a. Acceptable Means.
(1) GPS. TSO-C129(AR) defines an accep table means of com pliance for GPS equipment.
(2) GPS/WAAS. TSO-C146a defines an acceptable means of com pliance for GPS/WAAS equipm ent. RTCA /DO-229C is a com plete set of standards for GPS and GPS/WAAS and should not be used in conjunction with TSO-C129(AR).
Note: In a ddition to the senso r im provements, there are a num ber of differences between TSO-C129(AR) and TSO-C146a, most notably the compatibility of TSO-C146a equipm ent with RNP RNAV operations, m ore explicit pilot interface requirem ents (see RTCA/DO-229C, section 2.2.1.1), and a Vector-to-Final capability (see RTCA/DO-229C, section 1.8.2.4).
(3) GPS/WAAS/LAAS. TSO-C146a defines an acceptable m eans of compliance f or this e quipment when ope rating in the GPS/ WAAS m ode. When operating in the GPS/L AAS mode and conduc ting a GLS approach, the equipm ent must comply with the requ irements of section 2 of RTCA/DO-253A. When operating in the GPS/LAAS mode and not conducting a GLS a pproach, the equipment must comply with the sensor requirem ents of section 2 of RTCA/DO-253A (for PVT) and the navigation requirements of RTCA/DO-229C (section 2.2).
(4) GPS/LAAS. TSO-C129(AR) defines an accep table m eans of com pliance for this equipm ent when operating in GPS m ode (i.e., outside the coverage of a selected LAAS ground facility). W hen operating in the GPS/LAAS mode and conducting a GLS approach, the equipment m ust com ply wi th the requirem ents of section 2 of RTCA/DO-253A. W hen operating in GPS/ LAAS m ode and not conducting a GLS approach, the equipm ent m ust comply with the sensor requirem ents of section 2 of RTCA/DO-253A and the navigation requirem ents of TSO-C129a, Class A1 or A2 (except the RAIM requirements of paragraph (a)(3)(xv)).
b. Evaluation of Human Factors.a
(1) The applicant is encouraged to involve the ACO as early as possib le in the evaluation of the pilot interface.
(2) Controls.
(i) Appropriate feedback must be supplied during data entry, with confirmation of input action prior to activ ation based upon that input. Any equipm ent prompts must be easily understood. A suitable interface should be provided to allow data input, data output, and control of equipment operation.
(ii) Particular attention should be pl aced on the controls required to perform ope rations that occur with a high fr equency or that m ust be conducted under potentially stressful or busy conditions su ch as term inal area operations and m issed approaches. The workload associated with accessing the Direct-T o function, entering a Hold m ode, executing a m issed approach, and initiating an approach to a dif ferent runway at the sam e airport (or to the a lternate airpor t) f ollowing a m issed approach should be minimized.
Par 13 Page 21
(iii) Controls intended for use in flight should be designed to m inimize errors and, when operated in all possible com binations and sequences, should result in a condition whose presence or continuation woul d not be detrim ental to the continued performance of the equipm ent. Controls should be designed to m aximize operational suitability and m inimize pilot workload. The a mount of force required to activate knobs/buttons should be acceptable, feedback to the pilot should be adequate, and risk of inadvertent activation or deactivation should be minimized. Knob shape and size sho uld not interfere with equipm ent use and should he lp distinguish controls. Reliance on pilot memory for operational procedures should be m inimized. For those procedures that require pilot memory items, the memory item steps should be minimal and as intuitive as possible. The control/display should be operable with the use of only one hand.
(iv) Controls that are norm ally adjust ed in flight should be readily accessible and properly labeled as to their fu nction. Co ntrols th at are no t no rmally adjusted in flight shou ld not b e readily acces sible to th e flight crew . Labels sh ould be horizontally oriented, unobstructed by use of controls, descriptive of control function, and consistent across the equipm ent. The use of abbreviations should be minim ized. When abbreviations are used, they should conform to section 2.2.1.1.5 of RTCA/DO-229C.
