Par 7 13 NOTE 4: The above table is a result of an RSS combination of the below error elements. The accuracy tables are computed assuming that the waypoint is always at the tangent point and the aircraft is at the along-track distance measured from the same tangent point. This is important in the test procedures because the resulting crosstrack error from the course setting is a function of distance to waypoint.
Ground Equipment Error: Airborne Equipment Error:
VOR 1.4 degrees VOR 3.0 degrees DME 0.1 nmi DME 0.2 nmi + 1% CSE 1.6 degrees Computation Error 0.5 nmi
b. Flight Technical Error (FTE). Since FTE factors are normally beyond the control of equipment manufacturers or installers, these error sources are not included in the accuracy specifications of paragraphs 7(a)(1) and (2). The FAA has determined that when properly installed in an aircraft, multi-sensor equipment (except equipment incorporating a Class C() GPS sensor) meeting the operational and display characteristics contained in this AC, related discrete sensor AC’s, TSO-C115b, and related discrete sensor TSO’s, provides for acceptable values of FTE. FTE should not exceed 1.0 nmi for en route, 1.0 nmi for terminal or approach transition, or 0.25 nmi (for equipment using GPS data) or 0.5 nmi (for equipment not using GPS data) for approach operating modes on a 95 percent basis. Equipment integrating a Class C() GPS sensor must include display and system integration capabilities that provide an acceptable value of FTE, and is evaluated based upon the particular installation to determine the suitability of proposed operational and display characteristics.
c. VNAV Accuracy Requirements. For equipment that includes a vertical navigation capability (VNAV), see AC 20-129 for Vertical Navigation Requirements.
8. AIRWORTHINESS CRITERIA FOR MULTI-SENSOR EQUIPMENT LIMITED TO VISUAL FLIGHT RULES (VFR) ONLY.
a. Application Process. Persons wishing to use multi-sensor equipment for operations limited to VFR only may obtain approval of the installation by Type Certificate (TC) or Supplemental Type Certificate (STC). For equipment that has already obtained installation approval via the TC or STC process, approval may also be obtained via data approved by the FAA (responsible Flight Standards District Office) on FAA Form 337 (Major Repair and Alteration). The approval for return to service must be signed by one of the entities noted in 14 CFR part 43; i.e., repair station, manufacturer, holder of an inspection authorization, etc.
b. Airworthiness Considerations. Multi-sensor equipment approved for VFR use only does not require TSO-C115b authorization, however it must at least meet the en route/terminal system accuracy criteria contained in paragraph 7a of this AC. Each individual sensor must also meet any additional performance criteria (besides accuracy) applicable to VFR
14 Par 8 only approval that is contained in the respective AC applicable to that type sensor. The installation verification should ensure, but is not limited to, the following:
(1) Electromagnetic Compatibility. The multi-sensor equipment installation does not interfere with the normal operation of other equipment installed in the aircraft.
(2) Environmental Conditions. The multi-sensor equipment is appropriate to the environmental categories (or criteria) in which it is installed.
(3) Equipment Mounting. The installation of the multi-sensor equipment, including antenna, must be sufficient to meet all structural mounting, dynamic, and emergency landing loads appropriate to the aircraft category. It is acceptable to use the criteria in AC 43.13-2A, “Acceptable Methods, Techniques, and Practices - Aircraft Alterations.”
(4) Navigation Source Annunciator. A navigation source annunciator is provided on or adjacent to each affected display if the multi-sensor equipment installation supplies any information to displays such as a horizontal situation indicator (HSI), course deviation indicator (CDI), distance display, electronic map, etc., that can also display information from other systems normally used for aircraft navigation.
(5) Computer Software. The AC 20-115B, which refers to RTCA/DO-178B, provides an acceptable means for showing that software complies with applicable airworthiness requirements. The applicant should substantiate software levels in the safety assessment. As an alternative to substantiating software level(s) in a safety assessment, the applicant may develop all software that contains or affects navigation and integrity functions to at least the Level D criteria, as defined in RTCA/DO-178B. The FAA recommends that the standards for IFR equipment contained in paragraph 8b(1) be followed if the system is intended to be upgraded to IFR use in the future.
(6) Failure Protection. Any probable failure of the multi-sensor equipment must not degrade the normal operation of other required equipment or create a flight hazard.
