining what cables and connectors to use to connect the antenna to the receiver, including cable lengths that result in acceptable loss of signal power.
c. Ensure antenna and positioning/navigation equipment compatibility for aircraft equipped with SATCOM. Some equipment may have been qualified to less stringent interference levels intended for aircraft installations without SATCOM.
14-10. Interference - Electromagnetic Compatibility (EMC).
a. The equipment must not be the source of objectionable conducted or radiated interference, or be adversely affected by conducted or radiated interference from other equipment or systems installed in the aircraft.
b. Proper sensor equipment grounding is essential to ensuring EMC.
c. When possible, do not install positioning/navigation sensor(s) near a VHF radio.
d. The following paragraphs identify potential sources of interference and means of mitigating that interference:
(1) VHF communications harmonic, spurious, and local oscillator harmonics can cause interference. VHF interference can be mitigated by:
(a) Installing filters at the output of the VHF transmitter to prevent antenna-to-antenna interference. Such filters should have an insertion loss of 2 dB or less, or the installed VHF transceiver performance will have to be re-evaluated.
(b) Installing the positioning/navigation equipment as far away as feasible from any VHF transmitter equipment (to prevent case-to-case interference).
(c) Replacing the VHF equipment.
(2) An emergency locator transmitter (ELT) can re-radiate DME or VHF signals that can interfere with GNSS. Notch filters on the ELT antenna cable or replacing the ELT can eliminate this problem.
(3) DME has been known to cause interference to GNSS. Replacing the DME transceiver can eliminate the problem for some equipment.
(4) Direction finding equipment may cause interference. Moving the direction finding antenna to the belly of the aircraft has been found to eliminate this problem.
14-11. Interface Requirements.
a. Ensure sensor compatibility for aircraft systems using the position output (e.g., FMS, automatic dependent surveillance system, etc.). Systems should use the same bus standard (e.g., ARINC 429 with ARINC 743 data, RS-232 as applicable).
b. Operation of the positioning/navigation equipment must not adversely affect the performance of, or interfere with, other installed aircraft equipment.
c. Some applications will require the positioning/navigation sensors to provide a position output at a 5 Hz rate. However, sensors with a 1 Hz rate augmented by inertial aiding can be considered for use in applications requiring 5 Hz output.
d. Positioning/navigation equipment (particularly installations that use GNSS sensors) may provide output to a variety of applications, including TAWS/HTAWS, automatic dependent surveillance, and moving map displays. Output of navigation position, velocity and time must support the intended function for which it is used under any foreseeable operating condition. It is important to note for ground based navigation aids that the airworthiness approval alone does not guarantee performance since it is dependent upon the operational environment. This is significant when interfacing the positioning and navigation equipment to systems dependent upon the navigation accuracy and integrity statements.
e. The guidance material in this AC for sensors applies to all of these interfaces, and should be used in conjunction with the guidance material for the application. For specific information relating to:
(1) TAWS equipment, see TSO-C151(latest revision) and the latest revisions of AC 23-18, and AC 25-23.
(2) HTAWS equipment, see TSO-C194(latest revision) and Miscellaneous Guidance 18 in the latest revisions of AC 27-1 and AC 29-2.
(3) Electronic map displays, see TSO-C165(latest revision) and the latest revisions of AC 20-159, AC 23.1311-1, and AC 25-11.
(4) ADS-B equipment, see TSO-C166b, RTCA/DO-260B for 1090 Extended Squitter; and, TSO-C154c and RTCA/DO-282B for Universal Access Transceiver. AC 20-165 provides guidance information on ADS-B.
Chapter 15. Installation Considerations - GNSS.
15-1. Introduction.
a. GNSS equipment is particularly susceptible to out-of-band SATCOM emissions and in-band inter-modulation between multiple channel SATCOM installations. GNSS equipment must not be installed in aircraft with multiple SATCOM channels unless the simultaneous use of interfering frequencies can either be prevented or demonstrated not to interfere with the operation of the GNSS equipment. Note that some equipment is not compatible with SATCOM installations at all; this should be noted in the equipment manufacturer’s installation manual/instructions.
b. In addition, improperly used or installed GNSS re-radiators can present misleading information to GNSS equipment. Equipment manufacturers may provide mitigation against the use of erroneous data for GNSS position and navigation solutions. Possible measures include:
implementing or enabling cross-checks of GNSS sensor data against independent position sources and/or other detection monitors using GNSS signal metrics or data. It is incumbent upon airworthiness approval applicants to determine the method chosen by the equipment manufacturer and its adequacy for the aircraft integration.