(v) Controls sh ould be arranged logica lly acco rding to function al groups, sequence of use, and frequency of use. All displays and controls must be arranged to facilitate equipment usage.
(3) Messages.
(i) Standard pilot terminology should be used and their m eaning should be readily understood.
(ii) Messages should indicate what acti on should be considered or taken and should em ploy the m inimum num ber of wo rds required to conv ey its m eaning.
Messages or annunciation of m essages not rele vant to the execution of an approach, landing, rollout, or m issed approach shoul d be suppressed during the approach.
Messages not requiring pilot action during fl ight should not be sh own during flight.
Messages should be operationally relevant to current or upcom ing phases of flight. P ilot recall of m essages from the m essage storage queue should bring up the m ost important messages first. The hierarchy of messages should be determ ined by the im pact of the message on the saf ety of the f light f or th e current and upcom ing phases of flight.
Presentation of messages should not obscure safety-critical portions of guidance displays.
Messages requiring prompt pilo t action should stay up until action is taken or the pilot cancels the message. Safety-critical messages should be accompanied by an aural alert if that alert is not already provided by anothe r aircraft system. Messages of operational significance to the flight should not time out.
(4) Prompts. Where possible, prom pts should specifically indicate what programming action should be done next (e.g., “enter next waypoint”). Where possible, prompts should indicate the sp ecific control that should be us ed next, rather than relying on pilot m emory. Multiple prom pts requiri ng different actions should be presented sequentially rather than concurrently.
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(5) Cross-Track Deviation Display. Minimum discernible movement, accuracy of the cen tered display, resolution of the e lectrical output, linearity of the display and/or electrical output, and display latency sh ould be as specified in sectio n 2 of RTCA/DO-229C for a pplicable navigation m odes. Deviation display designs with display ranges and resolutions outside the bounds of the required values should be substantiated by dem onstration, by referen ce to appropriate em pirical data, or by similarity to previous certified range/resolution displays.
(6) For TSO-C146a equipm ent, the flight crew workload requirem ents in section 2.2.1.1.3 of RTCA/DO-229C are particul arly im portant and compliance to the requirements should be verified. The m anufacturer must also have specific and detailed rationale for deviating from any of the recommendations defined in Table 2-4 of RTCA/DO-229C.
(7) For TSO-C146a equipm ent, the manufact urer must identify any acronym s that do not com ply with the recomm endation in RTCA/DO-229C, section 2.2.1.1.7 for the terms listed. A specific and detailed rationale for deviating must be documented.
(8) For TSO-C146a equipm ent, complianc e to the requirem ents should be demonstrated by the m anufacturer under the test procedures specified in section 2.5.11.3 of RTCA/DO-229C. Som eone not fa miliar w ith the d evelopment of the equip ment should cond uct these tests to p roperly evalua te the pilot in terface, docum entation, and training information. This person should have minimal experience with the equipm ent, other than the opportunity to review the docum entation and training m aterials to be supplied with the equipment.
c. Navigation Database. For area navigation equipm ent, the navigation database i s an essential com ponent of th e ability of the equipm ent to perform its intended function.
Guidelines pertaining to the navigation da tabase can be found in RTCA/DO-200A. The manufacturer should pay particul ar attention to the specifica tion of the form at of data loaded into the navigation database, as described in RTCA/DO-200A, Appendix C.
(1) Database Process. The equipm ent manufacturer is usu ally the la st link in the aeronau tical data processing ch ain, as the for mat for t he data loaded in the final database is typically pro prietary. The applic ant for a TSOA must def ine the data qua lity requirements for the database, and there m ust be an aeronautical da ta process which will be accessible to the equipm ent users. RTCA/DO-200A provides an acceptable m eans of compliance for defining the data quality re quirements, and for the aeronautical data process.