Likewise, normal operation of the multi-sensor equipment cannot adversely affect the performance of other aircraft equipment. The interfaces with other aircraft equipment must be designed such that normal or abnormal multi-sensor equipment operation does not adversely affect the operation of other equipment. Operation of other equipment shall not adversely affect the multi-sensor equipment operation.
(7) System Controls, Displays and Annunciators. All displays, controls, and annunciators must be readable under all normal cockpit conditions and expected ambient light conditions (total darkness to bright reflected sunlight). Night light provisions must be consistent with other cockpit lighting. All displays and controls should be arranged to facilitate equipment usage. Controls that are normally adjusted in flight shall be accessible and properly labeled as to their function. System controls and displays should be designed to maximize operational suitability and minimize pilot workload. System controls should be arranged to provide
Par 8 15 protection against inadvertent system turnoff. Reliance on pilot memory for operational procedures shall be minimized.
(8) Navigation/Integrity Annunciation. The multi-sensor equipment should indicate, independent of any operator action:
(i) The following conditions by means of a navigation warning flag on the navigation display:
(A) The absence of power required for the navigation function.
(B) Probable equipment malfunctions or failures affecting the navigation function.
(C) Loss of navigation function.
(ii) If an integrity monitoring function (such as GPS RAIM) is provided, an appropriately located annunciator shall be provided to indicate loss of the integrity monitoring function.
NOTE 1: For equipment not incorporating an external navigation display (CDI, HSI, etc.), complete blanking of the control display is acceptable.
NOTE 2: Presentation of a failure/status annunciation (flag or integrity annunciation) does not require removal of navigation information from the navigation display.
(9) Autopilot/Flight Director Coupling. The multi-sensor equipment may be coupled to an autopilot and/or flight director provided the multi-sensor equipment has a deviation or steering output that is compatible with the autopilot/flight director system and no unusual interface is required.
(10) VFR Limitation Placard. A placard stating “GPS not approved for IFR” must be installed in clear view of, and readable by, the pilot.
(11) Pressure/Barometric Altitude Inputs. If the multi-sensor equipment incorporates Class B1, B2, C1, or C2 GPS sensor(s) and uses a barometric pressure altitude input, that input must be appropriate to the equipment.
(12) Manufacturer’s Instructions. The multi-sensor equipment, including antenna(e), must be installed in accordance with the instructions and limitations provided by the manufacturer of the equipment.
c. VFR Airworthiness Approval. There are two types of VFR airworthiness approval which differ significantly as to test requirements and data analysis.
16 Par 8
(1) First-Time VFR Airworthiness Approval (for a Particular Type of Multi- Sensor Equipment). This type of approval refers to the first time an applicant presents a particular (hardware and software configuration) model of multi-sensor equipment integrating multiple navigation sensors for FAA airworthiness installation approval and certification for VFR navigation use. The first approval of particular multi-sensor equipment must be accomplished through the TC or STC process. Each new model of multi-sensor equipment or significant changes (hardware or software) to existing equipment shall undergo the same approval process as the original equipment unless it can be shown by analysis and tests acceptable to the Administrator that the new model will function as well or better than the approved equipment. A first-time approval is conducted in four phases:
(i) Lab/Bench Tests and Equipment Data Evaluation. This phase consists of the following:
(A) Review of the manufacturer’s procedures for development of software and review of supporting documentation in accordance with the guidelines of paragraph 8b(5) of this AC.
(B) Analysis of failure modes and annunciations.
(C) Review of reliability data to establish that all probable failures are detected and that hazardously misleading failures are shown to be improbable.
(D) Review of installation and maintenance manuals.
(E) Evaluation of the operator’s manual (pilot’s guide).
(ii) Aircraft Installation Data Evaluation. Normally the manufacturer of the multi-sensor equipment will provide an aircraft as a test bed for the first time installation approval. This approval may serve as a basis for subsequent installation approvals. The following assessments are made:
(A) Review of the equipment installation in the aircraft.
(B) Verification that the multi-sensor equipment is appropriate to the environment in which it was installed. (DO-160C testing is acceptable to meet this requirement.)
(C) Analysis of a data flow diagram in order to review which equipment provides what data to which other equipment.
Par 8 17 (D) Verification that the installation of the multi-sensor equipment, including antenna, is sufficient to meet all structural mounting, dynamic, and emergency landing loads appropriate to the aircraft category.
(E) Verification that a placard stating “GPS not approved for IFR” is installed in clear view of, and readable, by the pilot.