15-2. Navigation Displays.
a. Navigation Display(s) for Retrofit Installations.
This guidance reflects the AC 20-138A clarification policy memorandum signed on July 30, 2007 and replaces that memorandum.
(1) Table 8 in paragraph 12-2.a classifies misleading information during GNSS CAT I and LP/LPV approaches as a hazardous failure condition; and paragraph 12-2.b states applicants must conduct a safety assessment of the installation to verify that design errors and failure modes meet the probability requirements for that failure class. The guidance in paragraph 15-2.a only applies when installing retrofit GPS/SBAS LP/LPV or GPS/GBAS CAT I GNSS landing system capability in aircraft that have a previously certified horizontal/vertical course deviation indicator(s) or an electronic display (or displays) approved for ILS approaches.
The intent is to apply this guidance when introducing GPS/SBAS LP/LPV or GPS/GBAS GLS capability into aircraft equipped with ILS course deviation indicator(s) or display(s) that were approved (TSOA/LODA, STC/PMA or type approval (TC/amended TC)) on or before July 30, 2007.
(2) Applicants may use the existing display(s) or course deviation indicator(s) for GPS/SBAS LP/LPV or GPS/GBAS GLS course deviations and annunciation data when adding GPS/SBAS LP/LPV or GPS/GBAS GLS capability into existing approved aircraft type designs, either by retrofit (STC, Form 337) or forward fit in the factory (amended TC), that were previously approved with ILS. It is not necessary to demonstrate compliance with the end-to-end design assurance levels of a ‘hazardous’ failure condition for the mechanical or electronic course deviation displays. Using the same path and device that was previously used to display ILS data for displaying GPS/SBAS LP/LPV or GPS/GBAS GLS data yields an equivalent level of safety to the original ILS certification basis. This equivalency provides 114
relief from paragraphs 12-2.a, 12-2.b, and data submittal requirements related to a ‘hazardous’ failure condition during LP/LPV or GLS approaches for the existing display(s) or course deviation indicator(s).
(3) This guidance may be applied to aircraft equipped with a single or dual horizontal/vertical course deviation indicator, or electronic display(s) with level C software.
However, this guidance cannot be applied to the equipment that computes and outputs the GPS/SBAS LP/LPV or GPS/GBAS GLS deviation data, as GPS/SBAS and GPS/GBAS do not have the same failure modes as ILS in generating the deviations. The installation is considered ‘not significant’ with respect to 14 CFR 21.101 when installing GPS/SBAS LP/LPV or GPS/GBAS GLS capability per this guidance.
b. Displays used for waypoint sequencing, start of a turn, turn anticipation, active waypoint, distance to active waypoint, desired track and actual track (track angle error), and automatic mode switching should be located within the pilot’s primary field of view, or on a readily accessible display page.
(1) Displays used for loss of integrity monitoring, TO/FROM indication, and approach mode annunciation should be located within the pilot’s primary field of view.
(2) Traditionally, 14 CFR part 23 airplanes with “classic” analog instrumentation in the “basic T” arrangement have included the center radio stack within the allowable field of view to satisfy this guidance. There is no intent for this AC to change that long-standing guidance.
Note: Primary field of view is being incorporated as a standardized term across all documents. The primary field of view definition should be broad enough to include the center radio stack on 14 CFR part 23 airplanes with “classic”, basic ‘T’ instrumentation.
c. For some “stand-alone” equipment, the display can be remote from the navigation computer and GNSS sensor. These displays must be compatible with expected outputs from the navigation computer to provide the proper deviation indications to the pilot. Both GNSS and display manufacturers are encouraged to provide a list of compatible GNSS/display equipment, as appropriate, in their installation manuals as this has been shown to greatly aid installation approvals. The installation instructions approved as part of the STC or the installation manual approved by the TSOA for GNSS equipment and displays should provide appropriate equipment compatibility lists and/or compatibility standards. Additional combinations can be established later through a simple amendment to the STC installation instructions or TSOA installation manual.
Note: In addition to or instead of compatibility lists, equipment manufacturers may decide to provide detailed compatibility standards information on their equipment’s interface characteristics.