(2) Reference System. The operations manual or A/RFM(S) should explain that GNSS navigation is referenced to the W GS-84 coordinate system. The following text is recommended: “Navigation information is re ferenced to WGS-84 reference system , and should only be used where the Aeronautical Information Publication (including electronic data and aeronautical charts) conform to WGS-84 or equivalent.”
(3) Government-Provided Electronic Database. Electronic databases sponsored by a governm ent organization should follow the same database assurance processes as described above and in RTCA/DO-200A. GNSS equi pment compatibility w ith government-provided electronic databases must be verified.
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d. Conversion of data for WAAS L NAV/VNAV approaches . As des cribed in section 2.2.4.3.1 of RTCA/DO-229C, a final approach segm ent (FAS) datablock is not provided for standalone L NAV/VNAV approaches. The vertical path for such approaches is defined by the threshold loca tion, threshold crossing height, and glidepath angle. However, the altitude of the threshol d is published as the height above m ean sea level (MSL ), rather than as the height above the W GS-84 ellipso id. Therefore, the published MSL height (interpreted as a he ight above the W GS-84 geoid) m ust be converted to height above the WGS-84 ellipsoid prior to bein g used. This conversion is not considered alteration of data per RT CA/DO-200A section 2.4.2, since it changes the reference system but not the actual vertical location. It is recom mended that this conversion be accomplished within the GNSS equipment itself so that there are consistent navigation database requirem ents for GN SS LNAV/VNAV equip ment and baro metric VNAV equipment.
14. EQUIPMENT PERFORMANCE – LAAS VDB RECEIVER.
a. Section 2 of RTCA/DO-253A defines an acceptable m eans of com pliance for LAAS VHF data broadcast receivers.
b. The receiver should be tested using the te st procedures specified in section 2.5.2 of RTCA/DO-253A.
15. INSTALLATION ISSUES – GENERAL . The following issues apply to the installation of all GNSS equipment intended for IFR use.
a. Failure Classification.
(1) From en route and term inal operations , the loss of navi gation function is typically considered to be a major failure condition for th e aircraft (see AC 25.1309-1;
AC 23.1309-1; AC 27-1; or AC 29-2 as appli cable to the aircraft ). Other aircraft navigation system s should be considered when determ ining the loss of the GNSS navigation function, which could be m ajor (no other navigation systems) or minor (other applicable navigation system s). GNSS naviga tion data is c onsidered to be m isleading when unannunciated po sition errors exis t. For en rou te, term inal, an d non-precision approach (NPA) and LNAV/VNAV approaches, presenting misleading information to the flight crew is also considered to be a m ajor failure condition for the aircraft. For LPV and GLS approaches, presenting m isleading information to the f light crew is conside red to be a hazardous failure condition. Thes e failure classifications are summarized in Table 3.
Table 3. Typical Hazard Classifications
En route/ Terminal Area/ NPA (LNAV) NPA with Vertical Guidance (LNAV/VNAV) LPV Approach GLS Precision Approach (Cat. I) Loss of Navigation Major Major Major Major Misleading Information Major Major Hazardous Hazardous Page 24 Par 13
(2) The applicant m ust conduct a safety assessm ent of the GNSS equipm ent installation to v erify tha t des ign erro rs and failure modes m eet th e prob ability requirements for that failure class. AC 25.1309-1, AC 23.1309-1, AC 27-1, or AC 29-2, per the latest revision levels as applicable, provide an acceptable m eans for showing that the hardware complies with pertinent airworthiness requirements.
b. Software Considera tions. AC 20- 115, curren t edition, d efines an acceptab le means of qualifying so ftware. The applicant is encouraged to subm it the Plan for Software Aspects of Certification (PSAC) ear ly in the software developm ent process.
Early submittal will a llow tim ely resolution of issues such as partition ing and determination of software levels.
c. Installation Instructions.
(1) The equipment m ust be installed in accordance with the in structions and limitations provided by the manufacturer of the equipment.