(F) Verification that the multi-sensor equipment does not interfere with the normal operation of other equipment installed in the aircraft. (May interface with IFR equipment.)
(G) Evaluation of the antenna installation. A critical aspect of many multi-sensor equipment installations is the installation of the antenna(e). It is important that the antenna for each sensor be one that is approved for the particular type (make and model) sensor installed.
(iii) Ground Test Evaluations. For multi-sensor equipment incorporating a Class B() or C() GPS sensor, static ground tests are conducted to verify the installed GPS equipment configuration (including antenna) provides position data meeting the accuracy criteria specified in paragraph 7a(1) of this AC. These tests shall cover a continuous period of 12 hours with a maximum sample interval of five minutes.
NOTE: The 12-hour ground accuracy test may be performed on the aircraft or by use of a representative mock-up configuration. If a mock-up test fixture is used, the entire installed multi-sensor equipment configuration, including antenna(e), must consist of the hardware to be used in the installation and be representative of the installed system configuration.
(iv) Flight Test Evaluations. Flight tests are conducted to verify proper operation and accuracy of the multi-sensor equipment as installed in the aircraft. Flight tests should include at least the following:
NOTE: Required flight evaluations for the first-time airworthiness approval of particular multi-sensor equipment are accomplished by the cognizant Aircraft Certification Office unless specific tests are delegated by the ACO to a flight test pilot designated engineering representative (DER).
(A) Evaluation of installed multi-sensor equipment to verify that it is functioning safely, and operates in accordance with the manufacturer’s specifications.
18 Par 8 (B) If coupled to an autopilot, evaluation of steering response while autopilot and/or flight director is coupled to the multi-sensor equipment during a variety of different track and mode changes. Additionally, all available display sensitivities shall be evaluated.
(C) Evaluation to verify the multi-sensor equipment installation does not adversely affect other onboard equipment (this test may be partially accomplished as a ground test).
(D) Evaluation of the accessibility of all controls pertaining to the multi-sensor equipment installation.
(E) Evaluation of the visibility of the controls, displays, and annunciators relating to the multi-sensor equipment installation during day and night lighting conditions. No distracting cockpit glare or reflections may be introduced, and all controls must be illuminated for identification and ease of use. Night lighting shall be consistent with other cockpit lighting.
(F) Validate multi-sensor equipment navigational accuracy in each operating mode (e.g. oceanic, en route, and terminal) for which approval is requested. In addition to overall system navigation performance, particular test requirements for navigational accuracy will vary depending upon the particular sensors integrated in the multi-sensor equipment and whether sensor accuracy performance data has previously been obtained. As an alternative to demonstrating the required accuracy with each specific sensor noted below, the multi-sensor equipment can be shown to have the requisite accuracy with various combinations of sensors.
(1) GPS sensor accuracy should be verified in each operating mode by at least 5 low altitude over flights of one or more surveyed locations (ensure survey point coordinates are relative to WGS-84 or NAD-83). An acceptable method of conducting this accuracy demonstration is to accomplish low altitude (less then 100 feet AGL) overflight of a runway threshold and record the GPS position as the aircraft crosses the threshold. The system accuracy is the distance between the coordinate position determined by the GPS and the coordinate position of the surveyed location (runway threshold). Runway threshold coordinates may be obtained from the airport operator or regional flight procedures office. If coordinate data conversion to WGS-84/NAD-83 is necessary, contact the National Flight Data Center at (202) 267-9277.
(2) Initial certification for systems including an Omega/VLF, Loran-C, VOR/DME, multiple (scanning) DME, INS or IRU sensor that has not previously been certified shall be based upon a demonstration of system accuracy by recording (at not greater than 15 minute intervals) the sensor position and comparing it to the actual
Par 8 19 position during evaluation flights representative of the area in which approval is desired. This flight test should be at least two hours in duration in order to provide a sufficient data set.
(2) Follow-On VFR Airworthiness Installation Approvals. This type of approval refers to installation approvals in any model or type of aircraft after a first time airworthiness approval of the particular multi-sensor equipment has been issued. Follow-on approvals may use the first time airworthiness approval, which was either a TC or an STC, as a basis for installation approval. Follow-on installation approvals may be accomplished by TC, STC, or data approved on FAA Form 337, if the flight standards inspector determines that there are no design changes as a result of the installation. The determination of a design change will be dependent upon the integration with other avionics and not on the basis of aircraft make and model. Since the GPS equipment operates on principles that are not dependent upon the aircraft make and model, the engineering evaluation of one aircraft model is applicable to a different aircraft provided the integration with the other equipment is equivalent.
The applicant or installing agency requesting a follow-on multi-sensor equipment installation utilizing this method of data approval should:
(i) Unless otherwise provided, contact either the manufacturer or organization responsible for obtaining the first time airworthiness approval of the multi-sensor equipment in order to:
(A) Obtain a sample airplane or rotorcraft flight manual supplement (or supplemental flight manual, if appropriate), if required for the aircraft.
(B) Obtain verification of the equipment approval status, including antenna, software, autopilot/flight director interface, and system integration requirements, etc.
(C) Discuss any problem areas and seek assistance in their solution.
(D) Verify that the maximum operating speed for which the multi-sensor equipment is qualified is compatible with the maximum expected ground speed of the aircraft.
(ii) Conduct a similar data evaluation as outlined in paragraph 8c(1)(ii) of this AC.
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(iii) Conduct a functional flight evaluation covering the items listed in paragraphs 8c(1)(iv)A through 8c(1)(iv)E of this AC. In addition, the flight test should demonstrate multi-sensor equipment navigational performance (including, as applicable to the sensors integrated in the system, Loran-C chain selection/switching, P-static protection, etc.) has not been affected by the installation in the aircraft.
20 Par 8 NOTE: Required flight evaluations for follow-on equipment installations approved via the Form 337 process may be conducted by the installer.
9. AIRWORTHINESS CRITERIA MULTI-SENSOR EQUIPMENT USED UNDER
INSTRUMENT FLIGHT RULES (IFR)
a. Application Process. Persons wishing to obtain approval of multi-sensor equipment integrating any combination of GPS, Omega/VLF, Loran-C, VOR/DME, Multiple DME, or INS/IRU sensors for IFR operations may do so via the Type Certificate (TC) or Supplemental Type Certificate (STC) process. For equipment approved via the TC or STC process, approval may also be obtained via data approved by the FAA (responsible Flight Standards District Office) on FAA Form 337 (Major Repair and Alteration). The approval for return to service must be signed by one of the entities noted in 14 CFR part 43; i.e., repair station, manufacturer, holder of an inspection authorization, etc.
(1) The initial (first-time airworthiness approval) certification of multisensor equipment requires extensive engineering and flight test evaluations and must be accomplished via the TC or STC approval process.
(2) Subsequent (follow-on) installations of the same multi-sensor equipment (hardware and software) in other aircraft are approved using a less extensive evaluation process since the basic engineering design has already been evaluated. Approval of follow-on installations may be accomplished via the TC, STC, or FAA Form 337 process. The extent of required evaluations depends upon the degree of integration of the navigation system with other aircraft systems, the similarity between the initial and follow-on aircraft models, and other changes that may have been incorporated in the multi-sensor equipment. The decision to allow an applicant to use FAA approved engineering data in support of an FAA Form 337 approval is left to the field inspector’s judgment. The FAA Airworthiness Inspector’s Handbook (FAA Order 8300.10) provides guidance applicable to multi-sensor equipment installations. Changes to software accomplishing navigation, integrity, or availability functions; changes in the number, type, or mix of sensors integrated in the system; changes to/addition of approved operating areas; or significant changes to operating limitations cannot be accomplished using the FAA Form 337 process and must use TC or STC procedures.
(3) Approval of multi-sensor equipment integrating a Class C() GPS sensor with any other combination of sensors for IFR operations must use the TC or STC process.
Because of the unique enhanced display and system integration requirements applicable to this integration of sensors and displays, appropriate test requirements and procedures must be determined based upon the particular application. These tests must establish the equivalent display capability of the installation and the display requirements for Class A() sensors.
Reference AC 20-138 for the specific requirements.
b. Airworthiness Considerations. Multi-sensor equipment to be approved for oceanic and remote, domestic en route, terminal, and non-precision instrument approach
Par 9 21 (except localizer, LDA and SDF) operations must meet the minimum navigation performance and operation standards for the appropriate equipment specified in TSO-C115b and this AC (including GPS equipment classes in TSO-C129 as referenced in TSO-C115b for multi-sensor equipment incorporating a GPS sensor). Additionally, each individual sensor must at least meet the navigation performance and operation criteria (besides accuracy) contained in the respective TSO and AC applicable to that type sensor.