(2) Equipment should be inst alled with separation be tween wires of redundant equipment (see AC 43.13-1B).
d. Environmental Considerations.
(1) Most equip ment will have been tested by the m anufacturer to the environmental categories described in RTCA /DO-160(AR), with resu lts described in an environmental qualification form . For this equipment, the environm ental categories (or criteria) to which the e quipment has been te sted m ust be com patible with the a ircraft environment in which the GNSS equipment is installed.
(2) For equipm ent which has not been test ed to the environm ental ca tegories described in RTCA/DO-160(AR), comparable testing must be conducted and a thorough definition of the aircraft environm ent must be developed through testing of the norm al and abnorm al environm ents. At a m inimum, the environm ental perform ance requirements identified in section 2.4 of RTCA/DO-229C m ust be m et for all components in the airborne GNSS equipment.
16. INSTALLATION ISSUES – GPS ANTENNA . This paragraph provides guidance related to the installation of a GPS antenna.
a. Antenna Placement.
(1) The antenna installation should m inimize the potential for signal blockage by any portion of the aircraft during airc raft m aneuvering. Shadowing by aircraft structure can adversely affect the operation of the GNSS equipm ent. Typically, a GNSS antenna is located forward or aft of the wings on the top of the fuselage.
(2) The installer m ust ensure that a 5 degree mask angle (or lower if the receiver ins tallation ins tructions id entify a lower m ask angle) is provided by the installation. This includes consid eration of antenna location and blockage, with the aircraft at typical cruising attitude. The antenna should also be located to m inimize the effects of airplane shadowing during typical maneuvers.
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(3) The antenn a should be separated as m uch as practical from transm itting antennas (e.g., VHF, TCAS, SATCOM, and HF). For small aircraft, the antenna should also be separated as m uch as practical fr om the windscreen to pr event case-to-antenna coupling.
(4) For insta llations on rotorcraft, th e e ffects of the rotor b lades on antenna performance must be considered.
(5) For multiple-sensor installations, th e installer should reduce the likelih ood that a single lightning strike affects all the sensors (i.e., do not plac e them in a stra ight line from the front to the rear of the fuselage).
b. Anti-Ice Protection. If the aircraft is approved for flight into know n icing conditions, the antenna must not be susceptible to ice buildup (i.e., is installed in a nonicing location on the aircraft, or is of a suf ficiently lo w profile that ice does not accumulate on the antenna). Alternatively, the equipm ent 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 possible ingestion into the engine of accumulated ice or degradation in aerodynamic performance. The effects of ice accumulation on the antenna, if any, can be found in the m anufacturer's installation instructions. See AC 23.1419-2A and AC 25-1419-1 for additional guidance.
c. Antenna Cables. Double shielded cables should be used to prevent interference coupling into the cable.
d. Structure. Any m odifications to th e aircraft to insta ll a ne w antenna n eed to be evaluated f or th eir im pact on aircraft s tructure. Fo r s pecific gu idance relatin g to modifications in the f uselage s tructure f or Part 23 aircraft, see AC 23-5, Cutouts in a Modified Fuselage of Small Airplanes.
17. INSTALLATION ISSUES – GNSS SENSO R. This paragraph provides guidance related to th e installation of a GPS sensor , including those sensors that integrate WAAS or LAAS. This paragraph also applies the insta llation of st and-alone navigation equipment, since that equipm ent has an em bedded sensor. This guidance applies to all installations, but is only expected to be sp ecifically ev aluated as part of the in itial airworthiness approval.
a. Antenna/Sensor Compatibility.
(1) Using the guidelines in the installati on instructions, verif y the antenna installation is compatible with the G NSS equipment. This m ay be accomplished by one of the following:
(i) The antenn a (pa rt nu mber) is id entified in the GNSS equipm ent installation instructions as compatible;
(ii) The GNSS equipm ent installation instru ctions state that the equipm ent is com patible with any standard, passive TSO-C144 antenna. In this case the antenna should have authorization to TSO-C144. Compare the RTCA/DO-160(AR) category for lightning-induced tr ansients spe cified in the a ntenna in stallation in structions with the category defined in the receiver installation instructions; or Page 26 Par 16
(iii) The GNSS equipm ent installation instru ctions state that the equipm ent is compatible with any standard, active TSO-C144 antenna. A signal power budget must be defined that takes the output power at the low-nois e am plifier of the antenn a (a s defined in the antenna installation instructi ons) and matches that to the receiver dynam ic range and sensitivity (as de fined in the GNSS equipm ent installation instructions).
Compare the RTCA/DO-160(AR) category for light ning-induced transients specified in the antenna installation instructions with the category defined in the receiver installation instructions.
(2) Use the gu idelines in the ins tallation instructions when d etermining what cables and connecto rs to use to co nnect th e antenna to the receiver, includ ing cable lengths that result in acceptable loss of signal power.
(3) Ensure antenna and GNSS equipm ent co mpatibility for aircraft equipped with SATCOM. Som e equipment may have been qualified to less stringent interf erence levels for aircraft installations without SATCOM.
b. Interference - Electromagnetic Compatibility (EMC).
(1) The equipm ent must not be the s ource of objectionable conducted or radiated interference or be a dversely affected by conducted or radiated interference from other equipment or systems installed in the aircraft.
(2) Proper grounding of GNSS and other eq uipment is es sential to ensu ring EMC.
(3) When possible, do not install the sensor near a VHF radio.
(4) The following paragraphs identify poten tial so urces of interference and means of mitigating that interference:
(i) SATCOM. Out-of-band SATCOM e missions and in-band intermodulation between m ultiple channel SA TCOM installations can affect GNSS.
GNSS equipm ent m ust not be ins talled in aircraft with multiple SA TCOM channels unless the sim ultaneous use of i nterfering frequencies can either be prevented or demonstrated not to interfere with the operation of the GNSS equipment. Note that some equipment is not compatib le with SATCOM installations at all, as noted in the installation instructions.
(ii) VHF Communications. Harm onic, s purious, and local oscillator harmonics from VHF equipm ent can cause inte rference to GNSS. In terference can be mitigated by:
(A) Insta lling f ilters at the output of th e VHF tran smitter to p revent antenna-to-antenna interference. S uch filters s hould have an insertion loss of 2 d B or less, or the installed VHF transceiver performance will have to be re-evaluated.
(B) Installing th e GNSS equipm ent as far away as feasible from any VHF transmitter equipment (to prevent case-to-case interference).
(C) Replacing the VHF equipment.
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(iii) ELT. An ELT can re-radiate DME or VHF signals that can interfere with GPS. Notch filters on the ELT antenna cable or rep lacing the EL T can elim inate this problem.
(iv) DME. DME has been known to cause interference to GPS. Replacing the DME transceiver can eliminate the problem for some equipment.
(v) Direction Finding (DF) equipment. Moving the DF antenna to the b elly of the aircraft has been found to eliminate this problem.
c. Interface Requirem ents. Ensure sensor compatib ility f or airc raft system s utilizing the position output (e.g. flight m anagement system , autom atic dependent surveillance system). Systems should use th e same bus standard (e.g., ARINC 429 with ARINC 743 data, RS-232 with RTCA/DO-229 Appendix H data).
(1) Operation of the GNSS equipm ent m ust not adversely affect th e performance of other aircraft equipm ent. Likewise, operation of the GNSS equi pment must not interfere with the operation of other equipment installed in the aircraft.
(2) Additional inf ormation on inte rfacing a GNSS sensor with a m ulti-sensor system can be found in AC 20-130A.