(1) System Integrity and Software Development. Loss of navigation or flight management information is considered to be a major failure condition for the aircraft as defined in AC 25.1309-1A, AC 23.1309-1A, AC 27-1, or AC 29-2A, as applicable to the aircraft. Hazardously misleading information to the flight crew is also considered to be a major failure condition for the aircraft. Navigation data is considered to be hazardously misleading when unannunciated position errors exist that are greater than those specified by the multi-sensor equipment or individual sensor requirements (e.g., GPS position integrity performance requirements in Table 2-1 of RTCA/DO-208). The applicant should conduct a system safety assessment to verify that design errors and failure modes that produce major failure conditions are improbable.
(i) AC 25.1309-1A, AC 23.1309-1A, AC 27-1, or AC 29-2A, as applicable, provides an acceptable means for showing that the hardware complies with pertinent airworthiness requirements.
(ii) AC 20-115B, which refers to RTCA/DO-178B, provides an acceptable means for showing that software complies with pertinent airworthiness requirements. The applicant should substantiate software levels in the safety assessment. As an alternative to substantiating software level(s) in a safety assessment, the applicant may develop all software that contains or affects navigation and integrity functions to at least the Level C criteria, as defined in RTCA/DO-178B.
(iii) The cognizant Aircraft Certification Office may grant a deviation to TSO-C115a or TSO C-129 (Classes B and C) to permit the applicant to use the procedures of RTCA/DO-178B instead of RTCA/DO-178A.
(iv) If software was developed using RTCA/DO-178A procedures (as is specified in TSO-C115a), the applicant may need to further evaluate some features of the software. RTCA/DO-178A does not address some applications of digital technology commonly found in multi-sensor equipment (e.g., use of user-modifiable software including data bases, option-selectable software, software development and verification tools, previously developed software in modular architectures, and field loadable software capabilities). In these cases RTCA/DO-178A does not provide adequate procedures and the applicant must include in the software aspects of certification plan the means for showing that these features comply with pertinent airworthiness requirements. One acceptable means for demonstrating such features comply with the pertinent airworthiness requirements is to comply with pertinent
22 Par 9 portions of the criteria contained in RTCA/DO-178B, which would supplement the basic criteria contained in RTCA/DO-178A.
(v) The FAA strongly recommends that the applicant use the procedures described in RTCA/DO-178B (or later revision), as referenced in AC 20-115B, to show that the software aspects of a system comply with pertinent airworthiness requirements.
For software developed prior to the availability of RTCA/DO-178B, Section 12.1.4 of RTCA/DO-178B provides a method for upgrading a baseline for software development so that changes can be made in accordance with the criteria contained in RTCA/DO-178B.
(vi) If application specific integrated circuits (ASIC’s) or other complex logic devices are used, they must be developed using a structured process similar to that used for software developed under RTCA/DO-178B. If deterministic tests exhaustively consider all possible input values and verify that the ASIC completely meets all pertinent requirements, then a structured development process is not required. Either methodology should be approved in advance by the cognizant ACO.
(2) Display Format/Operating Procedure Changes. Changes to navigational display formats and navigation/function operating procedures (implemented through hardware or software) may constitute major changes requiring additional evaluation.
(3) Location of System Displays. Each display element (i.e., the course deviation indicator (CDI), horizontal situation indicator (HSI), map display, etc.), used as a primary flight instrument in the guidance and control of the aircraft, for maneuver anticipation, or for failure/status/integrity annunciation, shall be located where it is visible to the pilot (in the pilot’s primary field of view) 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 CDU will most likely not be acceptable for IFR operations.
NOTE 2: Flight technical error (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. The use of non-numeric cross track data integrated with non-numeric track angle error data into one display may provide the optimum of situation and control information for the best overall tracking performance.
(4) Failure Protection. Any probable failure of the multi-sensor equipment shall not degrade the normal operation of other required equipment or create a flight hazard.
Likewise, normal operation of the multi-sensor equipment installation shall not adversely affect
Par 9 23 the performance of other aircraft equipment. The interfaces with other aircraft equipment must be designed such that normal or abnormal multi-sensor equipment operation shall not adversely affect the operation of other equipment nor shall operation of other equipment adversely affect the multi-sensor equipment operation.
(5) Environmental Conditions. The aircraft environment in which the multi-sensor equipment is installed shall be found to be compatible with the environmental categories (or criteria) in RTCA/DO-160C to which the equipment was tested.
(6) Electromagnetic Compatibility. The multi-sensor equipment installation should not be the source of objectionable electromagnetic interference, nor be adversely affected by electromagnetic interference from other equipment in the aircraft.