(3) GNSS sensors m ay provide output to a variety of applications, including terrain awareness and warning s ystems (T AWS), autom atic dependent surveillance (ADS), and m oving m ap displays for situati onal awareness. The guidance m aterial in this AC for sensors applies to all of these integrations, and should be used in conjunction with the gu idance m aterial f or th e applicat ion. For spe cific inf ormation rela ting to TAWS equipm ent, see TSO-C151a. G PS, GPS/ WAAS, or GPS/LAAS posi tion information m ay be used as the position determination as addressed in Appendix 1, paragraph 5 of TSO-C151a and paragraph 11 of AC 25-23. Guidan ce for electronic m ap displays can be found in RTCA/DO-257A, Minimum Operational Performance Standards for the Depiction of Navigation Inform ation on Electronic Maps . Guidelin es f or interfacing with ADS-broadcas t equipment can be found in RTCA/DO-242A, Minimu m Aviation System Perfor mance Standards for ADS-B. Failure of the GNSS equipm ent should not cause failure of the interfaced equipment.
d. GPS Equipment (with or without LAAS) . Equipm ent that uses TSO-C129(AR) as the bas is for the GPS f unctions has a lim ited ability to meet navigation requirements.
This limitation should be docum ented in the A/ RFM(S) as follows: “T he aircraft mus t have other approved navigation equipm ent in stalled and operating appropriate to the route of flight”.
18. INSTALLATION ISS UES – S TAND-ALONE NAVI GATION EQUIPME NT.
This paragraph provides guidanc e rela ted to the installati on of stand-alone navigation equipment. In additio n to the guidance in this paragraph, stand-alone navigation equipment incorporates an antenna and a sens or and should comply with the guidance in paragraphs 16 and 17 of this AC. This guidan ce applies to all ins tallations, but is only expected to be specifically evaluated as part of the initial airworthiness approval.
a. GNSS Configuration Strapping. Often, GNSS equipm ent uses software configuration straps or installation set parameters entered either manually by the installer Page 28 Par 17
or by using an Aircraft Persona lity Module (APM). These so ftware configuration straps or parameters disable/enable functionality, configure the in put/output formats, determine the displayed units of m easure, and m ore. The software configuration programm ing documentation must become a part of the inst allation package and shou ld be included in the Master Drawing List. W hen using an Approved Model List (see paragraph 8), multiple configurations may be specified.
b. General Human Factors Considerations.
(1) Control Accessibility. Controls inst alled for in-flight operation should be readily accessible from the pilot's seated position. Only single-hand operation should be required, the controls should be readily identifiable, and the use of c ontrols should not obscure pertinent displays.
(2) Inadvertent Turnoff. System contro ls should be arranged to provide adequate protection against inadvertent system turnoff.
(3) Control/Display Visibility.
(i) The appropriate flight crew m ember(s) should have an unobstructed view of displayed data when in the seated position.
(ii) Compliance with lim itations in the installation instructions for display readability should be verified.
(iii) All displays, controls, and annunciato rs must be easily readable under all norm al cockpit conditions and expected am bient light c onditions (total darkness to bright, reflected sunlight).
(4) Annunciations.
(i) Visual annu nciations m ust be cons istent with the c riticality of the annunciation and m ust be readable under a ll norm al cockpit illum ination cond itions.
Visual annunciations must not be so bright or startling as to reduce pilot dark adaptation.
(ii) Audible alarm s should be sufficien tly loud and of a ppropriate pitch quality, duration and pattern. Alarm s shoul d be easily deactivated (but not easily deactivated inadvertently). Audible alarm s m ust not in terfere with o ther aircraft audio alarms or alerts of higher priority (e.g. stall warnings, gear warning, or chip-detect).
c. Source Selection. For installations where GNSS outputs can drive a display that is shared with other navigation equipm ent (e.g., VOR/DME, ILS, MLS), the pilot must be able to select the source of guidance, an d the selected source must be clearly annunciated to the flightcrew, on or near th e affected display. GNSS- based navigation (GPS, GPS/WAAS, GPS/LAAS) need not be dif ferentiated. W hen applica ble, annunciation of the selected source should be driven by the same relay switch to preclude erroneous information in case of switch failur e. Autopilot o peration should be inhib ited if the displayed source is not the sam e as the source driving the autopilo t. This is particularly critical for approach operations (i.e., autopilot in approach mode).
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d. Navigation Display.
(1) The horizontal (and vertical) deviation(s) display(s) and failure annunciation should be located within the pilot' s prim ary field of view, as should any indication requiring immediate aircrew action. For the pur pose of this AC, the primary field of view is within 15 degrees of the pilot’s primary line of sight.
(2) Displays used for loss of integrity m onitoring, waypoint sequencing, start of a turn, turn anticipati on, active waypoint, di stance to active waypoi nt, desired track and actual track (track angle e rror), TO/FROM indication, appr oach mode annunciation, and automatic mode switching should be located w ithin the pilot’s normal field of view. The normal field of view is such that the pilot w ould notice an annuncia tion during norm al aircraft operation. Guidelines for the norm al field of view include: the lateral normal field of view is from the center of the airspeed indicator to and including the equipment if installed in the center radio stack. Alternatively, if the equipment is installed to the left of the airspeed indicator, the lateral normal field of view is the center of the altimeter to and including the equipm ent. The vertical norm al field of view incl udes immediately above and below the basic “T ” instruments (may also be anywhe re with in the lateral field of view).
e. Failure Indications.
(1) It is important to install the equ ipment such that all f ailure conditions result in appropriate displays.
(2) For installations not incorporating an external navigation display (CDI, HSI, etc.), complete blanking of the Control Display Unit (CDU) display is acceptable.
(3) Presentation of a failure/status annunc iation (flag or inte grity annunciation) does not require removal of navigation information from the navigation display.
(4) A unique annunciation of the loss of integrity m onitoring should be provided.
f. Dual GNSS Installations.
(1) Synchronization. Synchronization refers to the ability for control inputs to one RNAV equipm ent to autom atically update the other equipm ent. Dual installations should be synchronized whenever possible to reduce crew workload and prevent confusion over which system/flight plan is driving the naviga tion data. If the Functional Hazard As sessment requires du al GNSS or is o perationally required, then synchronization is required. If the navigation equipm ent are not synchronized, then the following issues should be evaluated:
(i) The pilot and co-pilot (if applicable) should be able to view and enter data in the of fside equipment. This m ust not interfere with throttle le vers, flap handle, etc.
(ii) The workload associated with m anually updating both equipm ent to maintain consistency between them should be evaluated.
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(iii) There should be no possible confusi on as to the equipm ent that is driving the autopilot and navigation displays.
(iv) Displays and annunciations should be evaluated for possible inconsistencies that result from diffe rences in the equipm ent (e.g., different flight plans could result in one equipment going into approach mode while the other does not). These inconsistencies m ust no t cau se a misleading situation. Thes e are of particu lar concern when the co ckpit architecture allows for cr oss coupling (e.g. GNSS- 2 switched to drive side 1 displays).
(2) Intermixing of RNAV equipm ent. W here multiple RNAV systems are intended to be used simultaneou sly (typically in two-pilot a ircraft), ins tallation of area navigation equipm ent with different crew inte rfaces can be very confusing and lead to problems since they m ay have conflicting m ethods of operation and conflicting display formats. There can be problem s even when interm ixing different versions of the s ame equipment. At a m inimum, considerations should be given to the following potential incompatibilities, especially wh en the cockpit arch itecture allows for cross coupling (i.e.
GNSS-2 switched to drive side 1 displays).
(i) Data entry: The equipment m ust have consistent methods of data entry, and similar pilot procedures for accom plishing common tasks. RTCA/DO-229C section
2.2.1.1.3 identifies a minim um set of comm on ta sks that should be considered. Any differences should be evaluated for pilot workload. If the wrong procedures are used (the procedures for data entry on the offside are used), th ere m ust be no m isleading information and it should be easy to identify and recover from the mistake.