(i) For systems incorporating a GPS sensor, the GPS equipment should be shown to meet the 2D accuracy requirements of paragraph 7a(1) of this AC when subjected to a radiated signal with continuous wave (cw) modulation at a frequency of
1.57542 GHz and an electric field strength of 20 mv/meter measured at the exterior case of the GPS receiver. The radiated susceptibility test procedures of RTCA/DO-160C, Section 20, should be followed when conducting this test. The test should be conducted with simulated satellite inputs and should not result in the loss of track of any satellite used for navigation.
The duration of the test must be sufficient to determine if tracking has been lost (20 seconds should normally be long enough, depending on the coasting features used by the GPS equipment). This test will usually be conducted by the GPS equipment manufacturer, but if not, the applicant is responsible for conducting the test.
(ii) For systems incorporating a GPS sensor, intermodulation effects are possible between multiple channel SATCOM installations and GPS. GPS equipment should not be installed in aircraft with multiple SATCOM channels unless the SATCOM equipment is modified to prevent simultaneous use of interfering frequencies. Current multichannel SATCOM equipment (as of 1/1/94) has not been modified in this manner and should not be installed with GPS equipment. In addition, certain GPS manufacturers have been granted a deviation to the requirements of TSO-C129 and will include a limitation in their installation instructions that states that the GPS equipment can not be installed in aircraft with SATCOM equipment. If such GPS equipment is installed in an aircraft the TC, STC, or FAA Form 337 should appropriately limit the future installation of SATCOM equipment in the aircraft.
(iii) For systems incorporating a GPS sensor, harmonic interference from VHF transmissions on 121.150, 121.175, 121.200, 131.250, 131.275, and 131.300 MHz may adversely affect reception of the GPS signal if less than 100 dB isolation is provided.
Low pass or notch filters installed at the output of the VHF transmitter to attenuate the undesired VHF signal should have an insertion loss of 2 dB or less to preclude the need for reevaluation of installed VHF transceiver performance.
24 Par 9
(7) P-Static Protection. If an E-Field antenna (whip, plate, or blade type) is used (such as with an Omega/VLF or Loran-C sensor), the aircraft should be protected by acceptable bonding techniques and installation of static dischargers. These protective devices should be specified as part of the approved design data for the multi-sensor installation. The capability to provide satisfactory P-static protection for the system should be demonstrated as part of the initial certification program. This testing may be accomplished by ground or static testing if sufficient data is provided to demonstrate that the proposed technique is equivalent to flight testing. If a flight demonstration is selected it must be conducted at speeds up to Vne, Vmo, or Mmo through known P-static conditions such as a cloud of ice crystals.
Momentary loss of signal when encountering heavy P-static conditions may be acceptable provided the equipment is capable of providing acceptable navigation information during such conditions. P-static charging of the aircraft can cause degradation of the signal-to-noise ratio by one of three major mechanisms:
(i) Sparkover of Isolated Metal Panels. Sparkover of isolated metal panels can be handled by appropriate bonding. This bonding needs to occur on all control and trim surfaces as well as isolated access panels. Bonding should be evaluated by a careful ohmic survey (an electrical bonding limit of 10 milliohms is considered acceptable) of techniques.
(ii) Streamer currents. The effects caused by streamer currents can be reduced by placing the receiving antenna as far as possible from any nonconductive surfaces such as windshields. The nonconductive surfaces may be coated with a conductive coating.
Temporary spray coatings are not satisfactory.
(iii) Corona discharge. Corona discharge can be reduced by the appropriate placing of orthodecoupled static dischargers on the extremities of the aircraft. A number of recent studies have shown that the frayed-wick types of dischargers constructed with a high resistance rod and metal pins are recommended although other types may also be used if they can demonstrate ability to provide protection from radio frequency (RF) coupling to the sensor antenna. The number, type, and location of these static dischargers to be installed on a particular aircraft model should be determined by following the instructions provided by the manufacturer of the static discharger for P-static protection.
(8) Anti-Ice Protection. If the aircraft in which the multi-sensor equipment is installed is approved for flight into known icing conditions, any antennae must have anti-ice protection or be found not to 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 provided there are no harmful effects, such as possible ingestion of accumulated ice or degradation in aerodynamic performance. (The effects of ice accumulation on the antenna, if any, can be found in the manufacturer’s installation instructions.)