(ii) CDI scaling: Sensitivity m ust be consistent or annunciated. Standalone nav igation equip ment that com plies with RTCA/DO-229C will have cons istent scaling provided the flight plans be tween the equipm ent are identical (see synchronization issues above).
(iii) Display symbology and m ode annunc iations: There should be no conflicting sym bols or annuncia tions (e.g., a common symbol used for two different purposes), and differences should be specifi cally evaluated to evaluate the potential confusion they may cause.
(iv) Mode logic: The modes internal to the equipment and interfaced to the rest of the aircraft should be consistent.
(v) Equipment f ailure: The ef fect of fa ilure of one equipm ent m ust not result in misleading information on the other equipment.
(vi) Displayed data: The display of primary navigation param eters (waypoint distance, bearing, de sired track/actual track, track angle error, num eric crosstrack deviation) must use consistent units and a consistent notation. Any inconsistency in the display of the primary information should not be approved.
(vii) Database differences: Di fferences i n the area navigation database and RNAV equipment response can resu lt in contradictory guidance between two dissim ilar types of RNAV equipm ent. A m eans of ensuring compatible updates between the equipment must be defined and documented in the operations manual or A/RFM(S).
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g. Interface to Flight Guidance System (FGS). This paragraph describes guidelines for interfacing the GNSS stand-alone naviga tion equipment with a flight guidance system. These issues may be addressed th rough operating lim itations in the A/RFM(S).
When used in this paragraph, the word “consistent” m eans that the installed GNSS equipment does not alter the cockpit desi gn philosophy or intended function of an FGS mode, switch or annunciation.
(1) Connection to the autopilot must be consistent with the autopilot installation instructions and the GNSS installation instructions.
(2) Operational com patibility be tween the GNSS equipm ent and th e F GS should be verified. Protection should exist in the GNSS or the FGS to keep the aircraft within the approved operational env elope (e.g. roll, pitch, speed, and altitud e). See the relevant guidelines for the particular aircraft (e.g., AC 23-17A and AC 25.1329).
(3) A positive, continuous and unam biguous indication should be provided of the FGS modes actually in operation, as well as those that are arm ed for engagement. In addition, where engagement of a m ode is automatic, clear indication must be given when the m ode has been arm ed by ei ther action of a m ember of the flight crew, or automatically by the system.
(4) The GNSS equipment must be compatible with the FGS modes of operation.
For example the arm, engage and disengage sequence of the GNSS equipm ent should be consistent in the annunciation and engage/d isengage tim ing of the FGS. This is particularly im portant during W AAS approa ches, which include different levels of service (reference section 2.2.4.7.4 of RTCA/DO-229C).
(5) If the GNSS equipm ent is connected to a reduced bank selector, the GNSS bank angle, like the FGS, m ust not be lim ited by this switch selection during approach.
Consistency in the operation must be maintained.
(6) Some aircraft operate in areas of th e operational flight envelope where buffet onset can occur when norm al maneuvering load factors are pr esent (e.g. at higher altitudes). This is a co ncern, s ince the GNSS equipm ent can autom atically sequ ence waypoints and initiate turn maneuvers which may or may not have the pilots attention. If buffet protection is no t provided for in th e GNSS or FGS (e.g. bank an gle limiting, or limiting of cruise altitude based upon buffet margin), an A/RFM(S) limitation should be added to the performance section, prohibi ting cruise operation at weight/a ltitude combinations that result in buffet margins less than 1.4g.
(7) For those aircraft that have incorpor ated bank angle lim iting, consideration should be given to whether the GNSS equipment is defining a path that cannot be flown.
h. Interface to Magnetic/True Switch. If a MAG/TRUE heading reference switch is installed in the aircraft, the GNSS equipm ent should be driven by the sam e switch to maintain consistency in the displays and operation for both m anual and automatic heading reference changes.
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